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18 The PCB Magazine • April 2014 LEAD-FREE REFLOW FOR HIGH-LAYER-COUNT PCBS continues ently. In theory, take a 1153 pin (34x34) BGA. It will have 132 possible routing escapes per layer (one trace between vias) plus 20 traces in the channel (five traces). This means that eight lay- ers would be required (plus five plane layers) to connect this BGA to the rest of the circuit. The TH vias create a "fence" that makes routing very layer- intensive. If we create more routing channels on the innerlayers by placing the blind vias on the surface, we connect more traces per layer and reduce the total layers. Channel routing uses blind microvias to form four or more additional cross-shaped, L-shaped or diagonal channels in a BGA fanout pattern. These new channels al- low up to 48 extra connections per layers for this BGA (8x6 traces). Four routing layers and four plane layers can be eliminated in this ex- ample. In some cases, the channel is not to escape the inner-pins of a BGA but to allow access to an adjacent BGA. The blind vias can be via-in- pad, near-via-in-pad or traditional dogbones. If working with a FPGA where pin swapping is possible, then the channels should be assigned to ground and power first. Contribution of New SMT Connectors Press-fit connectors require the use of through-holes. For thick boards, this might ne- cessitate the use of back-drilling (Figure 4a) to reduce the inductance of these vias. Because of the sensitivity of high-speed circuits to para- sitic inductance, even SMT connections might require their through-holes to be reduced to blind-via stubs (Figure 4b). But by using micro- vias, either in pad or off-pad, the need for larger through-holes is reduced as well as the resulting parasitic inductance, as seen in Figure 4c. Figure 5 shows the reduction in overall insertion loss due to the use of BGA connectors and blind- microvias instead of through-holes. If BGA con- nectors are used on the backplane or midplane, a 33% reduction is possible. If blind-microvias are used, then a 67% reduction can be realized. This also facilitates higher-density routing and fewer signal layers and their reference layers. High-frequency, controlled-impedance SMT matrix-connectors are now becoming available. These are BGA types. Typical is the header for a backplane or midplane daughter card. These are remarkable for helping to reduce the thickness of multilayers. Figure 4: press-fit connectors create a need for thick boards that require higher reflow temperatures and durations. (a) many times, the increased inductance require backdrilling. (b) even with surface- mounting, the through-hole plating may have to be eliminated. (c) microvias increase density, reduce thickness and reduce inductance [5] .