Reworkable edgebond boosts board-level reliability of area array packages

A new approach to enhancing the board level reliability of area array packages was presented at the recent SMTA International, an industry-leading conference on electronics assembly and advanced packaging. John McMahon, et. al. presented work, performed at Celestica, that demonstrates that a reworkable edgebond adhesive, manufactured by Zymet, takes a 55mm LTCC CBGA out to over 3000 cycles of 0C to 100C cycling without failure - a dramatic improvement over a non-reinforced component. In one comparison, a 47.5mm CBGA with 2115 I/O's had a nominal 4% failure rate (B4%) of 1170 cycles. With the reworkable edgebond adhesive, no failures were encountered at 3517 cycles, end of test. Unsurprisingly, a larger 50mm CBGA with 2303 I/O's, without adhesive, had a lower B4% of 850 cycles. Using the reworkable edgebond adhesive on a 55mm CBGA with 2892 I/O's, no failures were encountered at 3010 cycles, also end of test. CBGA's made from LTCC, which has a coefficient of thermal expansion of 12 ppm/C, incur a considerable amount of stress when assembled on an organic board. However, Mr. McMahon states that 'use of a modern (reworkable) edgebond material can significantly increase chamber cycling life by a factor of 3X or more'. In addition to extensive life-testing and failure analysis, Celestica also developed a precise dispensing process, as illustrated. And, it also confirmed the adhesive's reworkability, having developed and refined a rework process. The benefits of using a reworkable edgebond adhesive, over a reworkable underfill, to enhance the board level reliability of BGA's and other area array components such as WLP's, WLCSP's, CSP's, POP's, and QFN's, 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.