Hi Doug,
Seems to me that idea of not having to be extremely cautious about
component cleanliness whenever the boards are cleaned after soldering is
widespread, and even taught by IPC. Here are some quotes from
IPC-TP-1115 "selection and implementation strategy for a low-residue
no-clean process":
"Cleanliness.
The ionic cleanliness of bare boards, components and any other material
used as part of the finished product is a most critical issue affecting
the short and long-term reliability of products manufactured with a
low-residue or "no-clean" assembly process. The residues on incoming
materials will remain through to the end product. Standard tin-lead
deposition processes in board fabrication (e.g. HASL or tin-lead fusing)
can leave detrimental levels of electrically conductive and corrosive
residues on and in the surface of the laminate or solder mask. Component
leads and surfaces may be contaminated with residues left from the
chemicals used in the lead plating process. Many of these residues are
potentially corrosive. With the use of low solids flux and the
elimination of post solder-defluxing step, if the incoming materials are
not clean and the cleanliness level are not controlled by purchase
specifications, electrical leakage and metal migration can occur after
exposure to humid (> 50% RH) operating conditions."
"Components.
Not many assemblers have ever had to be concerned about the cleanliness
of their incoming components. With a no-clean process, component
residues can play a significant part in electrochemical failures.
Tinning flux and other detrimental lead preparation materials, such as
methane sulfonic acid (MSA), can result in corrosion and electrical
leakage. Component manufacturing residues on high-impedance devices can
have a major impact on device functions. Ion chromatography is
recommended for determining the base cleanliness of components."
Daan Terstegge
Thales Communications
Unclassified mail
Personal Website: http://www.smtinfo.net
>>> [log in to unmask] 02/11/04 02:33pm >>>
Good morning Daan,
Where did you ever get that idea? Must have been one of those
mornings
before I had my Diet Dew(s).
Residues on components or bare boards is always an issue. If the
residue
on either is a purely surface residue, then it is less of a concern if
they
are going into a no-clean assembly operation. It is more of a concern,
or
critical, in no-clean assembly because you have no way of addressing
those
residues.
If the components or boards have absorbed residues, you are toast.
You
"might" be able to do rescue cleaning in mild cases, but most often
not.
The laws of physics are not subject to repeal or negotiation.
Doug Pauls
"d. terstegge"
<[log in to unmask] To:
[log in to unmask]
SGROUP.COM> cc:
Sent by: TechNet Subject: Re:
[TN] Contamination on a SOT-23 component
<[log in to unmask]>
02/11/2004 03:16 AM
Please respond to
"TechNet E-Mail Forum.";
Please respond to "d.
terstegge"
Hi Brian,
I understood from earlier posting of you and others that the lack of
cleanliness of incoming components is considered a risk ONLY for
no-clean technology. Now Cheryl obviously has a problem with
contaminated parts in a process that does include cleaning. This makes
me wonder how (in general) the chances are that a cleaning process
designed mainly for cleaning flux residues from pcb assemblies will
clean the variety of contaminants present on incoming parts. Perhaps
you
can comment on this ?
Daan Terstegge
Thales Communications
Unclassified mail
Personal Website: http://www.smtinfo.net
>>> [log in to unmask] 02/11/04 08:56am >>>
Amen! Lord Doug has hit it on the head.
It may interest you (academically) to know that the erstwhile company
I
owned manufactured the Microcontaminometer 20 years ago, through to
1991. It was capable of measuring the ionic contamination on parts as
small as a single diode. It was not a popular instrument: I think we
probably sold less than 50 of the beasts. What we did find was that
many
components were hopelessly badly contaminated. There is a whole
chapter
in my book on this subject, with photos. One IC, which is shown,
measured at 12.4 µg/cm2 eq. NaCl and this is by no means exceptional.
My experience in my former professional life was that component
manufacturers cared even less about cleanliness than they did about
solderability and I know no reason why this state of affairs should be
any different today.
Brian
Cheryl Tulkoff wrote:
> I've been working on a tough but challenging contamination problem.
The
> contamination causes electrical leakage fails on a precision test
> instrument. Ion chromatography analysis points to an incoming
cleanliness
> problem on a SOT-23 component. Elevated choride and sulfate levels
were
> found on the components (5x - 10x greater ug/in2 levels than
anything
> found elsewhere on the boards). High levels were found even on our
"good"
> boards.
>
> We are trying to work with our supplier to identify and fix the
problem
> long term but are looking for some short term fixes as well.
Repeated
> water washing of the boards (high pressure spray cleaning) improves
the
> situation but does not fix it. Alcohol scrubs in the area further
improve
> it but don't eliminate the problem entirely.
>
> We have a only a DI water wash process (no surfactant/saponifier
> capability).Does anyone have any board or component cleaning
> recommendations?
>
> Are there any parts makers out there willing to tell me a little
more
> about parts cleaning after the lead plating process?
>
> What tool(s) do part makers use to verify cleanliness of parts?
>
> I don't have a tool capable of measuring what was found on the part
so I
> don't know where to go.
>
> Thanks, Cheryl Tulkoff
>
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