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July 1997

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Sun, 6 Jul 1997 04:55:37 -0400 (EDT)
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Having been involved as a solder mask supplier for almost 20 years, one of
the technology changes that has tended to simplify my life (most others have
made it harder) has been the switchover from reflowed Sn/Pb to SMOBC. Let me
explain,

Going back 20 years, a high percentage of PWB's were made with fused Sn/Pb
circuitry. This was back when surface mount was in its infancy, most boards
were strictly plated through holes (double sided), high density was 10 mil
lines and spaces and holes were big (ie, low aspect ratios). The general
feeling was that Sn/Pb was more reliable than SMOBC and most end users felt
more comfortable using it.

Generally, these fused Sn/Pb boards were coated with solder masks (back then
the most common were thermal or UV cure types, screen printed). Solder masks
do not adhere very well to fused Sn/Pb (compared to copper) and when boards
were run across the wave solder line, the solder would melt and break the
bond of the solder mask to the solder. At that point, the sodler mask tended
to blister, flake, fall off, etc. When this happened, a nasty phone call
would be made to the board supplier. Since you know what flows down hill, the
next phone call (even nastier) would be made from the board supplier to the
solder mask supplier (poor little ol' me).

As technology changed, densitites got tighter (5 mil lines/spaces),
multilayers came on the scene, holes got smaller (especially vias) and aspect
ratios increased and surface mount came on like gangbusters. These changes
aggravated the problems with solder masks on fused Sn/Pb designs.

I really believe that solder mask problems were the biggest single reason
that there became a massive switchover to SMOBC away from fused Sn/Pb and
surface mount technology was the main catalyst. With SMT, you have a minimum
of two times where the solder will melt (once for the fusion of surface mount
components on the component side and once for the wave soldering on the
solder side). Whereas, you might get lucky and the solder mask will survive
one "melting" of the solder without flaking, lifting, etc., it won't survive
two. Therefore, Sn/Pb on SMT designs became a disaster with solder mask
flaking, fusion of adjacent circuit traces, etc.

By now, at least 95% of all circuit board designs are built using SMOBC
technology.SMOBC has been accepted by most of the major end users who are
very concerned about long term reliability (IBM, DEC, Hewlett-Packard, AT&T,
Delco Electronics, Compaq, etc.). 

One of the real pioneers of SMOBC technology has been Odi Cardenas of Solder
Station One in Santa Ana, CA. I'm sure Odi could give you a lot of
information about the reliability of SMOBC and hot air solder leveling (he
was instrumental in getting IBM toi switchover to SMOBC in the mid 80's), Odi
can be reached at:

     Odilon Cardenas
     Solder Station One
     714-558-1019

Hope this helps.

Larry Fisher
Dexter Electronic Materials
[log in to unmask]

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