Bond-strength studies for 1-mil-diameter gold wires bonded to hybrid microcircuit substrates

Detailed studies of the failure probability for gold wires bonded to multiplexer substrates under severe acceleration in the W79 artillery shell environments were made. The studies included: the calculated resultant pull forces exerted on the bond joints due to the W79 acceleration environments; the suitability of the loop-hook pull tests and the use of the normal Gaussian distribution theory for statistical description of bond strengths; and the probability of failure for gold wires bonded to multiplexer substrates under artillery shell accelerations using fixed angle pull tests and a Weilbull distribution theory for the statistical description. Preliminary statistical analyses of the bond strength data obtained from the conventional loop-hook pull tests for a multiplexer substrate HMC, have shown that the ball bond is strong enough to withstand the 0.17 gram design limit load due to the W79 gun barrel environments with a very low probability of failure. For the wedge bond, however, the results of a statistical analysis for the bond strength agree with experience which shows that the wedge bonds are generally much weaker than ball bonds in multiplexer substrates, and the probability of failure may be high enough to cause a problem. The degradation of the wedge bond strength due to post-assembly testing is significant as indicated by the large increase in probability of failure for the post-assembly-tested wedge bonds. To illustrate this, consider a worst-case example which assumes all wires bonded to the hybrid microcircuit substrates have the same geometry and loading conditions. Assuming the W79 JTA has approximately 2760 of these bonded wires, the probabilities of failure are 0.0179% for virgin wedge bonds and 2.1% for post-assembly-tested wedge bonds