Ng Kuan Kuan,

Back in the good old days when I used to work for a very large blue chip
company, we did a lot of SM assembly onto flex circuits. We also did a lot
of chip on board and flipchip on board assembly as well. For a number of
reasons, the COB and FCOB operations took place after the SM components were
placed and reflowed. Typical cure schedules for the globtop and underfill we
used were 155 degrees C for 2 hours. On one particular product we also did a
cure at 170 degrees C for 90 minutes. This cure took place after SM
component reflow, die attach and globtop cure. As part of the qualification
of any product, we took samples at different stages through the assembly
process and did constructional analysis. One of the things we measured was
intermetallic growth. This was found to increase through these cure stages
but the level of increase was so small as to be fairly insignificant. As
well as the constructional analysis, all products went through extensive
environmental testing before, during and after product release (process
health checks). Bottom line is we never had any failures attributable to
extended cures post surface mount reflow. Again, this testing was undertaken
on flex circuits (Dupont pyralux material) not FR4 with 63 / 37 solder.

A word of warning, if you do go with a 175 degree cure and plan to use this
in a production process, ensure that you fully profile the cure oven you're
going to use. You'll not have much of a process margin if you're going to
use 63 / 37 solder. When we went ahead with the 170 degree cure we evaluated
a number of ovens and found that in some there were hotspots in the oven
where the temperature exceeded 183 degrees C. Obviously, if you take the
solder joint above reflow temperature and hold it there for a long time then
it's a different story altogether. To the trained eye, it's fairly obvious
that the joints have reflowed when you inspect them after the cure (but may
not be so obvious to a production operator).

Regards,

Martin

===== Original Message from [log in to unmask] (TechNet E-Mail Forum.) at
29/09/00 02:53
>Appreciate is someone can provide some expert advice on this.
>
>I have an assembly that requires the SMD attachments to be done first
>before the
>die attachment. The die attached adhesive requires to be cured at 175
>degrees C
>for maximum efficiency for one hour. My susbstrate is FR4.
>
>My question is: are there any known problems on solder joint and PCB
>reliability
>issues if the assembly is additionally cured for one hour at 175 degrees C
>for
>the die attachment process? Are there any papers that have been published on
>this?
>
>Looking forward to your expert advice.
>
>Thanks & Best Regards
>Ng Kuan Kuan
>DSO National Labs
>Singapore
>
>##############################################################
>TechNet Mail List provided as a free service by IPC using LISTSERV 1.8d
>##############################################################
>To subscribe/unsubscribe, send a message to [log in to unmask] with following
>text in
>the body:
>To subscribe:   SUBSCRIBE TECHNET <your full name>
>To unsubscribe:   SIGNOFF TECHNET
>##############################################################
>Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
>information.
>If you need assistance - 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 subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
##############################################################