Creating custom footprints in PCB Design - Formula Evaluation Walkthrough

Sample Data

This data represents the pad layout for a custom PCB footprint with pad names, their X and Y positions in millimeters, and pad sizes.

Cell	Value
A1	Pad Name
B1	X Position (mm)
C1	Y Position (mm)
D1	Pad Size (mm)
A2	Pad1
B2	0
C2	0
D2	1.5
A3	Pad2
B3	5
C3	0
D3	1.5
A4	Pad3
B4	0
C4	5
D4	1.5
A5	Pad4
B5	5
C5	5
D5	1.5

Formula Trace

Calculate footprint bounding box width = MAX(B2:B5) - MIN(B2:B5)

Step 1: MAX(B2:B5)

Step 2: MIN(B2:B5)

Step 3: MAX(B2:B5) - MIN(B2:B5)

Cell Reference Map

    B   
  +-----+
2 |  0  |
3 |  5  |
4 |  0  |
5 |  5  |

The formula uses the X Position values in cells B2 to B5 to calculate the footprint width.

Result

    A       B       C       D       E
1 | Pad Name | X Pos  | Y Pos  | Size  |
2 | Pad1     | 0      | 0      | 1.5   |
3 | Pad2     | 5      | 0      | 1.5   |
4 | Pad3     | 0      | 5      | 1.5   |
5 | Pad4     | 5      | 5      | 1.5   |
6 | Footprint Width: 5 mm                   |

The footprint width calculated from the pad X positions is 5 mm, shown below the pad data.

Sheet Trace Quiz - 3 Questions

What is the maximum X position among the pads?

A 0

B 1.5

C 5

D 10

Key Result

Calculate footprint dimension by subtracting minimum coordinate from maximum coordinate in pad positions.

Practice

1. What is the main purpose of creating a custom footprint in PCB design?

easy

A. To match the exact size and shape of a specific part on the PCB

B. To change the color of the PCB

C. To add more layers to the PCB

D. To increase the power supply voltage

Solution

1. Step 1: Understand what a footprint represents

   A footprint is the physical layout on the PCB where a part will be placed, including pads and outlines.

2. Step 2: Identify the purpose of custom footprints

   Custom footprints ensure the pads and outlines fit the exact size and shape of the part for proper placement and soldering.

3. Final Answer:

   To match the exact size and shape of a specific part on the PCB -> Option A

4. Quick Check:

   Custom footprint = exact part fit [OK]

Hint: Custom footprints fit parts exactly on the board [OK]

Common Mistakes:

• Thinking footprints change PCB color
• Confusing footprints with PCB layers
• Assuming footprints affect power supply

2. Which of the following is the correct step to add a pad when creating a custom footprint?

easy

A. Use the text tool to write the pad name

B. Draw a rectangle and fill it with color

C. Select the pad tool, click on the workspace, then set pad size and shape

D. Import a 3D model without setting pad properties

Solution

1. Step 1: Identify how to add pads in footprint editors

   Pads are added by selecting the pad tool and placing them on the workspace.

2. Step 2: Set pad properties after placement

   After placing, you define pad size, shape, and other properties to match the part's pins.

3. Final Answer:

   Select the pad tool, click on the workspace, then set pad size and shape -> Option C

4. Quick Check:

   Pad tool + set size = correct pad addition [OK]

Hint: Use pad tool, then set size and shape [OK]

Common Mistakes:

• Trying to add pads by drawing shapes
• Using text tool instead of pad tool
• Skipping pad property settings

3. Given a custom footprint with 4 pads arranged in a 2x2 grid, each pad 1.5mm square, spaced 2.5mm apart center-to-center, what is the total width of the footprint from outer edge to outer edge horizontally?

medium

A. 7.0 mm

B. 5.5 mm

C. 6.5 mm

D. 4.0 mm

Solution

1. Step 1: Calculate distance between pad centers horizontally

   Two pads spaced 2.5mm center-to-center means distance between centers is 2.5mm.

2. Step 2: Add half pad size on each side for total width

   Each pad is 1.5mm wide, so half width is 0.75mm. Total width = 2.5mm + 0.75mm + 0.75mm = 4.0mm.

3. Step 3: Consider the 2x2 grid arrangement

   Since there are 2 pads horizontally, total width is center-to-center spacing (2.5mm) plus half pad width on each side (0.75mm + 0.75mm) = 4.0mm. But the question asks for total width of the footprint, which includes both rows. The footprint width horizontally is 2 pads spaced 2.5mm apart plus pad widths: 2.5mm + 0.75mm + 0.75mm = 4.0mm.

4. Step 4: Verify if the question expects total width including both rows

   Since pads are arranged in 2x2 grid, horizontally there are 2 pads, so total width is 2.5mm + 0.75mm + 0.75mm = 4.0mm.

5. Final Answer:

   4.0 mm -> Option D

6. Quick Check:

   Center spacing + pad halves = total width [OK]

Hint: Add pad half-widths to center spacing for total size [OK]

Common Mistakes:

• Adding full pad width twice instead of half
• Confusing spacing with edge-to-edge distance
• Ignoring pad size in calculation

4. You created a custom footprint but the pads do not align with the part pins when placed on the PCB. What is the most likely error?

medium

A. Pad sizes are too large

B. Pad positions are incorrect or not matching the part pin layout

C. The footprint name is missing

D. The PCB layer color is wrong

Solution

1. Step 1: Understand alignment issues

   If pads do not align with part pins, the positions of pads in the footprint are likely incorrect.

2. Step 2: Check pad positions against part datasheet

   Verify pad coordinates match the exact pin layout and spacing from the part's datasheet.

3. Final Answer:

   Pad positions are incorrect or not matching the part pin layout -> Option B

4. Quick Check:

   Misaligned pads = wrong pad positions [OK]

Hint: Check pad positions match part pins exactly [OK]

Common Mistakes:

• Blaming pad size instead of position
• Ignoring footprint naming importance
• Focusing on PCB color instead of layout

5. You need to create a custom footprint for a new sensor with 6 pins arranged in a single row, each pin 1.0mm wide, spaced 2.0mm apart center-to-center. You want to add a silkscreen outline 0.5mm larger than the pad area on all sides. What is the total length of the silkscreen outline along the pin row?

hard

A. 12.0 mm

B. 16.0 mm

C. 15.0 mm

D. 13.0 mm

Solution

1. Step 1: Calculate span between pad centers

   6 pins, 5 intervals x 2.0 mm = 10.0 mm.

2. Step 2: Determine pad area length

   Add half pad width (0.5 mm) on each end: 10.0 mm + 1.0 mm = 11.0 mm.

3. Step 3: Apply silkscreen offset

   0.5 mm larger on all sides adds 1.0 mm to length: 11.0 mm + 1.0 mm = 12.0 mm.

4. Final Answer:

   12.0 mm -> Option A

5. Quick Check:

   (n-1)xspacing + pad width + 2xmargin [OK]

Hint: Add pad widths, spacing, then silkscreen margins [OK]

Common Mistakes:

• Confusing center-to-center with edge-to-edge spacing
• Forgetting to add silkscreen margin on both sides
• Ignoring pad width in total length