This was really interesting, as it echoed a similar problem that one of our
customers was having. The 'nice flat' tin/lead is probably a very thin
coating on the substrate metal that may easily allow oxygen to permeate
through. You may be using nitrogen in the reflow oven, but the question is:
what is your oxygen level? If the oxygen level is moderate (~1000ppm or
so), it may be enough to permeate through the thin solder layer, and
oxidise the underlying intermetallic. The solder dewets from the
intermetallic, giving a thick patch of solder in one spot, and bare areas
elsewhere, where the highest concentrations of oxide are found.
The oxygen level in your wavesoldering machine is probably pretty low, so
that the flux that is applied to the board surface is sufficient to remove
the very thin layer of oxide on the intermetallic, and does not have to
deal with metal that is re-oxidised during the wavesoldering process. The
joints should be reliable, although the amount of intermetallic is thicker
than you would have obtained with just a single pass: don't take good
solderability as a sure indicator of joint reliability.
"d. terstegge" <[log in to unmask]> on 09/10/98 05:23:57 AM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to "d. terstegge" <[log in to unmask]>
To: [log in to unmask]
cc: (bcc: Andy Mackie)
Subject: [TN] Dewetting of Surface Mount pads
Hi Technet,
Today I did some tests on a sample-board from a potential future bare
board supplier. It was a thin flex-rigid boards with a smooth and shiny
leadtin-plating (probably HASL) on the SMT-pads.
After passing the board through our reflow oven (forced convection,
nitrogen atmospere, no solder paste, no components) I noticed excessive
dewetting on all of the lands, except for those that had been in contact
with the center board support rail.
During wave soldering (again no components used, RMA flux) the dewetted
pads picked up fresh solder from the wave without any problem, after which
the soldered surfaces look smooth and shiny again.
I don't understand how a smooth leadtin plating can dewet during
heating-up, and the dewetted surface can still have a good solderability.
Does anyone have ideas about what's happening here ? Will solder joints to
such a board be reliable ?
Regards,
Daan Terstegge
Signaal Communications
* Unclassified *
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