PCB Assembly Compatibility with In-Circuit Testing
RUSH PCB UK Ltd wants all its clients to be assured that we ship all products from our production facility only after testing them to be in perfect working condition. For this we have a variety of strict quality control procedures at each stage of our PCB assembly process. This includes not only standard services such as visual inspection and automated optical inspection, but also advanced test procedures such as X-ray inspection, and functional circuit testing or FCT. Although each method has its own advantages, for the most meticulous testing method, RUSH PCB UK Ltd recommends In-Circuit Testing or ICT.
In Circuit Testing (ICT)
Working at component levels, ICT allows localizing issues that may be present on the board under test. For instance, ICT can point to a specific device as the cause of the problem. With ICT, it is possible to test the individual voltage and current levels on the PCB, while including a step-by-step program execution.
The above helps in troubleshooting complex boards where the board is still a prototype and the design is not totally verified. Boards not passing the test may need reworking at component level to potentially save the batch. When this happens, our test engineers generate a Design for Assembly (DFA) recommendation to the client.
At RUSH PCB, we work closely with our clients and provide them flexible services tailored to their individual requirements. We have an engineering team to review the specific test requirements for a project, recommend the necessary equipment, and develop the test workflow. We even design test jigs if necessary. We have equipment to handle any type of project.
One of the advantages of using ICT is its speed of test. For instance, a few seconds is all it takes to test a complicated board. Therefore, projects involving large volumes of PCBs benefit exclusively from ICT. Apart from the speed of testing, detection of faults at component levels makes the diagnosis process faster and at the same time, does not involve a skilled operator.
However, for the ICT to be effective and accurate, the process requires a dedicated test fixture and a program. In addition, the PCB design must also allow the test machine and fixture to interface properly with the assembly. To maximize the test coverage and find the maximum number of potential faults, our clients must consider some points when designing the layout for their PCB assemblies. This not only reduces the redesign steps necessary at the prototype stage, it also helps in producing boards that perform right at the first attempt.
Making PCB Assembly Compatible with ICT
Test Pads—have a test pad on each electrical network on the PCB, including on unused IC pins. It should be possible to connect to the test pad via a spring-actuated test pin in the test fixture. For through-hole technology, the test pin can engage the component leg on the solder side.
It is usual to place 0.05-inch (1.27 mm) diameter test pads on a 0.1-inch (2.54 mm) grid, with the test pads spaced 0.1 inch (2.54 mm) from any component, and 0.125 inches (3.18 mm) away from the edge of the PCB. The above dimensions allow using long-lasting standard test pins. RUSH PCB UK Ltd does not recommend using test pads of reduced diameters, as thinner test pins are generally more expensive, requiring more frequent replacements.
Probing—place all test pads ideally on the solder side of the PCB to allow the test pins on the jig to access them from the bottom side. While it is possible to place test pads on the top, the construction of the test jig will become more complicated and expensive as it will require additional transfer probes and wiring.
Solder-Side Components—it is preferable to have no components on the solder side, other than small SMDs. Test fixtures usually have a vacuum plate to hold the PCB assembly from the bottom. It may be necessary to mill the vacuum plate for accommodating components on the bottom side. As milling is an expensive process, it is necessary to restrict the milling for bottom components to only a few millimeters.
Locating Holes—add locating or tooling holes to the PCB (not in the panel), to allow the test jig to locate the PCB in the fixture. Preferably use non-plated holes of 3 to 4 mm diameter. Locating two tooling holes in diagonally opposite corners will allow the test jig to accommodate the PCB unambiguously. Keeping a free space of 5 mm around each hole will ensure the tooling pins of the fixture will not cause shorting of components or tracks during the test.
Pull-Up Resistors—use pull-up or pull-down resistors on all floating pins, rather than connecting them directly to the power rails. For pins that hold other devices to a reset state or high impedance state, the presence of these resistors allows the test jig to control the pins. Tying the pins through pull-up or pull-down resistors also helps in product functioning, as the circuit can reject spurious signals. These resistors also help the test jig in isolating individual components when locating a fault.
Space for Pusher Rods—these are necessary to push down on the PCB when testing. ICT jigs usually have fixtures with 2 mm diameter pusher rods and necessary space should be available between components on the top-side of the PCB under test. Spacing them evenly around the PCB helps the jig manufacturer locate individual positions for the pusher rods.
Programming Devices—although capable of programming devices such as EEPROMs during testing by ICTs, the cycle time per board may go up. RUSH PCB UK Ltd recommends pre-programming such devices before assembly, and allowing the ICT to control them during testing.
Batteries—preferably, fit batteries only after the testing is over. As an alternative, use a removable link to connect/disconnect them during the testing.
Review—reviewing the design to ensure proper functioning is important before committing to a fixture. Moving test pads or components on a PCB can mean a new fixture, leading to time and cost overruns, as an ICT jig can be expensive and take some time to prepare.
We at the Electronic Manufacturing Services (EMS) from RUSH PCB UK Ltd offer advice to our clients on the above points and help them to design their PCB properly for compatibility with ICT. However, once the design meets DFA requirements, ICT provides fast and accurate testing, making the returns worth the investment.