Issue link: https://iconnect007.uberflip.com/i/895789
November 2017 • SMT Magazine 39 DFX ON HIGH-DENSITY ASSEMBLIES clear specification, the microvias can look dif- ferent in size and shape between different sup- pliers and between batches of PCBs. By filling the microvias with copper in the PCB fabrication process, the voids and bridging issues can be eliminated (Figure 5). Silkscreen Issue Depending on the quality of the silkscreen, the height on top of the solder mask could range from 15-40 µm (Figure 6). This can create a gap between the stencil and PCB of 15-40 µm, depending on the PCB design. The gap can cre- ate printing issues leading to both too little or too much solder paste height and volume. De- pending on the design of a product, the solder mask can add 10-25 µm in addition to the silk- screen, creating a gap of 15-65 µm from the bot- tom of the stencil to the pad. In this case, the top of the solder mask is 422 µm on top of the copper pad. When the stencil is placed on top of the PCB, the effect of the silkscreen becomes very visible graphically (Figure 7). By looking at the SPI data (Figures 8 and 9), it becomes very clear that it can have a big impact on SPI yields, and potentially overall yields. Silkscreen is normally not needed on today's products and is an extra cost of approximate- ly ≈$0.01/square-inch. There is also a risk that parts of the silkscreen are placed onto the pads. Figure 3: Typical unfilled microvia from top. Figure 5: Bridge and void issue solved on a 0.4 mm pitch CSP by using a copper-filled microvia. Figure 6: PCB silkscreen on top of solder mask and copper. Figure 4. No clear specification if the microvia should be measured top, center, or bottom.