Solder mask expansion in PCB Design - Cell-by-Cell Formula Trace

Sample Data

This data shows pad sizes and their corresponding solder mask expansion values. The goal is to calculate the final solder mask opening size by adding the expansion to the pad size.

Cell	Value
A1	Pad Size (mm)
B1	Solder Mask Expansion (mm)
C1	Final Mask Opening (mm)
A2	1.0
B2	0.1
A3	1.5
B3	0.15
A4	2.0
B4	0.2

Formula Trace

C2 = A2 + 2 * B2

Step 1: A2

Step 2: B2

Step 3: 2 * B2

Step 4: A2 + 2 * B2

Cell Reference Map

    A      B      C
1 | Pad  | Expansion | Final |
2 | 1.0  | 0.1      |  ?    |
3 | 1.5  | 0.15     |       |
4 | 2.0  | 0.2      |       |

Cells A2 and B2 are used to calculate C2. The formula adds pad size (A2) plus twice the expansion (B2) because expansion is on both sides.

Result

    A      B      C
1 | Pad  | Expansion | Final |
2 | 1.0  | 0.1      | 1.2   |
3 | 1.5  | 0.15     |       |
4 | 2.0  | 0.2      |       |

The final solder mask opening for the first pad is 1.2 mm, calculated by adding pad size 1.0 mm and twice the expansion 0.1 mm.

Sheet Trace Quiz - 3 Questions

Why do we multiply the solder mask expansion by 2 in the formula?

A Because expansion applies on both sides of the pad

B Because the expansion value is doubled by default

C To convert mm to microns

D To account for pad thickness

Key Result

Final mask opening = Pad size + 2 × Solder mask expansion