Hello Vikie. Top side board temperature is a process variable and the intent
of the requirement is to reduce the thermal transfer from the wave solder
pot to the board itself. As the preheat heats up the board the thermal delta
between the board and the wave is minimize. The intent of doing this was to
allow the solder pot to be able to heat the plated through hole enough so
the solder will flow to the top side of the board. 

Initially, this was a rather simple process as the boards were simply double
or four layer multilayer boards. With this type of products the heat could
be simply supplied from the bottom side of the board and it would get the
board hot enough so the solder would flow up onto the top side.  With the
increase in the number of layers in printed wiring assemblies and also
increases in the thickness of the ground and power planes, supplying heat
from the bottom side is no longer a viable process.  To accommodate these
types of boards thermal chambers have to be created in the wave solder
machine so it in fact creates an oven environment, where both sides can be
heated simultaneously. You should watch for the total temperature difference
between side 1 and side 2 with this type of system and the temperature that
you select should be enough to allow the solder to flow to the top side of
the board, but not high enough to ruin the effectivity of the particular
flux you are using.  Contact your flux manufacturer for the exact
temperature that this occurs with the fluxes you are using.

So the bottom line is heat it enough to get the solder to flow but not
enough to ruin the flux, the temperature number itself is truly not the
number you should be looking for, but for the process to work properly.
Once the process works properly check the temperature number and that is the
number you should be using.

I hope this helps.

Leo Lambert
EPTAC Corp
WWW.EPTAC.COM

At 08:59 AM 6/23/97 -0700, Vickie Chapman wrote:
>
>
>
>Two questions:
>
>1.  I would like to know what others consider an ideal topside 
>laminate board temperature during preheat. The "rule of thumb" we have been 
>using is that we want this to be between 190 and 220 F (measured using a Wave 
>Solder Optimizer and/or tape dots). We are using an alcohol-based water
soluble 
>flux and standard tin/lead solder at 495 F.
>
>2. We are considering changing over to forced air convection preheaters,
but are 
> concerned that this will have more of a tendency to dry out the flux.
Again, we 
>are using an alcohol-based water soluble flux. Any experiences here?
>
>
>
>
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