I'll add a few comments in waiting for others to comment.
>=======
>Why did you pick that goal? Why worry about educating people to use a
>technology that is years away from general adoption? Few enough know how
>to work with the existing tin/lead alloys.

I guess I'd like to pro-actively look at using an environmentally
safer product than tin/lead alloys. I have heard quite a bit
about lead-free paste and it seemed that adoption might be
within the next 5 years. If this is true, I'd like to educate future
technicians about the lead-free alloys. 

>=========
Do you have any idea exactly what temperature the device "sees" when you
use this particular air temperature setting? Depending on how close you
hold the tip to the piece, the length of the heating cycle, and so on,
all you really know is that the temperature reached is between the
melting temperature of the solder and the temperature of the hot air. In
other words, you have no heat damage prevention.
>===========

Hot air set at 900 deg F and a hot air system for reflow is 
a recommended "high reliability" technique according to
material from IPC and PACE. True there is no way to
measure the actual temperature the device "sees", but
I do not believe this is a problem. I believe the technique
can accurately and safely install SMD components.

>=================
Surface appearance generally has little to do with joint integrity. Why
should two different alloys have different appearances? I expect that
the rosin in your previous paste prevented reoxidation of the surface
until the solder solidified. Now you're using a different (synthetic)
flux that may not be capable of withstanding the same amount of heat.
This means, you're seeing heavy oxidation on the surface with the new
paste. And don't forget that you're heating longer because of the higher
melting temperature of the new alloy.
>========================

Surface appearance is one way to tell if a solder joint is
acceptable. For eutectic alloys, a non-shiny joint indicates
reheating too many times or insufficient heat (or vibration
during cooling which can happen for 60/40 tin/lead alloys).
IPC recommends that joints be "shiny" to pass inspection.
I'm just wondering if this still holds true for lead-free alloys

>============
Have you considered that your new flux dissipates before the solder
flows. If this is the case, you would be trying to solder over a
reoxidized surface. Naturally, the solder would not flow smoothly.
(Incidentally, we should all trash the term "cold solder" since it
rarely has anything to do with lack of heat.)
>==================

I suspect this "no clean" flux is part of the problem. Perhaps it
is not suitable for hand rework applications. Perhaps I should try
an RMA mixture. There are also other lead-free alloys out there.

Or should I abandon lead-free use until it gets mandated by gov?

Dave

ps: I seem to get a lot of "vacation auto-mails" from this list server.



==========================
Dave Crabbe, Faculty
Computer Technician Program
N.S.C.C - Burridge Campus
372 Pleasant St, Yarmouth, N.S., B5A 2L2
http://cst.burridge.nscc.ns.ca/cst

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