PCB is printed circuit board,it mechanically supports and electrically connects electronic components using conductive tracks,pads and other features etched from copper sheets laminated onto a non-conductive substrate,achieve electronic components automated assembly,welding,inspection,guaranteed the quality of electronic devices,improved productivity and reduced the cost,it’s favorable for repair.
PCB from single layer to multilayer,flex,keep ourselves strong development.These years tend to high precision,high density,high reliably and small size to reduce cost and improve functions,PCB industry keep strong vitality in future electronic devices development.
Analysis of PCB board common defects
Various defects happened in usage follow PCB circuit complicate and components integrated.We summarized four common defects:
- The PCB layout is inappropriate,electromagnetic interference from around components.
- PCB circuit damage lead to system out of work.
- Components unsteadiness lead to device unstable.
- Incomplete welding lead to PCB board opening or short.
Works before PCB board inspection
- Know about the environment of device working,main the electric parameter impact from external environment.
- Inquiry the phenomenon led to PCB board failure and analysis the reasons caused.
- C. Check up carefully components of PCB board,find out the important components.
- D. Do the electrostatic prevention and electromagnet interference prevention measure.
- PCB board inspection process and rules.
The PCB board inspection flow chart
The flow chart indicated common process,from graphic we know the first step is inspect PCB structure and appearance,then make test for the important discrete components and IC integrated circuit.Repair or replace failure components after found out defect parts,use instrument to re-test PCB.The discrete components,IC integrated circuit is the most important parts,we understand the flow chart into four items：from observe to measure,from outside to inside,from easy to complicated ,from static to dynamic.
From artificial observation to instrument measure
All components connected by surface circuit in PCB, So the fist thing we can observe appearance by magnifier or microscope when it happened errors.
- Whether all components connected completely? If power and ground position can work.
- Whether pins are correct welding of integrated chip,diode,BJT,resistance,capacitor,inductance etc.
- Whether components have incomplete weld,miss welding and other operation problems.
Most damages can find through above initial observation and differentiate. But some defects can’t be discover by naked eyes,then need use multimeter to check.The normal impedance is 70~80 ohm between ground and power layer, if the value is too small meant some components perhaps be punctured,then we must find out the parts be punctured.In general we touch the burning hot parts and find out damages when power cord works. If the parts works then we need check and debug each components compared input and output parameter .
From outside to inside
The ratio failure components lead to PCB defects is large from above analysis,so how to find out the failure components is very important.As PCB structure feature,the components at outside of edge are easy damage,the ratio reach 86%.Components at outside major used in driver,inverter,isolation,protecting and communication for easy connection with inner circuit.They always be impacted from current surging,vibrating and external power lead to noise and dust,thus failure happened to this outside components.Components placed inside are major used in generate,amplify,transmit signal.Check the inside components after eliminated components at outside.
From easy to complicated
There related some test technology in PCB inspection,we follow rules:from easy to complicated in checking process.
a.The works before PCB test
Circuit Simulation is an effective way to reduce design period and cost.But simulation is in perfection condition ,it ignored all interferences at actually works,so shield all interferences before test is necessary.In general way shielding interferences is short of crystal oscillator and dismantle CUP avoid testing hamper.
b.From easy to complicated in special test way
We use rule “from easy to complicated “way to check components,because easy components can easy find out problems. We use exclusive method to check it one by one and record it.Retest that error caution PCBs to guarantee accuracy. We consider it important suspect for the PCBs failed test.
c. Complementary test method between different method
Sometimes we can’t find out the problem if only use one test way.we need use all kinds of test method to check from easy to complementary, now PCB test way from MVI,ICT to BST, and now more new test technology apply in PCB test.
From static to dynamic
All the methods above mentioned multimeter and other methods are static inspection.But sometimes the reason is very complicated and we can’t diagnose in static status.We use multimeter to check when power is on if static inspection can’t work.Integrated IC always use this method to check.
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