Thermal relief for through-hole pads in PCB Design - Cell-by-Cell Formula Trace

Sample Data

This table shows through-hole pad properties including pad diameter, copper area connected, thermal relief width, and number of thermal relief spokes.

Cell	Value
A1	Pad ID
B1	Pad Diameter (mm)
C1	Copper Area Connected (mm²)
D1	Thermal Relief Width (mm)
E1	Thermal Relief Spokes
A2	P1
B2	1.2
C2	50
D2	0.3
E2	4
A3	P2
B3	1.0
C3	30
D3	0.25
E3	3
A4	P3
B4	1.5
C4	70
D4	0.35
E4	5

Formula Trace

Thermal Relief Area = (Pad Area) - (Spokes Area) Where: Pad Area = π * (Pad Diameter / 2)^2 Spokes Area = Number of Spokes * (Thermal Relief Width * Pad Diameter)

Step 1: Pad Area for P1 = π * (1.2 / 2)^2

Step 2: Spokes Area for P1 = 4 * (0.3 * 1.2)

Step 3: Thermal Relief Area for P1 = 1.130976 - 1.44

Step 4: Pad Area for P2 = π * (1.0 / 2)^2

Step 5: Spokes Area for P2 = 3 * (0.25 * 1.0)

Step 6: Thermal Relief Area for P2 = 0.785398 - 0.75

Step 7: Pad Area for P3 = π * (1.5 / 2)^2

Step 8: Spokes Area for P3 = 5 * (0.35 * 1.5)

Step 9: Thermal Relief Area for P3 = 1.767145 - 2.625

Cell Reference Map

     A       B       C       D       E
  +-------+-------+-------+-------+-------+
1 | PadID |  Dia  |  CuA  |  TRW  | Spokes|
  +-------+-------+-------+-------+-------+
2 |  P1   |  1.2  |  50   |  0.3  |   4   |
  +-------+-------+-------+-------+-------+
3 |  P2   |  1.0  |  30   | 0.25  |   3   |
  +-------+-------+-------+-------+-------+
4 |  P3   |  1.5  |  70   | 0.35  |   5   |
  +-------+-------+-------+-------+-------+

References used:
- Pad Diameter (B2:B4)
- Thermal Relief Width (D2:D4)
- Thermal Relief Spokes (E2:E4)

This map shows the cells used in the thermal relief area calculation: pad diameter, thermal relief width, and number of spokes for each pad.

Result

     A       B       C       D       E       F
  +-------+-------+-------+-------+-------+---------+
1 | PadID |  Dia  |  CuA  |  TRW  | Spokes| TR Area |
  +-------+-------+-------+-------+-------+---------+
2 |  P1   |  1.2  |  50   |  0.3  |   4   | -0.309  |
  +-------+-------+-------+-------+-------+---------+
3 |  P2   |  1.0  |  30   | 0.25  |   3   |  0.035  |
  +-------+-------+-------+-------+-------+---------+
4 |  P3   |  1.5  |  70   | 0.35  |   5   | -0.858  |
  +-------+-------+-------+-------+-------+---------+

Column F shows the calculated thermal relief area for each pad.

The final result column (F) shows the thermal relief area for each pad, calculated by subtracting the spokes area from the pad area.

Sheet Trace Quiz - 3 Questions

What is the pad area for pad P1?

A 1.44

B 1.130976

C 0.785398

D 2.625

Key Result

Thermal relief area = pad circular area minus total area of thermal relief spokes calculated as spokes count times width times diameter.

Practice

1. What is the main purpose of thermal relief in through-hole pads on a PCB?

easy

A. To prevent any electrical connection

B. To increase electrical resistance

C. To make the pad larger

D. To control heat flow for better soldering

Solution

1. Step 1: Understand thermal relief function

   Thermal relief uses thin copper spokes to connect pads to planes, controlling heat flow.

2. Step 2: Identify the main goal

   This control helps soldering by preventing heat from dissipating too quickly.

3. Final Answer:

   To control heat flow for better soldering -> Option D

4. Quick Check:

   Thermal relief = better soldering heat control [OK]

Hint: Thermal relief balances heat for soldering, not resistance [OK]

Common Mistakes:

• Thinking thermal relief increases resistance
• Assuming it makes pads bigger
• Believing it blocks electrical connection

2. Which of the following correctly describes the copper connection in thermal relief for through-hole pads?

easy

A. Thin copper spokes connecting pad to plane

B. Solid copper plane connection

C. No copper connection at all

D. Thick copper bars around the pad

Solution

1. Step 1: Recall thermal relief design

   Thermal relief uses thin copper spokes, not solid or thick bars, to connect pads.

2. Step 2: Match description to options

   Only thin copper spokes correctly describe the connection method.

3. Final Answer:

   Thin copper spokes connecting pad to plane -> Option A

4. Quick Check:

   Thermal relief = thin spokes connection [OK]

Hint: Thermal relief uses spokes, not solid or thick copper [OK]

Common Mistakes:

• Choosing solid copper connection
• Thinking there is no copper connection
• Confusing thick bars with spokes

3. Given a PCB pad connected to a copper plane with thermal relief, what is the expected effect on soldering time compared to a solid connection?

medium

A. Soldering time decreases due to heat dissipation control

B. Soldering time decreases because of more copper

C. No change in soldering time

D. Soldering is impossible with thermal relief

Solution

1. Step 1: Understand heat flow in thermal relief

   Thermal relief limits heat flow to the plane, slowing heat loss from the pad.

2. Step 2: Relate heat flow to soldering time

   Less heat loss means the pad stays hot longer, allowing solder to melt faster, decreasing soldering time.

3. Final Answer:

   Soldering time decreases due to heat dissipation control -> Option A

4. Quick Check:

   Thermal relief slows heat loss = shorter soldering [OK]

Hint: Thermal relief slows heat loss, so soldering takes shorter [OK]

Common Mistakes:

• Assuming soldering time increases
• Thinking thermal relief has no effect
• Believing soldering is impossible

4. A PCB designer notices that the thermal relief spokes are too thick, causing excessive heat loss during soldering. What is the best fix?

medium

A. Increase the number of spokes

B. Reduce the width of the thermal relief spokes

C. Remove thermal relief and use solid copper

D. Add more vias around the pad

Solution

1. Step 1: Identify problem cause

   Thick spokes conduct more heat, causing excessive heat loss.

2. Step 2: Choose appropriate fix

   Reducing spoke width lowers heat conduction, improving soldering heat retention.

3. Final Answer:

   Reduce the width of the thermal relief spokes -> Option B

4. Quick Check:

   Thinner spokes = less heat loss [OK]

Hint: Thinner spokes reduce heat loss, fix soldering issues [OK]

Common Mistakes:

• Increasing spokes increases heat loss
• Removing thermal relief worsens heat control
• Adding vias does not fix spoke thickness

5. A PCB has a through-hole pad connected to a large ground plane. The thermal relief uses four spokes. If the designer changes to two spokes, what is the likely impact on soldering and electrical performance?

hard

A. No change in soldering; electrical resistance increases greatly

B. Soldering becomes harder; electrical resistance slightly increases

C. Soldering improves; electrical resistance slightly increases

D. Soldering impossible; electrical connection lost

Solution

1. Step 1: Analyze spoke count effect on heat flow

   Reducing spokes from four to two reduces heat conduction paths, retaining more heat in the pad, improving soldering.

2. Step 2: Analyze spoke count effect on electrical resistance

   Fewer spokes slightly increase electrical resistance but usually remain acceptable.

3. Final Answer:

   Soldering improves; electrical resistance slightly increases -> Option C

4. Quick Check:

   Fewer spokes = improved soldering + slight resistance rise [OK]

Hint: Fewer spokes reduce heat flow (improve soldering) and increase resistance slightly [OK]

Common Mistakes:

• Thinking soldering becomes harder with fewer spokes
• Thinking electrical resistance decreases
• Believing electrical connection is lost