The Circuit Designer's Companion - diagramas.diagram...

Printed circuits 41material. The common designation of “FR4” refers to the American NEMAspecification for flame-retardant epoxy-woven glass board, which is offered by mostlaminate manufacturers; there are many flavours of FR4, having better or worsespecifications but all of them meeting the NEMA requirements. Flame retardant gradesare available for all common materials, and are shown by the manufacturers’ ID beingmarked in red.For flexible boards the base films are polyester or polyimide. Polyester is cheap butcannot easily be soldered because of its low softening temperature, so is used primarilyfor flexible “tails”. Polyimide is more expensive but components can be mounted on it.Table 2.1 PCB laminate material propertiesMaterialSurfaceresistanceMΩDielectricε rDielectrictan δDielectricstrengthkV/milTempcox-y ppm/˚CMax.temp ˚CStandard FR4 Min 1.10 4 Max 5.4,typ 4.6–4.9Max 0.035 1.0 min 13–16 110–150FR408(high quality)Epoxy-aramid(close tolerance)1.10 6 3.8 0.01 1.4 13 1805.10 6 3.8 0.022 1.6 10 180Polyimide (Kapton) 3.4 0.01 3.8 20 300Polyester (Mylar) 3.0 0.018 3.4 27 1052.1.2 Type of constructionMost circuit requirements can be accommodated on one of the following board types,which are listed roughly by increasing cost.1. Single-sidedCheap and cheerful, for simple, low-performance and/or high-volumecircuits2. Double-sidedAs above but different track pattern on each side of the board, throughconnections made on assembly; e.g. low density applications3. FlexibleBase material thin and flexible, may be covered with further layers toprotect track pattern. May be through-hole plated. Replacement forwiring harnesses