Zymet’s reworkable edgebond adhesive, UA-2605-B, has been shown to enhance the board level reliability of a large WLCSP. With the reworkable adhesive, no failures were encountered at 2000 cycles, end of test.
WLCSP’s have a coefficient of thermal expansion of about that of silicon, 2.6 ppm/°C, and the CTE of the board is about 17 ppm/°C. The mismatch in CTE results in a considerable amount of stress when the two are assembled, as evidenced bythe extensive strain induced grain recrystallization of solder that accompanied early failures. Use of the reworkable edgebond adhesive substantially reduced the damage accumulation, resulting inincreased thermal cycle performance.
The benefits of using the reworkable adhesive, over one of its reworkable underfills, are significant. No board preheat and dwell time are needed for capillary flow. The risks of underfill voids and flux-underfill incompatibilities are eliminated. And, when performing rework, there is no need to remove underfill residues from the entire footprint of the package, virtually eliminating the risk of pad damage.
