Pressure-Compensated Chip Attach: A Low-Stress Silicon Bump Reflow Process

Assembly of silicon onto FR4-based organic substrates using flip-chip process continues to pose substantial thermomechanical challenges largely due to the disparate coefficients of thermal expansion (CTEs) of silicon and substrate. In flip-chip packaging, this CTE mismatch causes mechanical stresses in the chip-to-substrate interconnection bumps and the mechanically fragile interlayer dielectric (ILD) within the chip, leading to fracture and delamination. A novel process called pressure-compensated chip attach (PCCA) is presented, where the silicon chip is attached to the substrate by reflowing the solder bumps while maintaining the chip and substrate in a controlled hydrostatic pressure environment. Experiments show that the new PCCA process reduced the silicon test-chip warpage by 40% and eliminated ILD cracking.