Factors affecting the interfacial adhesion between a silicon die and die attach paste of integrated circuit package

Abstract

Delamination at the interface between a silicon backside and the die attach paste has been reported, and this delamination led to package cracking, resulting in reliability problem. In this paper, a series of experiments was conducted to investigate factors affecting the interfacial adhesion between the die back side and the attach paste of an Integrated Circuit (IC) package. Poor interfacial adhesion potentially leads to a delamination and crack at the interface. The aim is to understand better the interfacial adhesion phenomena, and possible failure mechanism(s) of a Ball Grid Array (BGA) package with delaminatiodcrack propagation between the die back side and the die attach paste.Numerous analysis and explorations on delamination were made by CSAM and cross-section work on dummy packages. During the course of investigations, contact angles, Atomic Force Microscopy, (AFM) and Electron Spectroscopy for Chemical Analysis (ESCA) were used. Adhesion test using a Shear test mode was done on dummy samples after die attach cure.Based on the evaluation results, factors that affect adhesion between die attach paste and silicon die were identified. Factors include namely UV tape adhesive transfer to die hack side, surface energy of silicon surface, and die attach paste toughness. The amount of chemical residues from the UV tape affects the adhesion strength. Contradictory to a common knowledge, the presence of a group of siloxane materials (silicones) which formed part of the chemical residues does not significantly influence the adhesion behavior between the die back side and die attach paste. An attempt was made to explain the observation.It was found that interfacial adhesion improves with die paste with a higher Kj, value (higher toughness). High surface energy of silicon surface remains the most important factor. This means that processes that activate the silicon surface would improve the adhesion strength. This can he achieved by, for example, a plasma treatment. Samples were built and the findings tested with positive results. delamination, and cross sectional work was done to further understand the delamination. Occasionally, delamination at the die back side and the die attach paste was observed, as illustrated in Figure 1. r -l Die back Figure 1: Cross section of a package showing die to paste delamination.

Influence of plasma cleaning on silicon die back adhesion performance

“…This layer of silicon oxide is helpful to adhesion between die backside and paste due to its providing active site for paste cross-linking. [3] …”

“…[3] Here come out the simple moldel to describe contaminated silicon surface status. They are bare silicon, a monolayer of a silicon oxide film and organic contamination film in turn from bare silicon surface to outside.…”

“…[3] In this study, the surface roughness is characterized by Zgyo NV5022, not-touch instrument based on white light interference principle. The spot scanned is 2.11x1.58mm.…”

“…Poor interfacial adhesion probably comes into being due to contamination induced, which will result from many reliability issues such as package popcorn, pad corrosion, wire broken during manufacturing, test or application of devices. [1][2][3] A particular phenomenon was observed that attached silicon dies post cure were flied off from the pad of lead frame during the wedge bonding process in PQFN devices with occurrence rate at ppm level. See Fig.…”