Clearance rules for different nets in PCB Design - Dashboard Guide

Dashboard Mode - Clearance rules for different nets

Dashboard Goal

Understand and compare clearance rules applied to different nets in a PCB design to ensure proper spacing and avoid electrical shorts.

Sample Data: Clearance Rules by Net

Net Name	Net Type	Minimum Clearance (mil)	Maximum Clearance (mil)	Standard Clearance (mil)
Power	High Current	10	20	15
Signal_A	Signal	5	10	7
Signal_B	Signal	6	12	8
Ground	Reference	8	15	12
Clock	High Speed	7	14	10
Reset	Control	5	9	7

Dashboard Components

• KPI Card: Average Minimum Clearance
  Formula: Average of 'Minimum Clearance (mil)' column.
  Result: (10+5+6+8+7+5)/6 = 6.83 mil
• Bar Chart: Minimum Clearance by Net
  X-axis: Net Name
  Y-axis: Minimum Clearance (mil)
  Shows clearance needed for each net.
• Table: Clearance Details
  Shows all columns from sample data for detailed comparison.
• Line Chart: Clearance Range (Min to Max) by Net
  X-axis: Net Name
  Y-axis: Clearance (mil)
  Lines show min and max clearance for each net to visualize range.

Dashboard Layout

+----------------------+-----------------------+
|  KPI: Avg Min Clear  |  Bar Chart: Min Clear |
|      (6.83 mil)       |   by Net Name          |
+----------------------+-----------------------+
|       Table: Clearance Details                |
|  (All columns for all nets)                   |
+-----------------------------------------------+
|  Line Chart: Clearance Range by Net           |
+-----------------------------------------------+

Interactivity

Filter by Net Type to update all components:

• KPI card recalculates average minimum clearance for selected net types.
• Bar chart updates to show minimum clearance only for filtered nets.
• Table filters rows to show only selected net types.
• Line chart updates clearance ranges for filtered nets.

All components respond to the same filter to maintain consistent analysis.

Self Check

If you add a filter for Net Type = Signal, which nets remain and how do components update?

• Nets remaining: Signal_A and Signal_B.
• KPI card shows average minimum clearance: (5 + 6)/2 = 5.5 mil.
• Bar chart shows minimum clearance bars for Signal_A and Signal_B only.
• Table shows only rows for Signal_A and Signal_B.
• Line chart shows clearance ranges for Signal_A and Signal_B.

Key Result

Dashboard shows clearance rules for different PCB nets with average, detailed table, and visual range comparison.

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