The Quality of Die-Attachment and Its Relationship to Stresses and Vertical Die-Cracking

The residual stresses after die-attachment have been studied experimentally using a test chip with diffused resistor strain gauges. Extensive measurements are reported for the following dieattachment/leadframe combinations: Au-Si/Alloy 42, epoxy adhesive/ Copper 194, and polyimide adhesive/Copper 194. Two-dimensional distributions of the stresses in the device surface are shown, and additional thickness measurements of the adhesive layer are used to assess the amount of stress relaxation occuring in adhesive die-attachment. Large variations in the measured stresses were found for die-attachment with polyimide adhesive. Thickness measurements and radiography showed that the stress variations are caused by large voids formed during the drying cycle of the adhesive. The effect of voids on die-attachment stresses is analyzed, and its potential as a cause of vertical die-cracking is discussed.