Introduction:
Ball Grid Array (BGA) is a widely used high-density packaging technology. Its main characteristic is that the chip’s pins are arranged beneath the package as spherical solder balls. This design enables smaller device sizes, more pins, greater pin spacing, higher assembly rates, and improved electrical performance. As a result, BGA packaging has become increasingly popular in the market.
Currently, there are limited methods for inspecting the quality of BGA soldering. Common inspection techniques include visual inspection, flying probe testing, X-ray inspection, dye penetrant testing, and cross-sectional analysis. Among these, dye penetrant and cross-sectional methods are destructive, suitable primarily for failure analysis but not ideal for routine soldering quality inspections. In non-destructive testing, visual inspection can only detect issues on the surface of the solder balls, while flying probe testing tends to have a high false-positive rate. X-ray inspection, on the other hand, is the most effective non-destructive method for evaluating BGA soldering quality, as it can detect issues hidden beneath the component’s surface.
Detection Process of BGA Soldering Quality using X-Ray Inspection:
BGA devices typically contain hundreds of solder balls, and multiple defects can occur simultaneously. Therefore, engineering experience and a systematic inspection process are critical. The common inspection workflow is outlined below.
Solder Ball Bridging and Solder Ball Loss:
These defects are easily detected through two-dimensional X-ray inspection. Solder ball bridging and loss can typically be identified by simply observing the BGA device as a whole.
Solder Ball Shifting:
Shifting of the solder ball is characterized by a distortion in the alignment of the BGA solder balls in a particular direction. This defect is detectable through X-ray inspection. More importantly, the extent of the solder ball shift must be quantified. To achieve this, the BGA image is magnified, and parameters like X-ray intensity and image contrast are adjusted for better clarity. The deviation of the solder ball’s center from the pad’s center is then measured. The diagram below illustrates a top-down view of shifted solder balls. Here, “L” represents the distance from the center of the solder ball to the center of the solder pad, while “D” is the diameter of the solder pad. The solder ball shift offset is calculated as L/D. Generally, a shift ratio (L/D) of less than 25% is considered acceptable, but this may vary depending on customer specifications.
Solder Ball Void (Cavities):
The internal voids or cavities within solder balls are easily visible through two-dimensional X-ray imaging. The figure below shows an example of a solder ball cavity as seen under X-ray. The area indicated by the arrow is the cavity inside the solder ball, appearing as a bright white spot against the dark background of the solder joint. Most X-ray inspection systems include software tools to calculate the size of these voids. If the total void area exceeds 25% of the solder ball’s total area, the BGA is considered defective and must be repaired.
Pseudo Soldering:
Two-dimensional X-ray inspection is often used for preliminary detection of pseudo soldering, enhancing the overall inspection efficiency.
Pseudo soldering defects are often difficult to detect using two-dimensional X-ray imaging. Therefore, 3D computed tomography is typically used to detect whether such defects are present.
Conclusion:
The quality of BGA (Ball Grid Array) soldering can be impacted by issues such as missing solder balls, solder ball misalignment, and pseudo soldering. While some defects may be immediately visible, others may cause failures during later use. To address this, it’s essential to balance detection efficiency during production. Combining 2D imaging with 3D tomography allows for more flexible and comprehensive fault detection, providing reliable quality assurance for BGA devices.
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