Solder mask expansion in PCB Design - Real Business Scenario

Scenario Mode

👤 Your Role: You are a PCB manufacturing analyst at an electronics company.

📋 Request: Your manager wants you to analyze solder mask expansion data to ensure quality control and optimize the manufacturing process.

📊 Data: You have data on solder mask expansion measurements for different PCB batches, including batch ID, measured expansion in micrometers, target expansion, and pass/fail status.

🎯 Deliverable: Create a report showing average solder mask expansion per batch, highlight batches that failed quality standards, and visualize trends to help the manufacturing team improve processes.

Sample Data

Batch ID	Measured Expansion (µm)	Target Expansion (µm)	Status
B001	120	100	Fail
B002	95	100	Pass
B003	105	100	Pass
B004	130	100	Fail
B005	98	100	Pass
B006	110	100	Fail
B007	102	100	Pass
B008	99	100	Pass

1

Step 1: Import the solder mask expansion data into your BI tool.

Load the table with columns: Batch ID, Measured Expansion (µm), Target Expansion (µm), Status.

Expected Result

Data table with 8 rows and 4 columns loaded successfully.

2

Step 2: Create a calculated measure to find the average measured expansion across all batches.

Average Expansion = AVERAGE('Data'[Measured Expansion (µm)])

Expected Result

Average Expansion value calculated as 107.375 µm.

3

Step 3: Create a filter or slicer to show only batches that failed the quality check.

Filter where Status = 'Fail'

Expected Result

Filtered view showing batches B001, B004, and B006.

4

Step 4: Build a bar chart with Batch ID on the X-axis and Measured Expansion on the Y-axis, color-coded by Pass/Fail status.

X-axis: Batch ID; Y-axis: Measured Expansion (µm); Legend: Status

Expected Result

Bar chart showing each batch's expansion with red bars for Fail and green bars for Pass.

5

Step 5: Add a reference line on the bar chart at the Target Expansion value (100 µm) to compare measured values.

Add constant line at 100 µm

Expected Result

Reference line visible at 100 µm on the chart.

6

Step 6: Create a summary table showing Batch ID, Measured Expansion, Target Expansion, Status, and a new column 'Difference' calculated as Measured Expansion minus Target Expansion.

Difference = 'Data'[Measured Expansion (µm)] - 'Data'[Target Expansion (µm)]

Expected Result

Table with difference values: B001=20, B002=-5, B003=5, B004=30, B005=-2, B006=10, B007=2, B008=-1.

7

Step 7: Interpret the results to identify which batches need process review due to high solder mask expansion.

Review batches where Difference > 5 µm and Status = 'Fail'

Expected Result

Batches B001, B004, and B006 identified for process review.

Final Result

Solder Mask Expansion Report

Batch ID | Measured Expansion (µm) | Status | Difference (µm)
-----------------------------------------------------------
B001     | 120                      | Fail   | +20
B002     | 95                       | Pass   | -5
B003     | 105                      | Pass   | +5
B004     | 130                      | Fail   | +30
B005     | 98                       | Pass   | -2
B006     | 110                      | Fail   | +10
B007     | 102                      | Pass   | +2
B008     | 99                       | Pass   | -1

Bar Chart: Measured Expansion by Batch (Red=Fail, Green=Pass)
Reference Line at 100 µm

Average Expansion: 107.375 µm

Batches B001, B004, B006 require process review.

✓Average solder mask expansion is higher than the target of 100 µm.

✓Three batches (B001, B004, B006) failed quality due to excessive expansion.

✓Batches with expansion difference greater than 5 µm mostly failed.

✓Visualizing data helps quickly identify problem batches for manufacturing review.

Bonus Challenge

Create a trend line chart showing solder mask expansion over time if batch dates are available, to detect any improvement or degradation in the process.

Show Hint

Add a 'Batch Date' column to your data and plot Measured Expansion against Batch Date with a line chart.

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