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February 2002

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From:
Guenter Grossmann <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 19 Feb 2002 10:25:57 +0100
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Antonio

To answer your question you have to ask yourself first what reliability is:
Reliability = Ability of a given identity to fulfill the required mission within a given time frame in a defined environment under a given load with a required probability.

This definition gives you the whole bandwidth on the parameters you have to look at.
- Thermal 
- Mechanical
- Thermomechanical
- Electrical (Including EMC)
- Humidity
- Chemical attack (including corrosion)

Depending on the application you have to choose the test sequence that has to be applied. No easy job.

But be careful. 

- You are not only talking about testing. You talk about accelerated testing. This means you talk about physics of failure. For each test you choose you have to think about the failure mechanisms you are about to activate and you have to understand the influence of the testing parameters on that particular mechanism.
Examples:
. Chemical processes can be regarded at as thermally activated. This means, in a first attempt you can run a tests with at least 3 temperatures to evaluate the activation energy for the Arrhenius law you can apply to your problem. This activation energy might incorporate several mechanisms activated. However, for practical purposes this is precise enough. Based on this knowledge you can design the accelerated test that allows you an estimation of the reliability in the real world. 
. Solder joints. A problem that Werner an I work and discuss on for many years now. Assuming that our model holds, tin based solder joints deform with two deformation mechanisms. Grain Boundary Sliding and Dislocation Climb. Both contribute their own part to the degradation of the solder joint. Senseless accelerated testing will activate large amounts of DC that never occur in reality. It will therefor be very difficult to extrapolate (anyway always a dangerous thing to do) the results found to the real environment.

- Reliability is a probability. This means, you need enough samples to find that number. Just ask the people finding failure rates of IC's they might give you an idea.

However, you are not alone. As a matter of fact all these questions are subject to research and I only mentioned how difficult the business is to show that we scientists are real big cracks. Seriously, there is a lot of data around in literature, but it's not a small job to find them. On the other hand you might even not ask for reliability but comparative testing. In this case, where you have a known specimen that fulfills your reliability requirements you can design a test where you activate the interesting degradation mechanism(s)  in both specimen to the same extend and compare the results of the two. Still not a peace of cake but a lot easier than starting from scratch.

Still, it's a fun job. Most of the time.

Best regards

Guenter



EMPA
Swiss Federal Institute for Materials Testing and Research
Centre for Reliability
Dipl. Eng. Guenter Grossmann

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1823 4054
mail:     [log in to unmask]

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