DEFECT OF THE MONTH
Solder Squishing
When BGAs move during reflow, intermittent shorts can result.
THIS MONTH WE look at ball grid array (BGA) opens and solder compression. Intermittent joints and shorts can be caused by package warpage at elevated temperatures. Hence the interest in lowering soldering temperatures commonly used for SAC alloys.
FIGURE 1 was part of an experiment to chart the movement of a BGA package during reflow soldering. Using our reflow simulation, we can see solder ball compression by the package laminate in the image. In many of our video experiments, we see package warpage causes solder shorts and open connections during second reflow. Intermittent open connections have been experienced on double-sided reflow and package rework of adjacent parts. This procedure has been helpful to demonstrate why and how this problem can exist, particularly with smaller packages.
close view of compressed solder balls
Figure 1. The BGA solder balls compressed during reflow. A modified reflow process might be needed.
Depending on the available equipment and budget, it is possible to fully characterize components during reflow simulation. This allows engineers to find ways to deal with these issues and compensate for warpage through process modification. Alternately, this information can be shared with designers to demonstrate some components are not worth using.

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Bob Willis Caricature
Bob Willis
is a process engineering consultant; bob@bobwillis.co.uk. His column appears monthly.