[AR] Re: Braze joint design guidelines?
- From: Peter Fairbrother <peter@xxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Fri, 9 Aug 2019 03:09:19 +0100
On 08/08/2019 23:44, Henry Vanderbilt wrote:
Wild thought: If you have a way to finely powder your braze material,
you might try mixing some with your flux, then experiment a bit. It's
possible this may have the same amateur-enabling effect it has in
plumbing applications, making thoroughly wetting the joint surfaces a
lot less dependent on finicky details of tools, materials, and technique.
It's possible too you'll (re)discover why this isn't usually done...
Good luck!
Actually it is done a lot, especially in soft solder paste and some
(most?) types of ball grid soldering of electronic components. It is
done with higher temperature fillers too, but not as often.
Sometimes braze pastes with flux are used for furnace brazing in air,
but this is not all that common - flux tends to get trapped in the joint.
More often the part is fluxed separately and a preformed shape of filler
metal is placed so that after the flux melts the filler then melts and
flows into the gap, theoretically at least.
That doesn't always work well, with flux inclusions etc caused by eg too
much oxidation of the metal during brazing - generally flux is mostly
used up removing oxides formed during the hot brazing rather than
removing existing oxides from the surfaces, so taking too long causes
the flux to get gloopy as it fills up with oxides, and not get out of
the way when the filler metal starts flowing - or flux dripping off, or
bubbling causing the preforms to move before they melt and flow, or the
many problems caused by uneven melting rates.
More often furnace brazing is done in a vacuum or controlled atmosphere,
without flux, at a somewhat higher temperature (and sometimes for
longer) than when brazing in air. Vacuum brazing is more expensive, but
it is easier to control and get repeatable and reliable results.
Preformed shapes - washers, flats, cut and shaped wires etc - of filler
metal are made and placed appropriately. If the parts are clean enough,
on heating the filler preforms then melt and flow into the joint gaps
(without any flux to get in the way, and they just start flowing
whenever they happen to get hot enough so uneven heating of eg complex
assemblies is much less of a problem) and we'll all be merry and bright!
Sometimes brazing pastes rather than preforms are used for vacuum
brazing, but then the binder just lightly holds the filler particles
together and then evaporates and dries, it is not a flux, no flux is
used. In good practice paste is not applied inside the joint itself, but
to somewhere where it will flow into the joint gap when the dried paste
melts.
Peter Fairbrother
Other related posts:
