Hi gang! The common thread that all of the examples that have been cited
is that high purity water is aggressive to material surfaces, especially
metals. As Rich described - "ion hungry" character of the water results in
surface species reactions which can degrade a number of pwb
characteristics depending on the type of metal and the quality of any
surface finishes. Doug and I have experienced the same impact on printed
wiring assembly solder joints. If you run an assembly thru a cleaning
system enough times, you can eventually etch the tin phase out of the
solder joints leaving a Pb rich surface. That Pb rich surface gives the
assemblies a purple appearance - we dubbed them "Barney boards" in honor
of the purple cartoon dinosaur. In another situation, ENIG assemblies
reacted with aluminum cleaning trays in a cleaning system - again, the
purity of the water played a role. Having ultra purity water isn't
necessarily a good thing for some situation.
Dave Hillman
Rockwell Collins
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Brian Ellis <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
03/07/2008 08:57 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Brian Ellis <[log in to unmask]>
To
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cc
Subject
Re: [TN] Effects of rinsing with high purity water.
We didn't have the weld problem with our cleaning machines, as the
welding rods were 18-8-2-1 alloy (expensive but was chosen for that
reason).
Welding 18-8 or 18-8-2 (or lower Cr/Ni SS) with 18-8 (or lower Cr/Ni SS)
rods is a sure fire way of having the weld problem.
However, we had one customer who coupled the machine to the water from
their semiconductor plant and the 18-8-2 alloy of the sheet metal
developed a thin coating of a loose very fine red powder, known as
rouge, despite the molybdenum. See
http://www.corrosionlab.com/Failure-Analysis-Studies/rouging.htm for
more details. This caused a long-lasting complaint from the customer but
I kept telling him to use a less pure water.
We never saw this with other customers who used DI water from a
mixed-bed column.
Brian
Douglas O. Pauls wrote:
> Along these lines, but slightly different, when my esteemed colleague,
Jim
> Maguire, formerly of Boeing, now with Intel, was doing SIR testing at
> Boeing, he told me an interesting story. All of their SIR chambers had
> been plumbed with 18 megohm-cm water to make it as pure as possible.
After
> a while (don't remember the time frame) he started seeing very fine
metal
> needles on his test samples. The high purity DI water had attacked the
> stainless steel of the chambers and the needles resulted. He had them
> plumbed with 8-10 megohm-cm water thereafter.
>
> I know that ultrapure DI water is not desired for use in
microelectronics
> because it can attack aluminum bond pads, interfering with wire
> bondability. In most cases, a microelectronics facility may start with
18
> megohm water, to get out the impurities, but will then mix carbon
dioxide
> back in to bring the resistivity down to the 2-6 megohm-cm range.
>
> For the cleaning of circuit boards, yes, I imagine that some people have
> been able to clean with tap water. Keep in mind that what comes out of
> the tap in your area may not be the same as what comes out of mine. Tap
> water quality varies greatly as you move geographically. My biggest
> problem with tap water cleaning is that most industrial water supplies
are
> coming from the municipal water supply. They add chlorine and sulfur
> compounds to cut down on bacteria growth. What two ions are death to
> electronics? Sulfur and chlorine. That is why I have always
recommended
> that people at least use RO quality water (500 kilohm-cm or higher) in
> cleaning electronics. If you are using a saponifier, as we do here, tap
> water has a large load of minerals that reduce the effectiveness of the
> saponifier and can result in shorter saponifier bath life. Again, with
RO
> or DI water, I improve cleaning efficiency and extend chemical life
which
> is more cost effective.
>
> On the other side of the coin, I have never really seen the need to have
> the ultrapure (15-18 mgeohm-cm) water for cleaning either. Purity of
2-8
> megohm-cm resistivity is easily achievable with standard industrial DI
> column setups.
>
> Doug Pauls
> Rockwell Collins
>
>
>
>
> Richard Kraszewski <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 03/07/2008 07:20 AM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> Richard Kraszewski <[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> Re: [TN] Effects of rinsing with high purity water.
>
>
>
>
>
>
> I've heard the reason is that high purity water is a rather unstable
> substance and as such is "ion hungry".
>
> Recall a story (urban "technet" legend?) which recounted how years ago
> Wang (?) had a custom SS cleaning system that used ultra pure DI. Over
> time the welded seams were eaten away and the system dumped on the
> floor.
>
> Rich K / Kimball
> 847-621-7310
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
> Sent: Friday, March 07, 2008 4:27 AM
> To: [log in to unmask]
> Subject: Re: [TN] Effects of rinsing with high purity water.
>
> I'll offer you a guess, without any scientific knowledge to back it up.
> If the Au is porous, the Ni could be attacked with OA residues diluted
> during the first wash, leaving nickel halides in the pores. The high
> surface tension of the final rinse water may prevent them from being
> removed but the Au/Ni couple with the damp residues could make a nice
> corrosion cell. Plausible?
>
> Brian
>
> Peter Barton wrote:
>> Hello Technetters,
>>
>> There has been some comment on this forum recently implying that
> cleaning or
>> final rinsing of assemblies with high purity DI water can have a
> detrimental
>> effect on solderability. We very occasionally see a solderability
> issue when
>> processing second side surface mount after first side population and
>> cleaning. Because of product build times and the use of paste with an
> OA
>> flux that must be cleaned soon after soldering we have to wash the
> first
>> side before continuing with the second side.
>>
>> The surface finish of the PCB concerned is ENIG. We have had various
>> analyses carried out on the surface finish but have never had any
> conclusive
>> evidence that the finish itself contributes to the problems seen.
>>
>> Can one of the gurus please explain how high purity water could affect
>
>> solderability and, if there are any circumstances in which variation
> in ENIG
>> could be susceptible to degradation in solderability if cleaned this
> way.
>> Best rgds,
>>
>>
>>
>>
>>
>>
>>
> -----------------------------------------------------------------------
>> Peter Barton
>> Senior Process Engineer
>> ACW Technology Ltd
>> Dinas Isaf West
>> Tonypandy
>> Mid Glamorgan. CF40 1XX Wales
>>
>> Tel: 01443 425275 (direct)
>> Fax: 023 8048 4882
>> International Tel : +44 1443 425200
>> International Fax : +44 23 8048 4882
>> Website/URL: www.acw.co.uk
>>
>
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