Achieving the right test coverage requires balancing multiple factors: manufacturing volume, product complexity, quality targets, and cost constraints. This guide helps you develop a practical strategy.
Defining Coverage Targets#
Before diving into implementation, establish clear goals:
- Minimum acceptable coverage: What percentage of components and nets must be tested?
- Defect escape rate: How many defects per million can you tolerate reaching customers?
- Throughput requirements: How many boards per hour must your test station process?
Industry benchmarks suggest 95%+ coverage for high-reliability products and 85-90% for consumer electronics. Your specific requirements may differ.
Coverage by Component Type#
Different component types present different testing challenges.
Passive Components#
Resistors, capacitors, and inductors are typically easy to test via ICT. Probe access to both terminals allows value measurement and detection of shorts.
Active Components#
ICs require strategic test point placement. For devices with accessible pins, in-circuit testing works well. For BGAs and QFNs, boundary scan or functional testing may be necessary.
Connectors and Mechanical Components#
Connectors require presence detection and continuity verification. Some connectors can be probed directly; others need mating connectors on the fixture.
Calculating Actual Coverage#
Coverage percentage depends on accessible test points:
| Factor | Impact on Coverage |
|---|---|
| Test points per net | Higher = better coverage |
| Component side access | Dual-side access increases options |
| Test point size | Larger pads = more reliable probing |
| Via accessibility | Filled/capped vias reduce coverage |
Improving Coverage in New Designs#
The most effective time to improve coverage is during PCB design. Consider:
- Add test points early: Adding test points after layout is expensive
- Size appropriately: Minimum 0.9mm diameter for reliable probing
- Maintain spacing: Keep test points at least 1.9mm apart
- Document test strategy: Communicate requirements to layout engineers
When to Accept Lower Coverage#
Sometimes less-than-ideal coverage is the right business decision:
- Very low volume products where fixture cost dominates
- Prototype builds with design changes expected
- Products with extensive functional testing as backup
- Disposable or single-use products
Get a coverage estimate
Upload your Gerber files to Studio for an automated test coverage analysis.
Summary#
Effective test coverage requires intentional design choices, not afterthoughts. Start with clear coverage targets, design test points into your PCB, and choose testing methods that match your production requirements.