For as long as I can remember, people have described differing subjects with the idiom: “It’s like comparing apples to oranges.” I looked it up to make sure I was using the phrase correctly, and sure enough, “comparing apples to oranges” is used to describe items that are popularly thought to be incomparable. The problem I have with this, however, is that I like both apples and oranges. At the end of the day, they both taste great and are used in delicious treats and juices. That sure sounds comparable to me.
When your printed circuit board is assembled by your contract manufacturer, they will likely test it using either in-circuit test (ICT) or flying probe. These two systems can both accomplish the goal of testing your PCB, but they will do it in different ways. In the same way that apples and oranges are different, yet they both satisfy your nutritional need for fruit, these two test systems will also get the same job done. Here’s a closer look at the in-circuit test vs. flying probe test systems so you will have a better understanding of the strengths and weaknesses of each.
Comparing In-Circuit Test vs. Flying Probe: The Facts About ICT
In-circuit test, or ICT, is a test system composed of individual probes designed to make contact with the test points on your assembled printed circuit board. This will check for assembly defects as well as the functionality of the board. Each net of your board should have a test point on it for testing, which means that a test fixture could contain thousands of probes.
The test fixture itself must be custom built for each PCBA tested. Usually, boards are tested from the back, but ICT fixtures can be created for the top, or the top and bottom together, depending on the needs of the circuit board. The fixture is built using the PCB design data, a bare board that hasn’t been assembled yet, and a known “golden” board that is completely assembled and verified to be correct.
The advantages of ICT are that it can test for functionality as well as for assembly defects. It can do this because ICT systems can handle more sophisticated testing method, including those specified in the Joint Testing Action Group (JTAG) standard. ICT is normally much faster than other types of testing systems in that it is contacting all of the board’s test points at once. Because of this speed, the expense of setting up the ICT test fixture and program can be recovered very quickly on boards being assembled in large volumes.
The disadvantages of ICT center mostly around the creation of the test fixture. Each test fixture, which is built by you, the customer, can only be created after the PCB layout is finalized. The development of this fixture is effectively equivalent to the development of a PCB, and there is a significant investment of time and money associated with it. Also, any revisions to the layout will in turn force changes to the test fixture, and in some cases, result in a completely new fixture being required.
Flying Probe Testing: Advantages and Disadvantages
Flying probe testing is an automated system that controls two to six probes that maneuver (fly) around a PCBA to contact test points on both the top and the bottom of the board. Unlike ICT, a test fixture is not needed because the probes themselves are moving to the specified test point locations. This does require specialized programming, just like the ICT system.
The advantages of flying probe is that it has a low cost and short development time for its setup. Because it can be set up and used so quickly, it is often used for an extra level of test validation at minimal impact to the overall assembly and test process. It can also easily handle PCB layout revisions because there isn’t a test fixture that has to be modified, only the programming needs to be adjusted. Flying probe is also able to handle larger boards than the ICT system, making it the go-to choice for testing large backplanes.
The disadvantages of flying probe are that it is a much slower test. Because there are only a minimal amount of test probes doing the work, they have to cover all of the test points on the board. For smaller boards and prototypes, this usually isn’t an issue, but for large complex boards with high-volume production runs, it is undesirable. Another disadvantage is that with its limited amount of probes, flying probe cannot handle the functional and other sophisticated testing that ICT can. It is limited to mostly looking for assembly defects.
Identifying the Best Test Strategy for Your PCBA
The PCB contract manufacturer you are working should have a complete understanding of each test system and which one will be best for your specific needs. For smaller boards that don’t require a lot of testing, or boards that are being produced in low volumes, the flying probe system may be the best choice. On the other hand, large volumes of boards and boards that are complex will need the speed and extended capabilities of in-circuit test. Often, CMs will use a combination of both systems to give you the best results. Flying probe will be used for basic testing during the prototype phase of the board development, and then the bulk of the testing will be transitioned to the ICT system for full production.
At VSE, we have a staff of engineers ready to help you to make the right choice for how your board will be tested. Our years of experience working with both of these testing methods means we understand how to get the right testing done for your circuit board.
