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16 The PCB Magazine • April 2014 LEAD-FREE REFLOW FOR HIGH-LAYER-COUNT PCBS continues through-hole boards. Typically, microvias are 6 mil with 12-mil pads, while TH vias are 13 mils with 24-mil pads. This obvious increase in space is multiplied by the fact that a blind via only goes from the surface down to the second or third layer. This opens up additional channels on the rest of the innerlayer to route additional traces. The other opportunity with HDI is the reduction of trace widths to go along with the reduction in dielectric thicknesses. Three mil (0.003") are not uncommon as well as 3.5-mil lines and spaces. This provides the opportunity to route 80–100 traces per square inch. This increase in density is illustrated in the cost-density tradeoff chart (Figure 2). The first column (A) is traditional TH boards from 4–40 layers. The prices (RCI) have all been adjusted to a basis of the cost of an 8-layer TH board from China. The DEN is the aver - age density of the stackup in pins per square inch. To find the equivalent of a particular TH board, move diagonally, following the dashed lines. For example, if you had a 22-layer TH mul- tilayer (RCI=6.67), the HDI type II (D) would be a 14-layer board (RCI=3.32); the type III (E) would be a 10-layer, RCI=2.30. These all have approximately the equivalent DENsity, but the HDI boards are less costly by 50% for the type II and 65.5% for the type III. Figure 2: the price-density matrix compares the relative prices (rci) of through-hole (tH) boards to their equivalent Hdi-microvias boards, along with average density (den) of pins per square inch [3] .