Large low-CTE glass package-to-PCB interconnections with solder strain-relief using polymer collars

This paper reports the use of circumferential polymer collars as a strain-relief mechanism to improve the fatigue life of low-CTE package-to-PCB solder interconnections, while preserving SMT-compatibility and reworkability. Acting as a partial underfill, the polymer-collar serves to block shear deformation at the solder-package interface, and redistributes the load to reduce the overall plastic strain concentration in the solders. It also suppresses failure initiation from defective surface sites and, thus further enhances reliability. Ultra-thin glass 100μm interposers were fabricated in 18.4 mm × 18.4 mm size to model, design and demonstrate the reliability enhancement with the polymer-collar approach. The detailed interposer design and fabrication process with laminated dielectric and metallization layers on both sides is presented. A new class of epoxies with low modulus, without the incorporation of silica fillers, was used to act as the polymer collars. The polymer collars are formed by spin-coating with an optimized thickness to provide the best compromise between the effective strain relief and reworkability. Board-level assembly was performed using standard SMT processes for glass interposers with and without polymer collars. Thermal cycling reliability testing (-40°C to 125°C) of interposers, assembled on PCBs with and without polymer collars for various thicknesses of the collar was performed.