Effects of Temperature on the Adhesive Performance of High Tg Underfill in 2.5D Heterogeneous Integrated Packaging

Abstract

2.5D heterogeneous integrated packaging required better adhesive performance of underfill at high temperature. This study found that: a) underfill with a higher glass transition temperature (Tg) had the stronger adhesive ability, and b) proper silane couple agents (SCAs) could significantly improve the adhesive performance of the underfill at high temperature.With larger size, higher interconnection density, and higher power consumption, 2.5D heterogeneous integrated packaging encounters more severe reliability challenges at high temperatures. Typically, warpage becomes more serious, easily causing delamination between the underfill and the chips at the corners due to thermal loading. Therefore, it is important to enhance the adhesive performance of the underfill at high temperatures. First of all, Tg is one of the crucial factors determining the adhesive strength of the underfill. After Tg, molecular distances become larger as the epoxy changes into the glass state and the adhesion between the underfill and the packaging interfaces (Si, passivation layer, solder mask, etc.). Thus considering from this perspective, higher Tg is favorable. In addition, SCA is the bridge bonding the underfilling and the packaging interface. Using SCAs with high-temperature resistance, the underfills can well maintain their adhesive performance at high temperatures. In this study, underfills with SCAs and with no SCA were used, the Tgs of which were different. The adhesive strength of the underfills against temperature are investigated. The die shear test is carried out for adhesion strength measurements, with temperature varied from 25 °C to 260 °C.Effect of temperature on adhesion: As the temperature increased from 25 °C to 260 °C, the adhesive strength of the epoxy declined significantly. This indicates that under high temperatures, the epoxy composites became soft and the adhesion strength was also weakened.Effect of Tg on adhesion: The adhesive strength decrement can be divided into three steps: A slow decrement of the adhesive strength, the adhesive strength dropped sharply, after that, the adhesive strength almost kept a stable value. Low Tg underfill decreases rapidly near the Tg point, while high Tg underfill decreases rapidly before the Tg point. In addition, the underfills with higher Tg showed higher adhesive strength at the same temperature than the underfills with lower Tg. Therefore, the adhesion strength is highly dependent on the Tg of the underfill.Effect of SCA on adhesion: First, the underfills containing SCAs had a better adhesive performance from 25 °C to 260 °C than the underfill without SCA. In addition, the underfill containing amino SCA showed the highest adhesive strength at high temperature; and the underfill containing epoxy SCA performed better at low temperature.