īetter EMI/RFI rejection due to the image plane effect, which has been known since the time of Marconi.Other reasons for multilayer boards, however, may not be obvious or intuitive: Since the PWMLB failures in question are induced by a thermal stress, and since the poorer grades of Type E materials used in these structures are susceptible to premature failure under thermal stress, we propose the use of elevated temperature rupture and creep-rupture testing as a means for screening copper foil (or its PTH equivalent) in order to eliminate the problem of Type E copper microcracking in advanced PWMLBs. We suggest that the strength of some Type E copper can be very low when the material is hot and that it is the use of this poor quality material in a PWMLB that results in PTH and inner-layer microcracking. In this paper, we show that there are some distinctive differences in the quality of Type E copper and that these differences can be detected before its use in a PWMLB. The failure can occur in the foil that makes up the inner layer traces or in the PTH deposit that forms the layer-to-layer interconnections. PWMLB structures for high-speed, high-density circuits are prone to failure due to the microcracking of electrolytic copper interconnections. Louis Zakraysek, in Electronics Reliability and Measurement Technology, 1988 Summary and Conclusions
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