Creating custom footprints in PCB Design - Quick Revision & Summary

Recall & Review

beginner

What is a footprint in PCB design?

A footprint is the physical layout on the PCB where a component will be placed and soldered. It includes pads, holes, and outlines matching the component's pins and size.

beginner

Why do you need to create custom footprints?

Custom footprints are needed when a component is not available in the standard library or has unique dimensions or pin arrangements that require a tailored layout.

intermediate

What are the key steps to create a custom footprint?

1. Gather component datasheet dimensions.
2. Define pad sizes and shapes.
3. Place pads according to pin layout.
4. Add silkscreen outlines.
5. Verify spacing and clearances.
6. Save and test the footprint in a PCB layout.

beginner

How do silkscreen layers help in custom footprints?

Silkscreen layers show component outlines and reference designators on the PCB surface, helping with component placement and identification during assembly.

intermediate

What is the importance of pad clearance in footprint design?

Pad clearance ensures there is enough space between pads and other copper areas to prevent short circuits and allow proper soldering.

What information is essential to start creating a custom footprint?

A PCB board thickness

B Component datasheet dimensions

C Soldering iron temperature

D Component color

The component datasheet provides exact dimensions and pin layout needed to create an accurate footprint.

Which layer typically shows the component outline in a footprint?

A Silkscreen layer

B Mechanical layer

C Solder mask layer

D Copper layer

The silkscreen layer displays component outlines and reference marks on the PCB surface.

Why must pad sizes be accurate in a footprint?

A To match the component pins for good soldering

B To make the PCB look colorful

C To reduce PCB weight

D To increase electrical resistance

Pads must fit the component pins to ensure reliable electrical and mechanical connections.

What happens if pad clearance is too small?

A The PCB becomes flexible

B The silkscreen disappears

C The component will not fit

D Short circuits may occur

Insufficient clearance can cause solder bridges and short circuits between pads.

When should you create a custom footprint?

A When the component is very cheap

B When the PCB is very large

C When the component is not in the standard library

D When the PCB color is different

Custom footprints are needed for components missing from standard libraries or with unique shapes.

Explain the process of creating a custom footprint for a new component.

Think about the steps from datasheet to PCB layout.

Why is pad clearance important in footprint design and what problems can arise if it is incorrect?

Consider electrical safety and assembly quality.

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