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92 DESIGN007 MAGAZINE I APRIL 2022 • To assure no solder mask on any pad in an SMD array, the minimum solder mask clearance for a surface mount pad is 0.002" per side (not applicable for panel sizes over 18"x24"). As space permits, a clearance of 0.0025" per side is preferred. • Allow 0.030" per-side solder mask clearance for score lines. • To prevent solder mask from going into and/or plugging a hole, solder mask clearance should be 0.010" (0.005" per side) larger than the pad size on both sides of the board. Tenting of Via Holes with Solder Mask With via tenting, the annular ring and via barrel are capped with solder mask. On a board coated with LPI mask, the vias can be screened with an epoxy or acrylic solder mask material, creating a cap over the hole. Artwork modifications necessary for pro- cessing are performed as part of the initial tool- ing. A separate design file must be provided by the customer, which includes only those vias which are to be capped. e customer needs to provide master pad solder mask and via fills, i.e., solder mask and via pads that are the same size as the outer layer pads. Via Capping Design Constraints • e maximum finished hole size for via capping is 0.020" diameter (preferred drill diameter 0.021"). • Generally, the non-test vias are capped on the top side of the board. ermal (epoxy) via caps will have a raised surface of approximately 0.0017" ±0.004" above the outer layer copper pad. • is is to make sure the via hole being capped is not plated shut or plugged with solder during reflow (such as HASL). In this case, due to trapped air between the via cap and plug, the via cap may "dome" during the cure process, thus creating height 0.0035". • UV (acrylic) via cap will have a raised surface of approximately 0.0019" ±0.0004" above the outer layer copper pads. Unlike thermal (epoxy), UV (acrylic) is not influenced by plated or solder-plugged holes, therefore the via cap height will remain constant. Note: via cap height thickness measurements may include solder and/or permanent solder mask thickness. • For product that receives either immersion silver or tin, UV via cap material must be used and applied aer the surface finish. Legend Marking Design Guidelines • Modern legend technology uses an ink jet printer for black or white legend; traditional technology is to screen it on. • To ensure all letters, numbers and figures are legible on the finished board, character Table 2: Legend marking design guidelines.

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