Acoustic Imaging finds
01/01/1998
Acoustic imaging finds "halo" defect in flip chips
Acoustic imaging has identified a flip chip defect in which underfill material flows around but does not come in contact with solder bumps (Fig. 1). The defect was found by researchers at Sonoscan`s applications laboratory, Bensenville, IL.
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Figure 1. White "halo" around several solder bumps in acoustic image of flip chip represents a potentially lethal delamination. (Photo courtesy of Sonoscan Inc.)
"This defect consists of a tubular delamination partly or completely surrounding the solder bump," explained company president Lawrence W. Kessler. "Since the underfill material does not completely surround the bump as it is designed to do, the delamination leaves a space into which the bump can migrate.
The defect, which has been noted in multiple lots, typically affects several solder or polymer bumps on the same device. Although the precise cause of the defect is undetermined, it may result from incomplete wetting of the solder bumps due to flux residue (Fig. 2).
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Figure 2. Schematic side view of halo defect. Not to scale.
Since the tubular delamination is quite thin, it is extremely difficult to detect by either x-ray or destructive physical analysis. Since migration of bump material occurs slowly, this type of defect would also be undiagnosed during initial electrical testing. In the field, however, it would eventually cause a failure. - K.D.