An unexpected PCB blistering anomaly occurred on the SMT line, and the production line was urgently shut down
At 9:15 a.m. today, when the SMT Line 1 of the company was switching to a certain batch of PCBs for Customer A, the AOI inspection machine at the exit of the reflow oven suddenly gave an alarm. The operator retrieved the images and found that there were three bulges with a diameter of about 2 mm on the solder mask layer of the 3rd board, accompanied by slight delamination. The production line immediately stopped and isolated the 1,200 pieces of materials in this batch. The quality, process, and equipment teams quickly assembled and launched the abnormal analysis process.
Reflow soldering curve and thermal imaging verification: The parameters are fully compliant
The process engineer retrieves the reflow soldering temperature curve immediately: This batch uses lead-free solder paste. It stays in the preheating zone (120 - 150℃) for 120 seconds, in the holding zone (150 - 180℃) for 90 seconds, and the reflow peak temperature is 255℃ (the upper limit of the specification is 260℃). The coincidence degree with the previous batch's curve reaches 99.2%, and there is no jump or out-of-specification situation. The equipment engineer simultaneously exports the thermal imaging data after solder paste printing. The error of the PCB surface temperature uniformity is ±2℃ (far lower than the specification of ±5℃), completely ruling out the blistering caused by local overheating.
Backtracking of the baking process: There are no loopholes in the pre-treatment stage
Immediately afterwards, check the PCB baking records: Since the storage time of this batch exceeded 72 hours, dehumidification baking was carried out at 125℃ for 4 hours according to the SOP before being put on the production line. The temperature uniformity report of the baking oven shows that the deviation of each point in the oven is ≤3℃; the humidity card is blue (humidity <40%rh), which meets the requirements for being put on the production line. after baking, it was left standing for 2 hours (the specification is ≥1 hour), and there is no residual stress caused by "thermal expansion and contraction". all records are signed and confirmed, and there are no missing processes.
Warehousing condition verification: No abnormalities were found in the environment and packaging
The storage data provided by the warehouse administrator shows that during the storage period of this batch of PCBs, the temperature and humidity in the warehouse were stably maintained at 22°C ± 2°C and 40%RH ± 5% (meeting the IPC - 6012 standard). The air - tightness test report of the vacuum packaging shows that the oxygen residual amount is less than 1%, and there are no pinholes or damages. The stacking height is 1.2 meters (the upper limit of the specification is 1.5 meters), and the bottom pallet is not deformed, ruling out the possibility of substrate damage caused by heavy pressure.
PCB manufacturer collaborative investigation: The root cause was not identified through full - dimensional testing
At 11 a.m., the quality engineer from the PCB manufacturer arrived on-site and simultaneously carried out three verifications:
Production record verification: The parameters for inner - layer laminate pressing (180°C/300psi/90 minutes) and solder mask curing (150°C/30 minutes) all meet the agreement. The curing degree test result reaches 95% (specification: ≥90%).
Slice analysis: The blistering area is located between the solder mask layer and the substrate, with no impurities or uncured resin.
Substrate performance test: The Tg value (glass transition temperature) is 150°C, which meets the customer's requirement of ≥140°C.
No abnormalities were found in all the tests, and the manufacturer was also unable to explain the reason for the blistering.
Current predicament and subsequent actions: The production line has been stopped for 4 hours, and we are still looking for a breakthrough
As of 1 p.m., the production line had been shut down for 4 hours. After temporarily switching to the materials from the backup supplier, production resumed, but the 1,200 pieces from the original batch remained in isolation. The engineering team is implementing two emergency measures:
1. Accelerated aging test: Place the original batch of PCBs in an environment of 85°C/85%RH for 24 hours to simulate the blistering risk under extreme conditions.
2. Contact SGS to conduct Fourier Transform Infrared Spectroscopy (FTIR) analysis to investigate potential hidden resin degradation or contaminants.
Currently, the problem has not been located yet. The production capacity loss is approximately 150 pieces per hour. In the follow-up, the test results need to be continuously tracked.