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This paper describes the results of an experiment to determine practical screening and quality-assurance procedures to preclude excessive adhesion degradation, due to solder leaching, in gold alloy/solder systems in thick-film hybrid microcircuits. These results include a comparative rating between the materials tested, along with criteria for rating different materials.
Four environmental stresses were compared to determine which one to use as a quality-assurance test for adhesion decay. The test method consisted of soldering copper pins to conductor pads on the substrate, and then pull-testing the pins to the destruction of the conductor/substrate bonds, both before and after environmental stresses. This method provides a measurement of both tensile adhesion between the soldered conductor and the substrate and adhesion degradation promoted by the environmental stress.
Seven thick-film conductor materials and three solder compositions were used, making a total of 21 conductor/solder systems tested. Pins were soldered to each of the seven conductor materials with each of the three solders. Adhesion measurements were made immediately, again after a minimum three-day shelf life, and also after each environmental stress. The median force-per-unit-area values for each conductor/solder/environmental stress combination were tabulated and plotted. The results are presented to show the relative degradations of the different materials.
Based on the results of the experiment, the temperature cycling stress appears to be the most practical stress to use as a screening procedure for solder leaching. Guidelines are presented for performing the screening test and for determining a rating for any material combination.
thick films, adhesion, solder, indium, tension tests, leaching, scavenging, temperature cycling, thermal shock, pull tests
Engineer, Westinghouse Electric Corporation, Defense and Electronic Systems Center, Baltimore, Md.