TechArchive

This article was originally written in the period 1995-2000

Gabor Martell, Managing Director of Montford Instruments, explains the principle of HARASS testing of products incorporating electronics.

Thermal stress screening is an established way of exposing hidden faults which do not show up during normal testing. This happens because sub-standard components and connections often function correctly at first and so are passed by automatic test equipment (ATE), but fail soon after being subjected to the stresses of handling, transportation and normal use. So large sums of money are spent powering up equipment during thermal cycling in order to boost fault detection rates.

Thermal stress screening subjects devices to repeated rapid heating and cooling, causing expansion and contraction within their components and connections. This builds up internal stresses which cause potentially defective devices to fail prematurely, before they are shipped.

Even so, some hidden faults can still be missed. Until now, the most common methods to boost detection rates have involved powering up the boards and testing them during temperature cycling, in order to catch faults which are intermittent.

This requires expensive racking and cabling systems inside the stress screening enclosure, perhaps more than doubling the cost. But unfortunately, because the cable connectors themselves are subjected to constant temperature cycling and repeated insertions, after a while they often only make intermittent contact, rendering the whole stress screening facility unreliable.

HARASS, which stands for Highly Accelerated Rapid Airflow Stress Screening, is a new technique developed with a major international telecommunications company. it provides increased detection without the disadvantages of powered-up testing.

Conventional thermal stress screening cycles air temperature, but with some equipment, the boards can lag so far behind that components see very little temperature change. Getting the board temperatures to change more quickly results in more defects being uncovered. HARASS takes this to new limits, using highly accelerated airflow and sophisticated temperature control.

Because HARASS transfers temperature to the boards more quickly than conventional stress screening, it is much more effective at uncovering faults, including those which are intermittent. In many cases this removes the need for powered up testing.

One worry often expressed is that since a board consists of components of differing mass, there is a risk of over-stressing and damaging those which track the air temperature most effectively. HARASS overcomes this by directly monitoring the temperature of the fastest-responding component and automatically reversing the applied temperature the instant that the component reaches its set point.

HARASS testing is non-random and non-combined, which means that identical tests can be repeated on successive board samples and the cause of failures traced back to specific tests. These features make HARASS ideal not only for production screening, but also for confirming that the production process is under control, for finding out why failures and investigating how successfully they have been rectified.

The effectiveness with which temperature is transferred is measured by the speed at which the temperature changes in the centres of a number of sample blocks of solid aluminium. This value is expressed in C?lk/minute. Montford instruments has produced several 20Cdeg Blk/minute HARASS chambers.

Features include:

• Interactive control to eliminate wasteful dwell periods
• Inherently dry atmosphere which prevents condensation on the boards
• Small footprint requiring small floor area
• Quiet operation

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