Werner and anyone else not tired of this subject,

I ran out of steam this morning at about 0200 and, as it's too cold to enjoy
golf here, I thought I'd rejoin what always is, to me at least, an
interesting subject - HASL. I pose the following for consideration starting
with some paraphrasing of industry experts including you Werner:

When using a tin-lead solder eutectic, the tin reacts with the metal
surfaces (I still want to return to this term as meaning an object?s area
having not depth with regard to diffusion and intermetallic formation), to
be soldered, to form an intermetallic compound. This ?metal? (does it ever
stop being a metal?) layer can be a very high tin concentration on the
solder side and a very high copper concentration on the other side. These
intermetallics can become brittle and weak ? when excessive (how much is
excessive and under what conditions to they become so, specifically, and do
they REALLY become excessive?). When they are excessive, solder joints
becomes susceptible to failure under mechanical stress as vibration or shock.

To produce high quality/reliable solder joints, the IMC must be as thin as
possible. The HASL process simply is a ?coffin nail? that can and should be
avoided. Again, it is, in my opinion, wasted. I think of it this way in
light of so many alternatives as OSP?s, ENIG, etc.

If the HASL process is not well managed (as is often the case when
operators, reduce conveyor speeds attempting, futilely, to effectively cover
copper surfaces, the first step to a ?thick? IMC is begun. Then, in
subsequent soldering operations (wave, reflow, hand, touch-up, repair,
rework, and modification) the layer thickens and the failure mechanism is
promoted. Is this correct or possible? If so, the IMC must be as thin as
possible.

Most recently, over the past few years (more than 25 but I ain?t saying), I
have been asked by clients to help evaluate quick turn board shops promoting
lower costs and faster deliveries. Who doesn?t take advantage? The first
place I go, after the sit down pow wow, is the lab. In many smaller
operations, not well managed but still offering cheap boards with fast
deliveries, chemistry log-books still sit there dusty containing hand
written data never having been entered into even a simple Excel spread sheet
and turned into even simple range charts, never mind XbarR charts. Seldom do
I ever see evidence the HASL flux chemistry or solder purity has bee
analyzed. The reason I bring this up is because this is where it all starts
and often ends as poor quality PCB?s often incapable of supporting soldering
operations capable of producing initial quality solder joints or those
reliable in any manner.

So, again, when an excessively oxidized board is introduced to an ill
managed HASL process, creative attempts often are made to make acceptable
solder termination areas. The effect is either ?ruined? as a non
wetted/wettable solder surface OR one that has excessive IMC formations
(again, how much is too much?).

With all this, I?ve seen some of the most excellent HASL surfaces coming
from well managed operations. I mean they?re flat, ?shiny,? (pardon that
one), and totally support good solder joint formation. So, I guess, my
complaint is why risk it? It?s just not worth it, nor is it necessary.

Earl

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