The application of Mil-Std-217 and other associated standards is now very outdated. These standards were developed for very old technology when failures were easy to ‘stimulate’ and simple high temperature testing was considered suitable as an Accelerated Life Test.
Nowadays stimulating defects in complex consumer products is a significant challenge. HALT chamber makers will tell you they can find as much as 90% of the product’s ‘latent’ defects in a short high stress test with only Random Vibration and Thermal Cycling. Although HALT is a very useful tool it certainly does NOT stimulate the wide range of latent defects in a newly developed consumer product.
So what can the Reliability Engineer do ? Refer to some magic standard ? Of course not, the engineer’s challenge is to look at the sub-assemblies making up the complex product and develop strong stress tests for each sub assy. The reason for this is that different sub-assemblies have very different technologies and material, hence ‘one stress test fits all’ will be useless !
The Reliability Engineer must consider the Design Limits of each sub-assembly and the materials / components which are used to decide on most appropriate stress test required to transform any latent defects into patent defects that can be failure analysed. This may mean one sub-assembly may react best to Thermal Shock followed by Random Vibration for mechanical defect stimulation while another electronic sub-assembly with SMD components may require periods of thermal shock followed by slower ramp rate thermal cycling plus periods of high temperature with humidity and power cycling / voltage change to stimulate weak components and poor solder joints to point of failure.
Also the engineer must consider the ‘Test Strength, of the proposed stress test to maximise probability the stresses applied can stimulate the defects that may exist, but this discussion can be left to another day.
In summary, the Reliability Engineer’s task is regularly under rated and their contribution under valued by those without the power of knowledge in Reliability.
We all of course look to change this as Reliability and associated warranty failure costs become a far more important factor in the future profit of new consumer products.
If you would like any information on planning new product stress testing feel free to contact Reliability Solutions via their website, www.reliabilitysolutions.co.uk