Since the early 1980's there has been much discussion regarding various approaches and methods in the use of environmental stress screening and Halt/HASS to markedly improve product quality and life. Unfortunately, there was widespread misunderstanding and misapplication of the methodology . . . . and some still remains. Accordingly, we have listed the "Seven Deadly Sins of HALT/HASS".
1. Thermal cycling chamber air instead of product.
Environmental chambers with high rates of air temperature change may not qualify as a HALT/HASS chamber. Simply cycling chamber air temperature is not sufficient. HALT/HASS requires thermal stimulation of the product.
Thermal cycling must cause the product to physically expand and contract at a relative high rate of change over a number of stress cycles. That is why you cannot simply use traditional environmental simulation chambers or sell them with a "HALT/HASS" label.
2. Vibration levels measured on table instead of product.
As in product thermal stimulation, HALT/HASS requires a product vibration response. Measurement of the input is not measurement of the product response to 6dof vibration.
3. Putting latent defects into product.
HASS overstress, whether, thermal, vibration, humidity, or other, can well cause new latent defects, which were not there to begin with.
Each product is different. It is vitally important to determine the optimum stress levels empirically when establishing a HASS production screen.
4. Taking out too much product life.
HASS screening during manufacturing can take unneeded life out of a product. If the screen is set too high or the one screen fits all approach.
5. Not tailoring HASS stress to product.
It is imperative that the HASS screens be tailored to the product. The one screen fits all approach does not work. The screen level may be too low or to high for some products. If too low it can allow potential infant mortality defects to be undetected. If too high it can take unneeded life out of the product.
6. Not functionally testing product while undergoing HALT/HASS.
Intermittent part failures can go undetected unless they are functionally tested while undergoing HALT/HASS. The level of detectably must be high in order to obtain the results desired.
7. Over design of product as a result of improper HALT.
HALT is essentially an exploratory stress test to find part defects and to replace weak parts with robust parts. However, care must be taken that the product is not subjected to HALT undue overstress and consequential redesign. Misapplication of HALT can result in an over designed product that is not commercial viable.
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Our long term experience with HALT/HASS seminars, installations, and practitioners worldwide is a valuable resource that we welcome you to draw on. Please do not hesitate to contact us.