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September 23, 2013
Tutorial - Reflow Skillet
about 5 years ago
The toaster oven.. Here's something that MAY work for you..
There's 3 different kinds of heat.. Convection (Heat passing through a gas or liquid to indirectly heat something), Conduction (Heat passing through a solid much like a transistor to a heatsink) and radiation (Like IR - Infrared, or reflective heat. this is an invisible wavelength produced by things as they burn or heat up.. )
Toaster ovens are RADIANT heaters. they produce Infrared wave lengths that heat objects in the oven but not the oven itself. larger objects absorb (or reflect) the infra red at the same rate that smaller objects do. however smaller objects heat up faster because they have less mass. This is why the smaller plastic pieces fail..
Hot plate/skillet are CONDUCTION heaters. I think the reason you have so much success with this method is because everything heats up at almost the same rate (Give or take the eye of the heating element being the hottest part)
last is CONVECTION.. I think that if you wanted to revisit this Toaster oven idea you may have much better success with 1 minor tweak.. convection heating is preferred since it evenly (Or more even than most methods) heats everything evenly, and is fairly fast. I realize that the element is on top AND bottom on your oven.. If you ever decide to revisit this project, try attaching a heat shield above the PCB, but below the element, and another above the bottom element and below the PCB.. Allow room for air to circulate around these plates. The idea is to heat the air in the oven and reflect the infrared coming from the heating elements away from the PCB so that they don't heat the board. We only want the hot air via CONVECTION to heat the boards as this will prove to be a very even heating of the board by theory. (Consider it to be a huge hot air gun across the entire PCB.)
Let me know if you try this.. Gadgedizzle (at) gmail (dot) com
Also consider this.. In favor of the hot plate, any magnetic field created by the heating element in the hotplate would be minimized by the opposing eddy current in the plate tube and/or the heating element housing.. It would be interesting to see how strong of a magnetic force is created by the plate. However, while working with flooded plane PCB's I don't think induction is going to be a much of a concern. Especially when you take into fact that it would be 1 trace against how strong of an inductive force? It would be near impossible to convince me that secondary voltage in the board traces is high enough to cause a problem in this scenario.. Thanks for the write up!! Good info!! Perhaps, consider a coated piece of aluminum in the pan to spread heat? Also, allowing more TIME for the board to heat so the differential between board and oven/pan temp is minimized may help you with your silkscreen/board damage problem...? Or do I have that backwards?
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