Device Packaging 2019

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Evaluation of pressure free nanoparticle sintered Ag die attach on Ag and Au surfaces
Keywords: sintering, nanoparticle, die-attach
A commercially available silver nanoparticle based die attach material has been used in a pressure free process to bond 2.5 mm square Ag plated Si die to Ag and Au plated substrates. The assemblies were stored at 300 °C for up to 500 h and the morphology of the sintered Ag and the plating layers as well as the shear strength was monitored as a function of time. The paper begins by describing the manufacturing parameters and processes used to obtain intimate contact between the Ag nanoparticles and die and substrate surfaces. Bondline thickness measurements were carried out after following the paste manufacturer’s drying and sintering temperature profile. Two types of substrate were used; a 5mm square Ni/Ag plated silicon substrate of the same composition of the die, and a W/Ni/Au metallised cofired alumina package. In the Ag substrate case it was found that die shear strength increased with storage time. The fracture surfaces of the sheared die and substrate as well as cross sections of untested die were examined using electron microscopy. It was found that the Ag grains grew in size and porosity decreased over time. There was also a clear difference in morphology between sintered Ag at the edges of the die and at the centre. During shearing the Ni layer was found to have separated from the chip at the edges of the die after ageing. In the Au substrate case, it was found that die shear strength decreased with storage time. It was found that the Au diffused into the Ag, creating a low porosity Au-Ag layer. Ag also migrated towards the Au surface, leaving behind a layer of voids which contributed to weakening of the joint. Analysis of the failure mechanisms and material interactions will be presented.
Graham Lewis, Managing Director
Eltek Semiconductors Ltd
Dartmouth, Devon

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