Durability, Derating, and Circularity

Article By : Michael Kirschner

To extend a product's life and minimize waste, product modularity, design for repairability, and design for reuse are important.

Last month, we began our evaluation of the Circular Electronics Partnership’s (CEP) roadmap. To me, it appears to be focused on recyclability and design for recyclability. Product modularity, design for repairability, and design for reuse are equally important and relate strongly to design for recyclability.

As a reliability engineer (admittedly, an obsolete one), I have an understanding of why and how electronic components and systems fail. They will not work forever and, while “planned obsolescence” rarely is actually planned, the useful life of an electronic product can be — and usually is — designed to exceed its expected use life, if even just incrementally. Designing for a product’s life, extended by the demand for it to have a second, and maybe even a third, life by being modular, reparable, and reusable, can dramatically change the technology, component selection, and verification processes necessary.

The difference between enabling a longer product lifetime and a shorter one is encapsulated in the myriad decisions engineers make. For example,

  • Gold plating thickness on connector pins: the number of mating (insertion/extraction) cycles specified for signal connectors is directly related to the thickness of the gold plating on the contacts. Connectors and sockets can often be specified with different thicknesses of gold plating based on the expected number of cycles over the lifetime of the item: for instance, gold flash vs. 15 microinches vs. 50 microinches of gold, each adding more potential cycles. But each costs more, of course, as more gold is used and the inherent value of being able to produce a longer-lived product is higher. So product lifetime can, in this case, be directly related to material cost.
  • Semiconductor operating voltage vs. performance: reducing voltage reduces CMOS performance. However, it also extends the functional life of the device by reducing
    • Heat, which accelerates a wide variety of electrical and mechanical/chemical failure mechanisms, and
    • Current density in metal conductors; the failure rate due to electromigration is proportional to the square of current density.

  • Component derating: I recently disassembled a subwoofer (manufactured by a well-known audio electronics company that shall remain nameless) that failed after a few years of use. While my neighbors, wife, and children have been lauding the failure, my goal was to find where and how it had failed so I could try to repair it (using a third party to repair it would cost more than a new one…this represents another fundamental problem). After finding the schematics and rather inadequate repair manual online, I noted that 16V electrolytic capacitors had been used throughout 15V circuits! While they are rated for 105°C, I would never have approved this in a formal design review — particularly for a piece of high-power, high-temperature consumer electronics. Replacing those (with slightly larger 25V equivalents) unfortunately failed to solve the problem; there are other electrolytic caps used in similarly overstressed situations as well as power transistors without heatsinks that people have noted as problems with this particular model — so my quest continues.

Lesson: Derating guidelines (e.g., for electrolytic capacitors) must be defined and in place to enable achieving an expected product lifetime (at an acceptable mean time between failure rate, or MTBF). They become critical when that lifetime is driven by regulatory and market pressures to exceed what used to be the design life for the product. I will come back to this in a future column.


Kemet polymer electrolytic cap voltage derating curve. (Image source: Kemet Engineering Center)
  • Repairability: This is becoming a regulatory requirement in the European Union, with France leading the way. While my subwoofer is not one of the five product categories subject to the French requirement for manufacturers to define the reparability rating of their product, it would probably get a rather low score. The product effectively has three replaceable subassemblies: the speaker, the amplifier, and the enclosure. Replacement of the speaker doesn’t appear to be particularly difficult, but replacement of the amplifier/power supply subassembly, which was not designed for maintainability — it was designed to be readily assembled into the system, but not to be removed, repaired, and/or replaced — without significant damage is nearly impossible. Unfortunately, the cost of third-party repair would exceed the cost of simply buying a new unit (which is what I ended up doing).

These are not simply design choices a design engineer alone can make. A product strategy embodying an extended product life has broad implications for the company, its (forward and reverse) supply chains, required resources internally and externally, and its business model. A holistic approach to redefining the company to support circularity must include most, if not all, product-related organizations in the company along with executive and financial management. In other words, circular market requirements represent a fundamental shift in how a manufacturer must operate to remain viable.

This article was originally published on EE Times.

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