The growing popularity and popularity of ball grid array packages continues to present new challenges for manufacturers and equipment suppliers. Due to the high number of hidden solder joints, defects cannot be detected by visual inspection or electrical inspection, so other detection techniques are required.
In particular, the BGA open circuit is becoming one of the most critical types of defects in today’s complex PCBA inspections. Open circuit is a common problem in visual inspection systems. Although X-rays can “see” the difference in diameter of the solder joints, it is often difficult to detect if these differences are significant, ie, when these differences are true open and when the manufacturing process changes normally.
Selcom Group, a large European electronics contract manufacturer, pioneered the use of 3D automated X-ray inspection 10 years ago to improve process control and defect detection capabilities in complex board manufacturing. The growing popularity of BGA has brought new challenges to Selcom. They decided to combine the test design approach in the pad design with the automatic X-ray inspection technology using Open Outlier’s patent in a unique study to solve this problem.
Selcom’s test-oriented design philosophy is to change the shape of the pad so that when it comes into contact with the pad during reflow, the solder joint forms a shape, and when no contact occurs, another distinct shape appears. In an elliptical pad, the solder produces an elliptical shape when contact occurs and remains circular when no contact occurs. Maintenance personnel can easily distinguish between good (oval) solder joints and open circuits (round)
Solder joints and thus the defective solder joints can be quickly detected from the X-ray system (see figure).
In the figure, the pads form an elliptical shape when soldered well. If no contact occurs, the pad will remain round. By using Open Outlier’s patented 3D automated X-ray inspection technology, maintenance personnel can simply determine the open circuit by simply finding a circle in a series of ellipses.
However, there are a few details to consider when designing a test pad. One is the strength and durability of the solder joints. The strength of the solder joint depends on the size of the pad. If the pad is too small, it will damage the absolute strength and reduce the integrity of the solder joint. Second, board space affects the ability to accommodate through-holes, copper foil, and test points on the board. If the pad is too large, it will affect the board design. Finally, if the ellipticity is too small, the elliptical pad will be more difficult to distinguish from the open-circuit solder joint, thereby compromising system accuracy and may increase the test false positive rate.
When the solder is good, the pads will form an ellipse.
Selcom tested various device types using elliptical pads and non-elliptical pads. The results show that the elliptical pad further expands the accuracy of automated X-ray inspection, especially when combined with the patented 3D X-ray and BGA Open Outlier technology. The round BGA pad defect detection rate is 79% to 90% for various BGAs. When the elliptical pad is designed for the same BGA, the defect detection rate is increased to 93%~100%!
This is a test-oriented design research effort to compare different solutions in a controlled environment and evaluate their potential applications in a production environment. The results of the test results, X-ray defect detection function and maintenance effect indicate a promising new method for BGA open circuit detection. You can also consider conducting a similar study to see how test-oriented design and testing affects and significantly improves BGA open-circuit defect detection.