King Sun PCB

conformal coating

Why PCB Need Conformal Coating

Conformal coating is a specially formulated coating used to protect the circuit boards from environmental erosion. Conformal coating has good resistance in high temperature and low temperature. It is a transparent protective film after solidification. It has excellent insulation, moisture resistance, anti leakage, shock proof, dustproof, anti-corrosion, anti-aging and corona resistance.

We often see the metal parts are patina of circuit boards, , that was no coating of conformal coating. The copper metal is combined with water vapor and oxygen to cause the chemical reaction.

The composition of conformal coating

Acrylic acrylic conformal coating is flexible and provides comprehensive protection. Because it is a single component system with good adhesion and simple operation, the requirements and conditions of the equipment is not high, convenient construction, high transparency, high brightness, the characteristics of the short operation cycle, so they are easy to use and easy to clean. Some acrylic products meet military standards, they are dry quickly and not anhydration, organic solvent available supporting its elimination, so this kind of circuit board conformal coating is one of the most widely use on the market,also the most effective product.

There are four types for the use of conformal coating

  • Brushing – commonly used, can produce smooth coating on the surface .
  • Spray – spray can be conveniently applied to maintenance and small scale production, gun production suit for large scale.but the two sprayings require high accuracy of operation, and may produce a shadow (place where not covered with conformal coating).
  • Automatic immersed coating – it can ensure full film coverage without causing material waste due to excessive spraying.
  • Selective coating – coating accurate and not waste materials, suitable for a large number of film coating, but the coating equipment requirements is high.Using a set of XYZ programs reduces coverage. When boards are painted, there are many connectors which don’t need be painted. Stick tape too slowly and there is too much residual glue when tear, can consider do a fixture  according to the shape, size and position with mounting hole location. Covered area where no conformal coating.

Operation requirements of conformal coating

  • Clean and dry the board to remove moisture and vapor,should remove the dust,moisture and oil stain on the surface of object in order to give full play to its protective effectiveness.Thorough cleaning ensures that the corrosive residue is completely removed and that conformal coating is well adhered to the surface of the circuit board.Drying conditions: 60°C, 10-20 minutes,the effect is better hot coating when boards removal from oven.
  • With brush coating method, brush surface area should be larger than components to ensure that all covered of components and pads.  
  • The circuit board should be flat and no dew after be brushed, brush should be smooth and no exposed part, 0.1-0.3mm is appropriate.
  • Before brushing and spraying, make sure that the diluted product is fully stirred and placed for 2 hours before brushing or spraying. Use a high quality natural fiber brush and brush gently at room temperature. If use machine,should measure the viscosity of the coating(using a viscosity agent or a flow cup), and the viscosity can be adjusted with a diluent.
  • Circuit board assembly shall be immersed vertically in the paint lees. Don’t immerse the connector,unless it is covered carefully, the circuit board should be immersed for 1 minutes until the bubble disappears and then slowly take out. A uniform film is formed on the surface of the circuit board. Most coating residues should flow back from the circuit board to the film dipping machine. TFCF has different coating requirements. Circuit boards or components should not be immersed in too fast, so as not to produce excessive bubbles.
  • The second using after the end of the dip, if the surface has crust phenomenon, remove the epidermis and can continue to use.
  • After brushing, flat on the stent, ready to cure, need to use heating method is coating accelerated curing. If the surface of the coating is uneven or contains bubbles, it should be put in a high temperature furnace for more time at room temperature to allow the solvent to flash out.

Note of conformal coating：

  • If you wish to get a thicker coating, preferably by coating two layers of thinner coating – and requiring that the first coat be completely dried, second layers are allowed to be applied.
  • In the coating on the circuit board, the general connector, software socket, switch, radiator, heat sink area, plug plate region is not allowed to have coated materials, it is recommended to use tear resistant solder covering.
  • Film thickness: the thickness of the film depends on the application method. The addition of diluent is large, the viscosity of the gum is low, the thickness of the coating is thin; on the contrary, the viscosity of the glue is high, and the thickness of the coating is thick.
  • All coating operations shall be performed at a temperature of not less than 16 DEG C and relative humidity below 75%. The circuit board will absorb moisture as a composite material. If not to tide, three paint can not fully protect the role. Pre drying and vacuum drying can remove most of the moisture.

 

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ENIG PCB

The Difference Between Gold Plating And ENIG

The surface of the circuit board has several processes: Bare PCB (surface without any treatment), Rosin board, OSP (organic solder preservative, slightly better than rosin), HASL (a tin, lead free tin), gold plating, ENIG(Electroless Nickel/Immersion Gold), which is more common. We briefly introduce the difference between gold plating and ENIG.

ENIG is a method of chemical deposition, and a layer of coating is formed by chemical oxidation-reduction reaction. The thickness of the deposit is thicker, which is one of the chemical nickel, gold and gold layer deposition methods, which can reach a thicker gold layer.

Gold plating uses the principle of electrolysis, also known as electroplating. Most other metal surface treatments are plated.

In the actual product applications, 90% of the gold boards are the ENIG, because the poor weldability of the gold-plating is his fatal flaw, but also a direct cause of many companies to abandon the gold-plating process.

ENIG of printed circuit surface is stable color, good brightness, good coating and good weldability. Basically can be divided into four stages: pretreatment (degreasing, micro corrosion, activation, after immersion), immerse nickel, immerse gold, end treatment (waste gold washing, DI washing, drying). The thickness of ENIG is between 0.025-0.1um.

Gold used in circuit board surface treatment, because the conductivity of gold is strong, good oxidation resistance, long life, general applications such as key board, finger board, etc. The fundamental difference between gold plating and ENIG is gold plating is hard gold (hard wearing), ENIG is soft gold (not hard wearing).

Difference between gold plating and ENIG:

  • * As the circuit board processing requirements are higher and higher, the line width and spacing have reached below 0.1mm. Gold plating is easy to produce gold wire short circuit. The ENIG only has nickel gold on the pad, so it is not easy to produce a gold wire short circuit.
  • * The crystal structure is more dense of ENIG compare gold plating, so it is difficult to produce oxidation.
  • * The ENIG only has nickel gold on the pad compare gold plating. In the skin effect, the signal transmission is in the copper layer, which will not affect the signal.
  • * ENIG and gold plating crystal structure is not the same, It required thicker gold for ENIG than gold plating,and the gold will be golden yellow than gold plating,which is recognition method of ENIG and gold plating,the gold is slightly pale of gold plating like nickel color.
  • * ENIG and gold plating crystal structure is different,ENIG is easier to weld, will not cause welding bad. The     stress of the ENIG is easier to control, and it is more conducive to the processing of the bonding products. At the same time, it is because the gold deposit is more soft than the gold plating. Therefore, the gold finger of ENIG is not wearable (the disadvantages of the ENIG).
  • * The ENIG only has nickel gold on the pad. Therefore, the solder mask on the line is more firmly integrated with the copper layer. It will not affect the spacing when it is compensated.
  • * For higher requirements of flatness PCB, generally used ENIG, ENIG generally does not appear black pad phenomenon after assembly. The flatness and longevity life of ENIG is better than gold plating.

So most factories now use ENIG process to produce gold PCB. But the ENIG process is more expensive than the gold plating process (higher gold content), so there are still a large number of low-priced products using gold plating process (such as remote control board, toy board).

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PCB welding

PCB Manual Welding Skills Of Surface Mount Components

With the development of science and technology, technical requirements, and now more and more circuit board using surface mount components, compared with the traditional package, it can reduce the area of the circuit board, easy batch processing, high density wiring.The chip inductance of chip resistor and capacitor is greatly reduced, and it has great advantages in high frequency circuit. The inconvenience of surface mount components is not convenient for manual welding. To this end, the circuit board welding engineer takes the common PQFP packaging chip as an example to introduce the basic welding method of surface mount components.

First:The necessary tools and materials

Welding tools need to have small iron with 25W copper head, welding rod, conditional can use the temperature can be adjusted with ESD protection of the welding table, note the iron tip to fine, the top of the width can not be more than 1mm. A pointed tweezers can be used to move and fix chips and check circuits. Also prepare fine wire and flux, isopropyl alcohol and so on.The purpose of using flux is to increase the fluidity of solder so that the solder can be drawn by iron and wrapped smoothly on the pins and pads depending on the surface tension. Remove the flux from PCB with alcohol after welding.

Second: Welding method

1. The circuit board welding engineers remind readers coated all pads with flux before welding,treating with rion,so as to avoid tin plating bad or oxidized, resulting in bad welding, chips generally do not need treatment.

 2. Carefully put the PQFP chip on the PCB board with the tweezers. Be careful not to damage the pins. Align it with the pads and make sure the chips are in the right direction. The iron temperature to more than 300 degrees Celsius, the iron tip with a small amount of solder, use tools holding down the alignment position of the chip, plus a small amount of flux in the two diagonal position of the pin, still holding down the chip, welding the two diagonal position of the pin, the chip is fixed and can not move.Re-check the position of the chip after welding diagonally. If necessary, adjust or remove and re-position on the PCB board.

 3. When soldering all the pins, add the solder on the tip of the iron and apply all the pins to the solder so that the pins remain moist. Contact the end of each pin of the chip with the tip of the soldering iron until the solder flow pin is seen. When soldering, keep the iron tip parallel to the soldered pin to prevent overlap due to excessive soldering.

 4. After soldering all the pins, soak all the pins with flux to clean the solder. Remove excess solder wherever necessary to eliminate any short circuit and lap joint. Finally, Check the missing welding with tweezers.After the completion of the examination, removal of flux from the circuit board, use hard brush dipped in alcoho and wipe along the direction of pin, until the flux disappeared.

 5. the circuit board welding engineer remind you of patch resistor is relatively easy to weld, can be the first in a little tin solder, then put a head element, with tweezers element, after welding on the head, and then see if it’s perfect; if it is perfect, and then welded on another head. It need takes a lot of practice to really master welding skills.

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Multilayer PCB Manufacturing

PCB Failure Analysis Method

PCB circuit board has become the most important part of electronic products as its carrier of various components and signal transmission,PCB quality and reliability determines the quality and reliability of the whole machine.

With the miniaturization of electronic products and environmental protection requirements of lead-free and halogen-free,PCB also develops towards high-density, high Tg and environmental protection.However, due to cost and technical reasons, a large number of failures have been occurred in the production and application of PCB, and many quality disputes have been triggered.In order to find out the cause of the failure and to find a solution to the problem and to distinguish responsiblility,failure analysis of failure case must be carried out.

Basic procedures for failure analysis

To obtain PCB circuit board failure or poor accuracy or mechanism, must abide by the basic principle and the analysis process, otherwise the failure may miss valuable information, analysis can not continue or might get the wrong conclusion caused. The basic process is generally must first be based on the failure phenomenon, through the collection of information, function test, electrical performance test and simple visual inspection, determine the failure position and failure mode, the failure location or fault location.

For simple PCB or PCBA, the failure of the site is easy to determine, but for the more complex BGA or MCM package devices or substrate defects through the microscope, the moment is not easy to determine, this time need to be determined by other means.

Then it is necessary to analyze the failure mechanism, namely the use of various physical and chemical means to analyze the mechanism leading to PCB failure or defects, such as solder, pollution, mechanical damage, moisture stress, corrosion and fatigue damage, CAF or ionic migration, stress overload etc..

Then analyze the reasons for the failure, the failure mechanism and process based on process analysis, find the cause of the failure mechanism of the test should be carried out as necessary, general test may, through the test can find the reason of failure induced by the accurate.

This provides a definite basis for the next step of improvement. Finally, according to the analysis process to obtain test data, facts and conclusions, the preparation of failure analysis report, the requirements of the report of the facts clear, logical reasoning, strict, coherent, and should not be imagined.

In the process of analysis, attention should be paid to the basic principles of analytical methods, from simple to complex, from outside to inside, from never destroying samples to using them. Only in this way can we avoid losing the key information and avoid introducing new artificial failure mechanism.

The failure analysis of PCB or PCBA is the same. If an electric iron is used to repair the missing solder joints or to shear the PCB strongly, then the analysis cannot be done. The failure site has been destroyed. Especially in the case of less failure samples, once the damage or damage to the site of the environment, the real failure reason can not be obtained.

Failure analysis technique

Optical microscope

The optical microscope is mainly used for the appearance inspection of PCB, looking for the failure parts and relevant material evidence, and preliminarily judging the failure mode of PCB. The appearance inspection mainly examines the PCB pollution, corrosion, bursting plate location, circuit wiring and failure regularity, such as batch or individual, whether it is always concentrated in a certain area and so on.

X ray

For certain parts that cannot be visually examined, as well as through and through the hole and other internal defects of the PCB, the X ray fluoroscopy system is used. X-ray system is the use of different materials, thickness or density of different materials to X light moisture absorption or transmittance of different principles to imaging. This technique is used to check the internal defects of PCBA solder joints, the defects in through holes and the defects of BGA or CSP devices in high-density packaging.

Slice analysis

Slice analysis is the process of obtaining PCB cross section structure by means of sampling, mosaic, slicing, grinding, etching and observation. Through slicing analysis, we can get abundant information about microstructure of PCB (through hole, plating, etc.), and provide a good basis for quality improvement in the next step. But the method is destructive, and once it is sliced, the sample is bound to suffer damage.

Scanning acoustic microscope

At present, for the analysis of electronic packaging or assembly is mainly ultrasound scanning acoustic microscope C mode, which is reflected in the material discontinuous interface using high frequency ultrasonic amplitude and phase and polarity changes to imaging, its scanning mode is along the Z axis scanning X Y plane information.

Therefore, scanning acoustic microscopy can be used to detect defects in components, materials, and PCB and PCBA, including cracks, delamination, inclusions, voids, and so on. If the frequency of the scanning acoustic is sufficient, the internal defects of the solder joint can also be detected directly.

The typical image scanning acoustic is the red warning color represents the existence of defects, due to the large number of plastic packaging components used in the SMT process, by a lead into a lead-free process in the process of a large number of wet reflux sensitive issues, namely the internal moisture of plastic devices or substrate delamination phenomenon in lead-free reflow higher process temperatures, common in high temperature lead-free process under the PCB will often appear in the phenomenon of explosion.

At this point, scanning acoustic microscopy has highlighted its special advantages in multi layer high-density PCB nondestructive testing. In general, the obvious burst plates can be detected only by visual inspection.

Micro infrared analysis

FTIR analysis is infrared spectroscopy and microscopy analysis together, using different materials (mainly organic) different absorption principle of the infrared spectrum analysis, compound material, combined with the microscope can make visible light and infrared light, as long as in the field, you can find the analysis the trace organic pollutants.

If there is no combination of microscopes, the infrared spectrum can usually be used to analyze samples with more samples. And in the electronic process, a lot of situations are micro pollution, which can lead to the poor weldability of the PCB welding pad or lead pin. It can be imagined that it is difficult to solve the process problem without the infrared spectrum provided by the microscope. The main purpose of micro infrared analysis is to analyze the organic contaminants in the welded surface or solder joint surface, and to analyze the cause of poor corrosion or weldability.

Scanning electron microscope analysis (SEM)

Scanning electron microscope (SEM) is one of the most useful failure analysis of large electron microscopic imaging system, the most commonly used morphology observation, scanning electron microscopy, the function is very powerful, any fine structure or surface characteristics can be enlarged to hundreds of thousands of times were observed and analyzed.

The failure analysis of PCB or SEM solder joints, mainly used for the analysis of the failure mechanism, specifically is used to observe the pad surface morphology, microstructure, solder joint measurement of intermetallic compound, solderable coating and analysis of tin whisker measurement and analysis etc..

Different with the optical microscope, scanning electron microscope and the electron is like, so only black and white, and the scanning electron microscope sample requirements for non conductive conductor and semiconductor part need to spray gold or carbon, or charge accumulation on the surface of the sample on the influence of sample observation. In addition, the depth of image of scanning electron microscope is much larger than that of optical microscope. It is an important analysis method for metallographic structure, micro fracture and tin paste.

Thermal analysis

Differential scanning calorimetry (DSC)

Differential Scanning Calorimetry is a method of measuring the relationship between the power difference between a substance and reference material and temperature (or time) under programmed temperature control. It is a method to study the relationship between heat and temperature. According to this change relation, the physical, chemical and thermodynamic properties of materials can be studied and analyzed.

DSC is widely used, but in the analysis of PCB is mainly used for curing degree, glass transition temperature conversion of various high polymer materials used to measure the PCB, the two parameter determines the reliability of the PCB in the follow-up process.

Thermal mechanical analyzer (TMA)

Thermal Mechanical (Analysis) is used to measure the deformation properties of solids, liquids, and gels under thermal or mechanical forces for program temperature control. It is a method to study the relationship between heat and mechanical properties. According to the relationship between deformation and temperature (or time), the physical, chemical and thermodynamic properties of materials can be studied and analyzed.

TMA is widely used in PCB analysis. It is mainly used for the two key parameters of PCB: measuring its linear expansion coefficient and glass transition temperature. The PCB of the base material with excessive expansion coefficient often results in fracture of the metallized hole after welding assembly.

Thermogravimetric analyzer (TGA)

Analysis (Thermogravimetry) is a method of measuring the relationship between the mass of a substance with temperature (or time) under programmed temperature control. TGA uses a precision electronic balance to monitor fine mass changes in the process of programmed temperature change.

The physical, chemical and thermodynamic properties of materials can be studied and analyzed according to the change of mass with temperature (or time). In the analysis of PCB, mainly used for thermal stability or thermal decomposition temperature measurement of PCB materials, if the substrate thermal decomposition temperature is too low, PCB after high temperature welding process will happen when the explosion plate or delamination phenomenon.

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pcb quality

What Kind Of PCB Is Good Quality?

With the rapid development of mobile phone,electronics,telecommunications and other industries continue to grow and also prompted the PCB circuit board industry quantity at the same time ,people requirements are more and more high in components of number,weight,color,material,precison and reliability.

However,due to the fierce competition in the market price,the cost of PCB circuit board material is also in the rising trend,more and more manufacturers at a low price to monopolize the market in order to enhance the core competitiveness.However ,these ultra low cost behind is reduction of material cost and production cost, lead to the devices are usually easy to crack,scratch, comprehensive factors included the precision,performance were not certificated,the serious influence the solderability and liability of products use.

In the face of the market all kinds of PCB circuit board, a good circuit board can distinguish from two aspects; the first method is to judge from the appearance, the other is from the PCB board specification quality.

Method for judging the quality of PCB circuit board:

From the appearance of the circuit board to distinguish between good and bad

Generally, the appearance of the PCB circuit board can be analyzed and judged in three aspects;

Standard rules for size and thickness

Circuit board to standard circuit board thickness is different size, customers can measure and check according to their product thickness and specifications

Light and color

The external circuit board is covered with ink, and the circuit board can play an insulating role. If the board color is not bright and the ink is less, the insulation board itself is not good.

Weld appearance

Because of the more parts of the circuit board, if the welding is not good, the parts will easily fall off the circuit board, seriously affecting the welding quality of the circuit board, the appearance is good, carefully identified, the interface stronger is very important.

High-quality PCB circuit boards need to meet the following requirements

  1. 1.  Copper surface is not easy to oxidize, affect the installation speed, oxidation soon after the bad;
  2. 2. After the components are installed, the telephone is easy to use. That is to say, the electrical connection should meet the requirements;
  3. 3. By the high temperature copper skin is not easy to fall off;
  4. 4. The line width, line thickness, line spacing meet the requirements, so as to avoid line heating, break, and short-circuit;
  5. 5. No additional electromagnetic radiation;
  6. 6. And high temperature, high humidity and resistance to special environment should also be considered within the scope;
  7. 7. The shape is not deformed, so as not to install after the deformation of the shell, screw hole dislocation. Now all mechanical installation, the circuit board hole position and line and design deformation error should be within the allowable range;
  8. 8. The surface mechanical properties should meet the installation requirements;

The above is the PCB circuit board to judge the good and bad methods, in the purchase of PCB circuit board, be sure to polish your eyes.

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PCB storage time

How Long Is PCB Storage Time?

There is a storage time after PCB complete manufacture.it need be baked if more than storage time,otherwise there easy occur burst board problems when make SMT.The storage time of PCB and the temperature and time of baking PCB are standard in industry.

Regulation of PCB control

PCB unpacking and storage

  • (a.) PCBs can be directly use online if unopened within 2 months manufacturing date.
  • (b.) PCBs must be labeled unpacking date within 2 months manufacturing date.
  • (c.) PCBs must be finished use online within 5 days after unpacking within 2 months manufacturing date.

PCB Baking

  • (a.)PCBs in the manufacturing date within 2 months of sealing opened more than 5 days, please take 120 +/- 5 ℃ baking for 1 hour.
  • (b.)If PCBs exceeds 2 months of manufacture date, please bake for 1 hours at 120 + /5 ℃ before online.
  • (c.)PCBs are over 2 months to 6 months before the manufacture date, please bake for 2 hours at 120 +/- 5 ℃ before the online.
  • (d.)PCBs are over 6 months to 1 year after the date of manufacture, please bake for 4 hours at 120 + /-5 ℃ before on-line.
  • (e.)PCBs be baked should be used within 5 days (put into IR REFLOW), it need to bake one more hour before reuse.
  • (f.)If PCBs are more than 1 year before the date of manufacture, please refer to the 120 +/- 5 ℃  baking 4 hours, sending back to PCB factory to remake HASL.

The method of PCB baking

  • (a.)Large PCBs place with flat,a stack up to 30pcs,open the oven and take out PCBs flat to natural cooling( need press fixture to anti warp and twist) after baking completed.
  • (b.)Medium and small PCBs place with flat,a stack up to 40pcs,upright type is not limited,open the oven and take out PCBs flat to natural cooling( need press fixture to anti warp and twist) after baking completed.

Preservation and baking of PCB in different regions

The specific storage time and baking temperature of PCB are not only related to the manufacture ability and manufacturing process of PCB manufacturers, but also to the region.

OSP and pure gold PCB, the storage time is usually 6 months, for the OSP process is generally not recommended to bake. The region is a great related to PCBs preservation,in general moisture is heavy in southern region,it’s very damp in Spring.PCBs must be used up within 24 hours,otherwise easy to oxidation,it’s the best used up within 8 hours after unpacking.The weather is generally dry in the Northern region,PCB has a longer storage time and a shorter baking time.the baking temperature is generally 120+/5℃,baking time according to the specific circumstances to decide.

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2LPCBDESIGN

How To Design Double Sided PCB

PCB (Printed Circuit Board) is an important electronic components, it’s base board of electronic components and carrier of electrical connections. it is called “printed” circuit board because it is made using printing technology.

Double-layer circuit board is layout wiring on both sides, but to use the two sides of the wire, must have appropriate circuit connection between the two sides. The “bridge” between such circuits is called via. The vias filled with or coated with metal holes in PCB, it can be connected with the two sides of the wire. Use PROTEL to draw double-sided pcb board, in TopLayer (top layer) to draw wire connection components, is in the top drawing board; select BottomLayer (bottom), in the bottom of the wire to connect components, is the bottom of the drawing board. Above is to draw double-layer pcb, meaning a pcb board in the top and bottom are painted wire. The double-sided PCB solves the difficulty of wiring in a single layer (which can pass through the hole to the other side), that is, both sides of the front and back wiring, the components can be welded on the front, can also be welded on the back, it is more suitable for use Single-side  complex on the circuit.

The two-layer board ground wire is designed as a grid-shaped frame, that is, the parallel lines are printed on the side of the printed circuit board, the other side is the vertical ground of the copy board, and then connected with the metal vias( resistance to be small).

In order to take into account each IC chip should be installed near the ground, often every 1 ~ 115cm place a ground line, so dense ground to make the signal loop area smaller, is conducive to reducing radiation. The ground network design method should be in the layout signal line before, otherwise it is more difficult to achieve.

Double-sided PCB board signal line wiring principle

After Double-sided board rational distribution of components, followed by the first design of the network copy board power cord, and then layout important line — sensitive line, high frequency line, then layout the standard line — low line. The key lead line is best have a separate power supply, ground circuit, lead line must be very short, so sometimes design a ground line close to the signal line near the key lead line, let it form a smallest working loop.

Four-layer board top and bottom wiring principle is as same as double-sided PCB, also the first layout key crystal, crystal circuit, clock circuit, CPU and other signal lines, must follow principle the circulation area as small as possible.

PCB IC circuit working, above mentioned many times of circulation area, the actual its difference concept of differential mode radiation. Such as differential mode radiation definition: the circuit current flowing in the signal loop, the signal loop will produce electromagnetic radiation, because this current is differential mode, so the signal loop radiation is called differential mode radiation, the radiation Field calculation formula: E1=K1·f2·I·A/γ

In the formula: E1 — differential mold copy board circuit space γ radiation field intensity from the differential mode radiation formula can be seen, the radiation field strength and the operating frequency f2, circulation area A, operating current is proportional to, such as when the operating frequency f is determined, the size of the circulation area is the key factor that can be directly controlled in our design. At the same time, the working speed and current of the circulation are better meet reliability,not the bigger the better, the narrower the jump, the larger the wave component, the wider, the higher the electromagnetic radiation, the greater the power of its current must be large (the above has been pointed out), which we do not expect.

Several logic circuits are given below to meet the allowable loop area reference values for the radiation Class B standard. It can be seen that the faster the circuit switch, the smaller the area allowed.

The key to the line, if possible around it can be surrounded by the ground. After the PCB copy board is finished, the available ground will cover all the voids, but it must be noted that these covered ground lines should be shorted with the low impedance of the large strata, which can achieve good results (Note: Should meet the conditions, such as creepage distance, etc).

Some tips of double pcb board

The use of automatic router to design pcb is attractive. In most cases, automatic routing of pure digital circuits (especially low-frequency signals and low-density circuits) will not be a problem. However, when attempting to use the automatic routing tool provided by the cabling software to do the wiring of analog, mixed-signal or high-speed circuits, there may be some problems and may cause extremely serious circuit performance problems. There are many things to consider about wiring, but the more troublesome problem is the grounding,If the ground path is started by the upper layer, the grounding of each device is connected to the ground via the cable on the layer. For each device in the lower layer, a ground circuit is formed by connecting the upper hole on the right side of the circuit board to the upper layer. The immediate red flag that the user sees when checking the routing mode indicates that there are multiple ground loops. In addition, the lower ground circuit is at a level. Can reduce the digital switching δi/δt the impact of analog circuits.It should be noted, however, that the two sheets have a ground plane in the lower layer of the circuit board. This is designed so that engineers can quickly see wiring when doing troubleshooting, often in the device manufacturer’s demonstration and evaluation board. But the more typical approach is to lay the ground plane on the top of the board to reduce electromagnetic interference.

Steps of double side PCB

  • prepare the circuit schematic
  • create a new pcb file and load the component package library
  • planning circuit board into the network table and components
  • automatic component layout
  • layout adjustment
  • network density analysis
  • wiring rules set
  • automatic routing
  • manually adjust the wiring

PCB design experience of double side pcb board(embedded hardware experience)

  • clearance spacing is better more than 10mil, high-density wiring is at least 5mil.
  • The minimum line is 10mil out from welding line,no oblique line otherwise produce acute angle and unaesthetic.
  • The main power line(current is relatively large)through the hole with two vias in parallel way to prevent a via failure lead to circuit board don’work.
  • Power supply inlet capacitor with 100uf and 104 ceramic way the export capacitor capacity should be large enough to meet the circuit requirements (high current will not pull the voltage instantly). Turn off the diode from the power chip output pin closer the better.
  • Power part of the resistance and capacitance to calculate the power, the package to meet the power requirements.
  • A number of RF circuits, the  RF can be crossed on different layers, reducing interference.
  • Pay attention to the lead position, to meet the schematic, not the same signal can be any position can lead.
  • The same characteristics of the signal line wiring signal characteristics to the same, as long as the distance from the same length, the same number of vias.
  • Will be some of the power supply of the decoupling capacitor filter capacitor can be placed on the back of the pin, save space and shorten the wiring distance.
  • Wiring with latitude and longitude wiring, the upper and lower wiring is clear, but also to reduce the hole, reduce interference.
  • When drawing the schematic, it is necessary to strictly calculate the rated current of the power chip rated current to meet the actual load requirements.
  • When wiring, place the in – line elements around the core wiring area, which will cause interspersed and affect the latitude and longitude. To prevent interspersed, because the welding may scratch the line of the welding layer, so that when the welding pin may produce adhesion.
  • Network chip under the ban copper
  • Welding crystal crystal strict fall, because excessive shock will affect its performance.
  • The four corners of the board are best rounded to prevent scratches.

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PCB design

LED Switching Power Supply PCB Circuit Board Design(II)

The wiring switching power supply contains high frequency signals

Any printed circuit on the PCB can play the role of the antenna,the length and width of the printed line will affect its impedance and inductance, thus affecting the frequency response.Even the printed lines through the DC signal are coupled from the adjacent traces to RF signal and cause circuit problems(even re-emit the interfering signal).Therefore,all the lines passing through the AC current should be designed as short and wide as possible,which means that all components connected to the printed circuit and connected to other power lines must be placed very close.The length of the printed wire is proportional to the inductance and impedance it exhibits, and the width is inversely proportional to the inductance and impedance of the printed wire.The length reflects the wavelength of the printed circuit response,the longer the length,the lower the frequency of the transmission and reception of electromagnetic waves,it can radiate more radio frequency energy.according to the size of the printed circuit board current.as far as possible to add the power line width,reduce loop resistance.at the same time,make the power line,ground direction and current direction,this will help enhance the anti-noise ability.Grounding is the bottom of the four current loop switching power supply circuit,as the circuit’s common reference point plays a very important role,it is an important way to control interference.Therefore, the layout should be carefully considered the placement of the ground wire,a variety of grounding will cause the power supply instability.

In the ground design should pay attention to the following:

a. Proper selection if the single-point grounding, the filter capacitor common should be the other ground point coupled to the exchange of high current point,the same level of circuit grounding should be as close as possible, and the level of the circuit power supply filter capacitor should also be connected in this level of ground,the main part of the circuit to consider the current back to the ground current is changing,because the actual flow of the impedance of the circuit will lead to changes in the various parts of the circuit and the introduction of interference.In this switching power supply,its wiring and the inductance between the devices less affected,and the formation of the ground circuit on the interference of greater impact, and thus a little ground,that is,the power switch current loop( in several devices are ground connected to the ground feet),output rectifier current loop several devices of the ground is also connected to the corresponding filter capacitor ground pin,so the power supply is more stable,easy to self-excited. Can not be a single point, in a total of two diodes or small resistance,in fact,then a relatively concentrated in a copper.

b. As far as possible,if the ground wire is very fine,the ground potential changes with the current changes,resulting in electronic equipment,timing signal level instability,anti-noise performance deterioration,so to ensure that each of the high current ground short and wide printed wire,as far as possible to widen the power supply, ground width,preferably the ground than the power line width,their relationship is :ground>power line>signal line,if possible,the ground wire width should be greater than 3mm,can also use large area copper layer for ground.In the printed circuit board has not been used on the ground are connected with the ground as a ground. For global cabling, follow these guidelines:

(1.) Wiring direction: from the welding surface, the arrangement of components as much as possible consistent with the schematic diagram,the wiring direction is best consistent with the direction of the circuit diagram alignment,the production process is usually required in the welding surface for a variety of parameters,This is done to facilitate the inspection, commissioning and maintenance of the production (Note: refers to meet the circuit performance and machine installation and panel layout requirements under the premise).

(2.)Design wiring diagram as little as possible when the alignment, printing arc on the line width of the sudden change, the wire corner should be ≥ 90 degrees, and strive to simple lines clear.

(3.)The printed circuit is not allowed to cross the circuit, for the cross may be crossed, you can use “drill”, “around” two ways to solve. That is, let a lead from the other resistance, capacitance, the triad pin at the foot of the “drill” in the past, or from the end of a lead may be “around” in the past, in special circumstances how the circuit is very complex, to simplify the design also allows Cross the wire to solve the cross circuit problem. Due to the use of single-sided, the in-line components are located on the top surface, the surface mount device is located on the bottom surface, so the placement device can overlap with the surface mount device, but avoid pad overlap.

c. Input and output ground switch power supply for the low-voltage DC-DC, want to return the output voltage back to the primary transformer, both sides of the circuit should have a common reference, so the two sides of the ground after the copper, but also Connected together to form a common ground.

Inspection

After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules set by the designer. It also needs to confirm whether the rules are in line with the requirements of the PCB production process. Generally check the lines and lines, And whether the distance between the through hole, the element pad and the through hole, the through hole and the through hole is reasonable, and whether or not the production requirement is satisfied. Whether the width of the power cord and ground wire is appropriate, and whether there is a place where the ground is widened. Note: Some errors can be ignored, for example, some of the connectors Outline part of the board and frame, check the gap will be wrong; the other changes each time after the alignment and vias, have to re-copper once.

Review According to the “PCB Checklist”

Including the design rules, layer definition, line width, spacing, pad, vias settings, but also focus on review the rationality of the layout of the device, power, ground network routing, high-speed clock network alignment and shielding, decoupling Capacitor placement and connection.

Design the output light to draw the file to note

a.The output layer Including top layer silk screen, bottom screen printing, solder mask layer (bottom solder mask), drilling layer (bottom layer), in addition to the formation of drilling documents (NCDrill), the hole layer (bottom layer).

b.When setting the Layer of the Silkscreen Layer, do not select PartType, select Outline, Text, Lines of the top layer and the silk screen. When setting the Layer for each layer, select Board Outline and set the Layer, Select the top layer (bottom) and silk screen layer Outline, Text, Line.d. Generate a drilling file, the use of PowerPCB default settings, do not make any changes.

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PCB design

LED Switching Power Supply PCB Circuit Board Design(I)

Seven steps of LED switching power supply PCB circuit board design.

In the switching power supply design, if the PCB board design is incorrect will lead to too much electromagnetic interference.There are seven PCB design steps will great stable power supply.Through the various steps in the need to pay attention to analyze,step by step on the board to do a good job PCB board design.

From schematic to PCB design flow

Create component parameters—> Input principle netlist—>Design parameter setting—>Manual layout—> Manual wiring—>Verify design—>Review—>CAM output

Parameter Setting

Adjacent wire spacing must meet electrical safety requirements, and for ease of operation and production,the spacing should be as wide as possible.The minimum spacing at least able to withstand the voltage,the signal line spacing can be appropriately increase when the wiring density is low, the high and low disparity of the signal line should be as short as possible and increase the spacing. In generally, the distance should be more than 1mm between the edge of hole to printed circuit board edge avoid pad defect in the processing.when the traces are thin connected to the pads,the pad should design to droplet-like.the advantage is that the pads will not peeling and the traces are not be easily broken.

Component layout

Practice has proved although circuit schematic design is correct, if printed circuit board design is improper also impact the reliability of electronic equipment.For example,if two parallel lines of printed circuit board are close,will formed a delay signal waveform and reflection noise at the terminal of the transmission line.it will decline the performance of product due to power and ground considerations are not thoughtful.so should use the correct method in the design of printed circuit board.Each switching power supply has four current loops:

  • ) Power switch AC circuit
  • ) Output rectifier AC circuit
  • ) Input signal source current loop
  • ) Output load current loop

The input loop charge the input capacitor through an approximately DC current and filter capacitor acts primarily as a broadband energy storage.Similarly,the output filter capacitor is used to store the high frequency energy from the output rectifier and eliminating the DC energy from the output load loop.Therefore, the input and output filter capacitor terminals are very important,input and output current loop should be only from the filter capacitor terminals connected to the power supply.If the connection between the input/output circuit and the power switch/rectifier circuit can not be directly connected to the terminal of the capacitor,the AC energy will be filtered by the input or output filter and radiated into the environment.

Power switch AC circuit and rectifier AC circuit contains high trapezoidal current,these currents in high harmonic component,the frequency is much larger than the switching base frequency,the peak amplitude can be up to 5 times of continuous input/output DC current, the transition time is usually about 50ns.These two circuits are most susceptible to electromagnetic interference and must be routed before other wiring lines in the power supply.The three main components of each circuit filter capacitor,power switch or rectifier, inductor or transformer should be connected to each other adjacent to place, adjust the position of the components so that the current path between them as short as possible.

The best way to build a switching power supply layout is similar to its electrical design.The best design flow is as follows:

  • Place the transformer
  • Design the power switch current loop
  • Design the output rectifier current loop
  • Connect the control circuit to the AC power supply circuit
  • Design input current source circuit and input filter design Output load circuit and output filter according to the functional unit of the circuit,all the components of the circuit layout,to meet the following principles:

 (1.)First consider the size of the PCB. PCB size is too large, the printed line is too long will increase impedance and decrease anti-noise ability,then increase the cost. If the printed circuit board is too small will lead to bad heat dissipation,and adjacent lines are vulnerable to interference. The best shape of the board is rectangular,with an aspect ratio of 3:2 or 4:3.

(2.) Consider the subsequent welding when place the components,the better not too density.

(3.)With the core components of each functional circuit as center,layout around it.Components should be uniform,neat and compactly arranged on the PCB,minimizing and shortening the lead and connection between the components,decoupling capacitors as close as possible to the device.

(4.) In the high frequency of the circuit, to consider the distribution of components between the parameters.In generally should arrange the components as parallel as possible.This is not only beautiful,but easy to assembly and mass production.

(5.)According to the circuit process to arrange the location of the various functional circuit unit,so that the layout flow, and make the signal as much as possible to maintain a consistent direction.

(6.) The first principle of the layout is ensure that the wiring rate,pay attention the fly line connection of mobile devices,put the connection devices together.

(7.)As far as possible to reduce the loop area to suppress the switching power supply radiation interference.

 

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PCB assembly fixture

PCB Assembly Files- PCB Enginner Need To Know

Some complex PCBs design be finished,you must take PCB to assembly plant when PCBs get back from PCB manufacturer.That assembly plant need you provide some necessary documents for production.Where the captain guy gave you talk about the PCB assembly when production of documents required,and what the representative of the file is useful.

PCB assembly processing required documents are the following:

Stencil file

Stencil file is solder paste file, it used in brush paste when PCB assembly, it can be automatically welded when PCB through high temperature furnace. But stencil file didn’t be included plug-in components information,it’s surface mount components information.What kind of stencil is? The following is a kind of stencil.

stencil

Above aperture is exposed for each surface mount components,no components area are covered. And the stencil is a certain thickness,the exposed holes are filled with solder paste when brush paste, then pcb pad coated with solder paste. The solder paste melting to stabilize assembly when soldering in high temperature.

Pick Place file

When assembly is processed with a machine.you have to tell the machine how to paste the components,which requires the coordinates of the document.Coordinate file also call pick place file,it has the coordinates of each components, layer, rotation angle and other information,the machine will be able to read this information,according to the requirements to paste components. Pick Place file are generally shown below:

Designator Footprint Mid X Mid Y Ref X Ref Y Layer Rotation Comment
U2 USON8_(3×4) 1341.535mil 276.575mil 1294.291mil 229.331mil T 90 MX25R3235F
TP3 TP-L_3D 1066.929mil 60.039mil 1065.945mil 60.039mil B 90 JTAG_TMSC
TP5 TP-L_3D 1121.063mil 125.984mil 1120.079mil 127.953mil B 90 JTAG_TCKC
U4 GNS302uLP_-_BS 645.669mil 308.563mil 645.669mil 618.111mil B 0 GNS302uLP
U5 YFP0025BABD 641.732mil 161.417mil 641.732mil 161.417mil T 360 BQ25120YFPR
L3 0402L_3D 535.433mil 67.913mil 535.433mil 67.913mil T 90 BLM15HG102SN1
C27 0402C_3D 482.284mil 67.913mil 482.284mil 67.913mil T 270 10uF
u1 PVQFN32_3D 1143.701mil 184.055mil 1143.701mil 184.055mil T 0 CC2640RHB
C7 0402C_3D 1024.606mil 335.63mil 1024.606mil 335.63mil T 90 100nF
X2 2.0×1.2×0.6mm_crystal 936.024mil 135.827mil 936.024mil 135.827mil T 90 32.768KHz
C26 0402C_3D 560.039mil 264.764mil 560.039mil 264.764mil T 90 1uF
C25 0402C_3D 721.457mil 104.331mil 721.457mil 104.331mil T 270 1uF
U3 LGA14 1349.409mil 430.118mil 1349.409mil 430.118mil T 180 LSM6DSL
L7 6-0805_L 780.512mil 185.039mil 780.512mil 185.039mil T 270 2.2uH
C1 0402C_3D 1231.299mil 346.457mil 1231.299mil 346.457mil T 90 100nF
C2 0402C_3D 1066.929mil 25.591mil 1066.929mil 25.591mil T 360 100nF
C6 0402C_3D 1183.071mil 346.457mil 1183.071mil 346.457mil T 90 100nF
C13 0402C_3D 1390.748mil 531.496mil 1390.748mil 531.496mil T 90 100nF
C16 0402C_3D 1340.551mil 531.496mil 1340.551mil 531.496mil T 90 100nF
C19 0402C_3D 1380.906mil 117.126mil 1380.906mil 117.126mil T 180 100nF
C20 0402C_3D 1151.575mil 25.591mil 1151.575mil 25.591mil T 360 1uF
C21 0402C_3D 619.095mil 266.732mil 619.095mil 266.732mil T 90 1uF
C22 0402C_3D 710.63mil 272.638mil 710.63mil 272.638mil T 360 10uF
C23 0402C_3D 1380.906mil 167.323mil 1380.906mil 167.323mil T 360 100nF
L2 6-0805_L 1282.48mil 40.354mil 1282.48mil 40.354mil T 180 10uH
R3 0402R-v2_3D 1297.244mil 117.126mil 1297.244mil 117.126mil T 180 56K
R4 0402R-v2_3D 561.024mil 178.15mil 561.024mil 178.15mil T 270 56K
TP1 TP-L_3D 1245.079mil 54.134mil 1245.079mil 54.134mil B 180 JTAG_TDI
C4 0402C_3D 1380.906mil 70.866mil 1380.906mil 70.866mil T 360 10uF
C5 0402C_3D 1380.906mil 26.575mil 1380.906mil 26.575mil T 360 10uF

There are components labels.X,Y coordinates,rotation angle,where level.

Component bitmap file

The component bit map file,in fact is the component silkscreen. It’s provided to operate workers to comparison in the production processing.Inspect if there’s any wrong of assembly.The component bitmap is shown below:

Component bitmap file

PCB assembly fixture file:

PCB assembly fixture file, it’s used in covering surface mount components and expose plug pin when PCB through reflow welding machine.Because in the production of the first process is surface mount welded,and then the plug-in welding.the plug-in soldering is done with wave soldering, it must be through wave furnace. The fixture is avoid surface mount components be assembled don’t fall when through wave furnace.

PCB fixture-top side

                                                  Top side of PCB fixture

PCB fixture-botside

                                                  Back side of PCB fixture

Panel drawing file

If PCB is made from panel,it also need provides panel file.Because the assembly manufacturers need panel drawing to make coordinates files.

If you want more about PCB assembly,please visit www.kingsunpcb.com

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