When reworking BGA chips, the quality of ball mounting cannot be judged solely by the naked eye or AOI, because all solder balls are hidden under the package. Only X-Ray inspection can penetrate ceramic or plastic encapsulation to clearly present the true state of each solder joint. After completing ball mounting on the JiajinYuan rework station, we must immediately send it for inspection; otherwise, if problems occur during subsequent reflow, it will be impossible to trace whether the cold solder joint is caused by ball mounting deviation or abnormal furnace temperature.

If a single solder ball is found to be significantly small or blurred at the edge in the X-Ray inspection image, it is most likely caused by insufficient Solder Paste amount during ball mounting or hot air temperature not covering the entire area. If multiple adjacent solder balls show a drawn-out gray shadow, it indicates that the squeegee pressure is too high, causing the solder paste to be dragged and deformed. This type of defect is completely invisible during conventional visual inspection, but under X-Ray, it will form a continuous gray band accompanied by a local density increase.

We once encountered a batch of 676-pin BGAs with a functional test pass rate of only 73% after rework. Disassembly revealed that most failure points were concentrated in the 48 corner solder balls. X-Ray retesting showed that the void rate at these positions generally exceeded 35%, far exceeding the 25% upper limit allowed by IPC-A-610. Further investigation confirmed that the hot air nozzle of the JiajinYuan rework station was calibrated with an offset of 0.15mm, causing uneven local heating and explosive volatilization of the Flux.

For BGAs with a 0.4mm pitch, the X-Ray equipment must enable a tube voltage above 100kV and a 5μm-level microfocus; otherwise, ghosting will appear at the solder ball boundaries, resulting in a misjudgment rate as high as 40%. Our production line uses the JiajinYuan supporting X-Ray machine as standard, whose automatic recognition algorithm can individually mark the diameter, roundness, and average grayscale of each solder ball, and export CSV for SPC analysis, avoiding subjective errors caused by manual image reading.

In actual operations, it has been found that X-Ray inspection results are strongly correlated with the final soldering yield. When the average grayscale of solder balls in the central area of a certain batch image is 8% lower than that of the surrounding area, the open-circuit defect rate after reflow for that batch will inevitably rise to around 12%. This indicates that the solder paste has undergone slight oxidation during the ball mounting stage; although the appearance is intact, its activity has seriously decreased. At this time, this can of solder paste must be discontinued and replaced with a newly opened batch.

The repeated positioning accuracy of the ball mounting fixture of the JiajinYuan rework station is nominally ±0.02mm, but after long-term use, if quarterly calibration is not performed, the measured offset can reach ±0.07mm. The X-Ray image will show that the entire row of solder balls is regularly tilted. At this time, simply adjusting the furnace temperature is meaningless; it is necessary to first perform mechanical reset of the fixture and use X-Ray to verify whether the reference point offset returns to within ±0.03mm.