When my brother and I would go camping with my parents years ago, we used to tease my dad about packing way too much for the trips. And yet, when the rain came, Dad was prepared with tarps to cover the campsite and tools to dig trenches to channel the water away. He was prepared for whatever the elements might throw at us, and as a result, we stayed mostly dry when other campers were rained out. Preparation is always a good thing—even in the design of printed circuit boards. And that means preparing them to be fully tested.
When a printed circuit board is built, the assembly processes your contract manufacturer uses should be designed to produce the best quality. For full assurance, however, that all the solder joints are good and all of the parts are operating up to specification, the board must be thoroughly tested. To do this requires that the circuit board must first have been carefully prepared for testing during its design. Too often, a PCB can’t be fully tested because it wasn’t set up for it in the first place. This lack of testing leaves the door open for missed assembly problems that should have been corrected before the board was shipped back to you. To prevent problems like this, here are some design for testability guidelines to ensure your next PCB design is fully prepared for testing.
Following PCB design for testability guidelines prepares your project for efficient manufacturing.
Design for Testability Guidelines to Consider When Designing Your PCB
The first step that a contract manufacturer will take when reviewing a design for test is to consult the customer test specification that has been provided. With this information, a test method can be developed to cover the specifications. Although the CM test engineers can create their own specification if necessary, it is a good practice for you to give your CM as much information as possible so the board is fully tested to the customer’s expectations.
The CM will include marking up the schematic with test and connection points as part of their test review, as well as examining the layout. It is another good design practice to include specific test points in the schematic to help the CM with this preparation. The design review combined with the test methodology will help determine the type of fixture that is needed and if any modifications to the design are warranted to increase the effective fault coverage of the test procedures. All of this information is then incorporated into a test instruction (TI) document accompanied by descriptions, illustrations, and expectations for the test technicians to use during actual testing.
One of the goals of the review is to determine what type of testing will be done—in-circuit test (ICT) or flying probe. Each of these procedures has advantages and disadvantages that must be considered for the final decision:
|PCB testing||In-circuit test (ICT)||Flying probe|
|Speed of testing||Fast||Slow|
|Apply power for functional and other tests||Yes||No|
|Board size||Standard size boards||Large boards and backplanes|
|Time to develop test fixtures and programming||Lengthy depending on board complexity||Minimal|
|Cost to develop test fixtures and programming||Expensive depending on board complexity||Minimal|
|PCB layout revisions||Time and cost to modify or replace test fixture||Minimal|
Flying probe testing lends itself very well to prototyping and low-volume production runs due to the ease of setting up the testing. With the constant layout changes made to prototypes, the ability to easily modify the test programming of the flying probe is an additional benefit. With an individual flying probe being used to contact each test point on the board, however, flying probe is slow when compared to ICT. The ICT fixtures and fixed programming are ideal for larger production runs of established board designs.
Laying Out Your PCB for Maximum Testability
When it comes to laying out the printed circuit board for testability in design, there are some important considerations to remember:
- ICT points must be on each net of the design for full test coverage.
- These test points must have a 50 mil clearance to components and pads.
- These test points also must have a 100 mil clearance to the edge of the board.
- ICT can be conducted on both sides of the board at once, but you should set this up with your CM in advance.
Another important aspect of testing a circuit board is the time the board will be tested on the bench for functional and/or measurement testing. This requires the addition of adequate probe points for manual testing of critical circuitry. Whereas test points usually refer to in-circuit or flying probe testing, probe points are vias designed large enough to give a technician access with a probe, wire, or header pin to conduct manual testing.
Working with Your CM to Get the Best PCB Test Results
By providing a test specification to your CM along with noted test and probe points in the schematic, as well as obeying the minimum spacing rules for test points in the layout, you will be well on your way to having a fully testable PCB. There is something else that you can do, however, which will also be helpful, and that is to consult early with your CM on your design. This way, you can get your questions answered, and you will find out what they need to best test your board.
At VSE, we have experienced design, layout, and test engineers that are ready to work together with you on your design. We can help you with any design questions or issues related to getting your board set up to be fully testable.