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.
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
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.
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.
I have tried various slow heating stages thinking that the
leads were not hot enough, but there was no improvement
in the end result.
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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