It is generally thought that uniformity and repeatability are essential to vibration and temperature screening and other testing activities. It has been found by the author that neither uniformity nor repeatability of vibration nor thermal profile is necessary in a comprehensive and correctly performed HALT & HASS program. HALT is Highly Accelerated Life Tests and HASS is Highly Accelerated Stress Screens.
First, uniformity will be considered. A year-long experiment was run at Santa Barbara Research Center in 1980 on a program which is reported in Reference 2. Approximately 12,000 production units were used in the experiment. It is important to note that the product that was screened had been run through Design Ruggedization, a precursor to HALT, and had been ruggedized so that the electrodynamic shakers and thermal shock systems available at that time could not damage flaw free units even with long exposures to the maximum stresses that these equipments could generate. This extreme robustness is not essential to the conclusions to be drawn but is generally beneficial for high field MTBF and is readily attained using HALT techniques.
When production Enhanced Environmental Stress Screening, a precursor to HASS, was begun, we obtained a new type of shaker and combined thermal system and actually obtained serial numbers 1 and 2. In that program, all axis pneumatic impact shakers and liquid nitrogen thermal chamber systems were used for Enhanced Environmental Stress Screening of an electro-optical device, a proximity fuse for a Sidewinder Missile. The fixture, mounted on the shaker and within the thermal chamber, held twelve target detectors–the proximity fuses. The thermal and vibration profiles and overall levels at each of the twelve screening positions were very different. The thermal profile varied by ±20° Centigrade and the ramp rates varied by +/- 50% from one position to another. The RMS vibration level varied by +/-50% and the PSD varied up to 60 dB from position to position. This variation was not expected nor thought to be acceptable as the classical paradigm of “uniformity and repeatability is essential” was in place in the author’s mind at that time. Since we had to use the equipment, a Proof of Screen (see Reference 2) was run. The same products were run through four screens sequentially with the vibration set at maximum level, only 11 GRMS quasi-random. The thermal system was set at the maximum possible ramp rate, which was 15°C per minute on the product. This repeated pass through the maximum possible screen did not break anything so more units were run through many screens in sequence to see if damage to the units under test were possible with this screening equipment. It was found that no matter what was done to the units, the screening system could not damage good units–at least in the time allowed!
Comfortable that we were safe in terms of product damage due to the variations in the thermal and vibration environments, we proceeded to begin production screening with the customer’s approval. In order to learn what affect the various environments had on the units under test, we kept track of failures at each position on the twelve position fixture for a complete year and then drew up Pareto charts for each position. The Pareto charts were quite similar, which meant that each of the twelve very different screens was equally effective! Please note that approximately 1,000 units had been run at each position, so a good statistical significance was present.
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