In an effort to further the understanding of the physics of MIB failure extensive tests were conducted to determine the effects of MIB-PTH reliability of: 1) long-term thermal cycle environments 2) selected MIB design parameters, and 3) selected MIB processing parameters. This paper presents the most significant results of these tests.Each of the eighteen MIB's tested contained approximately 1,300 PTH's divided evenly among thirty-four circuits. Three thermal cycle environments were selected for testing with each of these environments being within severe but realistic military use conditions. Test acceleration was achieved by a new experimental concept termed “enhanced defect” testing. With careful choice of test article design and manufacture, failure mechanisms which occur only rarely in a normal product sample may be observed and analyzed in large numbers, in a controlled defect-enhancement sample.Implementation of the enhanced-defect concept on the eighteen test MIB's resulted in six MIB types differing in the degree and type of defect enhancement. Some MIB's were manufactured with excessive amounts of “epoxy-smear”, others were purposely poorly laminated and still other MIB's were produced with thin hole plate, irregular hole plate or brittle hole plate to determine how these variables affected the PTH failure processes. Finally, a number of MIB's were manufactured with optimum characteristics to serve as experimental control samples.Circuit resistance measurements were periodically made throughout environmental exposure on all test MIB's. Microscopic examinations were performed on hundreds of PTH's during and at test completion. Point-to-point continuity profiles of numerous test circuits were completed after environmental test. The data acquired during the 60-million PTH-hours of testing, the 10,000 electrical measurements, the numerous microsections and the hundreds of circuit continuity profiles were analyzed in depth to determine the effects of the programmed design and processing parameters on MIB reliability.