To The Editor:
I have read with interest the articles on plastic vs. ceramic ICs. Normally I have the opinion that ceramics are needed for very large parts, severe temperature requirements (over 100 degrees Celsius), space applications, or anything with high radiation environments.
However, the article by Dr. Noel Donlin in the July 1997 issue of Military & Aerospace Electronics got my attention. Having recently worked on a missile program which was supposed to use the Perry Initiative directions [concerning the use of commercial off-the-shelf components] I have to say that Dr. Donlin`s views can affect actual designs with value deflating directions.
The matter of plastic vs. ceramic ICs must be tied down to specific applications. Saying all systems can or cannot use plastics is impractical. The application we designed was limited in temperature requirements and the plastic ICs met all the environmental specifications. The system we designed had both ground and missile circuit cards. For ground unit applications, everyone agreed plastic ICs would work fine.
Two principle ICs on the missile were only manufactured in a plastic flat pack and a ceramic leadless chip carrier (LCC). They only cost $28 per part for the plastic and about $280 per part for the ceramic part. The parts were 28 pins which required leads to be attached to the LCCs due to the different expansion heat rates of the part and printed wiring board. This probably doubled the $280 in handling cost.
Being part of a large manufacturing organization gave me access to data that showed the hermeticity of the ceramics were being lost at a high failure rate by the companies attaching the leads to parts on other programs.
While questioning the need for the ceramic part, it was revealed that the direction for ceramics had come from Dr. Donlin`s office. The reason for ceramics was to prevent plastic "breathing" which could possibly cause the bonds in the part to be contaminated by chlorine and failing. This mechanism was said to be due to the parts` not being powered up repeatedly due to storage requirements. This prevented the parts from "cooking" the moisture out by elevated internal temperatures.
After researching several sources, I found two studies which indicated this problem had been solved in the late 1970s by use of modern plastics. The missile is stored in a sealed container and the parts are coated with conformal coating. The manufacturer claimed the parts would be OK, but could not verify this since they were new technology.
Furthermore, I found from other projects (outside the company) that the plastic would meet the shock requirements much better than the ceramic due to lower weight and profile. It was disheartening to find that the actual logic for selecting ceramics was at fault. Finding the increased cost of some parts by 20 times and reducing the reliability (due to leads being attached) depressed me even more. Perhaps Dr. Donlin`s office can assist designers with value-added suggestions in the future.
Ed Larosche, senior engineer
Huntsville, Ala.
205-882-4107, ext. 9727
Company name withheld on request
