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HALT your reliability

It is often said that you can’t test quality into a product, you have to design it in. However, there’s a particularly powerful way of using testing to improve a product’s design so that it exhibits much greater reliability.

‘HALT’ – Highly Accelerated Life Testing – was invented by the pioneering American quality engineer Gregg Hobbs and developed over the last 30 years. Here is a greatly simplified summary:

HALT works on the basis that high levels of stress applied to a product for a short time will result in exactly the same failures as low levels of stress applied for a long period of time. This may seem counter-intuitive, but huge amounts of theoretical research and empirical testing have proven it to be true. So by applying increasing – and apparently unreasonable – amounts of stress to a product you can reveal hidden defects or shortcomings in the design that would normally only show up after months or years of normal operation.

Then you improve that part of the design so it’s no longer a weakness, and test again. And again.

It’s important to understand that you are trying to break the product, not pass the test. You keep increasing the stress until it does fail. Only then can you identify the weakest link and improve it.

halt2The stresses include operating over a wide thermal range (often from below -50 to above +100 Celsius), rapid thermal shock, random physical shock (often up to 50g or more), combined thermal and physical shock, and sometimes excessive supply voltages or electrical loads. Ordinary environmental chambers can’t do this, you need a special HALT/HASS chamber like the one shown in the photo; you can visit a test house and hire these by the day.

The unit under test is put it into a continuous, monitored test mode so that intermittent faults are immediately identified, and is kept running throughout the tests. Thermal cut-outs or sensitive components are disabled or moved outside the HALT chamber.

The full sequence of testing is too complex to describe here, but it involves increasing the stresses on the unit step-by-step, singly and in combination, until a fault condition occurs. The fault is diagnosed and repairs made – including design tweaks to make it more robust – so that testing can resume and the next weakest link found and fixed, and so on.

There is no pass/fail point; you just decide to stop testing and iterating because you have gone far enough (which is a lot further than you think, but beyond the scope of this short article!).

A full set of tests usually takes 3 to 5 days including set-up and some design ‘tweaks’ and re-testing; if re-testing is needed after a substantial design change this can usually be done in just 1 or 2 days.

Many design engineers mistrust HALT when they first hear of it. They say their equipment was designed for stationary use at -10 to +40 Celsius so subjecting it to -50, +100, and 50g is ridiculous. But when they fully understand the technique they rightly accept it as a powerful design aid.

I have been using HALT for six years and the results have been impressive; I have seen it make substantial improvements to the reliability of products from test and measurement equipment through industrial computers to microwave communications systems.

One note of caution, though – you can’t guess at how to do HALT, it mustn’t be done in a half-baked way. As Gregg Hobbs says: “Consistently, completely and correctly used HALT always works to the benefit of the manufacturer and the end user. Incorrectly used, nobody wins and some may lose more than reputation and money. Do it correctly, or do not do it at all!” It’s more complex than I have space for in a blog like this, so do read up about it and make sure you get good advice.

In a future blog I’ll talk about a matching technique for use on the production line – HASS.

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2 comments to HALT your reliability

  • Hello Tom,

    I find this blog occasional. Maybe we will find some business chance.

    Could you please show some reliability case to let me know your background and competence? Especially in telecom system device area.

    It’s better if you could send your or your consultants CV to my email.

    I’m interesting in reliability which cover from design stage to mass produce.

    Best Regards

    Bin Feng

    Chief Reliability Engineer
    Huawei Corporate General Technology Office
    Huawei Technologies Sweden AB
    Haukadalsgatan 3, PO Box 54, Kista Sweden
    Email: fbin@huawei.com

  • Tom G

    Hello Bin Feng

    Thank you very much for reading my blog and sending this comment. I shall be in touch with you by email shortly.

    Kind regards

    Tom

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