A Guide to PCB Testing Methods
WHAT IS PCB TESTING?
PCB testing is done only after the assembly process is complete and a printed circuit board has passed all of its inspections. While these inspections are fairly rigorous, including AXI testing that uses X-rays to examine a board, it cannot account for all of the defects present on a board. Generally speaking, PCB inspection is best suited for detecting defects related to soldering, while PCB testing can help indicate component defects.
While electrical defects account for a smaller fraction of errors than structural defects, they are just as important for ensuring that a PCB can operate as it is meant to function.
In-Circuit Testing
In-circuit testing, or ICT, is an extremely robust testing method that measures each component on a printed circuit board to ensure it is correct and in place. It is intended specifically for printed circuit boards that are unlikely to have revisions, and while it is an extremely effective method of PCB testing, it is not ideal for all applications.
ICT is conducted by testing a populated printed circuit board with electrical probes, testing aspects like open and short circuits, resistance, and capacitance. This is often done with a tool called a bed of nails tester, which are laid out in a way that matches the design of the printed circuit board.
In-circuit testing provides some of the best results to ensure that your PCB is functionally sound, and is well suited for larger connections and ball grid arrays. It is also one of the most expensive testing methodologies available due to the cost of assembly bed of nails fixtures, and must be chosen early in the manufacturing process. Thus, it is mostly used for products for design-for-manufacturing as the objective.
ICT also does not test for continuity through connectors. However, this can be resolved by pairing it with the correct inspection methodologies.
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Flying Probe Testing
Flying probe testing, or FPT, is an alternative to ICT that comes with its own set of strengths and drawbacks. This is done by attaching the printed circuit board to a fixture while a probe moves around it, making contact as required. Instead of costs being poured into the assembly of a bed of nails, the movement of the flying probe is dictated by software design, meaning that changes can easily be implemented.
FPT has significantly lower up-front costs to ICT, as it does not require the assembly of a bed of nails. However, for large-scale manufacturing this can actually be a drawback. Testing periods for FPT are significantly longer than ICT, meaning that for high volume production, these hidden expenses can scale exponentially. It also has a much shorter development lead time. Thus, it is a great alternative for early-stage, low volume production of orders
Functional Testing
Functional testing is the old-fashioned way of testing a printed circuit board, with an individual manually testing a board by uncovering problems with analog and digital circuitry. This type of testing is almost exclusively done because a customer requests it.
While this method is required for some standards of testing, it is one of the most expensive and time-consuming options available. The parameters of the test are typically determined by the customer, and is a variable process that is specific to each product.
PCB TEST METHODS SHOULD BE BASED AROUND YOUR OBJECTIVES
As a rule of thumb, unless a customer requests functional testing, you should opt to use either in-circuit testing or flying probe testing to determine that your product is functional. Which method you choose should be determined by several aspects: