Low Warpage Flip-Chip Under-Fill Curing

Mechanical stress in flip-chip assemblies continues to be a significant problem both for the reliability of the component and for the assembly of a flat component to the next board-level assembly. This paper describes the combination of unique low-temperature multi-step curing profiles with the use of variable frequency microwaves (VFM) to produce lower warpage components both on the die side of the package as well as on the board carrier side. This lower warpage compared to standard convection cure is maintained even after three sequential lead-free solder reflow conditions. Statistical data support this improved co-planarity by Shadow Moire measurements at various stages of processing from as-received parts through prebake, cure, and three reflow cycles. Typical co-planarity improvement in the 12-65 percent range is observed and verified by confirmation sample sizes used for microwave cured parts and conventional box oven cured parts. Thinned and larger die, and reduced thickness substrate boards showed the most warpage improvement with VFM. Two under-fill chemistries show the same effect despite lower cure temperatures and faster cure cycle times. A reduction of the elastic modulus above Tg was found in the VFM step-cured samples which may account for the some of the reduction in stress of the under-filled packages.