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

Practice

1. What is the main purpose of solder mask expansion in PCB design?

easy

A. To adjust the size of mask openings around pads

B. To change the thickness of the PCB substrate

C. To increase the number of layers in the PCB

D. To modify the copper trace width

Solution

1. Step 1: Understand solder mask expansion function

   Solder mask expansion changes the size of openings around pads to control soldering quality.

2. Step 2: Identify the correct purpose

   Adjusting mask openings prevents solder bridges or insufficient coverage during assembly.

3. Final Answer:

   To adjust the size of mask openings around pads -> Option A

4. Quick Check:

   Solder mask expansion = Adjust mask openings [OK]

Hint: Think about mask openings size control [OK]

Common Mistakes:

• Confusing mask expansion with PCB thickness
• Mixing mask expansion with copper trace width
• Assuming it changes PCB layers

2. Which of the following is the correct way to specify a solder mask expansion value to increase the mask opening size by 0.05 mm?

easy

A. solder_mask_expansion = -0.05

B. solder_mask_expansion = 0

C. solder_mask_expansion = 0.05

D. solder_mask_expansion = +0.00

Solution

1. Step 1: Understand sign convention for expansion

   Positive values increase mask opening size; negative values decrease it.

2. Step 2: Choose the positive value for increasing size

   0.05 mm positive value means mask opening grows by 0.05 mm.

3. Final Answer:

   solder_mask_expansion = 0.05 -> Option C

4. Quick Check:

   Positive value increases opening size [OK]

Hint: Positive value means bigger opening [OK]

Common Mistakes:

• Using negative value to increase size
• Using zero which means no change
• Confusing plus sign usage

3. Given a pad size of 1.0 mm and a solder mask expansion of -0.1 mm, what will be the effective mask opening size?

medium

A. 0.8 mm

B. 1.1 mm

C. 1.0 mm

D. 0.9 mm

Solution

1. Step 1: Understand the effect of negative expansion

   Negative expansion reduces the mask opening size by the given amount.

2. Step 2: Calculate effective mask opening

   1.0 mm pad size minus 0.1 mm expansion = 0.9 mm opening.

3. Final Answer:

   0.9 mm -> Option D

4. Quick Check:

   Pad size + expansion = opening size [OK]

Hint: Add expansion (negative) to pad size [OK]

Common Mistakes:

• Adding negative expansion instead of subtracting
• Ignoring sign of expansion
• Using pad size without adjustment

4. A designer sets solder mask expansion to 0.0 mm but notices solder bridging on the PCB. What is the most likely error?

medium

A. Expansion value is too small, should be positive

B. Expansion value is too large, should be negative

C. Expansion value syntax is incorrect

D. Expansion value has no effect on solder bridging

Solution

1. Step 1: Analyze effect of zero expansion

   Zero expansion means mask opening matches pad size exactly, no extra clearance.

2. Step 2: Understand solder bridging cause

   Without extra clearance (positive expansion), solder can bridge between pads.

3. Final Answer:

   Expansion value is too small, should be positive -> Option A

4. Quick Check:

   Positive expansion prevents bridging [OK]

Hint: Use positive expansion to avoid bridging [OK]

Common Mistakes:

• Thinking zero expansion is enough
• Assuming negative expansion prevents bridging
• Ignoring solder mask role

5. A PCB has pads of 0.8 mm size. To ensure no solder bridging but avoid excessive mask exposure, the designer wants the mask opening to be 0.9 mm. What solder mask expansion value should be set?

hard

A. -0.1 mm

B. 0.1 mm

C. 0.9 mm

D. -0.9 mm

Solution

1. Step 1: Calculate required expansion

   Desired opening (0.9 mm) minus pad size (0.8 mm) = 0.1 mm expansion.

2. Step 2: Confirm sign of expansion

   Since opening is larger than pad, expansion must be positive 0.1 mm.

3. Final Answer:

   0.1 mm -> Option B

4. Quick Check:

   Opening - pad size = positive expansion [OK]

Hint: Expansion = desired opening - pad size [OK]

Common Mistakes:

• Using negative expansion when opening is larger
• Confusing pad size with opening size
• Setting expansion equal to opening size