Chuck
In answer to your questions:
1 Ionic cleanliness levels for fine-line fine-pitch are commonly set at less
than 0.2microgrammes/cm2. Levels above this may well cause problems
2 No comment, not qualified to say.
3 Do NOT attempt to coat over this. The coating seal IN as well as out so
your problem will only be delayed rather than avoided.
Will a coating help in the longer term? Yes, but cleaning the device will
make life better.
No clean is not no residue, you probably have excessive flux residues in the
particular area that are causing the problem.
The coating spec you want is IPC-CC-830
Regards, Graham Naisbitt
[log in to unmask]
www.concoat.co.uk
Concoat Limited www.concoat.co.uk
<http://www.concoat.co.uk>
Alasan House, Albany Park
CAMBERLEY GU16 7PH UK
Phone: +44 (0)1276 691100
Fax: +44 (0)1276 691227
Mobile: +44 (0)79 6858 2121
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Chuck Mays
Sent: Monday, May 28, 2001 05:04
To: [log in to unmask]
Subject: [TN] Ionic contamination limit for PWA with 0402 sized
components
I've checked the technet archive and have not found a similar problem, so
here goes.
I have a problem with a small mobile product that is failing in field
trials. One area we have considered is a combination of RH and the level
of ionic contamination affecting high impedance areas of the PWA to cause
the problem. (The problem exhibits itself as a RAM corruption and the S/W
engineers are busily making sure it isn't their problem.)
An external lab checked a sample of the PWA's using the Omega method and
found the average level to be about 18 ug/sq.in. (I'm aware that 10
ug/sg.in. is the recommended limit.) The PWA uses 0402 sized resistors and
is the finest pitched assembly that the manufacturer in China who builds
this and other boards for us uses. A no clean process is used. The other
products built for us by this manufacturer are OK in terms of ionic
contamination and field performance.
Most of the failures in field trials have occurred not too far from the bay
in one of the suburbs of Sydney where the field trials are in progress.
The temperature is not high (15 - 20 degrees C), but we are now in the
rainy season. One day, when it rained in the suburb nearer the bay, and
not in the other field trial areas, 6 of the field trial units failed in
the one area.
My questions are:
1. Does anyone have experience that indicates that the 10ug/sq.in. level
for ionic contamination is OK for PWAs that use 0402 sized components? Or
does experience indicate that a lower level is more appropriate and if so,
what level? (It don't know the exact pitch of this board, but am waiting
for the answer.)
2. Is it possible to determine an impedance above which one should be
concerned with a level of ionic contamination for a given voltage?
3. If it turns out that for this design, RH and the level of ionic
contamination are causing the problem, can the situation be improved by
conformally coating the board or areas of the board? (If so, can someone
please point me to the applicable standards for conformal coating? Thanks.)
Best Regards,
Chuck Mays
Invetech Operation Pty Ltd
[log in to unmask]
Tel 613 9211 7700
Fax 613 9211 7702
The information contained in this message and any attachments is
confidential and intended for the named recipient(s). If you have received
this message in error, please contact the sender by return e-mail and
destroy the message and any attachments.
Any opinions or undertakings expressed in this message are those of the
individual sender except where the sender expressly and with authority
states them to be the opinions of Invetech.
----------------------------------------------------------------------------
-----
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
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 delivery of Technet send the following message: SET
Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases >
E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
----------------------------------------------------------------------------
-----
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
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 delivery of Technet send the following message: SET Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
