Bob Kondner is correct.
I measure the moisture content of bare PWBs on a regular basis using a precision balance. They are readily available at many scale companies both for sale or lease (typically about $50-$100 for a month depending on the size required). A single small drop of water is a huge amount on these precision scales. I use them to determine the optimum bake time to remove at least 80% of the saturated INTERNAL moisture content, in order to safely solder the board without having to worry about delamination, internal damage to via connections from Z-axis expansion, and CAF. It does not matter if soldering is done via reflow oven, selective solder, or wave solder or hot gas rework, any of these methods can lead to PWB and component damage if the board and parts are not reasonably dry, which from experience I have found to be below 20% of saturated level.
Please read the newly released IPC-1601 that our team put together. It has all of the methods for determining the saturated weight and the absolute dry weight of any given PWB with the aid of a precision scale such as this.
You will be shocked at the amount of SURFACE moisture still present after wash, air knife, and blow-off. It typically requires a minimum of 30 minutes at 105 deg. C. to get all of the entrapped SURFACE moisture off of a CCA, even after all that. And that is for a very typical CCA with a few large QFPs, a few 1208 caps or similar size parts, and 1 or 2 connectors.
It requires a MINIMUM of 6-8 hours at 105 deg. C to remove enough INTERNAL moisture so safely reflow most PWBs.
Because we use some immersion silver PWBs, it is important to know exactly how long of a bake time is required to remove sufficient moisture to solder safely, but no more. Immersion silver rapidly tarnishes from unnecessarily long bake times. However, even slight tarnishing does not usually reduce solderability significantly.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards
Sent: Tuesday, February 01, 2011 6:20 PM
To: [log in to unmask]
Subject: Re: [TN] PCBA Water Wash
Bob,
When I say "I have seen"... I literally mean I have seen droplets of water under QFNs, micro-BGAs and in sealed F/O transceivers after air knife blow-off at 0.7ft per min....
I look for water intrusion in parts and under components at 150X to 300x...
There are few instruments that are capable of measuring the weight of a single water droplet intrusion capable of providing generating dendritic growth in a single component on a average size PCA...
With the lab tools that were available to me, the measurement noise far exceeds the resolution required to make the material measurement.
Paul
Paul Edwards
Process/Quality Engineering
Surface Art Engineering
-----Original Message-----
From: Robert Kondner [mailto:[log in to unmask]]
Sent: Tuesday, February 01, 2011 4:09 PM
To: 'TechNet E-Mail Forum'; Paul Edwards
Subject: RE: [TN] PCBA Water Wash
Paul,
Well, try it. Go ahead, tell me the weights of the PCB when dry and then
after wash and blow off. You can easily calculate the equiv ug / in sq.
And / Or do an ionic contamination test of a DI and that same board you
measured above. I would love to see the numbers. My problem was I did not
have a scale with the required sensitivity.
Instead of anything with an "I have seen ...." do an "I did this experiment
and this is what I measured" type statement.
Enquiring Minds Want to Know! :-)
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards
Sent: Tuesday, February 01, 2011 2:02 PM
To: [log in to unmask]
Subject: Re: [TN] PCBA Water Wash
I disagree with Bob...
I have never seen an air knife remove water from all the spaces that can be
on a board...
The work fine if you have plenty of clearance between ALL surfaces...
I have seen air knives drive water into spaces where to water's natural
surface tension would NOT allow it to go...
When the water evaporates, if it evaporates, it leaves the organo-halogen
complexes that supply base materials for dendritic growth...
Paul
Paul Edwards
Process/Quality Engineering
Surface Art Engineering
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Tuesday, February 01, 2011 10:23 AM
To: [log in to unmask]
Subject: Re: [TN] PCBA Water Wash
City water is fine if it is removed (air knife) prior to drying.
Run the number on dissolved solids content and calculate or weigh the amount
of later left on the board. With a simple blow off you will find the
remaining ionic content is very tiny.
Get ionic contamination tests done. City or DI that test results are the
important issue.
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Forrester, Michael (H
USA)
Sent: Tuesday, February 01, 2011 12:08 PM
To: [log in to unmask]
Subject: [TN] PCBA Water Wash
I have three questions I hope someone can help me with:
1) We have a vendor that uses "city water" directly into their SMT PCA
washer. I believe DI water is industry standard.
Does anyone know if it is documented anywhere? Spec?
2) We had an issue where there was Chlorine contamination and gold
dendrites found inside some standard gull-wing 6 pin SMT ICs.
This issue seems to be batch related. The chip manufacturer claims
that chlorine is never used in their process. Should the SMT
process expect to handle the possibility that the wash solution may
get into a part? The PCA vendor uses unfiltered "city water",
with no cleaner, in the wash process and does a ROSE test on one
board of each lot. Since the ROSE test passes, the PCA vendor
is blaming the part manufacturer for having "leaky" parts, since the
thinking is the wash is getting into the part and trapped.
3) One issue I have is the water pressure in the wash is 80-100 psi.
I think this is too strong, and should be half of that? You want to
flood the board with water, not hit the board and bounce off?
Thank you in advance.
Best Regards,
Michael Forrester
Sr. Product Engineer
______________________________________________________________________
This email has been scanned by the MessageLabs Email Security System.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
---------------------------------------------------
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.aspx?Pageid=E-mail-Forums for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
ext.2815
-----------------------------------------------------
______________________________________________________________________
This email has been scanned by the MessageLabs Email Security System.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
---------------------------------------------------
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.aspx?Pageid=E-mail-Forums for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
ext.2815
-----------------------------------------------------
============================================================================
==
This email and any attachments thereto may contain private, confidential,
and privileged material for the sole use of the intended recipient. Any
review, copying, or distribution of this email (or any attachments thereto)
by others is strictly prohibited.
If you are not the intended recipient, please contact
[log in to unmask]
immediately and permanently delete the original and any copies of this email
and any attachments thereto.
============================================================================
==
______________________________________________________________________
This email has been scanned by the MessageLabs Email Security System.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
---------------------------------------------------
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.aspx?Pageid=E-mail-Forums for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
ext.2815
-----------------------------------------------------
==============================================================================
This email and any attachments thereto may contain private, confidential, and
privileged material for the sole use of the intended recipient. Any review, copying,
or distribution of this email (or any attachments thereto) by others is strictly prohibited.
If you are not the intended recipient, please contact [log in to unmask]
immediately and permanently delete the original and any copies of this email and
any attachments thereto.
==============================================================================
______________________________________________________________________
This email has been scanned by the MessageLabs Email Security System.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
---------------------------------------------------
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.aspx?Pageid=E-mail-Forums for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
______________________________________________________________________
This email has been scanned by the MessageLabs Email Security System.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
---------------------------------------------------
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.aspx?Pageid=E-mail-Forums for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
