Reflow on the large scale is achieved by applying solder paste to the printed circuit board using a laser-cut stencil with cutouts placed precisely where the pads are located. The solder paste itself is a mixture of flux and tiny balls of solder. A pick-and-place machine lifts the components from their packaging, e.g. a tape and reel dispenser and places them on to the board with their pads resting in the little gobs of solder paste.
The board then gets placed into the oven where a carefully controlled temperature profile is applied over the course of about 5 minutes. During this time the solder paste melts and the components ‘sit down’ into place before the solder sets as the board cools.
We can apply this basic technique to the hobbyist world with a simple plan of action. The pick-and-place machine will be replaced by my right arm and a pair of tweezers. A cost of zero so far, great stuff. Stencils and solder paste are both available to the hobbyist but the cost of the stencils are relatively high if you’re going to be making only a few boards and the solder paste needs refrigerated storage and only has a short shelf life. I’ll replace this part with a simple tinning of my boards using a soldering iron. It’ll take longer but should work just as well.
Finally we have the oven itself. Small ovens in various forms are available on ebay, amazon and they might even sell them in real shops made out of real bricks and staffed by real people. The fun part of the question is how do we make our oven follow a pre-programmed temperature profile instead of just powering up to a target temperature and staying there until the dinger goes ding and your chicken is roasted.
Of course we’re going to use a microcontroller to do it and that’s what the bulk of this article is about. Please read on.
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