| Hi Jason! | I can probably find something documented for you, but the easiest way to convince your QA people is just to show them. | If you have a board around that has some 1206 locations all in a row, print the board with paste and then place the parts incrementally off the pads...you know, place the first one dead on, the next one 10% off, the next one 25% off, etc...and then reflow it. | They'll be able to see with their own two eyeballs that you're not tryin' to blow smoke up their butts...it's simply a matter of the physics of surface tension, wetting forces, and component weight...and they'll also see how far off a 1206 component can be and still pull back into alignment. | Another illustration that I've used before trying to explain and show why this works, is to take two flat pieces of glass (like microscope slides) drop some water between the plates, and then set one on top of the other. If you try to separate them in the vertical direction, it takes quite a bit of force. You've just shown them how the components remain on the bottom of a board during double-sided reflow. | Then you try and separate the two plates in the horizontal direction, they'll snap back into place (provided that you don't go overboard in the distance that you slide them apart) and that will show them the forces that are at work "self-aligning" the part during reflow when the solder is liquid...it behaves the same way as the water working on the two glass plates. | One thing about this phenomena, is for it to work as advertised, you need to have the correct pad geometry and footprint for the part, and the part needs to have good solderability. | I wish you luck...(I know some QA people can be a little hard headed sometimes....GRIN) | -Steve Gregory- |
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