Effect of Thermal Mechanical Solutions on Solder Joint Reliability of Bare-Die BGA with Substrate Stiffener

Abstract

BGA package stiffener on bare-die substrate has become more prevalent to mitigate ball grid array (BGA) substrate warpage during the assembly process. The adoption of this technology for large-size BGAs may post challenges to thermal mechanical solutions (mechanical retention, heatsink, etc.) on motherboards and cards, especially the Peripheral Component Interconnect express (PCIe) cards or Open Compute Project (OCP) Accelerator Module (OAM) cards.In this work, temperature cycle test of an OAM card with a typical thermal mechanical design shows BGA solder joint cracks at early cycles. Failure location starts at BGA corners and spreads to edges with increasing cycles. A structural system model is established to include both the BGA on the card and the thermal mechanical solution. The effects of temperature change, thermal interface material (TIM) load, and thermal mechanical retention are investigated. Root-cause of the solder joint temperature cycling failure is identified through the finite element analysis. The interaction of the substrate stiffener and the thermal mechanical solution is shown as the primary factor to the solder joint temperature cycle failure. It can generate significantly high solder joint stress at BGA corners during temperature cycling. The finite element analysis shows that increasing stiffener thickness increases solder joint axial force during temperature cycle tests. It also shows that heatsink compressive TIM load effect is secondary.In conclusion, large-size BGA thermal mechanical design needs to adapt and accommodate to bare-die BGA with substrate stiffener, while substrate stiffener thickness needs to be addressed for thermal mechanical solutions during package design.