DFM 101: Solder Mask and Legend

Introduction
One of the biggest challenges facing PCB designers is understanding the cost drivers in the PCB manufacturing process. This article is the latest in a series that will discuss these cost drivers from the PCB manufacturer’s perspective, and the design decisions that will impact product reliability.

Solder Mask Availability
Most shops provide a variety of solder masks to satisfy the needs of all customers. Table 1 is a description of typical solder masks currently available. The need for closer tolerances has driven the implementation of laser direct imaging (LDI) solder masks (laser-defined solder mask pattern instead of screening).

DFM101-Table-1.jpg

DFM101-Fig-1.jpg

  • Solder mask is a protective coating that shields selected areas of a PCB from oxidation, handling and unwanted solder during assembly.
  • Solder mask allows a 0.003″ web to be placed between pads in an SMD array, provided the minimum spacing between these pads is 0.007” (by design).
  • Green solder mask allows a 0.003” web to be placed between pads in an SMD array, provided the minimum spacing between these pads is 0.007” (by design). All colors cannot maintain this dam size.
  • 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.

To read this entire article, which appeared in the April 2022 issue of Design007 Magazine, click here.


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