AEM, Inc. did have a small booth at APEC 2018 this year and I had the pleasure of meeting Jeff Montgomery, General Manager, AEM. High-Rel fuses and tin whiskers were our topics of discussion. Both of these topics are frequently after-thoughts by circuit designers.
AEM has a Hi-Reliability facility in beautiful San Diego, CA with product lines spanning high-rel fuses and fuse modules; high-rel chip beads; and a Tin/Lead (Sn/Pb) conversion process second to none.
I appreciate the need for good, Hi-Rel components in space since I was a circuit designer for some military products in my career. I also applied to NASA as a Mission Specialist on the Space Shuttle program as well, and although I was not accepted among the thousands of applicants, had I gone I would have wanted the comfort level of knowing that the likelihood of component failure was small on the Space Transportation System (STS) electronics systems.
On March 9, 2018, AEM’s surface-mount Solid Body fuses attained Qualified Product List (QPL) status, making AEM the only components in its class to be included in the Defense Supply Agency’s QPL. AEM’s devices have been in orbit for more than 35 years with zero failures.
Their Solid-Body fuse architecture fuses are top performers in a vacuum and spacecraft environment. Montgomery explained that Air Cavity fuses do not work as well in this harsh environment. Here’s why.
A Cavity Style (“wire-in-air”) fuse (Image courtesy of AEM)
AEM’s Solid-Body style fuse (Image courtesy of AEM)
Here is what happens in a fuse clearing waveform comparison of an AEM Solid-body fuse vs. a Cavity style fuse.
Fuse Clearing Waveform Comparison Under Vacuum Conditions and Rated Voltage (Image courtesy of AEM)
Below is a blown fuse under vacuum conditions and rated voltage.
The gold fusible element melts at the “neck-down” region and migrates in an arc-suppressive cover glass. The arc and plasma are contained within the element/substrate subassembly ports of the fuse. (Image courtesy of AEM)
AEM Solid-Body fuses have guaranteed min and max clearing (blow) times; safe interruption of overload currents at rated voltage; steady-state current de-rating based on the fuse case temperature which is independent of the vacuum condition; and greater mechanical vibration and shock resistance.
Hi-Rel fuse mechanical shock/vibration
Since solid-body fuses are designed and constructed with fusible elements which are completely surrounded by an arc-suppressing media (no movement permitted), they are less likely to experience a vibration induced failure. Given the relatively high levels of shock and vibration that are often associated with spacecraft launch, the increased mechanical ratings afforded by solid-body fuses are very desirable. See the Table below.
(Image courtesy of AEM)
Tin Whiskers in space
Why is the FDA interested in Tin Whiskers? Short answer is pacemakers. See this article on Tin Whiskers Problems, Causes, and Solutions.
There are well-documented Tin Whisker cases from NASA, Center for Advanced Life Cycle Engineering (CALCE), Food and Drug Administration (FDA) and others (Image courtesy of NASA)
Aside: Did you know that there is a Tin Whiskers Brewing Company run by engineers? But I digress.
AEM’s plating + fusion Sn/Pb conversion process involves Sn/Pb plating and subsequent fusion processing to ensure that resultant component terminations are a homogeneous mixture of Sn/Pb.
Hi-Rel Ferrite chip beads
When I was a circuit designer, I used to take a small piece of bus wire and slide it though a small hole in a round ferrite bead, then solder this small creation in series with the power supply voltage going to an IC so that high frequency noise would be attenuated with a neat, low-cost solution.
In space, this same type of solution requires traceability, extended service life, Sn/Pb terminations where the Pb is 5%+ to prevent Sn whisker growth, and sometimes Au terminations. That is an AEM High-rel Ferrite bead chip.
An AEM Hi-rel Ferrite bead chip (Image courtesy of AEM)
A far cry from a commercial Ferrite bead, AEM’s devices use 100 % Silver/lower frit loaded for external terminals; Tin/Lead (5% minimum lead content) or gold solder finish along with High-rel screening.
Even the impedance tolerance needs to be tighter than standard commercial products. AEM has 5%, 10%, 20% or even a 25% (commercial grade) tolerance available. This range of tolerances give the designer a broader selection for impedance matching to EMI interference.
Hi-rel and Space qualified needs are understandably necessary for both human safety and high dollar system investments in space. AEM has an excellent track record and I got a really good education and update on this type of circuit protection product, ferrite beads, as well as an update regarding tin whisker problems and a reliable solution for that occurrence.