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Reflow soldering lead onto much larger pad

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Reflow soldering lead onto much larger pad | 7 December, 2015

So I have an issue coming up that I'm not too sure how to handle. A board was designed with the wrong footprint for a 3-lead transistor in which the single-lead side has a much larger (think thermal pad for qfn) copper land than what is meant for the lead. When depositing paste and reflowing the board, what are my options here? Applying one large deposit of paste and hoping for the best?


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Reflow soldering lead onto much larger pad | 8 December, 2015

Apply the correct amount of paste for the pad size should work ok, as long as the part starts out in the paste. If you have a reduced aperture and the leg doesn't quite touch any paste could result in non-wetting or insufficient solder issues.

I guess it depends on quantity of boards and how much extra labor you want to put into making it fit. You can tape off part of the pad so it won't spread all the way during reflow, but that's time and labor intensive.

I'd probably have to see the actual part placement to see what you are dealing with.

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Reflow soldering lead onto much larger pad | 9 December, 2015

I've put down a SOT223, close enough to your 3pin transistor, in the same situation you have. The large pin was to be soldered on a large exposed ground plane in the corner of a board, over 2"x2" exposed area. We left the paste file as is, so wherever that pin landed, we applied the standard amount of paste.

As long as your paste is touching the lead you will be fine during reflow. The lead should keep most of the solder there, unless your doing class 3 stuff you shouldn't have any rework. But if your paste doesn't touch that lead that solder will pull or wet out leaving that pin insufficient.

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