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May 2015 • The PCB Design Magazine 51 article THE COMPOSITE PROPERTIES OF RIGID VS. MuLTILAyER PCBS continues of the filler property is exponential with regard to loading. For example 50% of TiO2 (Dk~100) in PTFE resin (Dk~2.09) gives a product with a final Dk of about 10, whereas if the effect were linear it would be expected to be about 50. In addition, with many resin systems copper peel strength falls off substantially with higher filler loadings, and where higher levels are essential, resin chemistry adjustments may have to be made to maintain acceptable peels. Copper foils used in laminates normally include a variety of finishes and "tooth struc- tures." For material like polyimide, a rougher surface results in improved peel strength, while in many microwave/RF products the lower profile coppers are used to minimize the copper foil effect on transmission line loss. We only touch briefly on the impact of copper on physical properties in this paper, but will not try to look at the electrical impact. Where ap- plicable, the assumptions made in this paper as- sume standard electrodeposited copper foil. II. Calculating the Composite Properties The following table of properties shows the effective values of key computational values for the resins and reinforcements covered in this paper. These effective values have been derived based on actual test results for both thin and rigid laminates, and represent a best estimate of how they behave in making a variety of lami- nate products. The calculation of dielectric constant from the data shown is done using a standard for- mula based on the volume ratios of each of the components. For this purpose we have used a composite dielectric constant value for the filled resin systems. The calculation of CTE values from the data uses the Schapery Equation, which relates com- posite CTE to weighted values of CTE, modulus and volume. Schapery Equation: Where: a = component effective linear coefficient of thermal expansion in ppm/oC v = volume of each component m = effective modulus of elasticity (Young's Modulus in mpsi) n = the number of components making up the composite (resin/reinforcement systems are binary but composites such as PWBs may have multiple layers that contribute individually) For each resin reinforcement system, we cal- culated the CTE and the dielectric constant for a range of resin content values that can be re- lated roughly to the typical resin content used Figure 1: Chart showing critical properties of various materials' effective values.