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Crimping versus soldering, in avionics

All flux has to be aggressive or it doesn’t do anything.

That statement is correct as put, but for decades there has been a range of fluxes, and most of the ones in common use today are minimally aggressive and can be left in place permanently. That was probably not the case 30-40 years ago.

the end of the solder forms a weak point for flexing

Obviously, the wiring has to be strain relieved with a connector shell

If you don’t do that, it will eventually break in any vibration environment, although crimping will last a lot longer.

I have seen a lot of avionics work which wasn’t strain relieved. And a lot more uses a simple snap-on shell which doesn’t hold the wires in place.

In automotive ECUs we are going away from soldering and crimping where possible towards Compliant Pin connectors

Sure… this is not GA though

Also compliant pin will still work loose eventually if you shake it hard enough and the connector is not well secured altogether.

Shoreham EGKA, United Kingdom

Speaking of stranded wire breaking points at the boundary of wicked-up solder, couldn’t this be managed by using solder with high ductility and low surface tension?

LKBU (near Prague), Czech Republic

Neil wrote:

Compliant Pin connectors

How can a unit using this be repaired if the cable from the pin to wherever it goes breaks? Is is possible to re-do them without damaging the PCB?

Biggin Hill

The sort of scenario where I have seen compliant pin tech is this

where the whole (rigid) connector is forced into the PCB holes using a press. Here is more

and it is blindingly obvious that it does rely on the connector body being pretty rigid. That 9-pin single row header would work loose if you yanked it enough times.

I don’t know if anyone uses this technology for attaching flexible wires to anything. I reckon it would not work.

Speaking of stranded wire breaking points at the boundary of wicked-up solder, couldn’t this be managed by using solder with high ductility and low surface tension?

Maybe, but are there solders which really are that soft? One does need strength too…

Shoreham EGKA, United Kingdom

Peter wrote:

Maybe, but are there solders which really are that soft? One does need strength too…

One family of solders that comes to mind is In-Pb, though it is said to have a low corrosion resistance. Another likely candidate could be >90% Sn + some of (Cu, Ag, In, Sb, Bi). Don’t quote me on that, though – it’s merely my scientific intuition.

LKBU (near Prague), Czech Republic

Hmmm; interesting. I have not seen these.

Lead based solders are nice and soft but are banned in the EU under ROHS although you can freely buy them and “everybody” still uses them for manual soldering, because

  • 60/40 is 1/4 of the cost of a really usable non-Pb solder*
  • you need a much less aggressive flux
  • 99.987654% of customers will not be checking your soldering with one of those handheld x-ray ROHS compliance testers
  • there are/were exemptions to ROHS e.g. military, avionics, “control and monitoring”, “internet switching”, which really p1ss off those wanting to make €€€ out of this compliance stuff

* we found the outrageously pricey Almit SR-37 (SAC305 metal but the flux is “special”) to be the only one which actually works and god knows what nasty flux it uses… we have been lead-free at work for years but that’s because of arm twisting by big customers to render the exemptions worthless

Shoreham EGKA, United Kingdom

Yes, SAC305 looks about right in terms of composition – it’s 96.5% Sn + 3% Ag + 0.5% Cu. I wonder whether one can use it with a flux that leaves no nasty residue. I can think of fluxes that would be hazardous to use but totally harmless and non-corrosive afterwards (e.g. hydrazine-based).

LKBU (near Prague), Czech Republic

We tested many SAC305 (and other) solders for hand soldering and none got anywhere near the SR-37 for usability. Some were OK if used at ~ 450C which is ridiculous. Lead-free soldering is already very hot.

Also avionics wire is not always easy to solder. The hi-temp insulation degrades the metal – threads here and here. I have found solderability issues with Type 55 wire which I bought personally from an “aviation” source a few years earlier! And when making connections to existing wiring you might have to solder 30 year old wire… in that respect, curiously, I found the blue Socata wire is much better than the standard white stuff used by everybody else. This may be a good case for crimping, unless the wire is corroded…

Hydrazine would be interesting. There may still be some left in here

Shoreham EGKA, United Kingdom

I think the solderability of avionics wire is largely to do with how damned rapidly it wicks heat away (especially the bigger gauge stuff). The small gauge stuff is easy enough to solder.

Andreas IOM

Hydrazine (N2H4) is a very strong reducing agent, which would restore metal oxides back into metal. It is a liquid about as volatile as water, and even if any liquid hydrazine remains at the joint, it is soon oxidized into nitrogen and water. Unfortunately, it is toxic, so one would need gloves and either a fume hood or a mask to work with it.

As far as I know, there are also hydrazine-based solid fluxes. They contain hydrazine salts, which decompose when heated, releasing hydrazine. These are more corrosive than rosin-based flux, but a lot less corrosive than classical active fluxes like NH4Cl or ZnCl2.

LKBU (near Prague), Czech Republic
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