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July 1997

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Sun, 6 Jul 1997 03:15:38 -0400 (EDT)
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In a message dated 97-07-04 01:59:40 EDT, you write:

>One of our customers has an engineer who believes that reflowed tin/lead
>has a significant reliability advantage over SMOBC.
>
I have no hard data on testing, but can tell you something about fused
(reflowed) Tin/Lead that may assist in understanding.  Fused Tin/Lead starts
out as electroplated Tin/Lead, and the ratio of Tin to Lead varies with the
current density in plating.  The implications of this are that the Tin/Lead
ratio will vary on the panel (board), and that the Tin will be lowest in the
lowest current density areas, which are typically the holes, and the greater
the aspect ratio (hole depth to diameter), the worse the problem becomes.
This clearly implies that the current density problems are going to be a
greater issue on modern boards.   Further this can mean that unless a
sufficiently high temperature is used, the high Lead areas will not fuse, and
thus will be very prone to corrosion.

63/37 Tin/Lead fuses, in theory at just under 400 F.   IF the ratio of Tin to
Lead varies from that 63/37 ratio, the minimum temperature required
increases.  However it increases much more, and much faster if the deposit is
high in Lead, versus being high in Tin.

And of course, SMOBC, or HASL solder is generally pretty uniform, and always
totally fused, or alloyed, as it starts out that way.

A side note, if the Tin/Lead is not perfectly fused, you can get real nice
galvanic corrosion forming, and the unfused areas will corrode quite quickly.

So, although Solder is Solder is Solder, my suspicion is that in the real
world, with current densities not closely watched, or controlled, the fused
(reflowed) panels would have much higher incidence of corrosion potential,
than SMOBC (HASL) panels, simply because you can depend on all the solder on
an SMOBC panel being alloyed, this is not necessarily the case on a fused
panel.

Rudy Sedlak
RD Chemical Company

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