Test engineering – an essential luxury?
01 August 2018
Many professionals lament that ‘test engineering’ is often considered a poor relation to the other engineering activities undertaken during an electronics product's lifecycle. But this is only true if you see test as the only ‘non-essential’ part of the design and production process...
After all, once an engineer has designed the circuits and the PCB layout, the production guys only need to assemble the product correctly. Assuming everything runs to plan, and the circuit has been prior simulated, then it should be a simple matter of power-up and away you go! However, this relies heavily on the assumption that everything has gone to plan, no mistakes have been made at any point, and also that the components used are both genuine and fault free.
And that’s a bit like saying that no road accidents will occur if everyone simply follows the Highway Code and pays full attention at all times to the prevailing driving conditions. However, as we all know, accidents will happen, due to human error and/or component failure – and the same is of course true during the design and production of an electronic assembly.
Testing = insurance
PCB assembly testing therefore assumes a similar role to insurance in this driving analogy. Through thorough testing, you can highlight and diagnose errors that may have crept in – not only from production, but also design. This can be in the form of: missing components, incorrect components, badly orientated components, solder bridges or open circuits (production errors); or incorrect component values, mistakes in schematic entry/layout design (namely missing power and grounds) and/or net name inconsistencies between hierarchy levels (design errors).
So-called ‘design for test’ procedures can ease the pain of board testing and also encourage designers to think, not only of the final function of the product, but also how readily it can be tested during production, and also field return (service). Regardless of your preferred method of testing – functional (hot mock-up), functional (via ATE), in-circuit (bed of nails) tester, JTAG (boundary-scan), or maybe a combination of techniques – you will find that there are design ‘rules’ (or at least guidelines) available to ensure your design has the right hooks in it to allow effective testing.
Test equipment vendors, along with test fixture vendors, will often provide helpful documents that outline such guidelines – and JTAG Technologies’ ‘Board DFT Guide’ is one such document that is freely available.
“But we don’t need to test…”
Naturally enough, most test equipment vendors are generous with their support for a potential customer’s test endeavours! But what about those who refuse to acknowledge their test responsibilities – the ‘reckless drivers’ among us? These are the people who will often tell you: “We don’t need to test our boards, because we demand that our contract manufacturer only sends us 100% working boards”.
But of course, ‘demanding’ and actually ‘receiving’ are two different matters. I would respectfully argue that no one could realistically achieve this directive – and if they could, what would be the cost? The OEM/designer may neither know nor care whether their boards are being tested – but of course they should!
For example, the CEM may only undertake a cursory functional test using some type of hot mock-up (system simulator): in other words, when the board powers up, does it do roughly what it should? If the board fails this test, then it is simply cast aside and another is built to replace it until the order is complete. This could mean that, depending on the CEM’s process quality and yield, any number of boards could be hitting the ‘bone-pile’ – and guess who is ultimately paying for that? The OEM, naturally.
If boards can be diagnosed and then fixed at a cost of less than their value (component costs plus build costs, namely labour and factory overhead), and the manufacturing process debugged also, then of course it makes sense to test more vigorously. But who actually decides that?
Added value for CEMs
It is easy to understand why, in the highly competitive word of contract manufacturing, CEMs may wish to keep their costs, and by extension, quotation values to a minimum.
Avoiding discussions over test (and who pays for it) is one way to do this. However, for any medium to high complexity design, this has to be considered ‘short-termism’.
Adding value to the sales proposition in terms of test can actually increase margins and offer confidence to the designer/OEM that you are equipped to deliver a high quality, thoroughly tested, product. So-called low ‘banner prices’ are always attractive, but once the costs of returns and rework are added into the mix, it’s often a case of ‘buy cheap, buy twice’. Ideally then, the chosen test methods will be a joint decision – based on knowledge of both the design and manufacturing process.
Both OEMs and CEMs, therefore, should review design for test (DfT), fault coverage, diagnostics resolution (test system performance) and more, before jointly determining the optimum (lowest cost effective) test path. Most reputable test systems now provide a fault coverage assessment figure, but what does that mean, how is it derived – and can it be trusted?
Independent fault coverage assessment systems are also available that can take inputs from a variety of test systems and aggregate the results to give users an overall test coverage. However, these do still require a great deal of understanding and maintenance: they generally suit large organisations with dedicated DfT staff members. For most designs, it will be beneficial for the project manager to include DfT and test specifications in the overall project programme.
By avoiding discussions over test, you are effectively driving without insurance: you can try it… but get caught out and the penalties can dwarf the initial savings. Even a simple process failure, such as a poor reflow profile, will be difficult or even impossible to diagnose without the use of test facilities. In this case, production can then cease, before revenue then stops – and in just a short time, an entire company can be in jeopardy.
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