Mark,
18 Megaohm DI water has essentially no ionic impurities. What is
happening is the DI Water is leaching metal ions from the solder.
Different metals can behave very differently. I understand that
aluminum for example can form a thin oxide layer protecting it somewhat
from this effect.
Most plastics, and metals will leach into the DI water over time. If
you want to maintain the ultrapure DI water you must use PVDF (Kynar)
lined plumbing etc. When you heat this ultrapure water the reaction
rate increases dramatically. I first learned this when I installed an
expensive brass valve on a hot DI water line. It worked great for about
3 days. After that the inside of the valve was gone and so was a whole
lot of DI Water. All over the floor.
As far as your difficulties with marking permanency. I doubt that an
inline spray washer could maintain ultrapure DI water long enough to
cause such an effect. However if you are using a closed loop system
with some big beefy ion exchange cylinders and are achieving very high
levels of resistivity even at high temperatures and the components are
exposed to this long enough then it may be worthwhile looking into. On
the other hand if the components have gold lids and were not cleaned
properly before marking then they are commin off. I'm surprised they
made it all the way to final rinse. I've seen plastic molding compounds
that were so bad the component would hit a hard stop and the marking
would just keep on going.
Regards, Russ Winslow
Six Sigma
1940 Concourse Drive
San Jose, CA 95131
[log in to unmask]
http://www.sixsigmaservices.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Mark Austin
Sent: Thursday, March 11, 1999 1:22 AM
To: [log in to unmask]
Subject: Re: [TN] FW: [TN] Dull Solder Bump
Russ
Why does hot DI water etch solder?
We have a water wash process for our avionics cards and we sometimes
find
that the idents have been removed from some of the components. Could
this be
due to the hot DI water?
===== Original Message from [log in to unmask] (TechNet E-Mail Forum.) at
11/03/99 00:18
>From: Russ
>Sent: Wednesday, March 10, 1999 4:02 PM
>To: 'Manish'
>Subject: RE: [TN] Dull Solder Bump
>
>
>Hello Manish,
>Guess what? The cleaner your DI water the worse the problem. Get some
>hot, very pure DI water and just try a quick test. The DI water will
>etch away the solder until you have no bump left.
>
>Also watch out for flux left on the wafer after reflow. Most water
>soluble fluxes are organic acid based and can still have some pretty
>good activity left in them even after reflow.
>
>Fast cooling, no vibration, cool DI water spray rinse (without
>recirculation), Hot clean DI (short time only) should have your bumps
>shinning pretty. Of course most of this is purely cosmetic.
>Solderability shouldn't be impacted by by the dull solder appearance.
>
>Regards, Russ Winslow
>
>Six Sigma
>1940 Concourse Drive
>San Jose, CA 95131
>
>[log in to unmask]
>http://www.sixsigmaservices.com
>
>
>
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]]On Behalf Of Manish
>Sent: Wednesday, March 10, 1999 2:58 PM
>To: [log in to unmask]
>Subject: [TN] Dull Solder Bump
>
>
>Hello Everyone:
>
>I am trying to design am experiment to study the surface appearance of
>solder bumps and its effect on the shininess/dullness of bumps. One of
>our customers recently reported an increased rejection rates at their
>placement machine as the dull solder bumps were not recognized.
>
>Although we have not conducted extensive experiments in house to study
>the
>effect of reflow profile, cooling rate and the flow of nitrogen etc., i
>would appreciate feedbacks regarding your experience with this issue.
>
>We also rinse our wafer lots with DI water after reflow. Is there a
>possibility of impurities in DI water attacking the surface of eutectic
>solder. All of the wafers we bump are near eutectic solder alloy
>composition. The issue of dull solder bumps seems to be a periodic
>issue,not surfacing regularly.
>
>I would appreciate your comments on factors responsible for causing
dull
>bumps.
>
>Regards
>Manish
>
>PS I am only refering to the occurence of dull solder bumps after wafer
>reflow. We use a water soluble flux during the wafer reflow.
>
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Mark Austin (Process Engineer)
ACW Technology Limited
Hylton Road
Petersfield
GU32 3XX
ENGLAND
TEL - +44 (0) 1730 300000
FAX - +44 (0) 1730 266045
Email - [log in to unmask]
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