I'll not go into a full rebuttal.  However, I can tell you that one engineer here has driven himself crazy solving "circuit design issues" that were nothing more than flux crap (OA and NC) left on the board after doing multiple modifications on his prototype boards, both SMT and PTH.

I convinced him to wash the board... problems solved!

Ooooooo...

So, if I wash my car will that fix the rough idle issues?
If I wash the dog will he stop barking when he goes out to pee at 11 o'clock at night?

Oh, my mind works in such strange ways.

Phil

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Thursday, April 26, 2012 10:45 AM
To: [log in to unmask]
Subject: [TN] 4 Questions: No Clean vs flux Deactivation. Possible Hogwash ???

Hi,

4 Questions (Please respond by number and elaborate if possible.)

 I have been listening to this thread about no clean fluxes and "Possible"
problems with flux residues that have not been "De-Activated" through complete heating during a soldering process.

First, I understand that a flux is "Activated", ie becomes chemically active, by heat. An example is ammonium chloride that when heated releases HCl (hydrochloric acid) which is a strong agent for cleaning a surface for good solderability. Below the temperature where a flux becomes "Activated"
the flux in not active. IE: "All Cold Fluxes are De-Activated".

 Question 1:  Are fluxes "Active" when they are cold?  Is a RMA active when it is cold?

  If the above is true what is all the stuff about "Active Flux" remaining on a PCB surface? Hogwash?

 Further, I know from experience that reflow temperatures that are too high or too long "Burn Off" the flux and provide very poor surface finishes. If so this implies a significant portion of un-used flux will remain on the PCB after the reflow cycle. Flux will not be "Activated" once the PCB cools but it will remain active during the liquid solder phases.

Question 2:  Is it true that we want flux active throughout the entire liquid solder phase and does this imply there should be un-altered flux remaining on the PCB surface after the reflow period?  (Me Thinks Yes?)

 A good example is an RMA flux. From experience I know RMA fluxes have a very wide processing window, You can "Touch Up" a joint covered with RMA without additional flux because of RMA robustness. It simply leaves a "Messy" board finish.

 Also, it has been my understanding that leaving an RMA flux on a PCB was a "Cosmetic Issue" but I do find that hard to believe. There must be "Bad Stuff" in all that gunk left behind.  But wire solder and RMA flux has been used for decades and is considered "OK" to remain on a PCB, I think.

Question 3:  Are any flux chemistries acceptable to be left on a PCB and not cleaned and would such PCBs pass an ionic contamination test?

Question 4:  Do all the companies that make "No Clean" Flux lie when they call them "No-Clean"?

Thanks,
Bob K.


> I have an issue with one of the assembly houses we use.  They are
> using a No-clean flux that contains adipit and succinic acids (per the
> MSDS) in an alcohol base.
>
> Is it allowed to leave flux on the PWB that HAS NOT BEEN DEACTIVATED
> BY HIGH TEMPERATURES OF SOLDERING?
>
> I am not talking about the flux used in the solder paste or wave
> soldered flus.  I am talking about the flux used at rework and
> touch-up of the PWB.
>
>


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