Fixture switches detect when the pressure frame is closed on a test fixture, ensuring proper DUT seating and probe contact before testing begins. Different switch types suit different applications based on cycle life requirements, safety needs, and fixture design.
Switch Types#
End-Stop Switches (Dev Fixtures)#
Standard switches with dual Normally Closed (NC) and Normally Open (NO) contacts. These provide flexibility in how the test system interprets fixture state.
Connections:
- S — Common
- G — Normally Closed
- V — Normally Open
Typically use JST XHP-3 connectors with 18-inch cables for connection to test instrumentation.
Probe Switches#
A cost-effective method leveraging existing probe infrastructure. Two test probes on the probe plate contact a conductive shorting plate mounted to the pressure plate, creating a detectable electrical short circuit when the fixture closes.
Advantages:
- No additional switch hardware required
- Simple electrical detection
- Low cost
Considerations:
- Relies on probe contact quality
- May require dedicated probe positions
Stroke Switches#
Similar to end-stop switches but designed for higher cycle life and demanding applications. Available with various actuation force options to match fixture mechanics.
Use stroke switches when cycle counts will exceed standard end-stop switch ratings or when consistent actuation force matters.
Inductive Switches#
Electromagnetic detection without physical contact. The switch senses a metal target on the moving plate without mechanical wear.
Benefits:
- Millions of cycles with no mechanical wear
- No contact bounce or debounce required
- Sealed construction for harsh environments
Requirements:
- Proper target material (ferrous metal typically)
- Calibrated spacing between switch and target
- Compatible mounting arrangement
Recommended for high-volume production exceeding 100,000 cycles.
Safety Switches#
Enhanced security features with interlock mechanisms to prevent accidental fixture opening during test. Meet production environment safety standards for operator protection.
Use safety switches when:
- Fixture contains hazardous voltages during test
- Accidental opening could damage DUT or equipment
- Regulatory requirements mandate safety interlocking
Contact Configurations#
| Type | Behavior | Best For |
|---|---|---|
| Normally Closed (NC) | Opens when fixture opens | Safety-critical applications |
| Normally Open (NO) | Closes when fixture closes | Simple presence detection |
NC contacts are preferred for safety-critical applications because a wire fault (broken wire) creates the same state as an open fixture, failing to a safe condition.
Integration Best Practices#
- Verify operation before production — Test switch function during fixture commissioning
- Use NC contacts for safety — Fail-safe design for critical applications
- Consider inductive for high volume — Avoid mechanical wear in production fixtures
- Maintain inspection schedules — Check switch operation regularly
- Wire consistently — Standardize switch wiring across fixtures for maintainability
Typical Integration#
The switch signal connects to test instrumentation (FixturCtrl, DAQ, or GPIO) where software monitors fixture state. Tests should not begin until the fixture-closed signal is confirmed.
Related Documentation#
- DUT Presence Detection — Board presence verification
- FixturCtrl — Instrumentation with fixture switch input
- Fixture Bases — Switch mounting in different bases