Taking a close look at voids in BGA solder balls:
Researchers are questioning the impact of voids in BGA solder balls. They may not cause defects, which may prompt a revision of standards that set defect levels for voids.
January 21, 2013
By Terry Costlow, IPC online editor
Manufacturing processes evolve as clever employees find ways to eliminate defects. Ball grid arrays (BGAs) are a case in point: Some defects like solder ball voids were once considered a major problem, but some researchers say they’re no longer an issue.
As ball sizes and spacing have shrunk and processes have improved, voids in solder balls may no longer be a problem. That’s a marked change from years past, and one that may call for a standard revision.
For years, J-STD-001, Requirements for Soldered Electrical and Electronic Assemblies, and IPC-A-610, Acceptability of Electronic Assemblies, had very specific guidelines for BGA solder balls. The standards say BGAs are defective when voids account for more than 25 percent of an X-ray image area. But many say that as more industry investigations have been completed, that criterion is no longer valid.
When those standards were written, lead spacings were around 1.27 mm and ball diameters were around 30 mils. Now spacings are 0.5-0.8 mm and ball sizes are 10-20 mils.
“At those dimensions, if you get a void in the solder ball, there is a high probability it’s going to be greater than 25 percent of the X-ray image area,” said Dave Hillman, principal materials and process engineer at Rockwell Collins.
Some say that voids degrade the solder ball joint, while others have done research showing that voids have little impact on reliability. Solder joints with voids of any size likely have other problems that would cause failure.
“There may be some confounding issues in test methodologies. When you’re seeing a wide diversity in industry BGA void test results, it can mean test vehicles, test parameters or testing condition setups are having an influence on the test results in contrast to other industry tests,” Hillman said.
Hillman will provide more detail at IPC APEX EXPO® in San Diego February 19-21. The title of his Wednesday morning technical conference session lays out his views on the topic: “The Last Will and Testaments of Tin/Lead and Lead-Free BGA Voids.”
Hillman, who has worked closely with Hewlett Packard’s Chris Troxel and Linda Woody of Lockheed Martin Missile and Fire Control on this issue, feels that it’s time to alter IPC standards to eliminate the defect criteria on voids.
“If all of the industry data is reviewed and BGA voids are determined to not be a defect, voids shouldn’t be in the standards as a defect,” Hillman said.
Instead of focusing on defects, manufacturers should tweak their processes when they see an increase of void occurrence or size. Changes in the appearance, size or shape of solder contacts often suggest that the process is beginning to go out of range. Monitoring the process can be more effective than testing for defects.
“J-STD-001E and IPC-A-610 use process control methodologies/protocols in a number of solder process areas rather than 100 percent visual inspection,” Hillman said.
While noting that there are many benefits to inspection with the broad range of tools now available, Hillman commented that there can be cases where visual inspections are no longer beneficial. That can be the case with solder voids, which aren’t visible without advanced inspection equipment.
“One of the negative aspects of X-ray inspection is that it lets you see things you couldn’t see before. A process engineer now needs to make a determination if the things being seen are a problem or an observation. Voids in BGAs are the poster child for this issue,” Hillman said. “You get more immediate benefits by establishing consistent solder processes than scanning for voids after the fact.”
Hillman feels that it’s important to update standards as more knowledge is gained. When J-STD-001 and IPC-A-610 were written, solder voids were thought to be a problem. But as technology has advanced and new test data is accumulated, researchers have found that they’re something of a red herring.
“A willingness to change workmanship criteria shows the flexibility of the industry and IPC,” Hillman said.
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