Scott,
Preheating can serve a couple of purposes;
1. To activate the flux should that be a part of the processes, as in
attachment of the component.
2.  To prevent thermal shock of the elements being exposed to the localized
wave/fountain.

Much of the criteria that is appropriate for wave soldering would apply here
since the process is quite similar, and as with wave soldering there is not
a single recipe that fits all situations.  We should never profile the
machine but should always profile the product for the machine.
Profiling is efficiently accomplished with the application of thermal
couples if the production resources warrant that effort.  Otherwise a
surface probe can suffice to achieve adequate results for occasional use.
Variation within the machine supporting the process can vary from a partial
contact to just maintaining the board position over the solder resource.
Where? I would recommend on the top side near the component leads of the
component.
How fast? Deals with capability to a great extent.  Component manufacturers
sometime recommend 2 degrees C per sec for sensitive components such as Chip
caps.

Mel Parrish
Soldering Technology International
102 Tribble Drive
Madison, AL 35758
256 705 5530
256 705 5538 Fax
[log in to unmask]

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Rougeux, Scott
Sent: Wednesday, September 26, 2001 12:21 PM
To: [log in to unmask]
Subject: [TN] FW: preheating assemblies prior to rework
Importance: High


> Your input is requested on this issue.
> In regards to preheating assemblies prior to reworking them on the Solder
> Fountain/solderpot.
> We have been asked to look into preheating components or boards during
> rework. This opens many questions:
> *     What is the desirable preheat temperature?
> *     How is it measured?
> *     Where is it measured?
> *     How quickly can the assembly be heated?
> Thanks.
>

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