Activation temperature is a handy term, we kind of understand what it means,
and use it often. But just talking activation temperature is limiting. 
What we are really talking about in this thread is the temperature range
over which the flux is effective in performing its various tasks. Some
fluxes can have a wide application window; others can be focused to give a
higher performance in a narrow zone and so on.
We also have to think of time. How long is it effective for example?  How
effective at its different tasks at different times?
In the case of BI/Sn we are looking at low peak temp reflow and quite
possibly a relatively long (for the peak) reflow time. The lower peak means
a lower energy input to power flux and joint formation. The tin content of
the solder is also low, making it effectively a more dilute solution. So it
it's likely going to do its job more slowly (Bi is a sleeping partner in
this alloy rather like Pb in Sn63c). It may also take longer to cook out the
flux to give acceptable SIR. Some fluxes may require a minimum temperature
to give good SIR and so on. (ref Karen's post).
So as well as moving the flux temp range down, we may well need to make it
so it can hang in a little longer. This is not to say other fluxes won't
work at all, just perhaps not as well. The importance or ranking of these
various factors will be supplier dependent according to how they formulated.


Regards


Mike 
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Friday, September 28, 2012 1:33 PM
To: [log in to unmask]
Subject: Re: [TN] Flux Activation Temps

Bob, 
The flux we use for SnPb and SAC is the same. 
However, the flux does not work well with SnBi. 
The solder looks good, but does not form a good bond to the Nickel (our
boards have NiAu finish).

We use a different flux that is, according to the supplier, formulated with
a lower activation temperature.
It does work better.

Guy


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Thursday, September 27, 2012 1:13 PM
To: [log in to unmask]
Subject: [TN] Flux Activation Temps

Hi,

 

I was looking though some paste datasheets which show a paste "Family"
available in different alloys. 

 

My Question:

 

Is a single flux formulation being used with these different alloys? It
seems so.

 

But the various melting point have a serious range. And I remember from my
college chemistry days that reaction rates double for every 10 C rise in
temp. (Is that still correct?) So I would expect fluxes to be relatively
"Inactive" al low leaded temps but then going super-fast at higher lead free
temps.

 

Can anyone clarify? Is it a real issue or hogwash-worthy.

 

Thanks,

Bob K.

 

 



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