Understanding die attach materials performance in electronic packages under harsh environments

The operation of electronic packages under increasingly harsh environments is a significant challenge for the microelectronics industry. The generally accepted microelectronic temperature limit is 125°C but there is a strong push to extend this up to temperatures as high as 250°C in down-hole and turbo-machinery applications. The present work concerns the investigation of two types of die attach materials: electrically conductive adhesives (ECAs) that offer relatively simple processing, low cost and low stresses imparted on the die and substrate but are generally considered unsuitable for harsh environments; and Au-2 wt.%Si eutectic that melts at <;;400°C, is less sensitive to moisture than ECAs, offers elevated temperature stability, is stiffer but requires high temperature processing and more complex manufacturing steps. Changes in the die attach materials were investigated by isothermal ageing at 250°C, thermal shock and thermal cycling treatments. Phenomena such as thermal migration and material deterioration due to interfacial reaction, outgassing, evolution of thermal strains, etc have been studied, supported by non-destructive scanning acoustic microscopy (SAM), shear strength testing and microstructural analysis. The advantages and disadvantages of each material have been rationalized in terms of their mechanical properties in the case of Si dies attached to metallized alumina substrates. Numerical modelling has also been used to help explain the reliability performance.