The PCB Design Magazine

PCBD-May2015

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48 The PCB Design Magazine • May 2015 article Abstract Most materials systems used in PWBs (aka PCBs) are composites of resins, fabric substrates and metal cladding. Each of these components has its own unique electrical and mechanical properties that contribute to the final charac- teristics of the finished laminates, prepregs and fabricated multilayer boards (MLBs). In most cases variables such as glass style and resin con- tent have offsetting impacts on physical vs. electrical properties. Data sheets often provide data in standard IPC formats, which may look at properties in rigid laminates (nominal 0.062" made using heavyweight glass at 32–40% resin content) rather than thin lams and prepregs used for multilayer PWBs (which more typically average 55% resin content or more). To further compli- cate the situation, some product lines are man- ufactured with consistent resin content for all product thicknesses, thus maintaining the di- electric properties while letting the mechanical properties be dictated by the constructions and resin content. by Chet Guiles arlOn MaTerIals FOr eleCTrOnICs The Composite Properties of Rigid vs. Multilayer PCBs An understanding of what such variations in properties mean and how they relate to what the users will actually encounter is essential to design, manufacturability, assembly and end- use viability. In this presentation we will look specifically at the effect of glass style and resin content on dielectric constant and CTE and dis- cuss how even at the product design level it is important for design engineers to take these into consideration. Conclusions In most cases, no single value of dielectric constant or in-plane CTE (representative lami-

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