Expansion Mismatch
Microelectronic assemblies experience high shear stresses at the boundary between different materials due to mismatched coefficients of thermal expansion. The resulting silicon substrate thermal shear concentrates stress at the solder joints and adhesive interfaces holding the chip to its carrier. This phenomenon is a primary driver of mechanical failure in packaged semiconductor devices.
Corner Concentration
Temperature swings during power cycling cause the silicon chip and the organic substrate to expand and contract by different amounts. Since the silicon substrate thermal shear is highest at the outermost edges and corners of the die, these regions are highly susceptible to delamination and solder joint cracking. These cracks can sever electrical connections or allow moisture to penetrate, leading to electrical shorts or corrosion.
Underfill Decoupling
Mechanical decoupling using underfill materials distributes the shear stresses over the entire area of the die rather than concentrating them on the peripheral joints. The underfill polymer must have a coefficient of thermal expansion that matches the solder and substrate as closely as possible. This matching reduces the stress concentration factor.
Polymer Softening
Limits of the assembly are dictated by the glass transition temperature of the underfill and the shear strength of the interfaces. Above this temperature the polymer softens and loses its protective capability.
