Introduction
The Importance of a Measurable & Repeatable Screening Process - by P J Mason MSEE
31KBMany latent product failures are exposed by natural temperature cycling in the products normal use environment. The various materials used for construction of the components and the finished item quite naturally expand and contract by different amounts when heated and cooled. They are said to have different thermal coefficients of expansion.
In service, such temperature cycling can be due to internally generated heat or from an operational environment which means that the product must survive constantly changing thermal conditions. The worst being when thermal changes are rapid as experienced for example by automobile electronics systems or aerospace components.
Well designed and well manufactured products withstand the repeated stress reversals of heating and cooling without failure. However, flaws can become concentration areas for the stresses which tend to concentrate around the defect, with the result that failure occurs.
How can latent defects be identified using thermal stress screening before the product leaves your factory ?
By performing thermal stress screening both during the design phase and in the production stage of a product, the same failure mechanisms seen by the productin real life can be forced to occur in the lab. It is important to remember that it is not necessarily the requirement for a product to survive and operate at an extreme temperature that is the problem. More commonly it is the stressing induced by temperature cycling from cold to hot to cold etc etc continuously which does the damage. Combine that with condensation, operational vibration and air born contaminants and the potential for shortened life can be anticipated quite easily.
Testing in controlled conditions can enable accelerated testing by the application of repeated cycling from one extreme to the other. The process is well established and as with most other technologies is being continuously refined. Processes which could consume fairly long testing periods can now be replaced by new methods which are more effective with fewer testing hours.
One of the new techniques is "very rapid airflow" With this process it is possible to uncover faults in a few hours which could have taken weeks to expose previously. Thermal Stress Screening is a major benefit in the production process, significantly reducing the level of field failures.
Although this text primarily addresses the use of production testing in the production process, there is an underlying requirement that the product has been properly designed and is capable of surviving in use. Thermal stress screening in the production line is a tool for exposing manufacturing or process control defects once a robust and suitable design has been achieved. Similar technology can be applied to testing in the design phase and production quality testing should always be considered from the earliest stages of product design. It is too late to discover fundamental design flaws once the product is in production.
Testing with thermal cycling is not achieved by moving the product from the oven to the fridge !
There are many ways to effectively thermally screen a product, consideration must be made to its size, structure operating environment and functionality. When anything is placed in a thermal chamber the forced change in temperature is absorbed by the product from the outside, in. Testing with the product powered up can for example increase the rate at which the internal temperature is forced to rise, turning off power might contribute to assisting the rate of a cooling down phase. Is that the result you need to achie ve ? If you think t hat thermal screening might help to improve your products you need to know which questions to ask and consider before answers from anyone can be trusted.
Where do I go to obtain impartial advice and guidance about setting up a thermal testing regeime ?
Talk to us at the SEE first, we run regular courses on all aspects of environmental engineering.