The Composite Properties of Rigid vs. Multilayer PCBs

Estimated reading time: 2 minutes

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 characteristics of the finished laminates, prepregs and fabricated multilayer boards (MLBs). In most cases variables such as glass style and resin content 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 complicate the situation, some product lines are manufactured with consistent resin content for all product thicknesses, thus maintaining the dielectric properties while letting the mechanical properties be dictated by the constructions and resin content.

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 discuss 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 laminate properties that are impacted by resin content variation—obviously there are others as well, but for purposes of keeping this work manageable, we will focus on these two) will hold for all thicknesses of laminate or build-ups for MLBs. In those cases where either or both are critical, it is necessary to control resin content in both laminate and prepreg to ensure that the finished properties fall within the desired and expected ranges.

In some cases, such as with 85N polyimide and 92ML thermally conductive epoxy products, the specific values of Dk and CTE are less critical than other things such as processibility and design flexibility (as for 85N) or thermal conductivity (for 92ML) and so resin contents are allowed to vary so as to meet these needs.

In other cases either CTE (as in 85NT) or dielectric constant (as for Multiclad HF or 25N/FR) are critical needs, and in those cases the resin contents must be held to controlled limits so that the desired properties are consistently held for most or all of the laminate configurations.

To read this article, which originally appeared in the May 2016 issue of The PCB Design Magazine, click here.