Surface Mount Technology (SMT) Substrate Material Requirements - A Brief Review

ABSTRACTThe state of the art for the printed circuit (pc) industry is the Surface Mount Technology (SMT) using leadless ceramic chip carriers (LCCCs). The SMT technique is used to design, fabricate, and assemble affordable high-speed, high-density electronic modules with reduced size and weight. However, to take full advantage of the SMT, new high-performance printed circuit board (PCB) substrate materials must be developed, especially if the goal is to satisfy the high reliability required in military applications. The most critical requirement is matching the in-plane coefficient of thermal expansion (CTE) with the SMT-PCB's and the chip carriers (LCCCs) to reduce the solderjoint stresses during thermal cycling. Solder-joint reliability can be improved significantly by tailoring the in-plane CTE to approximately 6–7 ppm/°C (of the alumina chip carrier) using Kevlar and/or quartz fabric-reinforced polymer composites, instead of conventional glass composites. Less attention has been focussed on other important requirements, e.g., out-of-plane (Z-axis) expansion, glass transition temperature (Tg), dielectric constant, and resin microcracking, which also play important roles in the overall performance of the substrate materials. For example, high Z-expansion puts additional strain on the plated through holes (PTH), thereby affecting PTR reliability. A low Tg significantly increases the amount of thermal stress imposed during thermal cycling on solder joints and PTH, and leads to failures. This paper contains a brief review of the requirements of a SMT-PCB substrate material, including such critical parameters as: in-plane CTE, out-of-plane CTE, Tg, dielectric constant, fiber-to-resin ratio, and resin microcracking, and their effects on solder joint reliability, PTH reliability, dimensional stability, and electrical performance.