Zymet’s reworkable edgebond adhesive, UA-2605-B, has been shown to enhance the board level reliability of a large WLCSP. The work has been performed and published in the Proceedings of the SMTA International 2016 conference, held in Rosemont, IL, “Reworkable Edgebond Applied Wafer-Level Chip-Scale Package (WLCSP) Thermal Cycling Performance Enhancement at Elevated Temperature” in a collaborative effort between Portland State University, Cisco, and the company. Additional work was published in the 66th Electronic Components and Technology Conference, in Las Vegas, “Effect of Local Grain Distribution and Enhancement on Edgebond Applied Wafer-Level Chip-Scale Package (WLCSP) Thermal Cycle Performance”. In the study, 8×8 mm WLCSPs assembled on organic substrate, were subjected to 0° C to 100° C thermal cycling. Absent adhesive, first failure occurs at 355 cycles and characteristic life is 638 cycles. With the reworkable edgebond adhesive, no failures were encountered at 2000 cycles.
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 by the extensive strain induced grain recrystallization of solder that accompanied early failures. Use of the reworkable edgebond adhesive substantially reduced the damage accumulation, resulting in increased thermal cycle performance.
The benefits of using the company’s reworkable edgebond 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.