On the Bonding Edge
A new reworkable edgebond adhesive enhances board-level reliability of large WLCSP.
A new reworkable edgebond adhesive* from Zymet has been shown to enhance the board-level reliability of a large wafer-level chip-scale package (WLCSP). The work was performed in a collaborative effort between Portland State University, Cisco, and Zymet, and published in the Proceedings of the SMTA International 2016 conference, which took place in Rosemont, Ill. (“Reworkable Edgebond Applied Wafer-Level Chip-Scale Package (WLCSP) Thermal Cycling Performance Enhancement at Elevated Temperature”). Additional work was published in the 66th Electronic Components and Technology Conference proceedings: “Effect of Local Grain Distribution and Enhancement on Edgebond Applied Wafer-Level Chip-Scale Package (WLCSP) Thermal Cycle Performance.”
In the study, 8 x 8 mm WLCSPs, assembled on an organic substrate, were subjected to 0-100°C thermal cycling. Absent the adhesive, first failure occurs at 355 cycles and characteristic life is 638 cycles. With the reworkable edgebond adhesive, however, no failures were encountered at 2,000 cycles, end of test.
WLCSPs have a coefficient of thermal expansion of about that of silicon, 2.6 ppm/°C, and the board’s coefficient of thermal expansion (CTE) is about
17 ppm/°C. The mismatch in CTE results in a considerable amount of stress when the two are assembled, as evidenced by the extensive strain-induced grain recrystallization of solder that accompanied early failures. By using the reworkable edgebond adhesive, testers were able to substantially reduce the damage accumulation, resulting in increased thermal cycle performance.
The benefits of using the reworkable edgebond adhesive over a reworkable underfill are significant. No board preheat and dwell time are needed for capillary flow. In addition, the risks of underfill voids and flux-underfill incompatibilities are eliminated. Also, 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. ASI