Process characterization of highly conductive silver paste die attach materials for thin die on QFN

Abstract

In this paper, die attach process characterization on two type of highly conductive silver paste die attach materials was discussed. The first silver paste die attach materials (DA1) was used as a reference which is silver-loaded epoxy adhesive with high thermal conductivity of 60W/mK and electrical conductivity of 16Ms/m. Second silver paste die attach material (DA2) can be sintered with low pressure or pressure-less at temperature of 220°C to 280°C. DA2 material acquires high thermal conductivity range of 100–170W/mK and electrical conductivity range of 12–15Ms/m. Process specifications were set at die tilt < 1%, average bond line thickness between 25μm to 50μm and full die attach materials coverage without overflow of materials on top of die's surface. Process was optimized with 70μm thin silicon daisy chain chip with die size of 5mm×5mm on Ag plated QFN lead frame for both silver paste materials and achieved the required process specifications. Process optimized on DA1 achieved average bond line thickness ranged from 24.5μm to 30.5μm with die tilt less than 0.24% and DA2 had average bond line thickness ranged from 32.6μm to 44.2 μm with die tilt less than 0.15%. There was further evaluation on die attach process with silver sintered paste for different die thickness (which 50μm, 70μm and 175μm were used) on a fixed die size of 5mm×5mm. Porosity after die attach cure is always a curial factor which affects the modulus and conductivity of the device. Investigation on porosity of cured die attached materials was carrying out on different die size range from 0.5mm × 0.5mm to 5mm × 5mm. This helped to understand the effect of die size on sintering process. Optimization of dispensing pattern and die attach process challenges of thin die attachment were discussed in details in this paper.