Ground pour on PCB in PCB Design - Cell-by-Cell Formula Trace

Sample Data

This data represents PCB copper pour areas for different nets and their copper thickness. Resistance is to be calculated for the ground pour (GND) based on area and thickness.

Cell	Value
A1	Net Name
B1	Area (mm²)
C1	Copper Thickness (µm)
D1	Resistance (mΩ)
A2	GND
B2	1000
C2	35
D2
A3	VCC
B3	500
C3	35
D3

Formula Trace

= (0.017 * 0.035) / 1000

Step 1: 0.017 * 0.035

Step 2: 0.000595 / 1000

Cell Reference Map

    A       B       C       D
1 |Net Name|Area   |Thickness|Resistance
2 |GND     |1000   |35       | 
3 |VCC     |500    |35       | 

Arrows: Resistance D2 depends on B2 (Area) and C2 (Thickness)

Resistance calculation for ground pour uses Area from B2 and Thickness from C2.

Result

    A       B       C       D
1 |Net Name|Area   |Thickness|Resistance
2 |GND     |1000   |35       |0.000000595
3 |VCC     |500    |35       | 

Resistance for GND calculated and shown in D2.

The resistance for the ground pour net GND is calculated and displayed in cell D2.

Sheet Trace Quiz - 3 Questions

What does the value 0.017 represent in the formula?

A Area in mm²

B Copper thickness in µm

C Copper resistivity in mΩ·mm

D Resistance in mΩ

Key Result

Resistance = (Copper Resistivity × Thickness) / Area

Practice

1. What is the main purpose of a ground pour on a PCB?

easy

A. To isolate components from each other

B. To increase the power supply voltage

C. To add decorative copper patterns

D. To create a large copper area connected to ground to reduce noise

Solution

1. Step 1: Understand the function of ground pour

   A ground pour is a copper area connected to the ground net on a PCB.

2. Step 2: Identify its effect on noise

   This large copper area helps reduce electrical noise and improves signal quality.

3. Final Answer:

   To create a large copper area connected to ground to reduce noise -> Option D

4. Quick Check:

   Ground pour reduces noise = A [OK]

Hint: Ground pour connects copper to ground to reduce noise [OK]

Common Mistakes:

• Thinking ground pour increases voltage
• Confusing ground pour with decorative copper
• Assuming ground pour isolates components

2. Which of the following is the correct way to assign a ground pour to the ground net in PCB design software?

easy

A. Assign the polygon to the power net

B. Assign the polygon to the ground net

C. Assign the polygon to the signal net

D. Do not assign any net to the polygon

Solution

1. Step 1: Identify the net assignment for ground pour

   Ground pour must be connected to the ground net to function properly.

2. Step 2: Choose the correct net assignment

   Assigning the polygon to the ground net ensures it acts as a ground plane.

3. Final Answer:

   Assign the polygon to the ground net -> Option B

4. Quick Check:

   Ground pour net = ground net = C [OK]

Hint: Always assign ground pour polygon to ground net [OK]

Common Mistakes:

• Assigning polygon to power or signal nets
• Leaving polygon unassigned
• Confusing ground net with other nets

3. Consider a PCB with a ground pour polygon set with a clearance of 0.2 mm from signal traces. If a signal trace is 0.15 mm away from the polygon boundary, what will happen?

medium

A. The polygon will maintain a 0.2 mm gap and not connect to the trace

B. The polygon will connect to the trace causing a short circuit

C. The polygon will ignore clearance and overlap the trace

D. The polygon will be removed entirely

Solution

1. Step 1: Understand clearance setting

   The clearance defines the minimum gap between polygon and other objects.

2. Step 2: Analyze the trace distance

   The trace is only 0.15 mm away, which is less than the 0.2 mm clearance.

3. Step 3: Predict polygon behavior

   Since the trace is too close, the polygon pour will retract inside the boundary to maintain the required 0.2 mm clearance, preventing connection.

4. Final Answer:

   The polygon will maintain a 0.2 mm gap and not connect to the trace -> Option A

5. Quick Check:

   Trace distance < clearance causes retraction to maintain gap = B [OK]

Hint: Polygon retracts to maintain clearance if distance < set value [OK]

Common Mistakes:

• Thinking polygon connects causing short when trace too close
• Thinking polygon ignores clearance and overlaps
• Believing polygon will be removed automatically

4. You created a ground pour polygon but it does not connect to the ground net pads. What is the most likely cause?

medium

A. The polygon is assigned to the wrong net

B. The polygon clearance is set too low

C. The polygon is too large

D. The PCB layers are not visible

Solution

1. Step 1: Check polygon net assignment

   If the polygon is assigned to a net other than ground, it won't connect to ground pads.

2. Step 2: Evaluate other options

   Clearance too low or polygon size does not prevent connection; layer visibility affects display only.

3. Final Answer:

   The polygon is assigned to the wrong net -> Option A

4. Quick Check:

   Wrong net assignment blocks connection = D [OK]

Hint: Check polygon net assignment first if no connection [OK]

Common Mistakes:

• Blaming clearance or size before net assignment
• Ignoring layer visibility settings
• Assuming polygon auto-connects regardless of net

5. You want to reduce EMI noise on a high-speed PCB. Which combination of ground pour settings is best?

hard

A. Assign polygon to power net, set clearance to 0.1 mm, and isolate from ground pads

B. Assign polygon to signal net, set clearance to 0.5 mm, and connect to power pads

C. Assign polygon to ground net, set clearance to 0.3 mm, and connect to all ground pads

D. Do not use ground pour and increase trace width instead

Solution

1. Step 1: Identify correct net assignment for EMI reduction

   Ground pour must be connected to ground net to reduce EMI noise effectively.

2. Step 2: Choose appropriate clearance and connections

   A clearance of 0.3 mm balances safety and connectivity; connecting to all ground pads ensures good grounding.

3. Step 3: Evaluate other options

   Assigning to power or signal nets or isolating ground pads reduces effectiveness; not using ground pour misses noise reduction benefits.

4. Final Answer:

   Assign polygon to ground net, set clearance to 0.3 mm, and connect to all ground pads -> Option C

5. Quick Check:

   Ground net + proper clearance + pad connection = B [OK]

Hint: Ground pour on ground net with good clearance reduces EMI [OK]

Common Mistakes:

• Assigning polygon to wrong net
• Setting clearance too low or isolating ground pads
• Ignoring ground pour benefits and only widening traces