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JANUARY 2024 I PCB007 MAGAZINE 75 3. Planar Printed Wiring Boards As designers are forced to put more function- ality into their PWBs, this leads to variation in layer thickness (impedance) copper weights between the power, ground, and signal layers. e addition of multiple functions on a single layer, such as split planes, can cause thickness variation in the printed circuit board. Dielectric thickness variations can cause a number of issues including inconsistent laser via quality, incon- sistent back-drilling depth, and inconsistent surface topography/flatness. Surface mount components are increasing in size. Increasing surface mount sizes/larger packages challenge the assembly process. Leading contract manu- facturers are increasingly demanding flatter sur- faces to ensure these components can be assem- bled and all the balls in a ball grid array package are planar to the surface of the PWB. Figure 14 shows that the subassembly layer and the first microvia layer have some amount of thickness variation which is quite typical for a thick subassembly and can measure 3–4 mils. e non-reinforced bonding materials act as leveling agents, but the inner-most microvia layer reflects the dielectric thickness inconsis- tency present in the subassembly. Figure 14 shows how the first layer of non- reinforced resin acts like a leveling agent and each subsequent lamination gives a very pla- nar surface. Testing was completed up to 13 laminations producing an extremely flat sur- face. Figures 15, 16, and 17 show the flatness of the printed wiring board surface that can be achieved with a non-reinforced bonding mate- rial that acts as a leveling agent. 4. Heavy Copper Resin Flow and Fill Another challenge facing fabricators is the lack of resin flow in some designs. is can be caused by some of the new ceramic-filled polyphenylene oxide resin systems having a very low flow rate of 10–15%, the high vis- cosities used to maintain consistency of the ceramic in the resin system, and the use of spread/flattened fiberglass to mitigate electric skew caused by inconsistency in the fiberglass weave. Figure 18 shows various spread weave Figure 14: Cross-sectional image showing the thickness variation of the sub and the first microvia layer. Figure 15: Cross-sectional image showing the flatness of a printed wiring board manufactured using only a non-reinforced bonding material. Figure 16: Cross-sectional image showing the flatness of a four-copper layer printed wiring board manufactured using only a non-reinforced bonding material. Figures 17: Cross-sectional image showing the flatness of a four-copper layer printed wiring board manufactured using only a non-reinforced bonding material.