Hi Rudy! Good information but Doug and I are looking for something more
specific - would you expect the DI water to increase the oxide thickness of
the aluminum bond pads or attack/remove material on the aluminum bond pads?
And if the attack/remove scenario is selected, how deep of an attack?
Angstroms or microinches in depth? The exposures we are dealing with is
room temperature and a maximum of 60 minutes exposure duration.
Dave Hillman
Rockwell Collins
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"<Rudy Sedlak>" <[log in to unmask]>@IPC.ORG> on 02/03/2001 05:57:46 PM
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Subject: Re: [TN] DI water and aluminum bond pads
In hopes of bringing just a LITTLE bit of chemistry to this discussion....
Pure water is not inherently more corrosive....it is probably less so,
HOWEVER,
Pure water lacks buffering power, the pH moves easily if anything affects
it,
the impurities, and especially water hardness, are pH buffers.
And so, when atmospheric Carbon Dioxide (which is an acid gas, forming
carbonic acid when it reacts with water) hits water which has no buffering
power, the pH falls like a rock.
Further, when water is purified by deionization, acid (H+) is introduced in
the cation column to replace the metals removed from the solution, and
Hydoxide (OH-) is introduced in the anion column, and if there is a SLIGHT
imbalance, the pH can swing widely.
And last, oxide-free Aluminum is actually attacked by pure, neutral water.
But we never see true oxide-free Aluminum. Aluminum depends on its
insoluble
oxide coat for protection, and if the pH gets even slightly acid or base,
that coating starts becoming soluble....and you got attack.
So, PLEASE stop saying pure water attacks Aluminum. Pure water will not
dissolve the oxide coat, so you do not get attack. Actually truly pure
water, at pH 7 (which is the pH of truly pure water) would not attack
Aluminum, because it would not dissolve the oxide coating, but obtaining
such
a critter (pure water) in the real world is just about impossible.
I will go away quietly now.
Rudy Sedlak---who tries to avoid even drinking pure water
RD Chemical Company
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