Is 60 Sn / 40 Pb better than 63 Sn / 37 Pb or 62 Sn / 36 Pb / 2 Ag for space grade soldering? I tend to prefer 63/37 because it's closer to eutectic and doesn't have as much of a "mushy" stage when it cools, and because it tends to contract onto the connection and seal gas out rather nicely. I also don't seem to have had any problems with 63/37 solder joints cracking like some of what I've seen from other solder types (which I suspect may be RoHS compliant stuff, and maybe wave- soldered), but I'm not sure how being in a spacecraft environment would affect that. :)
The anti-wicking and looping/gluing treatment sounds well thought out, and I can see how it would make the components hold up a lot better under launch vibration. I knew about the uncoated PCB's and the conformal coating already, as well as the clean room assembly, but hadn't heard of that particular wiring treatment. Now I know. :D
On Jan 29, 2008, at 5:10 PM, Howard Long wrote:
I've been reading about the construction of a few microsats and I see "Space Grade Soldering" mentioned a few times. So I'm curious as to what that entails ?
This is a question where you'll ask three different engineers the same question and you'll likely get three different answers.
I can't comment on the Microsats specifically, but in my experience at ESA there are a whole bunch of things we have adhered to in the past.
Firstly, 60/40 is good. Anything RoHS is bad. This is predominently due to 'tin whiskers' that the lead in the solder significantly resists. This leads to early failure in devices as the tin whiskers grow and short out previously insulated conductors.
I am not sure of the specific flux they use in the solder.
I am aware that reflow is often suggested as a way of soldering that prevents cracking of components by uniformly heating the whole device. This isn't universal practice though. Again this is to reduce early failure of devices due to weaknesses from stressing the part during conventional soldering.
We also use special anti-wicking pliers when soldering interconnecting wires (PTFE) to PCBs: that stops the solder running down into the wire behind the PTFE insulation. Then a small loop is made and the wire is glued a few mm (length dependend on the wire gauge) from the solder pad. This procedure is to provide a uniformly strong bond without weaknesses when it goes to the shake tests.
Generally PCBs have no coatings at all prior to mounting the components: no silk screen or resist. This is to reduce the chances of outgassing from small bubbles that may have formed during fabrication.
After the components have been mounted and the poulated PCB is tested, conformal coating is used, which is basically a guy with a tin of goo and a brush. The goo is generously brushed over the all the components and the entire pcb. Once hardened, it is designed to prevent outgassing and provide some resistance to vibration shock.
All this is done in a clean room, wearing disposable gloves and protective clothing to try to stop any foreign object damage including grease and other body gunge attaching itself to the parts.
73, Howard G6LVB
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