Novel high performance no flow and reworkable underfills for flip-chip applications

Abstract

Flip-chip interconnect is the emerging technology for the high performance, high I/O (Inputs/Outputs) IC devices. Due to the thermal mismatch between the silicon IC (CTE=2.5 ppm/0 C) and the low cost organic substrate such as FR-4 printed wiring board (CTE= 18–22 ppm/°C), the flip chip solder joints experience high shear stress during temperature cycling testing. Underfill encapsulant is used to couple the bilayer structure and is critical to the reliability of the flip-chip solder joint interconnects. Novel no-flow underfill encapsulant is an attractive flip-chip encapsulant due to the simplification of the no-flow underfilling process. To develop the no-flow underfill material suitable for the no-flow underfilling process of flip-chip solder joint interconnects, we have studied and developed a series of metal chelate latent catalysts for the no-flow underfill formulation. The latent catalyst has minimal reaction with the epoxy resin (cycloaliphatic type epoxy) and the crosslinker (or hardener) at the low temperature (≤180° C) prior to the solder reflow and then rapid reaction takes place to form the low-cost high performance underfills. The effects of the concentration of the hardener and the catalyst on the curing profile and physical properties of the cured formulations were studied. The kinetics and exothermic heat of the curing reactions of these formulations were investigated by differential scanning calorimetry (DSC). Glass transition temperature (Tg) and coefficient of thermal expansion (CTE) of these cured resins were investigated by using thermo-mechanical analyzer (TMA). Storage moduli (G') and crosslinking density of the cured formulations were measured by dynamic-mechanical analyzer (DMA). Weight loss of these formulations during curing was investigated by using thermo-gravimetric analyzer (TGA). Additionally, some comparison results of our successful novel generic underfills with the current commercial experimental no-flow underfills are reported. Additionally, approaches have been taken to develop the thermally re- workable underfill materials in order to address the nonreworkability problem of the commercial underfill encapsulants. These include introducing the termally cleavable blocks to thermoset resins, and adding additives to thermoset resins. For the first approach, five diepoxides containing thermally cleavable blocks were synthesized and characterized. These diepoxides were mixed with the hardener and the catalyst. Then the properties of these mixtures including Tg, onset decomposition temperature, storage modulus, CTE, and viscosity were studied and compared with those of the standard formulation based on the commercial epoxy: ERL-4221D. These mixtures all decompose at lower temperature than the standard formulation. Moreover, one mixture—Epoxy5—showed acceptable Tg, low viscosity, and fairly good adhesion. For the latter approach, two additives were shown that after added to typical cycloaliphatic epoxy formulation, do not interfere with epoxy curing, and do not affect the typical properties of cured epoxy system, yet provide die removal capability to the epoxy. Furthermore, the combination of the two approaches showed positive results.