Early Split-lot Experiments Helped Prove the Need for ESD Control Programs

An excerpt from Terry Welsher’s InCompliance article titled –
The “Real” Cost of ESD Damage about using “split lot” experiments to determine the effectiveness of ESD control when manufacturing electronics.

“At the time of the first EOS/ESD symposium in 1979 there were few mature ESD programs, but many companies were trying to establish them. Some of these companies were also developing their case for management through actual split-lot experiments.

Western Electric Denver Works (1981) – In this study the initial deployment of a basic ESD program was observed with careful collection of yield loss data for five key operations. Documented yield improvement up to 10.73% was observed, and with the assistance of the plant financial organization, the return-on-investment was estimated to be in the range of 900-2300%, depending on assumptions. This study was also significant in that it demonstrated that an effective program could be implemented in a very dry environment such as Denver, Colorado’s without humidity control.

Western Electric North Andover Works (1983) – This work included three separate definitive experiments on the effectiveness of ESD programs. Again, since ESD controls of any kind had not yet been implemented, simple split -lot experiments could be conducted. In these experiments as many as 1275 units were processed in single experiment with and without controls. Clearly, it would be difficult to justify taking these risks today. Ratios of the number of failures in the unprotected and protected lots ranged from 1.9:1 to 5.5:1. The return on investment (for implementation of wrist straps and some ESD-protective transport materials) was as high as 950%. The quality assurance organization also studied the quality of outgoing product. The controls instituted in the factory resulted in a 3:1 reduction of defect rates.

Lockheed Missiles and Space Company (1983) – In this study failure data before and after program implementation was collected and explicit cost avoidance estimates were made. A detailed itemization of implementation and maintenance costs was weighed against extrapolated expected failure costs and an annual savings of almost $2 million/year was demonstrated.”

Also, we have HBM Human Body Model, CDM Charged Device Model, MM Machine Model; he mentions CBE Charged Board Events and CDE Cable Discharge Events, noting “Recent data and experience reported by several companies and laboratories now suggest that many failures previously classified as EOS may instead be the result of ESD failures due to Charged Board Events (CBE). The reason for this is that boards may store considerably more charge than is stored in the standard CDM tests. The resulting failure signature shows more physical damage (Figure 2) than a stand-alone device failure would and thus FA experts unfamiliar with this phenomenon often make the wrong diagnosis. In addition, similar observations have been made regarding the misdiagnosis of Cable Discharge Events (CDE) as EOS. Some companies have estimated that about 50% of failures originally designated as EOS were actually CBE or CDE.”

To read the full article:
The “Real” Cost of ESD Damage by Terry Welsher, Dangelmayer Associates/ESD Association published in In Compliance Magazine

Leave a Reply