Functional Test Fixture Design Process

Tutorial on how to design a PCBA Functional Test Specification and Test Fixture based on a TeachMePCB badge.

1. Overview

Developing a functional test fixture is a multidisciplinary engineering project and has many parallels with the product development process. A simplified development process is shown below. 

Functional Test Fixture Design Process

A Test Specification is first created for the fixture. This specification is developed using the DUT’s schematic and PCB layout as the main inputs, and the various test cases can then be defined for the DUT. 

During the development of the test specification, a test fixture base will be chosen, along with the instrumentation that will be used to perform the testing. The choice of instrumentation also requires an idea of what programming language or tools will be used to implement the test plan. 

A complete test specification will contain a list of test cases and their expected results or limits, along with the following information: 

  • Testpoints used for each test case

  • Instrumentation Connections

    • What device and input/output is the testpoint connected to 

After a test specification has been completed, the fixture design process can then begin. A testpoint carrier board (TPCB) is designed to interface the test probes in the fixture with the test instrumentation. The mechanical fixture is also designed to have test probes in the correct locations to interface with the testpoints on the DUT. The mechanical fixture will also have guide pins to help locate the DUT within the fixture, and pressure pins to push the DUT against the test probes when the fixture is closed. Once the mechanical and electrical design of the system is complete, both the testpoint carrier board and the fixture cartridge are fabricated. 

After all of the components for the fixture have been received, the fixture can be assembled. This process involves assembling the fixture cartridge, inserting all of the receptacles and test probes, and attaching the TPCB. Once the fixture is completely assembled, then the test software can be developed.