Posted on 02 Jun, 2016 by Russell Poppe
Even the most competent of manufacturers can make mistakes during the build process; the nature of manufacturing means process deviations that can cause a margin of error. Therefore, testing should be a core aspect of your manufacturing operation, to ensure that products meet your customers’ expectations and work optimally in the field.
Best practice advice is to carry out testing at every stage of the build - and not just at the end when the product is ready to ship. That means starting with the "brains" of your electronics device: the printed circuit board (PCB).
In a previous post, we discussed the in-circuit test (ICT) method for PCB assemblies (PCBAs) and outlined why PCB design is crucial to a successful test strategy. In this post, we will look at another popular test solution for PCBAs: flying probe.
So what steps do you need to take to make sure you achieve the most accurate test results?
What is flying probe?
Flying probe is a type of automated test equipment (ATE) that offers full and accurate coverage for PCBAs. It carries out what is called "manufacturing defects analysis" (MDA), which covers the majority of the most common process faults that can occur. These include: open circuits (e.g. something not soldered), short circuits, passive component measurements (resistors and capacitors), diode and transistor orientation, and usually, basic supply voltage measurements.
Most systems also offer limited optical inspection, which can add coverage for those components that can’t be accessed electrically. Additionally, some systems have power up and functional test abilities.
Unlike ICT, this method doesn’t require a dedicated test fixture – therefore, it has a lower non-recurring engineering charge (NRE). While the flying probe machine itself can cost more than £150,000, if you are an OEM outsourcing to an EMS provider, you will only need to pay for each program you require, at a cost ranging from around £1,000 to £2,000.
However, despite the low initial cost, the cost per unit varies a lot and can be up to approximately £50 for complex boards. Therefore, this test solution is best suited to prototypes or lower volumes where the PCB design may change a number of times throughout the lifetime of the product. Rather than building a new fixture, as would be required if you were using ICT, you simply change the software program each time the PCB design is modified - a process that can be carried out with relative ease and at a significantly lower cost.
How to implement the best test strategy
Flying probe typically uses four probes (although it may use as many as eight), which can be programmed and reprogrammed for each board design. To achieve good 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.
It’s always recommended that you take design for test (DfT) into account when designing your PCB – however, with flying probe test this is less of an issue than it would be if you were implementing ICT. For example, dedicated test pads are not required with this test strategy.
Nevertheless, you should leave a border edge – usually around 3mm wide – that is free from components along two sides of the PCBA so that it can be held in the machine. And it’s a good idea to maximise test access on one side of the assembly if possible - i.e. have at least one probeable point for each network. Unless you’re using a double-sided machine, it costs more to turn the board over and test from both sides.
Furthermore, if your PCBA is particularly large, it’s better to keep the test access points close together. This is because this is a relatively slow test method and the further apart these are, the longer it will take to complete, thereby eating into your time and resources.
For more guidance, why not read our post: 9 PCB assembly design guidelines for flying probe test.
By following the advice outlined in this post, you will be able to achieve the best results from your test strategy. Flying probe is generally accepted as the best choice for prototypes and low volumes and enables both original equipment manufacturers and electronics manufacturing services providers to accurately detect faults in their PCBAs.
With the assurance that the "brains" of your electronics device has been manufactured to the excellent standards both you and your customers expect, you can continue the build process - safe in the knowledge that you won't encounter any unwelcome surprises further down the line.
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