I started to answer your questions but the predicates and assumptions
surrounding them are candidly a little wrong, [if all flux manufacturers
make untrue claims about their no clean products then >90% of the world's
electronics industry is in trouble. Clearly that is not the case. Equally
candidly I don't have the time to write a treatise on flux chemistry. 
So here are some facts about fluxes which you can apply to your assumptions
and questions. Hopefully this response will enhance your understanding.

Fluxes have 3 functions
De-oxidise surfaces to be soldered
Keep them deoxidized at soldering temperatures
Help the solder to spread and wet.

So the flux has some mechanical functions as well as the simple chemical
stuff you are addressing in your questions/statements

After soldering to be considered safe flux residues have to be
Non conductive
Non corrosive
Non hydroscopic

There are various ways to do this, simple chemical means, like breaking down
in to inert volatiles and disappearing up the chimney. Or mechanical,
encapsulating active residues in an insulating mass. Or only being active
when heated and so on. How significant the residues are depends very much on
the application - plumbing fluxes don't need to be washed off pipes but will
kill an electronics assembly.

Alternatively the user can elect to clean the residues away.


So if you apply the above to your questions and statements I think you will
get some answers. The rest you will have to read on line or in a text book.



Regards

Mike 



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Thursday, April 26, 2012 3:44 PM
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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