“Traditional space” programs are typified by larger spacecraft using a preponderance of high-quality, space-qualified components with expensive/irreplaceable mission payloads and the “failure is not an option” design mantra. In contrast small-satellite missions have more limited budgets, shorter mission lifetimes, are usually limited to operating in the more benign Low-Earth Orbit (LEO) environment, and often have lower mission criticality. A unique challenge to small-sat. system designers has been the largely bimodal distribution of available product grades. On one side, a huge universe of the consumer-off-the-shelf (COTS/COTS+) products that are relatively inexpensive, light, and with consumer levels of reliability and quality. On the other side, full space-grade QMLV/RHA and mil-grade QMLQ parts that use specialized materials, undergo extensive additional testing, operate over the wider military temp. range, utilize hermetically-sealed ceramic packaging, and, most importantly, come from single, controlled baseline (SCB) lots from QML (Qualified Manufacturers List) facilities certified and qualified by the Defense Supply Center for the production of devices to be used in military or space applications. There are no intermediate grades in between the two extremes, and largely, the small-sat. engineering community has focused on using predominantly COTS/COTS+ devices with limited success [1]. Given the recent growth in commercial, small-satellite missions requiring higher reliability and expectations for success, there is a need for higher performance and quality grade electronics somewhere between the two extremes.
Manufacturers are starting to release “intermediate-grades” to fill the gap in the grade “chasm” between COTS/COTS+ and space-grade. Most are encapsulated in plastic with extended operating temperature ranges and material changes. From some vendors, these intermediate grades are from a SCB lot flow and may also include additional quality and reliability screens. One of the most valuable features of some of the intermediate-grades is that they are from SCB lots guaranteeing that all parts are from the same lot and packaged in the same QML facility, minimizing manufacturing variation; thus what is tested and screened is what is actually flown, significantly reducing the risk of outliers from manufacturing variation. The extra quality and reliability provided by these intermediate grades come at a price point that is significantly lower than full space-grade but that is higher than consumer electronics. Additional test and quality screens added above the normal COTS/COTS+ processes also add cost, so pricing will naturally be higher than consumer parts. In the table an example of the different component grades offered by a single supplier (Texas Instruments) is shown. Other vendors provide similar intermediate grades under different names and each with slightly different features. Progressing from left to right on this table, product quality and reliability increase, along with the cost (from COTS to full Space-grade).
Beyond the need for extensive and specially designed up-screening programs, two major factors that increase the risk of using COTS/COTS+ in space are the growing use of copper bond wires and the pure tin often used for surface finishes. The reliability of Cu-wire has been optimized for commercial environments but poses some potential issues in space; bond integrity (Cu bond to Al requires tighter process controls), sporadic PPM-level corrosion due to plastic mold compound interaction, and bond wire neck breaks during temperature cycling can lead to higher fail rates [2, 3]. Most intermediate-grades do NOT use Cu-wire to avoid these risks. Even COTS/COTS+ that currently use aluminum may be converted to Cu-wire at any time without notification. The use of pure tin surface finishes is standard in many COTS flows that can lead to the formation of tin whiskers – conductive fibers that grow from finish surfaces and confirmed to have caused short-circuits when the whiskers bridged circuit elements [4, 5]. For this reason, intermediate-grades do NOT use any pure tin finishes. Most intermediate-grades will typically offer SOME of the test and qualifications of space-grade product, improving reliability and quality above COTS/COTS+ but enabling a more reasonable price point.
The space system engineer will ultimately have to balance the design and mission requirements with the overall budget and the availability and cost/performance of COTS/COTS+, intermediate, and space-grades grade components to ensure ultimate mission success. It is always better to have a broad selection of components to choose from, and the intermediate grades offer more choices.
[1] M. Swartwout, “CubeSats and Mission Success: 2017 Update (with a closer look at the effect of process management on outcome),” NASA Electronics Parts and Packaging Program Electronics Technology Workshop, June 2017. https://nepp.nasa.gov/workshops/etw2017/talks/28-JUN-WED/0900%20-%20swartwout%20etw%202017.pdf
[2] E. Rutkowski and M. J. Sampson, “Body of Knowledge (BOK) for Copper Wire Bonds”, NASA Electronics Parts and Packaging Program Electronics Technology Workshop, June 2015. https://nepp.nasa.gov/files/26611/2015-370-Rutkowski-Final-Paper-NEPPweb-Copper-Wire-Bonds-TN26444.pdf
[3] K. Rispoli, A. Lecomte, and A. DerMarderosian, “Assessment of Copper Bond Wire for Use in Long Term Military Applications”, Boston RS Meeting, May 2017. http://ewh.ieee.org/r1/boston/rl/files/boston_rs_meeting_may17.pdf
[4] NASA Tin Whisker (and Other Metal Whisker) Homepage. https://nepp.nasa.gov/whisker/background/
[5] J. A. Brusse, G. J. Ewell, and J. P. Siplon, “Tin Whiskers: Attributes and Mitigation”, 22nd Capacitor and Resistor Technology Symposium, 25-29 March 2002.
