When it comes to establishing contact for printed circuit board testing between the PCB that is being tested and the functional test fixtures, there are three primary methods used in the electronic testing industry today. Long-wire fixtures, which have been used for more than a generation; short-wire fixtures, which provides a more direct path for testing; and wireless functional testing fixtures, which were designed to overcome the issues regarding the limits associated with long-wire fixture testing. The functional test division at EMC Technologies works with discretely wired and wireless functional test fixtures for printed circuit board testing solutions.
What are Long-Wire Fixtures?
For over a generation, long-wire fixtures have been the industry standard for functional test fixtures. Nearly everyone who works in product development and the electronic testing industry is familiar with long-wire fixtures. They can be a very cost-effective option for lower node count printed circuit board testing, as they avoid the non-recurring costs that can be involved in the design and manufacture of a PCB. However, there are limitations. Long-wire fixtures have issues such as coupling problems and crosstalk due to the way that the wiring itself is routed. It can be quite costly to provide engineering change orders to accommodate these issues, especially in situations where high pin-counts are required. Also, it can be very difficult to control or even detect any unauthorized or undocumented wiring changes.
What are Short-Wire Fixtures?
The primary difference between long-wire and short-wire fixtures is that the hinged sheet metal box is replaced with lower profile rails. This change allows for the signal wires to follow a more direct path to connect the unit under testing (UUT) to the interface of the functional test fixtures. Overall, short-wire fixtures provide a much better quality of signal fidelity, but they are still outmatched by the abilities of wireless functional testing fixtures. The cost to design and manufacture short-wire fixtures is somewhere between the cost associated with long-wire fixtures and wireless functional test fixtures. It can be more complicated and costly to customize short-wire fixtures to improve performance than with long-wire fixtures, due to the wire density and compact design. As a result, this type of fixture is the most difficult to customize.
Why Wireless Functional Testing Wins
Compared to the other two types of functional test fixtures, wireless wins easily because it was designed specifically to address the limitations posed by long-wire fixtures. It also eliminates the issues experienced with trying to customize short-wire fixtures. With wireless functional testing, a multi-layer printed circuit board (PCB) is used to replace the signal wires. This special PCB is known as a T-board or translator board, and it was designed using typical fixture build files. The wireless fixtures can perform in a much more reliable fashion than the wired functional test fixtures because designers can control exactly how each trace is routed.
When reliability and consistency matter, wireless functional testing will always win out in the electronic testing industry. Duplicate fixtures are able to perform in a nearly identical manner and the electrical parameters are extremely consistent, due to the lack of moving wire. Issues like crosstalk, reflection, and noise characteristics are also improved in the use of wireless functional test fixtures. The signals are much cleaner, which allows the test to be run at a much faster rate, providing test-system speed in most situations. The compact design of the internal fixture electronics reduces fixture height to save space within the manufacturing environment.
Wireless Options Make a Big Difference
The advances experienced by the development of wireless functional testing have made a big difference in how printed circuit board testing is done. The misalignment of the probe or socket that is experienced in the wired fixtures, can create a higher false failure rate. Our functional test division has seen time and time again that wireless fixtures are able to improve probing accuracy simply due to their construction. While there are some limitations to wireless fixtures, such as not being good candidates for PCBs with low probe counts and not being able to trace the wiring visually, the advantages outweigh any concerns.
To learn more about wired and wireless functional testing or to work on the development of functional test fixtures for printed circuit board testing, contact our engineering and design team direct by calling 508-672-0808. We can answer any questions that you might have about our functional test division or help you create a custom solution that will meet your needs and industry requirements.