Solder flux residues

Hi everybody,

When using a no-clean reflow soldering process and the residues are still tacky and extend underneath almost any component on the PCB assembly is this considered a potential threat related to ionic contamination, pad, track and terminal oxidation and corrosion, electromigration etc?

In IPC-A-610 there is the following reference (I am talking about class 2):

Defect – Class 2,3
Wet, tacky or excessive flux residues that may spread onto other surfaces.

But right before this reference from IPC-A-610 the following applies:

Acceptable – Class 1,2,3
• Flux residue on, around or bridging between noncommon lands, component
leads and conductors.
• Flux residue does not inhibit visual inspection.
• Flux residue does not inhibit access to test points of the assembly.

And the picture right next to this criterion shows a SOIC with the flux - which to me seems to be tacky - flooding all the area between the IC pins.


In my case the supplied PCBAs have tacky residues that already extend underneath the SMD components nad between terminals.

In this case shall this be considered a defect (according to IPC 1st criterion) or further testing should indicate if the residues could do any harm in the long term?

In general what happens with the no-clean flux during reflow?
It gets more active due to temperature?
And when it cools down and it is not completely consumed (there are still non solid residues) what is its activity level?

Thank you for your time!

There is no such thing as a no clean flux.... we have discussed this in depth do a search of the forum.
Clean all flux residue from your PCB's.

Thank you marce for the answer.
Unfortunately the contract with our PCBA supplier does not include cleaning step in the process.
We are talking about 20K to 30K PCBAs per week used just on the manufacturing site I work.
So it is a decision that affects cost and is taken by the management...
So the question still remains.

Unfortunately the contract with our PCBA supplier does not include cleaning step in the process.

Click to expand...

Should be rephrased as "your company has a special contract with the assembly house that industry standard cleaning requirements are excluded". If so, there's little help. Otherwise claim industry standard.

Some flux residues can be cleaned and some cannot. Flux is basically activated rosin and a part of the flux melts away (visibly clear and slightly tacky) and another part is left as darker stain (contains metal oxide residue that was originally living on the metal surface). This is difficult to dissolve in solvent but can be removed with scrubbing.

Neither is corrosive (with respect to copper) in the short time frame but both must be removed if you plan to use some type of conformal coating. Regarding compliance with standards, a lawyer can tell better.

These days aqueous cleaning is used, last high volume placed did around the same number of boards, after an indecent with no clean flux, high humidity and a board failure the boss went out and got all the cleaning kit...
For general throw away products in a domestic environment there is probably no problem... but if you look around the general consensus is clean the boards.
And the residue left can be corrosive.

OK, I can understand that we all do not like residues on the PCBAs but if you are following standards like IPC-A-610
then you have to accept the fact that residues on a PCBA soldered with no-clean process for all three categories can be acceptable...

It must be called no-clean for a reason, I guess.

What I would like to know is under which conditions the no-clean residues are truly no-clean.

I mean what if you just leave wet/tacky flux on the PCB because the reflow temperature profile is not the right one and you get residues that have not reached a possibly critical temperature that is needed to disintegrate the flux to non corrosive residues.

Look up "there is no such thing as a no clean flux"
I do mostly Class 3 level designs and would never recommend not cleaning the boards, they have to anyway as conformal coating is the norm, and even if they are not going to be conformally coated they are cleaned the risk for these types of deigns is to great as is the expected life span. Also they are mostly tin lead soldered so the fluxes for class 3 tend to be rosin based.
At the end of the day everywhere I have worked has cleaned the boards and most customers I work for now clean the boards or use assembly houses that have an aqueous cleaning set-up at the end of the SMD lines.
If the reflow profile is not correct then you will have more problems than flux residue probably, at the end of the day with todays assemblies you have to be spot on with everything.

I agree marce to everything you write.
I had the chance to escalate this issue and I expect more attention at a higher level from my company even though I don't expect
to add a cleaning step in the assembly process.