Dave
Compare your results with the excellent cover photo of Soldering & Surface Mount
Technology Volume 12 Number 2 (2000) which has the caption SnAgCu soldered SOIC
joint showing typical surface roughness. I think you may find that high Sn
alloys are just simply more light-diffractive than near-eutectic SnPb ones.
Notwithstanding, you seem to be changing two parameters (alloy and flux type) at
the same time. It may be better to change just one, so that you can isolate
which is causing you grief. Also, may I suggest a metallographic section of such
a 'faulty' joint to see whether it has bonded well?
Brian
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.
>
> 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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