We're in a no-clean, water-based, lead-free world, and that sucks. Electronics
manufacturing is about getting product out the door with as little effort as
possible, so they leave the flux on the board rather than clean it off. This
"no-clean" flux isn't just a pretty pathetic flux, it's also a right pain to
clean off the board afterwards, so it should really be called "can't-clean".
All well and good if your standards are low, but if you care about the quality
of what you make, you'll want real rosin flux.
Decent rosin paste flux is getting harder to find every time I go looking, and
with the secrecy around formulations who knows what's in that stuff. The
solution is to take control of the process and make our own. Before I start
though here's a quick primer on what flux is, and what the various
formulations for electronics work are. You have to really understand your
needs to formulate a flux that's appropriate.
Flux is used to exclude air from the join while you're heating it, in order to
prevent the corrosion of metals that happens when you heat them up. It also
contains varying amounts and strengths of acids, which bind to the oxides
present on the surface of the metal and remove them from the join, so that
when you stick solder in it's able to wet the parent metal properly. You want
it in either a liquid form or a paste form. Liquid flux allows you to paint it
on with a brush (thick liquid) or dispense it with a pen (runny liquid) or
spraycan (very runny liquid). For board assembly I like to dispense with a
syringe. This allows me to use very thick paste flux that stays in place on
the board. I like it to be a little tacky, so when I place surface mount
components, they don't roll around. The solvents used between liquid and paste
fluxes are different, but the key ingredient is still the rosin.
The various types of flux are:
Rosin Flux. This is the grand-daddy of fluxes. It's made from tree sap,
that's had the volatiles (turpentine) boiled off. The resulting rosin is an
amber crystaline solid, that breaks fairly easily. It's mildly acidic, due
to the presence of abietic acid. Being a solid it needs some sort of solvent
to make it useable to coat the joint with. It's mild, and perfect for board
assembly where you're using good quality, reasonably fresh components. When
you solder the join, most of the solvent boils off, leaving the glassy rosin
behind. This is reasonably easy to remove using more solvent.
RA flux. Rosin Activated. This is rosin with various acids added to attack
the oxides on your circuit board and component leads. It's a bit on the
nasty side for most board assembly work, as we're usually working with
boards and components that have been processed well (HASL or ENIG coating
etc) and stored well to exclude oxides. This is used for those awful
terminals that have been out in the air or on the boat for decades. It is
imperative that you clean it off after soldering, as the acids will continue
eating the metal and after a few years it'll stop working. The residue
that's left after soldering is dark in colour, containing the oxides that
have bound with the acid in the flux. It's harder to remove than the oxides
from rosin flux, but still doable with a solvent and a bit of scrubbing.
RMA flux. Rosin Mildly Activated. Reduce the amount of acid in RA flux and
you get RMA flux. It's a compromise. Good as a substitute for rosin when
things aren't wetting well (old components, old boards). You need to clean
it off afterwards, as it'll eventually damage the equipment otherwise.
Cleaning wise, it's a mid-step between rosin and RA.
No-clean flux. This is a formulation made from random industrial chemicals,
gelling agents, what-have you, that's supposed to mimic real flux then dry
clear so the person contracting you for assembly work doesn't notice that
you haven't cleaned the boards. It's motive is to skip a step, not to
improve the product. Much worse though, it's often incredibly hard to clean
off. The manufacturers want something that doesn't get tacky or runny in
use, so they make it so that it dries hard. This equals very difficult ro
remove. Don't use it. Your boards deserve better.
Here's a recipe for real rosin paste flux just like your grandma made when you
were a kid, back when she worked as an industrial chemist. Nothing but the
finest free-range organic ingredients, and made with love. Cook some up and
gift it to that special engineer in your life, or just make a batch for your
Seriously, this doesn't just resemble commercial rosin paste flux. It's the
actual thing. It's not activated, which means that there are no acids added
besides the naturally occurring abietic acid in the rosin. You can add
additional acids if that suits your process, but I'm not particularly
interested in that. Ninety percent of the time I'm happy with straight rosin,
so I can use the commercial stuff for that occasional stubborn join.
This recipe makes enough to last a typical engineer a good few months. The
ratio of rosin to vaseline yields a flux that's just right for syringe
dispensing, with good tack.
- 15g of gum rosin, in chunks.
- 10g vaseline.
- Isopropyl alcohol (varies from 0g to 5g).
I just buy rosin online. It's got a huge pile of uses, both industrially and
for consumers. It's usually used for making things stick, so as a powder that
you can rub on your hands when doing rock climbing, playing baseball,
what-have you. It's generally advertised as gum rosin, which is made from sap
collected from pine trees, but there's also wood rosin, which is made from
grinding up the tree roots after harvesting the pine. I believe they're the
same, but I've only ever bought gum rosin. Please be sure to only buy rosin
that's certified free of angry bees.
The vaseline comes from the supermarket. Bunnings sells isopropyl alcohol.
Combine the rosin and vaseline in a 48ml Kilner hexagonal preserve jar. Break
up the rosin if necessary to get it into the mouth of the jar.
Heat the mixture in the oven at 120˚C for about 20 minutes. The rosin will
melt and form a thick treacle layer under the vaseline, which will go
Pull it out and stir to combine layers with a paddle-pop stick.
Pop it back in the oven for another few minutes, to encourage any trapped air
bubbles to rise to the surface.
Take it out of the oven and let it cool for ten minutes or so before sucking
into syringes, if that's your preferred dispense method. I find if I leave it
too long it thickens up and is hard to draw into the syringe.
If you find it's too thick, you can add a few percent isopropyl alcohol.
Isopropyl is the solvent in liquid flux, and it's probably also the solvent
you use for cleaning up your boards after assembly. Go easy on the isopropyl
though. As well as thinning the flux quite well, it also boils off very
rapidly on the PCB when you solder, resulting in spitting when you apply the
iron if you overdo it. I find the best time to add the isoproply is as the
mixture is cooling. It's easy to stir in then. Don't do it too soon after
taking it out of the oven though, as the temperature initially is above the
boiling point of the isopropyl, so you'll only waste it.
You can reheat the flux multiple times to get it just how you like. Warm it up
to 50˚C or so, so it's runny. Add a little isopropyl. Let it cool. If you
overdo the isopropyl then just warm it up over 80˚C, and the isopropyl will
start to boil off.
The stuff in the jar, or in the syringe, is just a lot of rosin in suspension
in the vaseline. When you heat that mixture, two things happen. Firstly the
rosin melts. It flows over the joint and does the good things that the rosin
does. The vaseline has a boiling point of around 300˚C, so a good amount of it
will boil off. This is mostly what you get in the little wisps of smoke that
come off the iron, plus of course some rosin that's caught up. It's not good
to inhale, so use some fume extraction. It doesn't have to be fancy. A simple
muffin fan sitting near to the work on the bench is usually plenty. If you
must use RMA or RA flux rather than straight rosin, definitely use fume
extraction because ingesting the acids is not at all good for your lungs.