PCB via hole problem in PCB proofing production

PCB via problem phenomenon description
During the trial production of the new intelligent conference equipment samples produced by circuit R&D, when the samples were subjected to strict aging test and the system performance test, it was found that two devices were disconnected (abnormal communication), accounting for 4% of the total tested products. After analysis, it is determined that the PCB via hole problem leads to abnormal product communication and dropped line.
Analysis process of PCB via hole problem
1. Voltage-PLL stability measurement
The failure of defective products is confirmed, and the measured power supply voltage of each functional module of the product meets the design requirements, and the stability of FPGA phase-locked loop strictly meets the design index.
2. Print judgment
By printing information, the fault is identified, and the fault area is locked to narrow the analysis range. The software prints and compares the data received by the abnormal device with the data sent by the connected device, and finds the communication line that causes the communication data error.
3. Resistance measurement
Conduct a comprehensive survey of the communication lines that cause communication data errors, and find the abnormal points of resistance. The factors that may affect the abnormal resistance of communication lines are: empty soldering of Pin pins on communication lines, tin connection between pins, bad via holes on lines, short and broken copper on PCB. Microscopic observation shows that the chip on the communication abnormal circuit is in good welding condition. The sectional measurement and comparison show that the resistance values of the two ends of the via holes on the communication abnormal circuit are about 500 mω, and the resistance values of the two ends of the similar via holes in the same position and nearby of the normal products are about 20 mω. It is preliminarily determined that the poor copper plating resistance of the via holes on the circuit leads to the communication abnormal circuit.
4. Via confirmation
Analyze the abnormal points of resistance value, and determine the causes of abnormal resistance value. When the temperature of PCB board rises, it will expand with heat and contract with cold. When the copper plating of PCB via hole is poor, the resistance of via hole will increase when the temperature of via hole rises. The via with abnormal resistance was heated by hot air cylinder (the temperature was controlled at about 100℃). When the via resistance was tested again, it was found that the via resistance increased with the increase of temperature, which proved that the via did have poor copper plating.
5. Flying line confirmation
The flying wires are used to connect the board surfaces at both ends of the via holes with the corresponding lines at the bottom of the board, so as to eliminate the influence of poor copper plating on the line resistance. After the product is powered on again for 72 hours, the system test shows that all functions of the product are normal, and there is no communication abnormality, which further indicates that the communication abnormality of the product is caused by poor copper plating on PCB via holes.
Treatment of PCB via hole problem
Inform the supplier PCB factory of the problem analysis results in time for improvement and send the improved products to the company for heavy volume test, and at the same time purchase the products of other PCB manufacturers for strict sample trial production and test. At the same time, the PCB board via hole defect inspection specification is added to the company’s PCB incoming inspection specification document and strictly implemented in incoming inspection.