King Sun PCB

rigid flex PCB

Can the rigid flex circuit board be routed at a 90-degree angle?

High-frequency PCB and high-speed signal transmission lines should avoid routing at 90 degree corners, which is strongly required in various PCB Design Guides.Because the high-frequency and high-speed signal transmission line needs to keep the characteristic impedance consistent, and the 90 degree corner routing is used, the line width will change at the corner of the transmission line.The line width at the 90 degree corner is about 1.414 times the normal line width. Due to the change of the line width, the signal will be reflected. At the same time, the extra parasitic capacitance at the corner will also affect the transmission of the signal.

Of course, when the signal propagates along the uniform interconnect, there will be no reflection and distortion of the transmitted signal. If there is a 90 degree corner on the uniform interconnect, it will cause a change in the width of the PCB transmission line at the corner. According to the relevant electromagnetic Theoretical calculation shows that this will definitely bring about the reflection effect of the signal.

This is the case in theory, but theory is theory after all. Is the impact of 90 degree corners on high-speed signal transmission lines important in practice?

According to King Sun rigid flex circuit board engineering, 90 degree corners will have a negative impact on high-speed signal transmission lines, which is theoretically certain, but is this impact fatal? The impact of 90 degree corners on high-speed digital signal and high-frequency microwave signal transmission lines Is it the same?

For high-speed digital signals, a 90 degree corner will have a certain impact on high-speed signal transmission lines. For our current high-density and high-speed pcb, the general trace width is 4-5mil, and the capacitance of a 90 degree corner is about 10fF. It is estimated that the cumulative delay caused by this capacitor is about 0.25ps, so a 90 degree corner on a wire with a 5mil line width will not have a great impact on the current high-speed digital signal (100-psec rising edge time).

For high-frequency signal transmission lines, in order to avoid signal damage caused by the skin effect, a wider signal transmission line is usually used, such as 50Ω impedance and 100mil line width. The line width at the 90 degree corner is about 141mil, the signal delay caused by parasitic capacitance is about 25ps. At this time, the 90 degree corner will have a very serious impact.

At the same time, microwave transmission lines always hope to minimize the loss of the signal. The impedance discontinuity at the 90 degree corner and the parasitic capacitance outside will cause the phase and amplitude errors of the high-frequency signal, the mismatch between the input and the output, and the possible existence of Parasitic coupling, which in turn leads to the deterioration of circuit performance, affects the transmission characteristics of PCB circuit signals.

Regarding 90 degree signal routing, our point of view is to avoid routing at 90 degree as much as possible. Didn’t it say that the impact of 90 degree corners on high-speed digital signals can be ignored?

According to King Sun rigid-flex circuit board engineering, of course, the influence of a single 90 degree corner on the signal quality of the high-speed digital transmission line, relative to the deviation of the height of the wire and the reference plane, the deviation of the uniformity of the line width and line spacing during the etching process of the wire itself, The change of the dielectric constant of the sheet on the frequency signal, and even the parasitic parameters of the vias, are much larger than the problems caused by the 90 degree corner.

However, today’s high-speed digital circuit transmission lines always have to be wound around the same length. When more than ten or twenty corners are superimposed, the signal rise delay caused by the cumulative effect of these 90 degree corners will become non-negligible. The high-speed signal is always transmitted along the impedance path, with 90 degree corners of equal length, and the final actual signal transmission path will be slightly shorter than the original.

And now the high-speed digital signal transmission rate is getting higher and higher, the current HDMI2.0 standard, the transmission bandwidth rate has reached 18Gbps, the 90 degree corner wiring will no longer meet the requirements, and it is now the 21st century, now The EDA software of 45 degree has been well supported even for those who use it.

At the same time, routing cables at 90 degree corners is not in line with people’s aesthetics in terms of engineering aesthetics. Therefore, for the current layout, whether you are using high-frequency/high-speed signal lines, we should try to avoid routing with 90 degree corners unless there are special requirements.

According to King Sun PCB engineering, for high-current traces, sometimes we will replace the traces with copper-clad copper sheets. Not only beautiful, but also there is no EMI hidden danger.

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PCB soldering

Circuit board soldering defects can be due to these reasons

Solder defects often delay new product launches,What factors will affect the poor soldering of the circuit board?

Solderability of circuit board holes affects soldering quality,The poor solderability of the circuit board holes in the PCB factory will cause virtual welding defects, which will affect the parameters of the components in the circuit, resulting in unstable conduction of the components and inner layers of the multi-layer board, causing the entire circuit to fail.

The so-called solderability is the property of the metal surface being wetted by molten solder, that is, a relatively uniform continuous smooth adhesion film is formed on the metal surface where the solder is located. The main factors that affect the solderability of printed circuit boards are:

The quality of the solder affects the quality of the soldering

Solder is an important part in the process of soldering chemical treatment. It consists of chemical materials containing flux. Commonly used low-melting eutectic metals are Sn-Pb or Sn-Pb-Ag. The impurity content must be controlled in a certain proportion to prevent the oxides produced by the impurities from being dissolved by the flux. The function of the flux is to help the solder wet the circuit surface of the board being soldered by transferring heat and removing rust. – White rosin and isopropanol solvents are generally used.

Soldering temperature and cleanliness of the sheet metal surface also affect solderability. If the temperature is too high, the solder diffusion speed will be accelerated. At this time, it has high activity, which will rapidly oxidize the circuit board and the molten surface of the solder, resulting in soldering defects. The contamination of the surface of the circuit board in the PCB factory will also affect the solderability and cause defects. , these defects include solder balls, solder balls, open circuits, poor gloss, etc.

Welding defects caused by warpage

Circuit boards and components are warped during the soldering process, and defects such as virtual soldering and short circuits occur due to stress deformation. Warpage is often caused by an unbalanced temperature between the upper and lower parts of the board. For large PCBs, warpage will also occur due to the board’s own weight falling.

Ordinary PBGA devices are about 0.5mm away from the printed circuit board. If the device on the circuit board is large, the solder joint will be under stress for a long time as the circuit board cools down and returns to its normal shape. If the device is raised by 0.1mm, it is enough to cause Weld open circuit.

Circuit board design affects soldering quality

In terms of layout, when the size of the circuit board is too large, although the welding is easier to control, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases; if it is too small, the heat dissipation decreases, the welding is difficult to control, and adjacent lines are prone to appear Mutual interference, such as electromagnetic interference of circuit boards, etc. Therefore, the PCB board design of the PCB factory must be optimized:

  • Shorten the connection between high-frequency components and reduce EMI interference.
  • Components with heavy weight (such as more than 20g) should be fixed with brackets and then welded.
  • The heating element should consider the heat dissipation problem to prevent defects and rework caused by large △T on the surface of the element, and the thermal element should be kept away from the heat source.
  • The arrangement of the components should be as parallel as possible, which is not only beautiful but also easy to weld, and suitable for mass production. The circuit board design is best with a 4:3 rectangle. Do not change the wire width abruptly to avoid wiring discontinuities. When the circuit board is heated for a long time, the copper foil is easy to expand and fall off. Therefore, the use of large area copper foil should be avoided.

Based on the above, in order to ensure the overall quality of the PCB board, in the production process, it is necessary to use excellent solder, improve the solderability of the PCB board, and prevent warpage to prevent defects.

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PCB heat dissipation

Ten tips of PCB Heat Dissipation

Why PCB Thermal management is important?

For electronic devices, a certain amount of heat will be generated during operation, so that the internal temperature of the device will rise rapidly. If the heat is not dissipated in time, the device will continue to heat up, and the device will fail due to overheating. performance will drop.Therefore, it is very important to have a good heat dissipation treatment of the circuit board. The PCB heat dissipation is a very important, so what are the heat dissipation skills of the PCB circuit board, let’s discuss it together.

Heat dissipation through the PCB board itself is currently widely used PCB boards are copper clad/epoxy glass cloth substrates or phenolic resin glass cloth substrates, as well as a small amount of paper-based copper clad sheets

Tips 1:

Although these substrates have excellent electrical properties and processing properties, they have poor heat dissipation. As a heat dissipation path for high-heating components, it is almost impossible to expect heat to be conducted by the resin of the PCB itself, but to dissipate heat from the surface of the component to the surrounding air.

However, as electronic products have entered the era of miniaturized components, high-density installation, and high-heat-generation assembly, it is not enough to rely on the surface of components with very small surface areas to dissipate heat.

At the same time, due to the large-scale use of surface-mounted components such as QFP and BGA, the heat generated by the components is transferred to the PCB board in a large amount. Therefore, the best way to solve the heat dissipation is to improve the heat dissipation capacity of the PCB itself that is in direct contact with the heating element. conduct or emanate.

Tips 2:

High-heat-generating devices plus radiator and heat-conducting plate. When there are a few devices in the PCB that generate a large amount of heat (less than 3), a heat-sink or heat-conducting pipe can be added to the heat-generating device. When the temperature cannot be lowered, use Radiator with fan for enhanced cooling.

When the number of heating devices is large (more than 3), a large heat dissipation cover (board) can be used, which is a special radiator customized according to the position and height of the heating device on the PCB or a large flat radiator. Cut out the high and low positions of different components. Fasten the heat dissipation cover on the component surface as a whole, and contact with each component to dissipate heat.

However, the heat dissipation effect is not good due to the poor consistency of the components during assembly and welding. Usually, a soft thermal phase change thermal pad is added to the component surface to improve the heat dissipation effect.

Tips 3:

For equipment cooled by free convection air, it is best to arrange the integrated circuits (or other devices) in a vertical arrangement or a horizontal arrangement.

Tips 4:

Use reasonable trace design to achieve heat dissipation. Because the resin in the plate has poor thermal conductivity, and copper foil lines and holes are good conductors of heat, improving the residual rate of copper foil and increasing thermal vias are the main means of heat dissipation. To evaluate the heat dissipation capacity of the PCB board, it is necessary to calculate the equivalent thermal conductivity of the composite material composed of various materials with different thermal conductivity—the insulating substrate for PCB.

Tips 5:

Devices on the same printed board should be arranged as far as possible according to their calorific value and degree of heat dissipation. Devices with low calorific value or poor heat resistance (such as small-signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) The uppermost stream (at the inlet), the devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed in the most downstream of the cooling airflow.

Tips 6:

In the horizontal direction, the high-power devices are arranged as close to the edge of the printed board as possible to shorten the heat transfer path; in the vertical direction, the high-power devices are arranged as close to the top of the printed board as possible to reduce the temperature impact of these devices on other devices.

Tips 7:

The heat dissipation of the printed board in the equipment mainly depends on the air flow, so the air flow path should be studied in the design, and the device or printed circuit board should be reasonably configured. When air flows, it always tends to flow where the resistance is small, so when configuring components on a printed circuit board, it is necessary to avoid leaving a large airspace in a certain area. The configuration of multiple printed circuit boards in the whole machine should also pay attention to the same problem.

Tips 8:

Devices that are more sensitive to temperature are best placed in the area with the lowest temperature (such as the bottom of the device). Never place it directly above the heat-generating device. Multiple devices are best staggered on a horizontal plane.

Tips 9:

Place the devices that consume the most power and generate the most heat near the best heat dissipation locations. Do not place high-heat components on the corners and edges of the printed board unless a heat sink is arranged near it. When designing the power resistor, choose a larger device as much as possible, and adjust the layout of the printed board so that there is enough space for heat dissipation.

Tips 10:

Avoid the concentration of hot spots on the PCB, distribute the power evenly on the PCB board as much as possible, and keep the temperature performance of the PCB surface uniform and consistent. It is often difficult to achieve a strict uniform distribution in the design process, but it is necessary to avoid areas with too high power density, so as to avoid hot spots affecting the normal operation of the entire circuit. If conditions permit, it is necessary to conduct thermal performance analysis of printed circuits. For example, the thermal performance index analysis software module added in some professional PCB design software can help designers optimize circuit design.

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Flex PCB1

Why flexible circuit boards is easy soldering failures?

The factors that cause the welding defects of flexible circuit boards have the following three reasons：

The solderability of the flexible circuit board holes affects the soldering quality

The poor solderability of the holes of the flexible circuit board will cause virtual welding defects, which will affect the parameters of the components in the circuit, resulting in unstable conduction of the components and inner layers of the multi-layer board, resulting in the failure of the entire circuit function.

Flex PCB

The main factors affecting the solderability of printed flexible circuit boards are:

(1.) The composition of the solder and the properties of the solder being soldered. Solder is an important part of the welding chemical treatment process. It is composed of chemical materials containing flux. The commonly used low-melting eutectic metals are Sn-Pb or Sn-Pb-Ag, and the impurity content must be controlled by a certain proportion. , to prevent oxides produced by impurities from being dissolved by the flux. The function of the flux is to help the solder wet the circuit surface of the board being soldered by transferring heat and removing rust. White rosin and isopropyl alcohol solvents are generally used.

(2.)Soldering temperature and cleanliness of the sheet metal surface also affect solderability.If the temperature is too high, the diffusion speed of the solder will be accelerated. At this time, it has high activity, which will rapidly oxidize the molten surface of the flexible circuit board and the solder.Soldering defects and contamination on the surface of the flexible circuit board will also affect the solderability and cause defects. These defects include solder balls, open circuits, and poor gloss.

Welding defects caused by warpage

Flexible circuit boards and components are warped during the welding process, and defects such as virtual welding and short circuits are generated due to stress deformation.Warpage is often caused by the unbalanced temperature of the upper and lower parts of the flexible circuit board.For large PCBs, warpage will also occur due to the weight of the board itself.Ordinary PBGA devices are about 0.5mm away from the printed flexible circuit board. If the device on the flexible circuit board is large, as the circuit board returns to its normal shape after cooling down, the solder joints will be under stress for a long time. If the device is raised by 0.1mm, the Enough to cause open soldering.

The design of the flexible circuit board affects the welding quality

In terms of layout, when the size of the flexible circuit board is too large, although the welding is easier to control, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases.If it is too small, the heat dissipation will decrease, the welding will be difficult to control, and the adjacent lines will easily interfere with each other, such as the electromagnetic interference of the circuit board.Therefore, the PCB board design must be optimized:

(1.) Shorten the connection between high-frequency components and reduce EMI interference.

(2.) Components with heavy weight (such as more than 20g) should be fixed with brackets and then welded.

(3.)The heat dissipation problem should be considered for the heating element to prevent defects and rework caused by large ΔT on the surface of the element, and the thermal element should be kept away from the heat source.

(4.)The arrangement of components should be as parallel as possible, which is not only beautiful but also easy to weld, and suitable for mass production.The flexible circuit board is best designed as a rectangle of 4:3. Do not change the wire width abruptly to avoid wiring discontinuities. When the flexible circuit board is heated for a long time, the copper foil is easy to expand and fall off. Therefore, the use of large-area copper foil should be avoided.

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PCB layer stack

How to design PCB layer stackup?

How to choose how many layers of boards are used for design and how to stack them, Today we are going to talk about this topic.

The following are some common stacked structures, let’s analyze them：

PCB Layer stackup of single-sided and double-sided boards

For two-layer boards, there is no stacking problem due to the small number of layers. Controlling EMI radiation is mainly considered from wiring and layout.The electromagnetic compatibility of single-layer and double-layer board is becoming more and more prominent.The main reason for this phenomenon is that the signal loop area is too large.It not only produces strong electromagnetic radiation, but also makes the circuit sensitive to external interference.The simplest way to improve the electromagnetic compatibility of the line is to reduce the loop area of key signals.

Key signals: from the perspective of electromagnetic compatibility, key signals mainly refer to signals that produce strong radiation and signals sensitive to the outside world.Signals that can produce strong radiation are generally periodic signals, such as low-order signals of clocks or addresses.Interference sensitive signals refer to those analog signals with low level.

Single and double-layer plates are usually used in low-frequency analog design below 10kHz:

(1.) The power lines on the same floor shall be routed radially, and the total length of the lines shall be minimized.

(2.)When layout the power and ground wires, close to each other.Lay a ground wire beside the key signal line,This ground wire should be as close as possible to the signal wire.then appear a smaller loop area,reduce the sensitivity of differential mode radiation to external interference.When a ground wire is added next to the signal line,It forms a circuit loop with the smallest area.The signal current will take this loop instead of other ground paths.

(3.)If it is a double-layer circuit board, a ground wire can be laid along the signal line on the other side of the circuit board, close to under of signal line, and the line shall be wider as possible,then the loop area thus formed is equal to the thickness of the circuit board multiplied by the length of the signal line.

PCB Layer stack up design of 4 layer PCB:

There are two types of layer stack up design:

  1. SIG－GND(PWR)－PWR (GND)－SIG.
  2. GND－SIG(PWR)－SIG(PWR)－GND.

For the above two stack up order designs, the potential problem is for the traditional 1.6mm (62mil) PCB thickness. The layer spacing will become very large, which is not only unfavorable for impedance control, interlayer coupling and shielding.In particular, the distance between the power ground layers is large, which reduces the board capacitance and is not conducive to filtering out noise.

For the first solution, it is usually used in the case of more chips on the board.This solution can get better SI performance, but it is not very good for EMI performance. It is mainly controlled by wiring and other details.

Note: The ground layer is placed on the connecting layer of the signal layer with the densest signal, which is conducive to absorbing and suppressing radiation; increase the board area to reflect the 20H rule.

For the second solution, it is usually used when the chip density on the board is low enough and there is enough area around the chip (where the required power supply copper layer is placed),In this scheme, the outer layers of the circuit board are all ground layers, and the two middle layers are signal/power layers.The power supply on the signal layer is routed with a wide wire, which can make the path impedance of the power supply current low, and the impedance of the signal microstrip path is also low, and can also shield the inner layer signal radiation through the outer ground. From an EMI control standpoint, this is the best 4-layer board structure available.

Note: The distance between the middle two layers of signal and power mixed layers should be widened, and the wiring direction should be vertical to avoid crosstalk. appropriate control panel area, reflecting the 20H rule.If you want to control the trace impedance, the above scheme is very careful to route the traces under the power and ground copper islands.In addition, the copper layers on the power or ground plane should be interconnected as much as possible to ensure DC and low frequency connectivity.

Layer stack up design of 6 layer PCB

1.SIG－GND－SIG－PWR－GND－SIG

For the design with high chip density and high clock frequency, the design of 6-layer board should be considered, and the stacking method is recommended.

For this scheme, this stack-up scheme can achieve better signal integrity, the signal layer is adjacent to the ground layer, the power layer and the ground layer are paired, the impedance of each trace layer can be well controlled, and the two The formation is able to absorb the magnetic field lines well.And in the case of complete power supply and ground layer, it can provide a better return path for each signal layer.

2.GND－SIG－GND－PWR－SIG －GND

For this scheme, this scheme is only suitable for the case where the device density is not very high. This kind of stack has all the advantages of the above stack, and the ground plane of the top and bottom layers is relatively complete and can be used as a better shielding layer. to use. It should be noted that the power layer should be close to the layer that is not the main component side, because the bottom plane will be more complete. Therefore, the EMI performance is better than the first solution.

Summary: For the scheme of the six-layer board, the distance between the power layer and the ground layer should be minimized to obtain good power and ground coupling. However, with a thickness of 62 mil, although the layer spacing has been reduced, it is not easy to control the distance between the main power supply and the ground layer to be very small. Comparing the first scheme with the second scheme, the cost of the second scheme is greatly increased. Therefore, we usually choose the first solution when stacking. When designing, follow the 20H rule and the mirror layer rule design

Layer stack up design of 8 layer PCB

This is not a good stacking method due to poor electromagnetic absorption capability and large source impedance. Its structure is as follows:

1.Signal 1

2.Signal 2

3.Ground

4.Signal 3

5.Signal 4

6.Power

7.Signal 5

8.Signal 6

It is a variant of the third stacking method. Due to the addition of the reference layer, it has better EMI performance, and the characteristic impedance of each signal layer can be well controlled.

1.Signal 1
2.Ground
3.Signal 2
4.Power
6.Signal 3
7.Power
8.Signal 4

The best stacking method, due to the use of multiple ground reference planes, has a very good geomagnetic absorption ability.

1.Signal 1
2.Ground
3.Signal 2
4.Power
6.Signal 3
7.Ground
8.Signal 4

How to choose how many layers of boards are used for design and how to stack them depends on many factors such as the number of signal networks on the board, device density, PIN density, signal frequency, board size and so on.We have to take these factors into consideration. For the more the number of signal networks, the greater the device density, the greater the PIN density, and the higher the frequency of the signal, the multi-layer board design should be used as much as possible.In order to get good EMI performance, it is best to ensure that each signal layer has its own reference layer.

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How to improve PCB circuit high-speed signals loss?

When the PCB design is completed, the laminate layer is determined, and the PCB board has been put into fabrication, the simulation evaluation found that the high-speed signal channel margin may be unsafe. Is there still a chance to improve?

Let’s first introduce the loss classification of PCB. There are three kinds of losses: conductor loss, dielectric loss and radiation loss.

You may hear more about the first two. In fact, the loss of PCB is mainly the first two. As for the radiation loss, it is also related to the dielectric constant DK. In addition, the radiation loss will only exist in the microstrip line. If it is properly designed, it can be reduced to a relatively low level, accounting for a very small proportion in the total loss.

Among them, the dielectric loss is mainly caused by the polarization phenomenon of the dipole. As shown in the figure below, the higher the applied voltage frequency, the greater the current, the greater the number of dipoles swinging in the material, the greater the amount of dipole movement under the action of the electric field, and the greater the volume resistivity of the medium. The power loss in the system is higher. In order to describe the material properties that measure the law of dipole motion, the concept of DF came into being.

PCB-DF

Let me talk about another principle of conductor loss. First of all, we must know that there is an important concept in the high-speed theory called the skin effect. At higher frequencies, the current will run along the surface of the conductor, that is, at high frequencies, the size of our resistance depends on the current flowing through. The size of the cross-section, the smaller the cross-section through which the current flows, the greater the resistance, so the conductor loss also gradually increases with the increase in frequency.

PCB Current Distribution

To sum up the loss factors mentioned above, the sheet material mainly determines the dielectric loss, that is, the size of the sheet DF that we often say has the greatest impact on the loss, and it is for this reason that we distinguish between different grades of sheets . In addition, the line width and copper thickness of the trace affect the conductor loss.

The above mentioned is very consistent with the scenario we summarized. The board is fixed, the dielectric loss is basically fixed, the stacking and design are fixed, the wiring structure is fixed, and the conductor loss is basically fixed.

So if we still want to improve the loss in this case, we must see if there are any factors that can affect our loss.

In addition to the above factors, we also introduced the influence of copper foil roughness. The surface of the copper foil is relatively rough (in order to increase the adhesion between the copper foil and PP), so the roughness of the copper foil needs to be considered at high speed, and the roughness of the copper foil will also affect the loss of the trace.

In fact, this loss can also be included in the conductor loss. The principle is probably like this. Due to the skin effect, the current will be transmitted on the copper teeth. When passing through the protruding copper teeth, the current transmission path becomes longer than the smooth copper surface, so it will further increase the DC and AC resistance at the same time, so as to increase the conductor loss.

We are familiar with several types of copper foils with different roughness grades, including ordinary STD copper foil, RTF reversal copper foil and HVLP ultra-low profile copper foil. Of course, there are now HVLP2 and HVLP3 copper that continue to be optimized on the basis of HVLP copper foil.

But the reality is like this. Many friends know that copper foil with different roughness will affect the loss, but they don’t know how much influence it has. Can there be quantitative data to give it.

In fact, everyone thinks it is not easy. The not easy point is that the loss is composed of several large parts, which mainly include the sheet DF, the trace width and copper thickness, the reference layer thickness and the copper foil we are talking about now. Roughness.

If you want to extract the effects of different types of copper foil roughness separately, then you must ensure that the factors are consistent in order to simply extract them. The specific point is to ensure that the board is the same, the wiring structure is the same, and the thickness of the wiring reference is also the same. Only when the roughness of the copper foil is different can the influence of the copper foil roughness be known separately. Do you think it is possible to do it?

Of course, since Mr. High-speed asked, it means that Mr.High-speed must have done it! Mr. High-speed also specially made a test board for this purpose, in order to compare the difference between the popular RTF and HVLP copper foils. Yes, it is simply the difference brought about by the difference in copper foil!

Mr. High-speed makes HVLP and RTF copper foils on the upper and lower parts of the same laminate. Because the upper and lower parts are symmetrical, the wiring structure and the thickness of the dielectric are exactly the same. If the same laminate is used, it means that the board is the same, and it is the same time. Processing will not produce processing errors, because you can successfully do this by walking the same length of traces on the trace layers of the designated RTF and HVLP copper foils!

PCB Layerstckup-1

Just like this test board, we take the RTF and HVLP copper foil traces on the L5 and L12 layers respectively, and the trace length is the same (the trace length is 3 inches after de-embedding), so in addition to the copper foil roughness, Other factors can be guaranteed to be the same. In this way, the two pairs of traces are tested, and the effect of two different copper foil roughness on loss can be obtained by pure comparison.

We immediately give the loss difference tested, as shown below: From the loss results, RTF copper foil is indeed worse than HVLP copper foil (everyone knows this). From the quantitative analysis, the effect of pure copper foil roughness on loss has reached the level of 15%-20%, which can be regarded as a big influencing factor.

 

RTF copper foil

This article is considered to be a relatively long article. First of all, we theoretically analyze several major factors that cause PCB loss. Then we shared with you a test method of how to separately extract the influence of different copper foil roughness on the loss, so as to quantitatively get the influence of different roughness on the wiring loss.

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RO4350B PCB

How to choose high frequency circuit board materials?

With the rapid development of electronic design technology and manufacturing technology, electronic products are gradually developing towards high density, high functionality, light,small, and high transmission rate; In addition, with the rapid development of chip miniaturization and the increase of the number of data transmission, the working frequency of the system is higher and higher, and the application of high-frequency circuit board is more and more widely.

High frequency circuit board circuit design, structure design and process technology are the three elements of electronic product design. To become an excellent electronic product, we need not only excellent circuit design, but also good manufacturability, because good manufacturability can reduce the problems in mass production, shorten the product development progress and reduce the design cost, so as to improve the competitiveness of products. Therefore, the selection of circuit board materials by product design engineers in electronic product design is particularly important.

With the development of communication technology from 2G and 2.5G to 3G, as well as the current 4G and 5G, the data transmission throughput is getting larger and larger, the bandwidth required is getting wider and wider, and the frequency is getting higher and higher; The miniaturization of equipment is also one of the development trends in the future. Once the equipment becomes smaller, the circuit board needs to have higher heat conduction capacity and higher dielectric constant.

High frequency circuit board materials are mainly used in high-power amplifiers, base station antennas, global positioning systems, weather radars and weather satellites, as well as automotive radars and sensors.

RO4003C PCB

When the working frequency of the circuit is in the RF frequency band, the range of plates that can be selected by the design engineer will be greatly reduced. Rogers RO4835 high frequency circuit board board special formula improves oxidation resistance. When used for a long time at a certain temperature, it can provide particularly stable electrical properties while maintaining the same processing advantages as FR4 thermosetting resin material.

At the same time, like RO4350B, the dielectric constant (DK) is 3.48 at 10GHz, the dielectric loss (DF) is 0.0037, and has a low z-axis coefficient of thermal expansion (CTE), ensuring the reliability of metal vias under various processing and operating conditions. The x-axis and y-axis expansion coefficients of the material are similar to those of copper and have excellent dimensional stability.

When talking about how to select high-frequency circuit board raw materials, Rogers company suggested that “the main parameters of circuit board materials are DK and DF. In the circuit boards used for high-frequency, the stability of DK value is the guarantee of plate reliability; DF value should be as small as possible to reduce signal loss. In addition, some high-frequency plates will also be used in some high-speed and large-scale data transmission systems.” It is particularly emphasized that all circuit boards of Rogers materials meet ROHS standards, and some products have been halogen-free. At the same time, she also said, “because there is no mandatory standard for the use of halogen-free plates in the country, there are not many companies in demand, but Rogers is reserving technology for future demand.

In addition, 2929 adhesive sheet is another high-performance circuit board product recently launched by Rogers company. It is a thin hydrocarbon adhesive sheet without glass red reinforcement, which can be used for high performance, high reliability and multilayer plate structure. The product has a DK of 2.9 and low DF (less than 0.003) at microwave frequency. It is very suitable for laminating multilayer boards with high-performance circuit board materials. It can be pressed with traditional flat press and vacuum chamber press. At present, it can provide thicknesses of 0.0015, 0.002 and 0.003 inches.

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Rigid PCB

How PCB circuit boards are tested?

Do you know how PCB circuit boards are tested?

In general, all circuit boards should be 100% open for short circuit test before shipment. Automatic detection of circuit board is not only basic, but also necessary. What are the common test methods in complex circuit board testing?

Needle bed test method

The open circuit and short circuit test of PCB circuit board is to put a test pin at the end of the line and power on. If the resistance of the same network (line) is greater than the minimum on resistance, it is displayed as open circuit. If the resistance of different networks (lines) is less than the minimum insulation resistance, it is displayed as short circuit.

The probe with spring is connected to each detection point on the circuit board. The spring makes each probe have a pressure of 100 – 200g to ensure good contact at each detection point. Such probes are arranged together, which is called “needle bed”. The equipment of needle bed tester is expensive and difficult to maintain. The probes are arranged differently according to their specific application.

Flying probe test

Flying probe test is a program-controlled moving probe. Flying needle test has two test methods: capacitance method and resistance method.

The principle is: capacitance method: a capacitance can be formed between two conductors, and the standard capacitance value can be obtained through the “learning” of the software on the “standard board”. If it is lower than the standard, it is a short circuit, and if it is higher than the standard, it is an open circuit; Resistance method: similarly, the resistance between two conductors can be regarded as a resistance, and the standard resistance value can be obtained through the “learning” of the software on the “standard board”. If it is lower than the standard, it is open circuit and higher than the standard, it is short circuit.

The flying probe tester does not depend on the pin pattern mounted on the fixture or bracket.Based on this system, two or more probes are mounted on tiny magnetic heads that can move back and forth on the X-Y plane, and the test points are directly controlled by CADI Gerber data.

The double probes can move within a distance of 4mil from each other. The flying pin tester is based on the measurement of capacitance. The circuit board is tightly pressed on the insulating layer on a metal plate as another metal plate of the capacitor. If there is a short circuit between lines, the capacitance will increase; If there is an open circuit, the capacitance will decrease.

Using observation instruments

The circuit board has small volume and complex structure, so the observation of the circuit board must also use professional observation instruments.

Generally, we use a portable video microscope to observe. Using a video microscope camera, we can clearly see the very intuitive microstructure of the circuit board. In this way, it is easier for us to design and test the circuit board.

Now many circuit board manufacturers use portable video microscopes msa200 and vt101, which can realize “observation, detection and discussion at any time”, which is more convenient than traditional microscopes.

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5G Anternna PCB

Why 5G antenna PCB board need pouring copper on bottom and top layer

Do you know the advantages of PCB pouring copper on the surface of the PCB? In the whole process of PCB design, engineers want to ignore the link of pouring copper on the bottom of the PCB in order to save time. Is this right? Is it necessary for the PCB to pouring copper on the bottom of the surface? What are the advantages of copper on the bottom of the PCB?

First of all, we need to be clear: the copper on the bottom of the surface is beneficial and necessary for the 5G antenna PCB, but the copper on the whole board needs to comply with some conditions.

Advantages of pouring copper on the bottom of the PCB:

From the EMC point of view, the whole board is covered with copper on the bottom of the surface, which provides additional shielding protection and noise suppression for the inner signal, and also has a certain shielding protection for the components and signals on the bottom of the surface.

  1. From the perspective of heat dissipation, as the current PCB boards are getting denser and small, the BGA main chip also needs tobe considered thermal issues more and more. The copper on the whole board improves the heat dissipation capacity of the PCB board.
  2. From the perspective of process analysis, the entire board is covered with copper, so that the 5G antenna PCB board is evenly distributed, avoiding the bending and warping of the board during PCB processing and pressing, and avoiding the different stresses caused by the PCB over reflow due to the unbalanced copper foil. The PCB is warped and deformed.

Reminder: For two-layer boards, copper pouring is necessary

  1. On the one hand, since the two-layer board does not have a complete reference plane, the pouring copper can provide a return path and can also be used as a coplanar reference to achieve the purpose of impedance control. We can generally pouring copper on the bottom layer as the ground plane, and put the main components on the top layer and use power lines and signal lines. For high-impedance loops, analog circuits (analog-to-digital conversion circuits, switch-mode power conversion circuits), copper plating is a good practice.

The condition of copper pouring on the bottom surface.

Although pouring copper on the surface and bottom layer is good for 5G antenna PCB, it also needs to follow some conditions:

1.Pouring copperby hand at the same time as much as possible, do not cover it all at once, avoid broken copper skin, and properly add via holes in the copper paving area to the ground plane.

Reason: The surface copper-clad plane must be separated by the surface components and signal lines. If there is a poorly grounded copper foil (especially the thin and long copper), it will become an antenna and cause EMI problems. .

2. Consider the small foot print, such as 0402 0603 and other small packages, to avoid tombstone effect.

Reason: If the whole board is covered with copper, if the component pins are fully connected with copper, the heat will be lost too quickly, and it will be difficult to desolder and rework.

3.It is best to pour copper tothe whole board continuously. The distance from the ground to the signal needs to be controlled to avoid discontinuity in the impedance of the transmission line.

Reason: The copper skin that is too close when laying the ground will change the impedance of the microstrip transmission line, and the discontinuous copper skin will also cause a negative impact on the impedance discontinuity of the transmission line.

4.Some special situations depend on the application scenario. PCB design should not be an absolute design, it should be weighed and used in conjunction with the theories of all parties.

Reason: In addition to sensitive signals that need to be grounded, if there are many high-speed signal lines and components, a lot of small and long copper fragments are generated, and the wiring channels are tight, it is necessary to try to avoid the surface copper skin to be connected to the ground plane through holes. thennot to pour copper on the surface

The above are the advantages of pouring copper on the surface of the PCB board, I hope to help you.

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PCB ground layer

Why Is The PCB Ground Plane Important?

If you do not pay attention to the ground layer during PCB design, the circuit board may not achieve the expected performance and may suffer from various forms of electrical noise and interference. Let’s take a look at what is a PCB ground plane and some tips on how to ensure a solid foundation for PCB design as a circuit board.

Why is the PCB ground plane important?

The ground plane on a PCB circuit board is usually a large area of ​​metal connected to the circuit ground. This metal area is sometimes only a small part of the board, or in a multi-layer design, it can be the entire board layer. Depending on the design needs, it may even occupy multiple layers.

The ground plane has three important purposes on the PCB circuit board:

(1.)Voltage return: Most of each component on the PCB will be connected to the power supply network, and then the return voltage will return through the grounding network. On a board with only one or two layers, the ground net usually must be routed using a wider trace. However, by dedicating the entire layer to the ground plane of the multilayer board, the process of connecting each component to the ground grid can be simplified.

(2.)Signal return: Conventional signals also need to return. For high-speed designs, it is very important to have a clear return path on the ground. Without this clear return path, these signals will cause great interference to the rest of the PCB.

(3.)Reduce noise and interference: With the increase of signal speed, there will be more and more switching states of digital circuits. This will generate noise pulses through the ground circuit, which may affect other parts of the circuit. A ground plane with a larger conductive area helps to reduce this interference because it has a lower impedance compared to the grounding net routed through wires.

By carefully planning the layer stacking configuration of a multilayer circuit board, PCB designers can use the ground plane to help control the electrical performance of the circuit board. By using a ground layer between the two active signal layers, the crosstalk between the signals on these layers can be eliminated. And by ensuring that there is an uninterrupted signal return path on the ground plane, the signal integrity of high-speed transmission lines can be improved. The ground plane is usually also connected to components that get hot to help dissipate heat

 Important elements of power supply or ground plane

As we said, the ground plane can be a designated area of ​​metal on the circuit board layer, or it can occupy the entire layer itself. How to accomplish this depends on the type of PCB design CAD system you use. Most CAD systems allow you to draw a plane so that it looks like a whole piece of metal, and you can specify any net for the plane, whether it is ground, power, or other shapes. CAD systems used to use negative image planes because they require less computing power, but today most PCB designers prefer to draw positive image planes because these tools now have sufficient functionality.

The PCB designer will draw the area of the power or ground plane on the specified layer according to the shape they want. The inner layer of the circuit board is usually the entire layer, and the outer layer of the circuit board is usually a small area to repair a specific component or group of components. In some cases, the designer may split the plane on the inner layer. This may prove to be very useful when reducing the number of layers of the circuit board to be manufactured. Usually, the power network is usually divided into planes, and the ground plane still retains a complete layer to help improve signal integrity and eliminate noise and interference.

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