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Design for Assembly: Components, Part 2
January 8, 2014 |Estimated reading time: 1 minute
Click here to read Part 1 of this column. Surface Mount Solder Joints
These seven factors are used to calculate the optimum land size per IPC-7351:
- Component body tolerance.
- Component terminal tolerance.
- Fabrication tolerance: ± 2 mil (0.05 mm).
- Placement tolerance: ± 1 mil (0.025 mm).
- Land size round-off.
- Land spacing round-off.
- Solder joint goals for toe, heel and side (Figure 16).
Figure 16: Image of toe, heel, and side.
The heel fillet in a gull-wing lead component is very important. Many reliability studies establish that 60 to 80% of gull-wing solder strength is in the heel. Land position or length that precludes forming the minimum heel fillet is a failure mechanism.
Plated Through-hole (PTH) Hole and Pad Size Calculations
Before you calculate the pad size, you must first calculate the hole size per Figure 17.
Figure 17: IPC specs for calculating hole sizes.
Once you figure out the hole size, the annular ring can be calculated by adding a minimum annular ring value of 2 mils (0.05 mm) plus a minimum fabrication allowance of 16 mil (0.4 mm) to the hole-size. The nominal fabrication allowance is 20 mils (0.5 mm) and the maximum fabrication allowance is 24 mils (0.6 mm). Read the full article here.Editor's Note: This article originally appeared in the November 2013 issue of The PCB Design Magazine.