Clearance rules for different nets in PCB Design - Cell-by-Cell Formula Trace

Sample Data

This table shows different nets in a PCB design and their required clearance distances in mils.

Cell	Value
A1	Net Name
B1	Clearance (mil)
A2	Power
B2	10
A3	Signal
B3	6
A4	Ground
B4	8
A5	Clock
B5	12

Formula Trace

MAX(B2:B5)

Step 1: B2:B5 values = {10, 6, 8, 12}

Step 2: MAX(10, 6, 8, 12)

Cell Reference Map

     A        B
  +-------+-------+
1 | Net   | Clear |
  +-------+-------+
2 |Power  |  10   | <--
3 |Signal |  6    | <--
4 |Ground |  8    | <--
5 |Clock  |  12   | <--
  +-------+-------+

The formula references clearance values in cells B2 through B5 to find the maximum clearance.

Result

     A        B        C
  +-------+-------+--------+
1 | Net   | Clear | Result |
  +-------+-------+--------+
2 |Power  |  10   |        |
3 |Signal |  6    |        |
4 |Ground |  8    |        |
5 |Clock  |  12   |  12    |
  +-------+-------+--------+

The result 12 in cell C5 shows the maximum clearance required among all nets.

Sheet Trace Quiz - 3 Questions

What clearance value does the formula MAX(B2:B5) return?

A 10

B 12

C 8

D 6

Key Result

MAX(range) returns the largest numeric value in the specified range.

Practice

1. What is the main purpose of clearance rules between different nets on a PCB?

easy

A. To make the PCB look more organized

B. To increase the speed of the PCB

C. To reduce the cost of manufacturing

D. To set safe distances to prevent electrical shorts

Solution

1. Step 1: Understand clearance rules

   Clearance rules define minimum distances between different nets to avoid electrical shorts.

2. Step 2: Identify the main purpose

   The primary goal is to prevent shorts and ensure electrical safety on the PCB.

3. Final Answer:

   To set safe distances to prevent electrical shorts -> Option D

4. Quick Check:

   Clearance rules = prevent shorts [OK]

Hint: Clearance = safe distance to avoid shorts [OK]

Common Mistakes:

• Thinking clearance speeds up the PCB
• Assuming clearance reduces cost
• Believing clearance is for aesthetics

2. Which of the following is the correct way to specify a clearance rule for a net class in a PCB design tool?

easy

A. Add a text note about clearance on the PCB

B. Set clearance value in mils or mm for the net class

C. Change the net class name to clearance

D. Assign a color to the net class

Solution

1. Step 1: Identify how clearance rules are set

   Clearance rules are numeric values defining minimum spacing, usually set in mils or millimeters.

2. Step 2: Match the correct method

   Setting a clearance value for a net class is the correct way to apply clearance rules.

3. Final Answer:

   Set clearance value in mils or mm for the net class -> Option B

4. Quick Check:

   Clearance rule = numeric spacing value [OK]

Hint: Clearance rules use numeric spacing, not colors or names [OK]

Common Mistakes:

• Confusing clearance with net color
• Trying to rename net classes as clearance
• Adding notes instead of setting values

3. Given the following clearance rules:
Net Class A to Net Class B: 10 mils
Net Class A to Net Class C: 15 mils
If a trace from Net Class A is 12 mils away from a trace from Net Class B, and 14 mils away from a trace from Net Class C, which clearance violations occur?

medium

A. Violation with Net Class C only

B. Violation with Net Class B only

C. Violations with both Net Class B and Net Class C

D. No violations

Solution

1. Step 1: Compare distances with clearance rules

   Distance to Net Class B is 12 mils, required clearance is 10 mils (12 > 10, no violation). Distance to Net Class C is 14 mils, required clearance is 15 mils (14 < 15, violation).

2. Step 2: Identify which clearances are violated

   Only the clearance to Net Class C is violated because 14 mils is less than the required 15 mils.

3. Final Answer:

   Violation with Net Class C only -> Option A

4. Quick Check:

   Distance < clearance = violation [OK]

Hint: Check if distance is less than clearance to find violations [OK]

Common Mistakes:

• Confusing greater than with less than for violations
• Checking only one net class
• Ignoring the exact clearance values

4. You set a clearance rule of 12 mils between Net Class X and Net Class Y, but the PCB design tool still shows clearance violations for traces 13 mils apart. What is the most likely cause?

medium

A. The clearance rule is not applied to the correct net classes

B. The tool rounds clearance values incorrectly

C. The traces are actually closer than 13 mils

D. Clearance rules only apply to pads, not traces

Solution

1. Step 1: Analyze clearance rule application

   If the tool shows violations despite distances greater than clearance, the rule might not be applied correctly to the net classes involved.

2. Step 2: Evaluate other options

   Rounding errors or trace measurement errors are less common; clearance rules apply to traces as well as pads.

3. Final Answer:

   The clearance rule is not applied to the correct net classes -> Option A

4. Quick Check:

   Incorrect rule application causes false violations [OK]

Hint: Check if clearance rules target correct net classes [OK]

Common Mistakes:

• Assuming tool rounding causes errors
• Ignoring rule assignment to net classes
• Thinking clearance applies only to pads

5. You have three net classes: Power, Signal, and Ground. The clearance rules are:
Power to Signal: 20 mils
Power to Ground: 15 mils
Signal to Ground: 10 mils
To optimize PCB space without risking shorts, which clearance rule should you consider increasing and why?

hard

A. Increase Power to Ground clearance because ground nets need more isolation

B. Increase Signal to Ground clearance because signals are sensitive to noise

C. Increase Power to Signal clearance because power nets carry higher current

D. Increase Signal to Signal clearance even though not listed

Solution

1. Step 1: Understand net roles and risks

   Power nets carry higher current and can cause shorts or interference if too close to signal nets.

2. Step 2: Identify which clearance to increase

   Increasing Power to Signal clearance improves safety and reduces interference risk, optimizing space safely.

3. Final Answer:

   Increase Power to Signal clearance because power nets carry higher current -> Option C

4. Quick Check:

   Power nets need larger clearance to signals [OK]

Hint: Prioritize clearance increase where current or risk is highest [OK]

Common Mistakes:

• Increasing clearance where not needed
• Ignoring power net current impact
• Assuming ground nets always need largest clearance