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To assist you with your selection process, here are some basic questions you should ask to determine which fixture kit is more suitable:
Q: Is the fixture for R&D or Production and if production, is it low, medium or high volume?
A: If it’s for R&D or low production volume, the clamshell is typically fine whereas medium to high volume warrants a Z-Axis because of the potential of constantly side loading probes. Z-Axis is also better for registration.
Q: Are the test points single or dual side probing?
A: Clamshells are usually fine for single side probing as the probes are usually mounted on the bottom pin plate. A Z-Axis kit is always advisable when there is dual side probing or when visibility is required and the probes must contact from the top.
Q: What is the smallest center to center spacing of the test points?
A: For anything less than 50 Mil spacing, it is recommended a Z-Axis is utilized.
Q: What is the test point count?
A: Anything more than 80 test points using standard to low force spring probes, should use a Z-Axis type fixture kit.
Q: How densely loaded is the top side of the PCBA?
A: When the PCBA has a lot of tall components, it can be difficult to strategically place push rods on a clamshell type of fixture kit due to the angle the upper plate actuates.
There are other factors that can weigh in to the determination of kit type however answering these key questions can point you in the right direction.
See Clamshell to Z-Axis comparison for more information which can support your decision and design process.
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