
Solder Alloys and Metallurgy - sohkamyung
https://hackaday.com/2019/05/30/the-fascinating-world-of-solder-alloys-and-metallurgy/
======
bocklund
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.

~~~
mmf
“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?

~~~
Armisael16
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.

------
Epopeehief54
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.

~~~
close04
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.

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

------
metaphor
> _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...](https://nepp.nasa.gov/whisker/reference/tech_papers/2010-Panashchenko-
IPC-Tin-Whisker.pdf)

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

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

