Effectiveness of ESS
The references contain many examples of the successful use of ESS. AT&T called its process environmental stress testing (EST) to emphasize the fact that the company used the results to make product improvements.15 The process combined temperature step stress and temperature cycling between -20(degree)C and +70(degree)C for circuit card assemblies.
Figure 3 shows a plot of failures vs the number of cycles in the EST process. From the data in Figure 3, the investigators concluded that the optimum number of temperature cycles was 16.
In addition to the improvement in outgoing quality, the investigators tracked field failure results. They reported a five-fold improvement in product which had been exposed to EST, compared to product which was not exposed to EST.
Although some ESS practitioners believe that the process should always be conducted on 100% of the product, a sample EST process has been implemented successfully.15
One two-stage ESS process for laser diodes was comprised of a steady-state burn-in at 165(degree)C and 10 kA/cm216 The results showed that unscreened lasers had a medium lifetime of about 600 h, compared to about 6,000 h for screened lasers. for 2 h prior to assembly, and a second steady-state burn-in at 70(degree)C for 150 h after assembly.
In another study on laser diodes, AlGaAs laser diodes were exposed to an ESS process consisting of operation under power in inert atmospheres.17 The results are shown in Table 5. Again, significant improvement in operating reliability was obtained for products which had been exposed to ESS.
If a product has a very low failure rate, the design and operation of the ESS process can be quite complex. McClean reported the use of a technique called highly accelerated stress audit to screen printed-circuit card assemblies.18 The screening stresses were temperature cycling and vibration, with power being applied during the process. As the name implies, the test was applied on a sample basis.
As noted in these examples, the development and operation of an ESS process must be highly customized to the product being screened. Perhaps the greatest benefit of ESS is the hands-on knowledge and experience about the product gained by those who design and manufacture it. For this reason, it is not a good idea to assign the ESS process to a reliability department or a third-party screening organization with limited capability to change the design or manufacturing processes.