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50 The PCB Design Magazine • May 2015 article THE COMPOSITE PROPERTIES OF RIGID VS. MuLTILAyER PCBS continues nate properties that are impacted by resin con- tent variation—obviously there are others as well, but for purposes of keeping this work man- ageable, 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 prod- ucts, 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 di- electric constant (as for Multiclad HF or 25N/ FR) are critical needs, and in those cases the res- in contents must be held to controlled limits so that the desired properties are consistently held for most or all of the laminate configurations. Experimental/Assumptions For purposes of this discussion we will look at three typical resins that are being used in Ar- lon products: Arlon's 85N pure polyimide resin ("pure" defined here as not containing flame retardant additives that might reduce overall thermal stability) used for high temperature and PTH reliability, 92ML thermally conduc- tive "green" epoxy resin used in boards requir- ing thermal management, and ceramic-filled MCHF resin (for MultiClad HF) designed for use in high frequency applications. Additionally, we will consider standard E- Glass (electronics grade woven fiberglass) fab- rics as well as nonwoven aramid (NWA) fabric as typical substrates for PWB applications. E- Glass is relatively inexpensive and has been the mainstay of the PWB industry for many years, while nonwoven aramid offers advantages in weight, registration stability and finished prod- uct thermal cycling life. Most properties of a composite such as a pre- preg or laminate are based on the volume ratios of the individual components, using formulas that have been refined empirically over many years. Dielectric constant, for example, is pro- portional to the volume ratios of the resin and glass components adjusted by a factor to give a Dk value that is accurate for the frequency at which the raw material variables are defined. CTE is computed using the densities of the raw materials, weighted by their volume per- centages and individual CTE values using the Schapery Equation (see below). The results giv- en in this work take into account such modify- ing factors as the weave of glass fabric (which is partially out-of-plane and hence has an effec- tive modulus lower than the theoretical fiber modulus) and the effectiveness of any filler in modifying the modulus of the material. A table of material "effective" properties (working as- sumptions established based on properties of actual control laminates) for the materials dis- cussed in this paper is included below. I. Materials Set • Arlon 85N is based on pure polyimide res- in (no added flame retardant or filler). • Arlon 92ML is a halogen-free high perfor- mance (lead-free) epoxy based system contain- ing high loading of thermally conductive ce- ramic fillers. • Arlon MCHF (MultiClad® HF Resin) is a second generation multilayerable microwave/RF system containing ceramic fillers consistent with desired electrical and mechanical properties. • PTFE (polytetrafluoroethylene) with and without ceramic filler is widely used in micro- wave/RF laminates. • Arlon 25FR is a first generation ceramic- filled multilayerable microwave system used here for reference, being a system whose build- ups are all held to a constant resin percentage resulting in the same Dk (dielectric constant) and df (dissipation factor, aka loss tangent) for all laminate thicknesses and MLB buildups. • E-Glass fabric has been the industry stan- dard for longer than this writer can remember. • Nonwoven Aramid "paper" reinforcement (NWA) is made up basically of short aramid fi- bers with an appropriate binder. A note here regarding fillers and the effect of fillers on material properties is in order. While the effect of fillers in modifying the properties of the basic resin is volume dependent, benefit