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52 SMT Magazine • November 2016 cording to drill size, board thickness, and plat- ing type. Proximity to adjacent layer vias is also critical to measure. If you are using backdrill technology, make sure that you have a means to check for back- drill depth tolerance so that the backdrill does not penetrate, or partially penetrate, the stop layer (Figure 7). Also check for backdrill-spac- ing clearance to copper features throughout the stackup as you are introducing an additional to consider these fabrication tolerances. Figures 1–4 show some examples of issues found at fab- rication that affect PCB reliability. A 3-mil spacing for a length of 10 mil might be acceptable, but it increases the risk of shorts when it spans 100 mil. This may result in the solder not wicking properly or evenly inside the hole, resulting in what could become a field failure. Your CAD software permits this type of spacing for the same net; however, a short here could result in a critical net (controlled imped- ance, clock net, differential timing) that fails to perform as designed. The challenge for many PCB designers is they do not see, at the design stage, the final sol- der mask openings the fabricator will use. These openings change in size and shape based on the fabricator's practice. Thus, potential bridging is- sues might not manifest until boards are built. One of the more challenging areas to ensure quality in today's PCBs is with vias. Vias can take the form of plated-through holes mechan- ically drilled, microvias that are laser-formed, and backdrills that are mechanically drilled but are a secondary process and serve as an alterna- tive to sequential lamination. Figures 5 and 6 il- lustrate a few rules to check for vias. A different annular ring is often specified for the top, bottom, and inner pads on a microvia. Additionally, allowable annular ring varies ac- HOW TO IMPROVE PCB RELIABILITY Figure 4: Circuits close to a pad must be fully covered by mask to avoid solder bridging during the assembly process. Figure 5: Via-to-via overlap in some cases is permissible and, in other cases, not. Figure 6: Stacked vias greater than three deep are at risk of collapsing without special processing. Figure 7: Check for backdrill depth tolerance so that the backdrill does not penetrate, or partially penetrate, the stop layer.