Sometimes while shopping for appliances, I’m faced with a choice: do I select a more expensive all-in-one device that will handle multiple tasks or the cheaper tool that exists for a singular purpose? The central question boils down to whether I prefer specificity or a jack-of-all-trades approach. Of course, this is not the sole determining factor; the cost, as mentioned above, and other issues such as space, storage, and total utility may also arise. When it comes to bare boards and in-circuit testing (ICT), printed circuit board fabrication and assembly face a similar dichotomy.
Flying probes have seen wide adoption over the past three-plus decades, especially in the realm of prototyping and other low-volume production runs. However, the advantages provided by a PCB flying probe test quickly erode when scaling for larger productions. An explanation of the PCB flying probe test, its benefits, drawbacks, and alternatives to the test are offered below.
What is a PCB Flying Probe Test?
A flying probe test checks for signal continuity in both bare boards and finished assemblies. The aptly named probes are programmed to contact the board at the nets’ test points of interest during runtime. A fixtureless design trivializes the time between a finished fabrication or assembly product and testing. While a flying probe test will be performed on both bare and populated boards, the content the tests are checking for differ:
- Electrical test: Uses the netlist and component location data to search the board for opens and shorts. It is performed after fabrication for quality assurance before beginning assembly, also known as PCB e-test.
- In-circuit test (ICT): Tests for circuit characteristics and opens and shorts to determine the success of the final assembly process.
The tradeoff for the flying probe test over the bed-of-nails testing during ICT can be considered a difference between manufacturing acceleration and top speed. As a flying probe does not require the use of a fixture to prevent flexure during operation, it is much quicker to test an individual or small run of boards than dedicating time to designing, molding, and drilling. Additionally, the lack of fixture requirement means any changes to the board that would otherwise require adjustments to the fixture are circumvented. With its smaller overhead and flexibility in the design process, flying probe testing has carved a significant niche in small batch runs.
Testing in Larger Production Runs
Thus far, flying probe testing has been presented as a highly convenient option that supports design modularity in prototyping and revision stages. Unfortunately, its flaws become apparent when production ramps up. While flying probe testing is more responsive to changes during development and the duration of an individual test on a small production lot is not noticeable, the speed of the flying probes becomes a hindrance on larger volume runs.
An individual bed-of-nails test may accomplish in seconds what might take a flying probe test minutes or even hours; the travel time of the probes from test point to test point can simply not compete with the ability to probe the entire board in a single pass. The magnitude of difference in speed increases the number of tests necessary in mid-to-high volume production. Essentially, there’s a point of intersection between the flying probe testing and bed-of-nails testing where the initial savings in run-up time from a fixtureless test give way to the test’s speed.
Further Testing Options
Flying probe and bed-of-nails testing cover electrical and ICT for varying production runs. However, additional options exist for quality assurance of bare and assembled printed circuit boards. While these methods provide a more supplementary mode of evaluation, they are nonetheless invaluable as assessment tools:
- Boundary-scan: An outgrowth of JTAG and codified into a standard by IEEE, boundary scan testing can check for opens, shorts, and runtime errors. An advantage of boundary-scan testing is that it can be performed entirely without probing, meaning that the test format holds greater future promise with the tendency of board dimensions to shrink while maintaining or increasing component density.
- Automated optical testing: Visual evaluation of bare or assembled PCB. While the test cannot diagnose manufacturing defects, optical testing proves itself beneficial in many ways. It is contactless and can provide feedback beyond a simple pass/fail check. Operators can hone in on processes that fail to meet specifications more precisely. Additionally, x-ray optical testing can be used to check underneath packages to ascertain the condition of solder joints – this finds wide use in evaluating the hundreds or thousands of connections of ball grid arrays.
- Function test: As opposed to previous tests, this gauges the functionality of the board, i.e., does the board work as intended from a circuit logic perspective. The environment of the board will also be simulated to establish that the board’s functionality will persist in field conditions.
Your PCB Contract Manufacturer Will Guide Testing of Your Boards
While PCB flying probe testing is a valuable tool, it is thoroughly outclassed in larger production lots by bed-of-nails testing and additional testing methods. When it comes time to manufacture your boards, you want the option that offers the best turnaround time and quality assurance before your boards reach customers and enter into service. Whatever testing your board requires, VSE and our fabrication partners will collaborate to ensure that every board has undergone rigorous evaluations with exceptional care.
