High reliability underfill encapsulated fine pitch system on flex packages

In system on flex (SOF) packages with high density I/O´s, larger die size, and fine pitch, the mismatch of material properties between the silicon die, underfill (encapsulant) and flexible substrate induces concentrated stress field around the bonded areas (Bumps & traces) during assembly process. The concentrated stresses result in delamination on various interfaces involving a range of length scales from hundreds of nanometers to millimeters; this delamination can be seen as halo after cross-section SEM analysis. This delamination of underfill increases the chances of moisture absorption and potentially results in electro-chemical corrosion of the stressed area. This corrosion could affect the reliability of the package. In this paper, a detail study on which factors affecting the generation of these micro-delamination in underfill is investigated. The delamination is driven by either the adhesion failure within the substrate and underfill or by the stresses induced in the interfacial sites. We developed an extra cleaning process to remove seed-layer Cr+ to avoid adhesion failure due to contamination of flex. Strength of underfill to avoid failure under shear stress depending upon various properties is analyzed. It shows that filler material have more strength that non-filler materials but at cost of flow rate and voids. It was observed after TCT and button shear test that underfill with higher tensile strength could absorb more stresses. Effect of peak temperature during assembly process was studied; this showed that underfill delamination across the interfacial sites increases with increase in temperature. So assembly process development with lower temperature is more beneficial for high reliability of SOF packages.