Void-Free Underfill Process With Variable Frequency Microwave for Higher Throughput in Large Flip Chip Package Application

Abstract

Moisture voiding in underfill materials can cause reliability issues for the flip chip packages. The bake-out step included in the assembly process flow to avoid this problem cannot be completely efficient for some large die size packages. This is due to complex substrate circuit designs and time delays subsequent to the bake-out step. This paper proposes using the variable frequency microwave cure to eliminate the moisture voiding of flip chip large packages assembled without any bake-out step. Results showed, for a given ramp rate, a decrease in voiding formation with decreasing VFM cure temperature. It was also found that, at low final cure temperatures, the hold steps promoted the voids formation more than the ramp steps. At high final cure temperatures, both ramp and hold steps induced voids formation, growth, and coalescence. Another interesting observation was that a slower ramp rate reduced void formation even at high cure temperature. Based on the voiding evolution study done here, two optimized cure profiles were proposed, one comprising a two-step approach and another using a one-step cure with a low ramp rate of 2 • C/min. These optimized VFM profiles demonstrated good adhesion and reliability results while providing a void-free underfill process without the need for a time-consuming bake-out step.

Using Confocal Microscopy and Digital Image Correlation to Measure Local Strains Around a Chip Corner and a Crack Front

“…Macroscopic fracture properties, such as the fracture toughness, adhesion strength and the critical energy release rate at the Si/polymer interface, can be experimentally measured, e.g. via the cantilever beam test [7][8][9]. Although these fracture properties can be applied as input parameters to models to evaluate the reliability of the package, it is still challenging to predict the lifetime of the devices due to the difficulties involved in estimating correctly the local stress or strain levels at some critical positions, such as the die corner, the solder joints and the BEOL.…”

Low temperature cure of epoxy thermosets attaining highTgusing a uniform microwave field

“…This technology eliminates the possibility of metal arcing found in single frequency systems. With these uniform VFM fields, it has been reported that polymerization of epoxy thermosets, thermoplastic films, and other polymer systems have been cured at temperatures well below the standard thermal cure, with high extent of cure. In addition, there is evidence of modified thermomechanical properties in these systems, including modulus, crosslink density, stress, and coefficient of thermal expansion .…”