Issue link: https://iconnect007.uberflip.com/i/948150
72 SMT007 MAGAZINE I MARCH 2018 For simplicity, we'll assume the solder paste deposition aperture is equal to the original pad size (often it is less). For the Cu-D pad, the trace to the pad adds relatively little additional copper area, creating only an additional 2% copper area to the pad. For the Msk-D pad, the surrounding copper plane adds signifi - cantly more copper area, 1.7X. Solder paste will occupy an area of 714 out of 1205 total pad area. This leaves 41% of the Msk-D pad area unaccounted for in solder joint volume. The post reflow results will be less intercon - nect volume at the termination. Without suffi- cient solder volume, the minimum fillet height may not meet your acceptance standards. The spoke width of thermally relieved pads can have the same effect on termination quality as an Msk-D pad. Pad size relative to mask expansion directly affects the magnitude of the issue. In the example above, the Msk-D pad is 1.7 times larger than the original pad design using 5 mil mask expansion. If the pad was 10 times larger, using the same mask expansion, the Msk-D pad would only be 1.1 times larger. The smaller the pad, the greater the impact of mask defined pads. Some variables to consider: 1) In design: a. Solder mask expansion or oversize value b. Thermal relief dimensions relative to pad size 2) Manufacturing: a. Stencil aperture size b. Stencil thickness Additional Considerations • Try to be consistent within a design layout using Cu-D or Msk-D pads, avoid mixing both. • Many suppliers are now supplying recom- mended footprint dimensions for Cu-D and Msk-D designs. You may have to maintain both in the system library and use them accordingly. • Telling your stencil fabricator to globally reduce aperture size by a percentage may produce undesired results. Take control, review the fabrication data yourself and edit to improve first run success. • Lead-free solder doesn't wet and flow as well as tin/lead solder did. This normally undesirable attribute may be a benefit for Msk-D pads since the solder doesn't flow away from where it is printed as much during reflow, keeping the volume at the termination. SMT007 Ken Horky is a process engineer at Peterson Manufacturing. Figure 1: Graphical representation of the pad definitions with example dimensions.