Posted on 12 May, 2016 by Russell Poppe

PCB_design Testing is a fundamental element of your manufacturing operation. Without implementing a robust test process, you run the risk of overlooking manufacturing defects that could potentially cause your products to fail in the field. This situation can wreak havoc on your reputation and customer relationships.

So testing is crucial.

But it is not a process you should simply carry out after a product has been made; it is not an "add-on". Instead, it is necessary to carry out specific test procedures at various stages along the way. Finding a fault in a product before it has been manufactured to its full value, and either rectifying or scrapping it, is a lot more effective than testing the finished goods, only to discover a problem and then reject them.

In this post, we will be focusing on testing printed circuit board assemblies (PCBAs) using the in-circuit test (ICT) method. This is a popular and cost-effective strategy that delivers component-level fault diagnosis, rooting out problems with PCBAs before they are inserted into larger assemblies.

However, none of these benefits can be realised without taking into account PCB design. Let's take a look at the best practice steps you should follow, to ensure that you obtain the optimum results. 

What is in-circuit test?

ICT requires a type of specialist automated test equipment (ATE) that carries out what is called "manufacturing defects analysis" (MDA). It checks each component on a PCB individually and verifies things like passive component measurements (resistors and capacitors), diode and transistor orientation, and basic supply voltage measurements and looks for faults such as open circuits (e.g. something not soldered) and short circuits.

Additionally, integrated circuits (ICs) can have some powered, basic functional testing, as well as "vectorless" testing to check the soldering of pins to the PCB through a non-contact, usually capacitive, probe or plate. And ICT can also provide limited analogue and digital measurements.

Get the "Design guidelines for in-circuit testability" technical paper

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The importance of PCB design

In order to carry out ICT, you will need to invest in a dedicated test program and a "bed of nails" test fixture, which together cost in the region of £10,000. Because of this high non-recurring engineering charge (NRE), this method is most suited to high volumes and stable designs. If you change your PCB design, you will need to purchase new equipment, meaning the costs can quickly ramp up. As a result, it is important that your PCB has been designed with test in mind. 

So how should you design your PCB? 

The best layout configuration is to have all the components on one side of the board and all test pads on the other. Fixtures are available that will allow probing to both sides of the board, but this will increase the fixture cost and program debug time.

ICT requires, at least, a 0.05" wide test pad per net, designed into the PCB up front, which is used as a target for the fixed test probe. Pads should also 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 - as well as at least 0.125" away from the PCB edge. 

Space should be left on the component side of the board to allow "pusher rods" to press the board down. 

ICT in practice

To achieve good test coverage, a bill of materials (BOM), computer-aided design (CAD) data and schematics are required. The CAD data is used to generate the basic test program, ensuring information is taken from the original design rather than a manual interpretation of other data.

Populated and unpopulated sample PCBs are then necessary for fine-tuning the test programs -"debugging" - and making the fixture, to ensure the assemblies will physically fit as they should. 

ICT is a highly accurate form of testing and, despite the initial investment required, the cost per unit is low. Typically, it takes around 30 seconds to test a medium-sized board, at a cost of less than £1.

PCBAs are the "brains" of electronic devices and, therefore, it is important that they are tested thoroughly. ICT remains one of the most widely used and successful methods of achieving this. However, without taking steps to ensure your PCBA is suitable for testing, you will not be able to get the most from your test strategy. 

And if you get testing wrong, you increase the chance of manufacturing defects slipping through your net, with potentially damaging consequences for your organisation. 

For more detailed information about the importance of PCB design to in-circuit test, why not download our technical paper here.  

Gui

Topics: Best Practice

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About the Author

Russell Poppe
Russell Poppe
After an early career designing electronics for engine control systems and hand held computers, Russell qualified as a Chartered Engineer and has spent the last 20 years in various production and engi...read more