Posted on 31 Dec, 2015 by Russell Poppe
In-circuit test (ICT) has been around for many years now. And despite the emergence of competing test technologies, it remains one of the best ways to test a printed circuit board (PCB) assembly.
ICT's enduring appeal comes from the speed of test, which is usually a few seconds. This speed makes it economical for larger volumes. Additionally, its precise, component-level detection of faults makes the diagnosis process faster and less skilled.
A dedicated test fixture and program is required to achieve this level of accuracy. The product must also be designed to allow the test machine and its fixture to interface with the assembly correctly. This maximises the test "coverage" - i.e., all the potential faults it could find.
So, if you want to get the most from your test strategy, here are 9 points to consider when designing the layout for your PCB assemblies:
1. Test pad access
Every electrical network, including "no connects" (such as unused IC pins), should have a test pad designed into the PCB that can be probed by a sprung test pin in the fixture. The exception is where conventional "through-hole" technology is used, where it is fine to probe onto the component leg on the solder side of the assembly.
The ideal pad size is 0.05” diameter and pads should be at least 0.1” apart from each other and any components, as this allows the use of robust test pins that will last longer. It is acceptable to use pads three-quarters or even half this size - though they might not last quite as long before needing to be replaced and are generally more expensive to buy. Pads should also be at least 0.125” away from the PCB edge.
2. Bottom-side probing
Ideally, the test pads should all be on the bottom - or solder side - of the PCB. It is possible to probe on both sides, but with additional transfer probes and wiring it makes the fixture much more expensive if top-side probing is required.
3. Solder side
There should be no components on the solder side, but if this is unavoidable, then keep them small. This is because the test fixture vacuum plate will need to be milled out to accommodate them, and this won’t be possible for anything more than a few mm thick.
4. Tooling holes
Don’t forget to add tooling holes to the PCB - ideally in the main PCB itself, rather than the panel waste - in case you want to test field returns. The holes are required for the tooling pins to locate the PCB in the fixture, and should be 3 to 4mm in diameter, with no plating. These should be located in diagonally opposite corners, with about 5mm of clear space around them, so the fixture tooling pins don’t short out to components or tracks.
5. Pull up (or pull down) resistors
These resistors should be used on all device signal pins, rather than tying them directly to power rails. This method allows the test machine to control the signals. This can be particularly important for pins that hold digital devices in a reset or high impedance state; this may not be required for the product to function, but is very useful in test when trying to isolate individual components in a circuit.
6. Leave space
The top of the fixture needs to be able to push down on the PCB, so allow at least a 2mm diameter space on the PCB between the components for "pusher rods". These need to be evenly spaced around the PCB area.
7. In-circuit test
In-circuit test systems are often capable of programming devices during the test cycle. While this can be convenient, it can also dramatically increase the overall cycle time. Consider whether these devices can be pre-programmed before placement. Of course, ensure the tester can then hold them in a reset state once power is applied.
Ideally, don’t fit batteries until after test, but if they are present then design in a removable link, so they can be disconnected during ICT.
9. Design stability
Finally, but perhaps most importantly, ensure your design is stable before committing to a fixture. These can cost anywhere from £4,000 upwards, so if components or test pads are moved on a PCB it can mean a new fixture is required.
You will probably find that your electronics manufacturing services (EMS) partner is well placed to advise you on these points, as it does take some effort to design a circuit and PCB correctly for ICT, and where space is at a premium it can be particularly challenging.
However, the reward of fast accurate testing can, and has for many years, provide a plentiful return on this investment and remains well worthy of consideration.
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