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May 2015 • The PCB Design Magazine 55 1. In-plane coefficient of thermal expansion (CTE) increases because the CTE of the resin (in normal, unfilled systems) is much greater than that of any of the reinforcements we use. In- crease in CTE (X,Y) can affect the artwork com- pensation needed to obtain proper registration; increasing CTE (Z)—perpendicular to the thick- ness of the board—will impact overall Z-direc- tion expansion, and while this is much harder to calculate theoretically, in general higher Z- expansion will mean more strain (and stress) in plated through-holes, and a decrease in PTH re- liability through thermal cycling. An interesting exception to this is in the case of nonwoven aramid reinforcement, where the Z-direction expansion is inherently greater be- cause of the constraint on X,Y due to the high modulus aramid fibers, and yet the PTH reliabil- ity through thermal cycling can be 2–3x better than with the same resin system on a woven glass substrate. The reason for this appears to be that with glass fabrics there are discrete fiber bundles that intersect hole walls and concen- trate stresses at those points, which are where most copper barrel cracking failures occur. The more uniform NWA reinforcements do not have the discrete bundles of fibers and as a result, do not tend to isolate or concentrate stresses. In instances where the value of CTE(X,Y) is important, such as in 85NT nonwoven ara- mid products where the end-use is for SMT ap- plications requiring a consistent reduced CTE, the resin content of both laminate grades and prepreg sales grades is held to a constant value for all thicknesses of product. This restricts vari- ability in CTE to that caused by normal vari- ability in resin content. The wide range of resin contents experienced with the normal range of glass styles used when using standard E-Glass reinforcement doesn't permit this degree of sta- bility of CTE, or, as we will see, in dielectric con- stant. 2. Higher resin contents are inherently more difficult to control and so variability in resin content and flow are more likely in pre- pregs with higher resin contents. The selection of a higher resin content for any specific glass style then will also affect resin flow and pressed thickness, sometimes in a hard to predict man- ner that will interact with board design and process variables such as pressure and heat-up rates. Too low a resin content, sometimes de- sired to control laminated thickness, may result in lack of adequate flow and associated dryness, especially in large areas of relative low pressure Figure 7. Figure 8. Figure 9. article THE COMPOSITE PROPERTIES OF RIGID VS. MuLTILAyER PCBS continues