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November 2016 • The PCB Magazine 29 Manufacturing Process The manufacturing process is shown in Fig- ure 12. The microvias are lasered or punched in the polymide adhesive and then filled with the TLPS paste. The structure can now take layer pairs from any other HDIS process (such as Shel- dahl's) and turn them into a multilayer struc- ture through sintering. The conductive pastes have to be sintered in a condensing vapor of flu- orocarbon at 215°C for 2 min. The structure is then postcured by baking for 40 min. at 175°C. Table 3 details the process. Power Mesh for Increased Layer Density Power mesh is a layer topology architecture that merges power supply routing with signal routings. The techniques use the old RF tech- niques of co-planar power. Conventional de- sign utilizes dedicated power planes as seen in Figure 13a. Increasing density and the number of voltage rails has required that split planes be introduced, shown in Figure 13b. But higher pin-count BGAs like FPGAs can have numerous voltages, and this is where utilizing blind vias minimizes both the plane perforations and in- sures coupling and a return path for the signals. A further experimental HDI technology, can in- crease the split planes to as many as eight differ- ent voltage rails. That is to use two, orthogonal layers to dis- tribute POWER as a 'mesh structure' and to place signals between the different voltages. This can be seen in Figure 13c and the struc- ture is called a 'Dual Offset Coplanar Stripline w/Separate GND Reference' (Figure 13d). Line widths and dielectric distances are given for the various impedances that are commonly used. This structure has the advantage of lower crosstalk but more importantly, it provides volt- age to all the components from 'LAYER_2' and Figure 10: The co-lamination (OrmeLink) multilayer structure [8] . Figure 11: Examples of TLPS cross-sections of three layer pairs with lasered-vias filled with TLPS paste vias and buried vias for FR-4 innerlayers [8] . Figure 12: The OrmeLink multilayer substrate fabrication process [8] . INNOVATIVE USE OF VIAS FOR DENSITY IMPROVEMENTS