As a former flux formulator, we would never expect to rely on heat to deactivate (neuter) any flux activators. Reduce risk, sure. Eliminate, no.   It was always part of good marketing and sales tool kit however, because it sounds plausible (& desirable). 

This kind of thinking related to thermal deactivation is not uncommon. Good example is the old Battelle Institute glutamic acid hydrochloride & urea concept. Heat releases the HCl for activity and the later & higher peak heat releases the urea activity to neutralize the free HCL.  Was touted as ~a No-clean water soluble flux at one time. Actually worked rather well, but not enough to always count on the residues being harmless. 

Rich Kraszewski/PLEXUS

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Thursday, April 26, 2012 9:45 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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