I've done prototyping with surface mount components where the leads at least reach the side of the package. I want to solder a part where the leads are completely underneath the package. How do you do hand assembly with such a package?
Solder up the pads, which you have designed to stick out from the package far enough to put a soldering iron tip on. Place the item, and reflow either one pin at a time or with hot air gun.
If the pads stick out a long way, you can set the item, drop the iron on the outside of the pad, and feed solder between the pad and the device. hold it down well so it does not shift while you do 2 pins at opposite corners, then go back and do the rest of the pins. Don't use too much solder.
If the pads don't stick out, you have to go to paste and reflow, I think.
I've done this, it's pretty easy. Look at the last few photos here:
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Part of the trick is to extend the pads out far enough so that you can still use an iron to heat up the copper enough to fix any problems. But getting it right the first time certainly helps.
You can use the hotplate to take the chip back off if needed, too.
I put blobs on the PCB, tin the leads of the package, put some flux on the PCB, plop the part on the PCB. Then I use a long narrow conical bent tip with a bit of solder on the tip, and solder one pin at a time under a good microscope (Carton something or other) with an illuminator. I put the board at an angle and I can see a bit under the package. The problem is that if the first pin is jacked up too high, the other pins won't bridge. Which is ridiculous, because usually the universe makes sure that all that solder does at those scales is bridge.
I did some uWave stuff with copper tape, or better still, foil*, positioned then trimmed with a scalpel. For soldering, a dab of solder paste is best, then heat the exposed part of the pad. I am searching for a high temperature adhesive which is still peelable. Normal contact adhesive turns to jelly with heat.
Foil, as used for manufacture of PCB's, is thinner, and a harder copper, so cuts cleanly.
I use a waffle iron with flippable plates. One side of the plates is for waffles, the other side of the plates are smooth for grilled cheese sandwiches. I use the smooth sides. Stick the board in, shut the lid, and in a minute or two your board should be ready. Put a dab of solder paste on a unused pad so you can tell when the solder melts. I've done those awful parts with thermal pads under the body using this technique.
You want to heat the board evenly as much as possible. The grill doesn't contact the top of the board or any of the components. This waffle iron technique has the advantage of creating a hot air cavity which heats up the components and gets the job done fast. I've used this method on a few of Linear Technology's DFN and QFN packages with the thermal tab under the part and 0.5mm pin pitch.
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