Reason for flux removal from PCB

Thread Starter

AlbertHall

Joined Jun 4, 2014
12,231
I have often seen that we should remove flux from a PCB after soldering but I have not seen any reason given.
It occurred to me that painting the finished DIY board with liquid rosin flux might work well to protect the bare copper from corrosion yet leave it easy to solder/desolder if necessary.

Is this a bad idea?
 

djsfantasi

Joined Apr 11, 2010
8,903
It occurred to me that painting the finished DIY board with liquid rosin flux might work well to protect the bare copper from corrosion yet leave it easy to solder/desolder if necessary.
It is a bad idea. Rosin flux is corrosive in its liquid state. When it becomes warm, due to ambient heat or heat from the components on the board, it will liquify slightly. The amount of liquigicsrion depends on the heat. Thus, it will slowly corrode traces, component leads and possibly components. So it will not protect the bare copped from corrosion. Quite the opposite. You can use lacquer or water-based paint to protect the copper. That can easily be removed to solder/desolder as needed.
 

Ya’akov

Joined Jan 27, 2019
7,473
No-clean tacky flux is good stuff and creates a really good environment for drag soldering and similar techniques. The layer of flux is very thick and the soldering operation—if not in a vacuum—is very well insulated from the air which would promote oxidation while trying to solder.

The “no-clean” aspect has value to the extent the manufacturer is asserting that there is nothing in there that will attack the PCB traces or substrate if left on the board. Not cleaning the mess of a TF soldered part would have other nasty side effects even if not those. One obvious thing is the attraction of dust and dirt.

Another concern is maintenance and sticking probes into the stuff. And of course there are going to be components that find the little conductivity of the TF enough of an excuse to start acting up in random and otherwise unaccountable ways.

So cleaning off the flux is just sensible, and as pointed out even aesthetic. But if you don’t manage to get every trace, the no-clean aspect is there to prevent those trace amounts from actively damaging your board.

IPA, and/or ultrasonic cleaning can get rid of it all, though. The latter being far more easier to make work but the former doable.
 

MrSoftware

Joined Oct 29, 2013
2,118
What would you recommend to protect the copper while still making rework easy?
It's not clear what you're asking. On a typical professionally manufactured PCB the copper is covered by the solder resist, exposed pads will be covered by solder (HASL finishing) or the plating of your choice such as gold. When it's time to rework, flood it with flux, rework then clean when done. If the board will be going into a very bad environment, such as a humid or salty marine environment, use a conformal coating or pot it once the board assembly is done. Rework is rare once a board is in service. If frequent repairs are needed, it's likely a design change is also needed.

I have often seen that we should remove flux from a PCB after soldering but I have not seen any reason given.
It occurred to me that painting the finished DIY board with liquid rosin flux might work well to protect the bare copper from corrosion yet leave it easy to solder/desolder if necessary.

Is this a bad idea?
Aside from the potential electrical issues such as parasitic leakage, especially on very high impedance circuits; flux can make a sticky mess that attracts dirt and other contaminates. Also if there's anything mechanical on the board, such as relays or switches, flux may potentially migrate into them causing problems, especially if the electronics get warm, either due to power usage or just being in a warm environment. Flux would surely not be helpful to decals that may be on the board, and could potentially be detrimental to other things that the board might come into contact with, such as an enclosure, or the finish on an enclosure. Many reasons to clean the flux.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
14,699
Most flux residue will absorb moisture and a lot of flux residue will collect dust. And, very interestingly, with some sort of soft drink spilled on a powered circuit board, can form some sort of explosive chemical mix.
I discovered that while servicing a "Superscope" brand clock radio MANY years back. Somehow the drink reacted with the flux and solder, and since many of the solder joints had to be redone because of the corrosion, I got a "POP" and a very small burst of flame upon heating many joints. So the whole board got an extended hot water wash and then days of drying. But eventually it was repaired.
 

Thread Starter

AlbertHall

Joined Jun 4, 2014
12,231
It's not clear what you're asking.
I mill my own PCBs which means that bare copper is left on the board. These are generally protype boards which, if they work well, may be put into service. I would like to be able to coat the boards with something to protect the board but, as they are prototypes, rework may well be needed and so the coating needs to allow this easily (so not conformal coating).

What product can I use for this, please?
 

nsaspook

Joined Aug 27, 2009
11,277
Big Clive offers his observations on flux:
It's unlikely that board was being used in a industrial environment for 26 years. If it's sitting in a nice clean, room temp space, OK, it's not critical but on that board in the video the copper still looks bright (maybe there's some sort of Conformal coating on the board) so IMO it's not a realistic example of what happens in the field with flux and exposed traces.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
14,699
It's unlikely that board was being used in a industrial environment for 26 years. If it's sitting in a nice clean, room temp space, OK, it's not critical but on that board in the video the copper still looks bright (maybe there's some sort of Conformal coating on the board) so IMO it's not a realistic example of what happens in the field with flux and exposed traces.
I see that there is a lot on yoo toob that does not reflect reality, especially in technical areas.
 

bassbindevil

Joined Jan 23, 2014
731
At one place we used to just coat boards with clear varnish or whatever was in the paint cabinet. It seemed to work OK; at least nothing ever failed because of it. I seem to remember that lacquer melted the printing off some components, so don't use that. Real conformal coating has extra magic stuff like UV indicators to help with QA, and it's maybe easier to remove for rework than varnish. If you leave the rosin core flux on because of possible future rework, it seems to deteriorate with time and doesn't work as well as fresh flux.
 

Ya’akov

Joined Jan 27, 2019
7,473
I see that there is a lot on yoo toob that does not reflect reality, especially in technical areas.
As it turns out, this particular YouTuber—Big Clive—is quite a good source of information. He has a very substantial practical experience and doesn’t claim to know everything. He happily corrects errors, which are uncommon even in his large body of work.

Clive is one of the Good Things™ about YouTube.
 
Top