On Thu, 14 Oct 2004, Sebastian Garcia wrote:

> I know that it works doing so. But again, the oven free response is 
> exponential, and it's impossible to fit in the paste manufacturer's 
> recommended error margin for the thermal profile. Moreover, the 
> plateau-shaped profiles were developed at the earlier days of reflow 
> soldering, when the ovens where provided only with IR heaters. Today, 
> the recommended profile for convection ovens replaces the plateau zone 
> with a slope shape.

The oven response is essentially linear if the heaters are significantly 
hotter than the oven (this is true when you turn the heaters full on in a 
toaster oven). It is exponential if the heater is controlled to a lower 
temperature (as it is in production ovens). So you are both right ... and 
you can 'fix' the slope between linear and exponential by setting the 
heater temperature. Very hot = more linear, cooler = exponential, exactly 
set to target temp = asymptotic (will 'never' reach target temp). I have 
plotted several ovens at several (constant) power settings and my 
conclusion is that you can approximate any (heating) temperature profile 
you wish by setting specific power levels for specific amounts of time, in 
open loop. For more rapid cooling slope opening the oven door for a 
specific opening width (using a ceramic wedge shim of the right size) 
works great. So with the right practice, you can 'be' the controller. I 
used my wristwatch as timer and a B&K thermocouple instrument for 
temperature reading.

> I wish to follow the manufacturer's recommendations, to guarantee
> long-term reliability of the soldered boards.

That's always good but can you achieve it with homemade equipment ?

Peter
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