To The Editor:
With regret I am writing this letter concerning the Viewpoint article, "Marrying military and commercial microelectronic production lines," by Greg Saunders in the November 1999 issue of Military & Aerospace Electronics.
My problem (and probably most of your readers) is that military microelectronic production lines and products are no longer a reality. A military equipment supplier using them is being required to do so by his contract. In the real world the system developer must be competitive and profitable. This is done by having the best system performance, the lowest production cost (meaning low reject rate, surface mount, automated pick and place, SPC, part availability, approved suppliers), the size and weight demanded by his market, high reliability and on time delivery. This means surface mounted plastic encapsulated microcircuits (PEMs).
Why is the application of PEMs questioned? In the late 1960s, when this new packaging scheme was introduced there were failure mechanisms which were not completely understood pertaining to molding materials, assembly processes and environmental limitations for the devices made at that time. To assure that this early technology, with its limitations, did not get into military equipment, MIL-HDBK-217, Reliability Prediction of Electronic Equipment, was subsequently revised, assuring that the calculated reliability using military-approved devices — QPL at that time — would be required to meet the equipment contract reliability level. In the 1980s, the demands by major customers in the automotive, avionics, and computer markets required the device suppliers to understand and eliminate those failure mechanisms that limited the application of PEMs.
The explosion we see today in electronic equipment (high performance, small size, light weight, high reliability and low cost) is the result of the steps taken by the microelectronic industry to assure meeting customer needs. Improvements occurred at wafer processing, package design, device assembly, molding process and materials. The result has been new package outlines and interconnect approaches and innovated printed circuit board assembly techniques to accept these huge numbers of solder connections. The world we know could never have been possible without plastic packaging.
The problem is that revisions to MIL-HDBK-217 have not corrected the major disparity between cavity and solid encapsulated packages. Revision F, December 1990 makes it clear that they can not be equal. This document assumes reliability improvement can be assured by additional testing, with some of those tests only being relevant to cavity packages where the particular failure mechanism exists. But HDBK-217 itself has been shown and most agree that it has NO validity in predicting field life of electronic systems.
PEM reliability assurance can be arrived at by several paths. Best commercial suppliers have an extensive periodic accelerated test database available that clearly demonstrates the elimination of earlier failure mechanisms. A survey I did of military equipment developers which compared their experiences of MIL cavity vs. PEM solid packages clearly shows the advantages of commercial PEMs. These results were reported initially in the February 1997 issue of M&AE and an in depth version, "Why Use PEMs in Military Equipment: Users` Response", published in Microelectronic Reliability, Vol. 38, No, 3, 1998.
Concerning Greg`s theme of his Viewpoint. Several years ago I was a member of the DESC QML audit (validation) team that went to TI`s high volume (nearly 2 billion in 1996) Kuala Lumpur, Malaysia commercial microcircuit assembly and plastic packaging facility for possible qualification. When the team left, the plant was QML qualified. The QML and the 0 degrees Celsius to 650 C commercial part leaving the plant were identical in die, mold component, SPC and test, ABSOLUTY NO DIFFERENCE! The QML microcircuits went to Taiwan where they are burned-in and screened then marked. They then went to Midland, Texas for distribution. A significant cost adder is placed on these units. I am sure this acceptance of best commercial practices is the case for the other nine QML PEM qualified suppliers.
If you look at the QML listings Greg suggests at www.dscc.dla.mil/Offices/Sourcing and Qualification, you will find the up to date QML PEM qualified suppliers and devices. An observation is that Standard Microcircuit Drawings (SMDs) are not developed by any of these qualified suppliers until an order is placed for a QML plastic product. It is not clear from this information if there are any QML PEMs that have been ordered and a SMD has been generated.
The question of radiation hardness is unique to terrestrial applications and nuclear warfare. In the past, special MIL Approved device processing facilities were required to meet these requirements, justification for QPL/QML. However, the Iridium satellite wireless communication system, both above and on earth, used COTS technology, including PEMs. It therefore appears that with careful design and vender/part selection, the costly infrastructure for space components may not be necessary.
Edward B. Hakim, consultant
The Center For Commercial Component Insertion
Spring Lake, N.J.