Douglas H Bennett said:
>I have to agree with Werner with regard to the combined use of IR and
>Forced Convection. I am also interested in why Eddie is prejudiced against
>the combination of IR and Convection.
If by "predjudiced" you mean he decided what sort of furnace to buy without
waiting to see what the Fraunhofer Institute said, then he is not alone.
>At Heraeus, we commisioned a study at Fraunhofer Institute in Berlin on the
>effects of various percentage combinations of forced convection and IR. As
>a result of the study we have modified our full forced convection oven
>design. (Yes, Heraeus also manufactures reflow and curing ovens in
>addition to solder paste and glues)
And numerous other companies. Yours is the only one I have heard recently
make any statement other than maximal forced convection is the best way to
go.
>Without going into a lot of detail, the report showed that a percentage
>combination of IR and Forced Convection helped the reflow process
>especially when it came to large mass components. The critical part
>however is that the IR must be well controlled. If the IR system drifts
>over the course of a day then the process will be non-repeatable.
This is the apparent difficulty of using IR heating. It is of course true
that any normal furnace operating at solder reflow temperatures will have a
measurable amount of IR emission, but the trend for the last 8-10 years has
been to maximize the convective heating.
Generally, I consider radiant heating to be a non-equilibrium process. In
other words, the preference would be to have a process that will only be
capable of bringing the work to a set temperature. Without a lot of detail,
a radiant heating process, a la the kitchen toaster, may give you a bit of
dark toast if the timing is wrong. But then, I have never been an
institution.
>If anyone has any questions about the report or the Heraeus Ovens, please
>contact me directly.
Thanks, but here we replaced two furnaces in 1997. The old equipment used
passive panel IR emitters, and the new equipment features fan forced
heaters. The delta T achieved is markedly better, with shorter heated zones
and faster conveyor rates. But next time I buy a furnace, I will try to
keep all this in mind. Your point sounds interesting, if unconventional.
regards,
Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com/
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