When performing BGA soldering quality assessment, we must rely on X Ray equipment to perform penetrating scans of solder joint cross-sections, as optical inspection is completely unable to identify void defects inside solder balls. The presence of voids directly reduces heat transfer efficiency and accelerates solder joint fatigue cracking; especially in automotive and industrial control products, excessive void rates often cause early failures.
The JiajinYuan production line currently uses a 2D X Ray system paired with automatic recognition software, set to a minimum resolution of 5 micrometers and a magnification of 1000x, ensuring clear capture of micro voids larger than 15 micrometers in diameter. After image acquisition, the algorithm automatically segments the solder ball region and void region, and calculates the void rate based on pixel statistics. The entire process requires no manual region selection, avoiding subjective errors.
In actual operation, we found that voids are not uniformly distributed at the center of solder balls, but often aggregate along the pad edge or on the offset side of Solder Paste printing. This is closely related to blocked Flux volatilization channels during reflow, insufficient nitrogen flow, or unreasonable stencil aperture design. Therefore, we feed X Ray inspection results back to the solder paste printing and reflow oven parameter teams, adjusting squeegee pressure to 4.2 kg, reducing nitrogen oxygen content to below 100 ppm, raising peak temperature to 238°C and maintaining it for 65 seconds. The void rate dropped from an average of 7.3% to 3.1%.
For different package sizes, the acceptable void rate criteria must also be dynamically adjusted. For example, QFN devices with 0.4mm pitch allow a single solder joint void rate of no more than 3%, while SOIC with 1.27mm pitch is relaxed to 8%. However, all devices must meet the hard requirement in IPC-A-610E Class 2 that the total void area proportion is ≤5%; otherwise, even if the appearance is good, the product cannot be released.
We once encountered a batch of GPU modules for a customer order that failed X Ray re-inspection three consecutive times, with voids concentrated in corner solder balls. The final root cause was identified as a 0.012mm height deviation in the stencil bottom support block, leading to insufficient local solder paste thickness. After replacing with a JiajinYuan custom stencil, the batch passed inspection on the first try. This shows that void X Ray inspection is not only a final inspection method, but also a key input for process root cause analysis.
During daily maintenance, the focus position and grayscale threshold of the X Ray equipment must be calibrated every day; otherwise, reduced image contrast will lead to missed detection of small voids. Last week, due to failure to clean the detector window in time, 32 chips were misjudged as having acceptable void rates, and the actual measured value after rework reached 9.7%. Therefore, a standard test board must be used for image consistency verification before inspection, and baseline image parameters must be recorded at least twice per shift.
