Resolution of the Gold Wire Grain Growth Failure Mechanism in Plastic Encapsulated Microelectronic Devices

Abstract

The relative statistical influence of bonding parameters and assembly materials on the failure frequency rate of microelectronic devices in plastic encapsulated packages due to grain growth, failure are resolved. Wire creep is a stressinduced break in a thermocompression gold wire ball bond which most frequently occurs within several wire diameters above the ball in plastic encapsulated microelectronic devices. Thermal Shock, Condition C, was used as the standard test by which to measure performance in this investigative program. A complete analysis was made of all materials and processing factors which would potentially contribute to the wire creep, grain growth failure mechanism. Over 75 test matrices were defined and executed with the demonstrated result that only four variables had apparent major influence on this failure mode: 1) the gold wire impurity constituents, 2) the type of wire bonding capillary, 3) the gold wire ball formation technique, and 4) the lead frame plating impurities. A full factorial experiment was conducted to delineate the relative influence of these four contributors. The results of this investigation show that: 1) lead frame plating impurities can significantly accelerate the wire creep, grain growth failure frequency; 2) hydrogen torch flame-off ball formation always results in wire creep, grain growth failure; 3) the type of capillary contributes to this failure to a lesser but still significant extent; and 4) the type of wire is the least significant contributor, but within this variable the level of grain modifying impurities is significant. Hypotheses, supported by additional experimental investigations, with foundations in metallurgy have been formulated. Corrective actions implemented as a result of this investigation have reduced the average cumulative failure frequency rate of 30 percent at 250 thermal shock cycles to zero cumulative failures at greater than 3000 thermal shock cycles.