Abstract
Printed circuit boards must meet stringent requirements imposed by elevated temperature processes required for mixed-solder and/or Pb-free assembly. To meet these requirements, laminate manufacturers offer a variety of resin formulations, reactive additives, and glass styles designed to impart specific properties. Both the coefficient of thermal expansion (CTE) and the glass transition temperature (Tg) have received considerable attention with respect to design of high-temperature laminates. CTE mismatch between the copper and the laminate within a PCB results in stress upon the copper that may manifest itself as opens within vias, at the interfaces between internal lands and plated-through hole barrels, as well as open traces. Since the CTE of resin materials below the Tg is typically on the order of 5X lower than the CTE above Tg, a typical laminate design strategy is to produce a resin that exhibits a high Tg without adversely impacting other properties. Numerous factors affect the ultimate Tg of the resin, including the functionality of the monomer(s), crosslink density, the cure profile, and absorbed moisture. Within the electronics industry, Tg is determined via differential scanning calorimetry (DSC) as per IPC-TM-650. However, due to the multilayer construction of current circuit boards coupled with sample size limitations, DSC has been shown to be an inadequate technique for measurement of the glass transition temperature. The endotherm in the DSC is often ill defined, of marginal quality, and may be convoluted with stress relaxation and/or volatile outgassing at elevated temperature. Dynamic mechanical analysis (DMA) has been demonstrated to provide far greater information relative to not only the Tg, but also physical property depression due to moisture plasticization and incomplete resin conversion in various high-Tg laminate systems. Several case studies regarding phenolic-cured epoxy resins, cyanate ester/epoxy blends, and/or polyphenylene oxide/triallylisocyanurate blends will be discussed.
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