Hi, Bev.
Reuven is correct in stating that the PWB is the biggest concern. But
the concern about adjacent parts and their solder joints is also great,
especially from a reliability standpoint.
When we develop and qualify our rework profiles, we put thermocouples on
the PWB, on the adjacent components, directly on one of the leads if
possible and on the adjacent component body. We then chart the
temperature of the leads of the component being reworked and the
adjacent component bodies and leads as well.
You can prevent the adjacent component body from exceeding 100 deg. C
and the solder joints from exceeding 150 C by masking them off, not with
tinfoil or Kapton tape which is not ESD-safe and is a thermal conductor,
but with an ESD-safe insulative hi-temperature masking tape. This tape
is commonly available in industry catalogs and is about ten times
cheaper than Kapton tape. We make sure the pwb never exceeds 140-150 C,
as we do not want to get into the Tg zone during rework.
We pre-bake the assembly if it has been out exposed to humidity for
longer than 12 hours. We have qualified this process by understanding
exactly what the moisture retention rate is for the standard PWB and the
rate for different PWB materials, and the time required to bake out the
moisture to a level less than 20% of saturated weight, and the length of
time before the moisture content has returned to above 50% of its
maximum (saturated) weight. We track the time the assembly has been out
after the original bake prior to assembly and reflow. The rest of the
time we keep the assemblies in dry chambers if they are not being worked
on, up to test.
We control the Open Floor Life of MSDs up to reflow. After that, for
rework, we pre-bake only for the sake of the PWB. If we have a component
that failed for something other than a workmanship issue (wrong part,
misplacement, damaged from mishandling, etc.) then we make sure the
assembly is baked long enough to remove the failed component safely for
purposes of failure analysis and/or RTV.
While this may not be practical for most PWB assemblies or for
commercial product, we do this as both our bare PWBs are extremely
expensive, and the finished assembly much more so. We build military and
space hardware, so we need to know that the adjacent component's solder
joints are not being heated up close to liquidus and the component
bodies are not being heated up to the temperature where water boils.
However, once the system is set up, it is remarkably easy to control all
of your product this way. We never, ever have a board delaminated in the
reflow oven nor as a result of rework. And we never have popcorned
components.
While it is a massive culture change to implement a system such as this,
once it is done there is a real culture of "reliability consciousness"
in the factory.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Reuven Rokah
Sent: Thursday, January 18, 2007 7:27 AM
To: [log in to unmask]
Subject: Re: [TN] Rework of circuit packs with MSL Parts
Hi Bev,
You are refer only to components.
The PCBs also can be delaminated during BGA rework and should be dried.
Best Regards
Reuven ROKAH
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Thursday, January 18, 2007 3:23 PM
To: [log in to unmask]
Subject: [TN] Rework of circuit packs with MSL Parts
Technetters,
We have required that circuit packs that are going to be reworked to be
dried before rework. My questions to all of you are:
1) Do you require this for all your boards irrespective of what is on
them?
2) Do you dry it to the requirements of the most sensitive MSL
component?
3) Do you use thermocouples to see how hot the MSL components are going
to get from radiated, convected and transmitted heat?
40 If the latter, what is the maximum temperature you allow the nearest
MSL component to reach before you require a pre-dry?
Thanks in advance.
Bev
RIM
---------------------------------------------------------------------
This transmission (including any attachments) may contain confidential
information, privileged material (including material protected by the
solicitor-client or other applicable privileges), or constitute
non-public information. Any use of this information by anyone other than
the intended recipient is prohibited. If you have received this
transmission in error, please immediately reply to the sender and delete
this information from your system. Use, dissemination, distribution, or
reproduction of this transmission by unintended recipients is not
authorized and may be unlawful.
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e 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 1.8e 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 1.8e
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
-----------------------------------------------------
|
