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.