Anyone aware of needs for Pb-free consulting? I might just go out on my own  --
how about a Pb-free newsletter??

John Lynch

Jim Smith wrote:

> My observations are interspersed in Mr. Crabbe's original text.
>
> Jim Smith
>
> Dave Crabbe wrote:
> >
> > As a new member I wonder if a news list wouldn't
> > be a better avenue. I hope the following has not
> > already been discussed and answered.
> >
> > The goal:
> > ===========
> > Finding a lead-free solder paste for use in
> > SMD rework using PACE hot air workstations,
> > in an educational setting.
>
> 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.
>
> > The problem:
> > ===========
> > I've experiemented very briefly with a sample
> > of solder from EFD which consists of Sn96Ag4
> > "no clean, low residue" paste for its suitability
> > in a Community College soldering program
> > contained within a Computer Technician program.
> >
> > I am looking to use the lead-free paste to replace RMA
> > eutectic paste which we use when teaching
> > installation and removal of 1206, 0805, SOT23, SOICs and
> > PLCC installation using a PACE
> > MBT250 hot air jet soldering station.
> >
> > When I used this paste I noticed some characteristics
> > which would seem to make it unsuitable for
> > my application.
> >
> > 1. The solder seems to melt fine with the hot air
> > jet. The jet is normally set to 900 deg F for eutectic
> > paste use and this works fine for melting the lead-free
> > paste sample I have. (900 deg F translates MUCH
> > lower when it reaches the components.) So the
> > higher melting point does not seem to be a problem
>
> 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 not heat damage prevention.
>
> > 2. When the joint cools, it appears dull and grainy.
> > If this were a eutectic joint, it would fail a visual
> > inspection as it would look like the joint were
> > reheated, overheated, or vibrated during solidification.
>
> 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.
>
> > 3. Wetting action seems minimal. I do not get anywhere
> > near the nice wetting action of eutectic solder. Although
> > it appears barely acceptable for 1206 and 0805 packages,
> > it is horrible for SOIC (gullwing) types. It does not seem to
> > bond well with the lead, and the leads seem to set more
> > on top of the solder rather than "sink" down to the pad
> > with the paste wetting all around the lead. At many points
> > the lead/paste junction looks similar to that on a "cold"
> > joint, although in prying the lead / pad apart, it does seem
> > that an intermetallic bond has formed.
>
> 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 have tried various slow heating stages thinking that the
> > leads were not hot enough, but there was no improvement
> > in the end result.
>
> As noted, your problem is almost certainly not related to lack of heat.
> Try going back to a rosin-based flux and see if it helps. And be aware
> that overheating of parts with your hot air method is incredibly easy --
> and invisible.
>
> > Comments & recommendations?
> > ============================
> > Has anyone successfully used any lead-free paste that
> > gives results similar to eutectic? If so, could you list
> > the actual product and manufacturer so that I could get
> > a sample and test for its suitability in our program.
> >
> > Any comments appreciated.
> >
> > Thanks
> >
> > Dave
> >
> > ==========================
> > 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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