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What are the reasons why (some) space-grade components are shaped like they are, with their characteristic long flat leads, golden finish, etc.?

Image: DP83561-SP from TI DP83561-SP

Are the assembly houses supposed to cut the leads to a correct length? What is so different in space that it forces the designers to have to have specific lead lengths?

What is the golden finish made of? What is its purpose?

Harnex
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  • _”What is the golden finish made of?” Gold. I talked to an EE who designs satellites. Putting an extra 1 kg in GEO costs about 100,000 €. Gold is about 40,000 €/kg. You do the math. – winny Jan 18 '22 at 13:57
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    Does this answer your question? https://electronics.stackexchange.com/questions/442549/why-do-radiation-hardened-ic-packages-often-have-long-leads?rq=1 – Adam Q Jan 18 '22 at 14:06
  • @AdamQ I don't know how this thread didn't show up during my initial research, thank you. It still leaves me wondering about the gold(en) finish, and thinking about the price of gold doesn't really answer my question. I'd rather send 0 kg of gold to GEO than 1 kg. – Harnex Jan 18 '22 at 14:18
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    @Harnex He's saying it is so expensive to launch that the cost of gold doesn't matter. Do you not know why gold is used everywhere in electronics, even on Earth? Look it up. In space everything already costs more so if anything goes wrong it costs even more so they use more gold. If your smartphone dies because you did not use gold you are lose one smartphone. If a part on your spacecraft dies you lose one spacecraft, possibly lost some astronauts, wasted one launch rocket, and might have lost your window to *ever* do the mission again. – DKNguyen Jan 18 '22 at 14:25
  • @DKNguyen well first of all it is unknown if it's a simple finish on the ceramic package or a thick pad connected to the substrate (I'm not talking about the leads' gold finish). Then it could be because it's non-corrosive, has higher emissivity than ceramic, not bad for proton shielding, a good thermal conductor for an eventual heat pipe attachment, but I'm looking for someone who actually knows the exact reason - I know gold is pretty good, but it's not the best at everything. – Harnex Jan 18 '22 at 14:45
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    @Harnex You're forgetting more mundane things like solderability or even storage prior to soldering. This came up *immediately* in my search and refutes your proton shielding: https://radiositysolutions.com/all-that-glitters-is-not-gold/ Reading that, the reasons really do sound more mundane than what you seem to have in mind. – DKNguyen Jan 18 '22 at 14:57
  • @DKNguyen Yes I'm familiar with gold finish on leads, but not on the ceramic packages, especially on top of the component. Yes about the proton shielding, I was actually thinking of Xray shielding when I wrote that. There aren't a lot of things that are effective at blocking protons haha. – Harnex Jan 18 '22 at 14:58
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    @Harnex That link says corrosion. Sounds like they use gold for corrosion just because they can. No need for cheaper alternatives. CYA. – DKNguyen Jan 18 '22 at 14:59
  • @Harnex DK has done a yeoman's job of listing reasonable things, general and specific. The corrosion facet is very likely. Partly, because here on Earth. But partly because of the space environment where high energy particles ionize molecules, which are charged and very much affected by the local environment to move around (both away from and towards the craft in space.) Some of these fragments will attach themselves to random locations and that includes metal surfaces. They will accumulate where there is more bonding, than less. Have you studied/found any papers from the ESA or NASA on this? – jonk Jan 18 '22 at 16:10
  • It seems my research skills are quite lacking, seeing most people here came up with a search engine answer at a moment's notice. Have to work on that. @jonk it does seem, from the link that DK provided, that the lid would not be pure gold but a certain alloy 42 or Kovar, with an outside gold finish and an inside nickel finish. With this in mind, it does seem that this done solely to reduce corrosion effects. I haven't studied anything related to charge deposition on exposed metals in space. – Harnex Jan 18 '22 at 16:27
  • @Harnex It's a serious problem, from what I hear. I worked with a physicist, more than decade ago, who talked about various problems he had to cope with when considering the environment in space for satellites (he worked on several different such projects.) The term he used with me was "brown crud." But the way he talked about it, it was at the top of his list for eventual time-in-space-related mission failures. I didn't bring it up, he did. And he pointed out that it was the central problem he faced when considering how to keep things working longer than 20-30 yrs. – jonk Jan 18 '22 at 16:36
  • @Harnex Look up what happens to cadmium, normally known for great corrosion resistance, in a hard vacuum. If you run into the same link I did, it also says sometimes good old nickel plating is used for contacts and passivated stainless steel hardware for screws. I found that surprising. – DKNguyen Jan 18 '22 at 20:41

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