Solder Alloys and Metallurgy | Hacker News Reader

Solder Alloys and Metallurgy

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53 pointssohkamyung9 comments

As a side note, intermetallic compounds can happen in any process; soldering, brazing, welding, casting, etc. Most are bad because they are brittle and are often crack nucleation sites, however some compounds can have various strengthening effects, though not typically in solders.

Eutectic alloys are also notable because they often have the lowest melting point (unless there are multiple eutectic points). The liquid/solid together explanation was kind of an unclear explanation in my opinion. What the author meant is that a eutectic alloy does not have a range of temperatures where liquid and solid can coexist in a stable state (the general case for multicomponent materials), upon reaching the eutectic temperature, the solid immediately melts into liquid.

“Eutectic alloys are also notable because they often have the lowest melting point”. Isn’t the very definition of eutectic the mix proportion that has _the lowest_ melting point?

Not precisely. It’s the lowest _local_ melting point on the composition-temperature phase diagram. One material pairing can have multiple eutetic points.

Also, the fact that everything melts at the same temperature isn’t particularly important -it’s easy to add more heat. It’s really that everything freezes at the same temperature.

I once read that lead-free solder is not allowed in medical equipment since someone’s life depends on it’s reliability. Is that still true?

As for consumer stuff you probably don’t see that problem because nobody keeps it long enough anymore. Everything is throw-away.

It's not a matter of "not allowed", the medical devices directives were more concerned with reliability than specific solders. 10 years ago the state of lead-free solders raised concerns whether they should be used in anything but low risk medical electronics. So the following provision was used:

> To avoid conflicts between the RoHS Directive and the medical devices Directives, it is essential to ensure that adequate field data is available to validate the laboratory data from accelerated testing

So they keep testing and periodically reevaluate all exemptions from the lead-free directive. The plan was to go full lead-free in every medical device class right about now, or next year.

The history of metallurgy is also very interesting and much more varied than bronze to iron age :)

> The advantage of an eutectic alloy is that it’s far easier to get a good-looking joint with it.

Strange how shiny joints were highlighted as opposed to repeatability of automated manufacturing processes and generally lower melting points when compared to alternatives in the same class.

> The application of a conformal coating...can help limit the formation of tin whiskers.

A very misleading statement with more than a few caveats[1].

[1] https://nepp.nasa.gov/whisker/reference/tech_papers/2010-Pan...

The presentation you linked concludes that 2mil nominal conformal coating is indeed highly effective...

...when using Arathane 5750, in storage, and at ambient temperature. The devil is in the details.

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