Daan,
Not only are parts manufacturers somewhat lean concerning cleanliness
(thin profit margins!) but the shipping, handling, and storage of parts
can be very problematic. Particularly when using a parts jobber that
re-packages odd lots. Very nasty stuff has been known to get on
components (even when stored by the assembly manufacturer).
David A. Douthit
Manager
LoCan LLC
d. terstegge wrote:
>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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