Filled No-Flow Under-filling - Process and Materials

摘要

A novel no-flow under-filling process and materials, NF220 series under-fill and PK002 epoxy flux, for flip chip assembly with eutectic SnPb solder bumps are developed and optimized. Here a significant content of filler is incorporated in the no-flow under-fill NF220. The assembly process can be described as below: (1) dispense filled no-flow under-fill NF220 onto footprint pads for flip chip, (2) form a thin film of epoxy flux PK002 on a flux tray, (3) pick up flip chip and place into the thin epoxy flux film, (4) lift the flip chip from flux tray, (5) place the flip chip with epoxy flux on solder bumps onto footprint pads with under-fill deposited, (6) re-flow/cure the flip chip system by sending through an adequate heating profile. The viscosity of epoxy flux should be low at dipping step to allow flux pick-up, but high when placing into no-flow under-fill to displace the filled under-fill. Dipping temperature at 45°C provided high yield due to balanced flux volume picked up and flux pot life. With increasing filler content in no-flow under-fill NF220, the CTE, the shear strength, the deterioration due to PCT, and the yield decrease, and the number of temperature cycling at onset of failure first increases, then decreases at filler content beyond 20%, while that at total failure increases. The temperature cycling behavior is affected by filler content via CTE matching enhancement effect and wetting interference effect. The wetting interference can be explained by a filler distribution model where the chance of having filler particle on pads follows a bell shaped probability distribution, and the distribution curve becomes wider with increasing filler content. The no-flow under-fill with 20% filler content, NF220-2, is optimal in terms of reliability and yield, and is better than leading conventional commercial capillary flow under-fill in temperature cycling performance, despite a slight under-fill delamination after 96 hours PCT and a slight filler settlement. This under-fill with 20% filler content is also adequate in solder joint wetting, solder joint non-voiding, and under-fill non-voiding. The overall performance indicates that this novel filled no-flow under-filling system, NF220-2 and PK002, is an advanced solution for low cost, high reliability flip chip assembly applications.