Brian, so we have three possibilities: deonized water, deweynized water and
polariced water. Which is best in your offpinion?
Inge
----- Original Message -----
From: "Brian Ellis" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, May 07, 2009 4:51 PM
Subject: Re: [TN] PB Surface Contamination
> To understand this problem, you also need to understand what happens
> when synthetic polymers are heated. What I am about to say applies
> equally to solder masks and to the substrates under them.
>
> When a polymer is heated, it reaches a temperature where its
> characteristics change, called the glass transition temperature. Above
> this temperature, the surface of a polymer becomes very porous and opens
> up like a sponge. To make things worse many of the chemicals that the
> polymer comes in contact with contain surfactants or other molecules
> which have a hydrophilic and lipophilic ends. If the molecule is
> linear, or nearly so, such as most of the common detergents and
> surfactants, another phenomenon occurs and the lipophilic end of
> hundreds of molecules rush into the pores and are trapped there when the
> polymer cools again. If, for example, the product is a flux, then some
> activators will probably become co-trapped and these can be released,
> not only with subsequent heating, but also at ambient temperatures over
> a long period of time.
>
> This is obviously a danger for the reliability of any printed circuit in
> critical applications. In addition, just to make things worse, many
> thermosetting polymers have ionic salts in their molecular makeup. For
> electrical applications, these salts are practically eliminated but
> there is always a risk of small quantities remaining and these can also
> leach out.
>
> Other posts have specifically mentioned HASL. This is because the
> fluxes used for this process are not only highly activated but nearly
> all the other chemicals in them are closely related to surfactants!
> You've got the worst of both worlds!
>
> What can be done about them? Not a great deal if you have a batch of
> circuits that leach enough to compromise the functionality. Perhaps a
> good wash in DI water after having baked the circuits may improve the
> situation if the problem comes from the fabricator. If it comes from
> your own workshop, then a systematic tracing will find the culprit.
>
> Hope this helps!
>
> Brian
>
> Kane, Joseph E (US SSA) wrote:
>> A cleaned bare PB is exposed to bake or reflow, and comes out with
>> measurable ionic contamination. The board is dirtier at the surface
>> after a heat excursion.
>>
>> The contamination isn't coming from the oven, handling, or anything
>> external.
>> Heat causes something to move from the inside of the board to the
>> outside.
>>
>> We haven't done any ion chromatography or other testing yet to
>> run this down, but has anyone heard of or experienced anything similar?
>>
>> In our normal process, we clean everything multiple times, but if the
>> contaminant builds up under solder mask, or in other areas that are
>> difficult to clean, we could have SIR or corrosion problems around fine
>> surface features or in high voltage areas.
>>
>> And if this ionic contamination doesn't just move up to the surface,
>> but is somehow moving around inside the board, could it collect in some
>> places and lead to CAF or other problems?
>>
>> Joe Kane
>> BAE Systems
>> Johnson City, NY ---------------------------------------------------
>> Technet Mail List provided as a service by IPC using LISTSERV 15.0
>> To unsubscribe, send a message to [log in to unmask] with following text in
>> the BODY (NOT the subject field): SIGNOFF Technet
>> To temporarily halt or (re-start) delivery of Technet send e-mail to
>> [log in to unmask]: SET Technet NOMAIL or (MAIL)
>> To receive ONE mailing per day of all the posts: send e-mail to
>> [log in to unmask]: SET Technet Digest
>> Search the archives of previous posts at:
>> http://listserv.ipc.org/archives
>> Please visit IPC web site
>> http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
>> information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
>> ext.2815
>> -----------------------------------------------------
>>
>
> ---------------------------------------------------
> Technet Mail List provided as a service by IPC using LISTSERV 15.0
> To unsubscribe, send a message to [log in to unmask] with following text in
> the BODY (NOT the subject field): SIGNOFF Technet
> To temporarily halt or (re-start) delivery of Technet send e-mail to
> [log in to unmask]: SET Technet NOMAIL or (MAIL)
> To receive ONE mailing per day of all the posts: send e-mail to
> [log in to unmask]: SET Technet Digest
> Search the archives of previous posts at: http://listserv.ipc.org/archives
> Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
> for additional information, or contact Keach Sasamori at [log in to unmask] or
> 847-615-7100 ext.2815
> -----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
