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56 The PCB Design Magazine • May 2015 such as areas where most of the copper is etched away in part of the board, but not uniformly across the panel. 3. As the resin content increases, dielectric constant normally decreases. There may be a few exceptions in the area of very high-di- electric constant systems where the resin has a higher Dk than the substrate, such as in Arlon's Dk 10 products—PTFE/ceramic filled systems—but that's not normally the case. This has an impact on the imped- ance calculations for any particular design, and if a variety of laminate and prepreg resin contents are used in the various layers of a PWB, some amount of guesswork may be neces- sary to "design" for a fixed impedance, often 50 ohms for transmission lines. What this means in practical terms is that in normal systems such as FR-4 and Polyimide made with E-Glass reinforce- ments, the Dk of each laminate and/or prepreg element will vary depend- ing on the "normal" resin contents for the different styles of glass fabric, typically quite high for thin glass such as 106 (72–75%) and much lower for heavyweight glass used to make rigid laminates such as 7628 (37–42%). As a result data sheets that report properties for rigid laminates (0.023" and above) may show maximum values for dielectric constant that are much higher than those actually obtained in multilayer constructions. A polyimide whose IPC/41 spec calls for a Dk of 5.2 max will actu- ally have a value of 4.0 or less in a high layer count multilayer board using thin laminates and high resin thin prepregs. It's easy to see than in such a case, if impedance is calculated based on the data sheet value, the results may be badly skewed. To compensate for this, some products such as Arlon's 25N and 25FR, which are pri- marily designed for use in microwave or RF de- signs where the principal concern is dielectric constant consistency, are designed such that the resin contents of laminate and prepreg are held to fixed limits such that the specified dielectric constant is held to within the tol- erance of resin content manufacturability, as low as +/- 1% or even less. If pre- pregs or laminates have to be used that do not meet the nominal target resin con- tent, recalculation of Dk may be necessary for lay- ers with active circuitry. V. PWBs vs. Raw Laminates: The Impact of Copper Copper foil has a rela- tively high density and a high modulus of elasticity (Young's Modulus of ~17 mpsi) and a CTE (effective) of roughly 10.7 ppm/ o C. (Pure copper CTE is actu- ally about 17 ppm/ o C and the 10.7 value reflects per- formance in actual con- structions.) This has a number of impacts on PWB design and function. 1. In plated through-holes (PTHs), copper's expansion co- efficient will be less than that of the resin in conventional epoxy or polyimide systems, and so any strain in the hole caused by expansion and contraction of the resin during thermal cy- cling will impact PTH reliability. This is partly due to "latent defects" caused by large thermal excursions during manufacturing and assembly operations, and partly to the actual amount of Z-direction expansion caused by the CTE of the resin. For example, copper with a CTE of 17 ppm/ o C (in PTHs) will expand 3750 ppm (0.37%) over 210 o C range (50 o C to 260 o C) while epoxy sys- tems will expand as much as 35000 ppm (3.5%) and polyimide systems as much as 16000 ppm as the resin content increases, dielectric constant normally decreases. there may be a few exceptions in the area of very high-dielectric constant systems where the resin has a higher Dk than the substrate, such as in arlon's Dk 10 products—PtFe/ ceramic filled systems— but that's not normally the case. " " article THE COMPOSITE PROPERTIES OF RIGID VS. MuLTILAyER PCBS continues

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