Thermal characterization of multilayer printed wiring boards during lamination

The results of a study to thermally characterize the laminating process for multilayer printed wiring boards (MLB´s) are presented. The temperature in MLB´s is determined both as a function of three-dimensional location and of time. The study is carried out experimentally using 1 mil (0.025 mm) diameter thermocouples imbedded into the epoxy/glass prepreg layers. MLB´s with 5 and 15 conducting layers are studied for the "hot press" laminating process, for which the MLB lay-ups are inserted between the hot press platens (360°F/182°C). The results show that whereas for 5 layer MLB´s the epoxy/glass prepreg layer temperatures are practically identical, the prepreg layers in 15 layer MLB´s show temperature differences in excess of 37°F (20.5°C). It is shown that these temperature differences are also strongly dependent on the time lag between lay-up insertion and the closing of the press. This time lag can be as small as 4 seconds under laboratory conditions and typically 80 seconds for multi-opening production presses. Other effects which are reported include those of padding thickness and production conditions. It is further shown that these temperature differences, which can be the cause of laminating problems such as voids, cured thickness variations, measle tendencies, and delaminations, can result in high proportions of defective MLB´s; but can be reduced by increasing the padding thickness or employing the "cold press" laminating process.