Assessment of thermo-mechanical stresses in Low Temperature Joining Technology

Abstract

The Low Temperature Joining Technology (L T JT) creates silver joints by a sintering process. It is an alternative die-attach technology to soldering. To initiate the sintering process the temperature has to be raised above � 215°C. The quality of a sintered joint is strongly enhanced by applying pressure to the specimen during the process, thus reducing the porosity in the sintered material. After the interconnect process, residual stresses occur in the sintered material, as well as in the joined parts.The examined specimen was a silicon MOSFET attached to a copper substrate at a sintering pressure of 50 MPa. The scope of this paper is to provide information about residual stress after production and thermo-mechanical stresses during thermal cycling. The focus is put on the investigation of the reliability of sintered silver by simulation.

Are Sintered Silver Joints Ready for Use as Interconnect Material in Microelectronic Packaging?

“…This simple correlation may be flawed, however, as high-strength materials typically have low fracture toughness, which is a measure of the resistance to crack growth during cyclic loading. 74 Another explanation for this observation in Table III is that the sintered nano-Ag joints had a higher density than the micron-Ag joints, although this postulate cannot be confirmed because the density for the micron Ag joints was not reported, 63 while the density for the nano-Ag joints was only 80.4 %. 64 In a related study, Suganuma and coworkers 66 researched the influence of Ag-filler shapes on the thermal-fatigue resistance of sintered Ag joints.…”

“…It has been reported that the die-attach thickness plays a more crucial role than the die size in thermal life-cycle studies of sintered Ag joints 63 because different ''stress-relief'' mechanisms predominate at different die-attach thicknesses. 71 According to that study, sintered Ag joints were found to be unsuitable for die-attach thicknesses of 75 lm or more because of their increased propensity for crack growth instead of stress relaxation within the sintered Ag layer.…”

“…Further analysis reveals that the die-attach thicknesses of these nano-Ag and micron-Ag joints were approximately 35-40 lm, while the die sizes used in the nano-Ag and micron-Ag joints were 14 mm 9 14 mm and 5.4 mm 9 3.4 mm, respectively. 63,64 A larger die is more susceptible to thermal fatigue failure than a smaller die because of the higher stress present in the larger die. In other words, the nano-Ag joints were subjected to more stressful conditions than the micron-Ag joints, but the former still performed better than the latter.…”

“…A porous sintered Ag joint behaves like a viscoplastic material at high temperature. 63,72 At high temperature, sintered Ag joints undergo plastic deformation, resulting in crack formation, and these cracks propagate during thermal cycling. A larger temperature range imposes a higher strain on sintered Ag joints, which results in a shorter life cycle.…”

“…In the case of ''life cycle to failure'' (N f ), the criterion for failure differs from one work to another; some researchers have used the loss of 20 % of the die-bonded area the failure criterion, while others have used 10 % of the die-bonded area. [62][63][64] Some researchers have defined a crack length equal to the thickness of the die-attach layer as the detection limit for observing crack lengths and failure criteria. 71 Therefore, caution should be exercised when comparing results from different sources.…”