Thermal Characteristics of Multilayer Printed Wiring Boards During Lamination

This paper presents the results of a study to identify the thermal characteristics of the laminating process for multilayer printed wiring boards (MLB's). MLB's with 5 and 15 conducting layers were studied by embedding thermocouples into the epoxy/glass bonding sheets during the "hot-press" lamination process. An effort was made to isolate the important parameters governing thermal variation during hot-press lamination and to determine the thermal timing "window" for cure-pressure application necessary for uniform resin flow. The results show that whereas the temperature of the bonding sheets in a 5-layer MLB are practically identical, the temperature in 15-layer MLB's differs in excess of 37°F (20.50°C). The study also concludes that the lag between the time the lay-up is inserted and the press is closed governs temperature differences for a given MLB. Under laboratory conditions, the lag can be as small as 4 s;typically in production, however, the lag is 80 s for multiopening presses. It is these temperature variations that can cause voids, cured thickness variations, measle tendencies, and delamination, resulting in high proportions of defective MLB's. These temperature differences can be reduced either by increasing the padding thickness or by employing a "cold-press" lamination process. The effectiveness of these methods is also discussed.