Understanding Bare Board PCB Testing

October 21, 2020 Cadence PCB Solutions

Key Takeaways

  • Learn why bare board PCB testing is needed.

  • Find out how bare board PCB testing is done.

  • Learn how to choose the best bare board PCB testing method.

You may not know this, but jigsaw puzzle manufacturers take the idea of missing pieces very seriously. I recently learned that periodically, some manufacturers will pull an individual puzzle out of production and manually assemble it. If they find any missing pieces then a big alarm goes off and they will often dispose of that batch of puzzles in production. They know how frustrating it is for the consumers of their puzzles to think they have finished only to find one piece of the puzzle missing. 

It can be said that PCBAs are a lot like jigsaw puzzles. They both have a bunch of pieces that must be carefully assembled with the overall aim or goal kept in mind at all times. And in this case, pushing your PCB into production with doing bare board PCB testing is a lot like being a puzzle manufacturer that isn’t checking for missing pieces. The result can still be angry customers, but in the PCB realm there is even the possibility of reworks, product recalls, or other collateral damage. So let’s look at everything you need to know about bare board PCB testing. 

Why Do You Need Bare Board PCB Testing? 

a machine doing bare board PCB testing of a large green circuit board

Bare board PCB testing helps detect post-fabrication issues.

PCB manufacturing is a complicated process where copper layers are etched and hundreds of holes are drilled according to a design. Even with the aid of machines, the results aren’t always perfect. Occasionally, you’ll have issues on a few PCBs that go undetected until they are assembled and deployed.

For example, the etching process may leave a tiny piece of copper connecting two pads, or a trace may be disconnected due to mishandling or process flaws. Multilayer PCB manufacturing is even more complicated, which increases the risk of flaws on the bare board PCB. 

It’s a mistake to assume that the PCBs are in perfect condition or that the manufacturer will automatically perform post-fabrication tests. I once missed out on a specific testing instruction when completing a work order for hundreds of PCBs. The result was catastrophic, as the manufacturer skipped the testing process and I ended with lots of problematic boards.

If you want to avoid the terrible fate of recalling dozens of products and compensating for the loss, you’ll need to test every single bare board PCB. It is the only way to ensure that they are schematically correct before the assembly process begins. Testing also drastically reduces the number of issues when you’re running functional tests later.

How to Test Bare Board PCBs 

a stack of green, bare board PCBs

Get the manufacturer to run a flying probe or fixture test on your bare board PCBs.

Before you get the idea that you can assess your PCBs with a visual inspection only, I can assure you that it’s impossible to do so. Many of the shorts and disconnections are too small to be seen with the naked eye. Besides, it’s impossible to check multilayer connectivity with a visual inspection.

To test a bare board PCB, you’ll likely need assistance from your manufacturer. There are two types of bare board PCB tests available: the fixture test and the flying probe test.

The Fixture Test 

This involves developing a fixture containing multiple pins that match the number of nets on the circuit. It means that you’ll have two bed-of-nails fixtures pressing the PCB from the top and bottom with the hundreds of probes making a connection with the testing points of the PCB. It’s a very fast testing method, but it is an expensive method and must be remade if the board undergoes any changes after the production of the fixture. 

The Flying Probe Test 

This uses two or more probes to test all the nets on the PCB. The probe moves along the X-Y axis according to the layout programmed on the testing panel. There is no need to develop a fixture for the flying probe test, which makes it the cheaper option. However, the flying probe test is comparatively slower.

What’s the Best Bare Board PCB Testing Method? 

A PCBA being tested via the flying probe test method

A flying probe test is ideal for small volume production.

Both the fixture test and flying probe test will save you the frustration of overlooking manufacturing issues in your bare board PCBs. The question is, which should you choose?

The answer lies in the volume of your order and the cost involved. If you’re producing a small number of PCBs, the flying probe test is the better choice. The cost of a flying probe test is calculated per PCB, which is economical when the number is small. However, you’ll want to opt for the fixture test if you’re manufacturing hundreds or thousands of PCBs. The bulk of the cost lies in the fixture setup, which is insignificant when spread over a very large number of PCBs. 

Regardless of your choice of testing method, you’ll still need a PCB design and analysis software with robust design and testing features to generate the manufacturing files with ease. OrCAD PCB designer allows you to do so and helps make sure the job is done right the first time without a headache. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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