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BGA Opens Detection: A DfT Approach
How an oval pad design aided defect detection using x-ray inspection.
For ball grid array packaging, the high number of hidden solder joints combined with the inability to verify defects with visual or electrical inspection leads to the need for other techniques.
Specifically, BGA opens continue to be one of the most critical types of defects to detect on today's complex assemblies. Opens are a common problem for visual inspection systems. While x-ray can "see" the differences in diameter between joints, it is often difficult to identify when those differences are significant - when the differences are legitimate opens versus when they are simply part of normal variations in the manufacturing process.
A large European electronics manufacturer, Selcom Group, first began using 3-D automated x-ray inspection 10 years ago to improve process control and defect detection on complex assemblies. The popularity of BGAs was a challenge for Selcom. The firm decided to approach the issue by combining a design for test (DfT) approach for pad design with automated x-ray inspection with patented technology in a unique study.
Selcom's DfT idea was to change the shape of pads so that, during reflow, wet solder would have one shape when it makes contact with a pad, and take a significantly different shape if it does not make contact. In the case of oval pads, wet solder will acquire an oval shape when it makes contact but will retain a circular shape if it does not (Figure 1). This makes it easy for repair operators to differentiate between good (oval) and open (circular) solder joints. Then they can quickly verify defect calls from the x-ray system.
 FIGURE 1: Wet solder takes the shape of the oval pad when it makes contact. If it does not make contact, it stays round. Opens by be spotted by operators or on certain equipment by simply looking for circles in a sea of ovals. |
There are, however, several items to take into consideration in the design of test pads. The first is robustness and durability of solder joints. The strength of joints depends in part on the size of the pads used. If pads are too small, sheer strength is compromised and joint integrity can be reduced.1 Second, the ability to fit vias, traces and test points on the boards is affected by board real estate. If pads are too large, board design is affected. Finally, if the degree of ovalization is too slight, then oval pads will be harder to distinguish from round open joints, so test system accuracy will be compromised and false defect indictments may increase.
A variety of component types were tested with and without oval pads. The results suggest that oval pads further expand the accuracy of automated x-ray inspection, especially when used in combination with novel 3-D software. Circular BGA pad defect detection capability ranged from 79 to 90% effectiveness for various BGA ball and pitch sizes. When oval pads were designed into those same BGAs, defect detection effectiveness improved to 93 to 100%.
This DfT study compared solutions in a controlled environment and evaluated their potential application in a production environment. In terms of test effectiveness, x-ray defect detection capability and repair effectiveness indicate a promising new approach to BGA opens detection. Readers may want to consider conducting similar studies to see how DfT and inspection can impact and substantially improve BGA opens defect detection capability.
References
S. J. Kim, C. H. Lee and S. G. Lee, "A Study of High Density and Reliability BGA Package with Solder Ball Lands of Oval Type," Electronic Components and Technology Conference (ECTC) Proceedings, May 1998.
Tamara Pippert s AXI product manager at Agilent Technologies Inc. (agilent.com); mtbu_om@agilent.com.
