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Electronic assembly thermal management has always been an issue but has become more significant as we pack more power and function into a smaller form factor. In recent years, the growing use of LEDs for illumination on a large scale has presented additional thermal demands. EMI control also benefits from generous copper use.
The first inclination for a designer is to add copper in the form of planes and flooding. How this copper is added with respect to the component footprint’s pads can alter the termination quality.
Calculating the area for each pad:
Cu-D = 63.0(L1), 11.8(W1)
Oval Pad = ((63-11.8) *11.8)+(PI()*(11.8/2)^2) = 714
+ Partial Rectangular Trace = 5.0*3.0 = 14.5, 14.5+714 = 729
Msk-D = 73.0(L2), 16.5(W2)
Unmasked Rectangular Copper Area = 73.0*16.5 = 1205
Cu-D = Non-solder mask defined or copper defined pads
• Pad features are created by the copper etching process in fabrication.
Msk-D = Solder mask defined pads
• Pad features are created by the respective solder mask pattern applied over copper, post etching in fabrication.
The Msk-D pad is 65% larger than the Cu-D pad.
Figure 1: Graphical representation of the pad definitions with example dimensions.
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 significantly 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 interconnect volume at the termination. Without sufficient 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:
- Solder mask expansion or oversize value
- Thermal relief dimensions relative to pad size
- Stencil aperture size
- Stencil thickness
• Try to be consistent within a design layout using Cu-D or Msk-D pads, avoid mixing both.
• Many suppliers are now supplying recommended 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.
Ken Horky is a process engineer at Peterson Manufacturing.