A Primer on Manufacturing Tests for Your Electronics
I Don’t Always Test My PCBAs, But When I Do I Do It In Production
That’s a lie. I do always test my PCBAs.
I make sure prototypes, bare boards, and assembled units fresh off the new product line get the testing they need.
In this post, I’m focused on the latter. There’s a whole host of testing options to make sure that your freshly assembled electronics are built correctly and performing up to your specifications.
More Than Just A Functional Test
The most familiar kind of testing is functional testing.
At the end of the line, you want to make sure every function and feature of your product is working as you expect it to. More often than not, though, you want to be able to spot problems before it gets to that point.
If you’re only testing your product at the end of the production line, finding a defect can lead to expensive rework or scrapping a costly product. If we can catch problems earlier, we can deal with problems at a less expensive stage of the assembly process.
It’s especially valuable to run a variety of tests on your electronic assemblies right after your PCBAs come out of the SMT line but before they get assembled into mechanical enclosures. That way you can rework faulty PCBAs without having to disassemble them from an enclosure.
So What Else Is There?
Great question! I’m so glad I asked that.
There are all sorts of fantastic testing options that you can include in your electronics assembly process. Here are the common options:
Manual Visual Inspection (MVI)
Simple. Straightforward. Exactly as advertised. And like most manual tasks, fairly expensive. But the setup costs are pretty low.
This is as basic as having a technician manually inspect PCBAs that come off of your SMT or other assembly line.
Usually, this will include some optical enhancement to make the small parts and solder joints more visible.
This is something you might do during a New Product Introduction (NPI) process or when setting up a new line. But is probably not something you’re going to do in mass production.
Automated Optical/X-Ray Inspection
Automated Optical Inspection (AOI) and Automated X-Ray Inspection (AXI) are the automatic cousins of the MVI process described above.
The automation allows these inspection processes to be faster and cheaper.
AOI is just as straightforward as MVI, but instead uses computer vision to automatically examine a PCB or PCBA.
Because of it’s simple setup and low cost, it often gets used in multiple places along the production and assembly line. (It’s worth noting that AOI processes are also used during the PCB fabrication process to examine inner copper layers before outer layers are added.)
It’s common to use AOI at the end of an assembly line to check component placement and solder joints. But it’s also common to use AOI at earlier stages.
You might, for example, use AOI after applying solder paste to your PCB but before you place components, to verify correct solder paste placement.
The main drawback of AOI is that it only catches faults that are visible on the surface of a board.
For example, if you have BGA parts on your PCBA, you won’t be able to see the solder joints underneath those parts to verify good connection to all of the part’s pins.
This is where AXI comes into play.
AXI is more expensive to setup and operate than AOI, of course. But it does offer some great insights into what’s going on in your circuit board.
You get to see inside your board, inside the parts on your board, and all of the solder joints, even the hidden ones, to truly verify that all of the connections have been properly made during the assembly process.
Unless you have a highly complex board with yield issues or very high testing requirements (such as in aerospace or medical applications), you’re probably not going to want to pay for using AXI in mass production. However, you might use it during the NPI process to better understand your yield and your common failure points.
After that, as long as your yield is high enough, you’ll probably want to ditch the AXI process, save your money, and catch failures somewhere else in the test process.
In-Circuit Test (ICT) and Flying Probe
While the visual/optical inspection tests are a great first pass test to check your assemblies, they don’t check that everything on your board is working.
We’ll need more advanced testing to catch certain electrical problems.
In-Circuit Test (ICT) is customized to each PCBA design.
Hopefully, you designed your boards to have good test point coverage (good test point coverage is usually defined as >90% of all of the electrical nets on your board).
Based on this test point coverage, you will build a “bed-of-nails” test fixture.
The “nails” are test point probes — usually spring-loaded pins or “nails” — aligned to the test points on your board. The fixture will hold your PCBAs in place, align everything so that the test point probes make good contact with their designated test points.
Depending on your needs, ICT can cover a lot of different electrical tests.
At a very basic level, it will test for shorts and opens in your assembly.
ICT can also measure impedances, charge and discharge capacitors, and test for other analog behavior.
You can power up your board in the ICT fixture and verify power draw. At the more advanced end, you can inject signals and test for correct behavior in the subsystems, just a step below functional testing, described below.
Similar to ICT is a Flying-Probe test.
The goals of the test are the same, but in this case, you’re not building a fixture specific to your PCBA.
A typical Flying-Probe fixture will have multiple test probes on motorized arms which can place them precisely on key test points on your board.
Flying-Probe is more expensive and more time-consuming than ICT because of this customizability, but it does not require that a custom test fixture be put together; it only requires some up-front programming to operate the existing device.
This makes Flying-Probe testing great for small volume runs of PCBAs, when you can’t necessarily justify the cost of an ICT fixture.
JTAG Boundary Scan
It’s easy to think of JTAG as primarily a programming interface for the digital electronics on your PCBA.
But if you set everything up correctly in your design by including a Test Access Port and making sure your digital ICs are boundary scan-enabled, you can use this powerful test process to check the functionality of your digital electronics.
A boundary scan will use the built-in test circuitry of your digital ICs to test PCB connections, monitor the state or voltage of individual pins and test the subsystems on your ICs.
Boundary scans can be used in conjunction with ICT, or in the case where your board is almost entirely digital, it can be used in place of ICT. They can be used to test digital interconnects and basic functionality of all of your digital ICs.
And the real beauty of this is that boundary scans need only a 4-pin Test Access Port wired into your PCBA; you no longer have the complexity of all of the test pins of ICT.
Specialized software uses the JTAG interface to automatically test devices that are boundary-scan enabled, and you can get a full boundary scan in a few seconds.
In those few seconds, a boundary scan can achieve similar or better test coverage than ICT and can diagnose any faults or failures with your digital electronics.
System Functional Testing
Functional testing. This is what most people think of when it comes to testing.
This is the core “your boards do what you want them to” test.
This test is usually done after some level of electromechanical assembly. It may be before you seal everything up, but it’s after you’ve attached buttons, control panels, lights, motors, or the other mechanical devices that your board might interface with.
Once you have your PCBA assembled into its mechanical enclosure, or assembled to the point where it’s effectively functional, you can start your testing.
The exact setup of your functional test is, of course, highly dependent on the designed functions of your boards.
It’s important in a functional test to include all of the key behaviors as well as all of the key failure modes of your board.
You should, for example, make sure that the board behaves to specification with bad inputs or mechanical failures. This is especially true for high reliability devices.
It’s also important to remember that while functional testing seems like a complete test, it lacks a lot of test coverage that is included in other tests mentioned here.
So while it is an important test, you will almost certainly want to include some of the other testing methods mentioned in this post for more complete coverage.
A good example of a possible failure mode that may not be caught in functional testing is the board failure scenario that Adam described in his blog post in July.
Functional testing is important, but as I mention often, it is just one of many tests.
Accelerated Life Testing
Sometimes called a “Soak Test,” accelerated life testing is focused on running your board through as many cycles of normal operation as possible in a short period of time.
This could include things like rapidly cycling power systems; starting, stopping and reversing motors; cranking microprocessors to full speed; charging and discharging batteries; and much, much more.
Similar to functional testing, Accelerated Life Testing is very specific to your circuit board and what it’s trying to accomplish.
This test is mostly focused on how a product survives in the normal usage case in the real world. It is often combined with environmental stress testing, described below, which focuses more on the extremes a product might encounter.
Environmental Stress Testing
Environmental stress testing involves taking your product to the extremes: temperatures, humidities, pressures, and other environmental conditions that could damage or affect the operation of your circuit board.
There are usually specialized test chambers designed to simulate these environments.
You may not need to run every single unit through accelerated life testing and environmental stress testing; it very much depends on your reliability requirements.
You might run a small sample of manufactured units through stress testing to validate your design, but then not necessarily run every single manufactured unit through the same test when you get to mass production.
This one is my favorite tests because there can be sparks and explosions, but if your PCBA is designed and manufactured correctly, there shouldn’t be.
High-potential testing, sometimes called “hi-pot” testing is focused on the ability of your PCBA to resist extremely high voltages across different parts of the system.
It’s a common test in power electronics that routinely deal with kilovolts or more. Your power systems need to be able to withstand extreme voltages, and this is the test for that.
It often involves an enclosed test chamber (to manage any potential sparks and explosions that could happen in the case of failure).
Contact leads are connected to points on the PCBA under high-potential testing (for example, the power terminal leads of a device), and then a high-potential tester is used to generate anywhere from 10kV to a megavolt or more.
Leakage current is measured and if it’s within spec, all is good. If it’s out of spec, some dielectric or isolation is not performing as needed.
If your device explodes, welp… enjoy the fireworks.