Many integrated circuits nowadays have a pad on the underside that needs to be soldered down. It can (for instance) serve as a ground connection or it might be needed to be soldered for better heat dissipation. Since the pad is not exposed on the edges of the device package, it can be awkward for prototyping. Ordinarily the solution is to use a reflow oven, a hot plate or a hot air tool. But, it is also possible to just use a soldering iron. I'm not sure how common this technique is, but it works well for me so I thought I'd do a brief write-up. I don't always use this technique, but is still a useful thing to know.
Here is the part to be soldered:
The photo above shows the underside of the integrated circuit, and it can be seen that it needs to be soldered on the pad marked U1 on the PCB.
Solder Paste and Tape Technique
The technique I use involves first applying some solder paste to the center pad on the PCB. The pads for the pins do not have solder paste applied for this method to work, just the center pad.
Next, take a short length of Kapton tape, and pick up the integrated circuit with it. Then, it is an easy matter to hover it over the PCB at the correct landing position, and tape it into position! If it goes on wonky no problem, just peel the tape and reattempt; the tape is sticky enough that you can re-position several times. The photo below shows one device taped down. To the left of it, you can see two more center pads which have been pasted but I have not taped parts down yet.
The paste was applied using a splint from a matchstick, since the area to paste is so tiny,
Once you're happy with the positioning of the taped parts (use a magnifier to check it is perfectly square and aligned), flip the board onto the other side. You may need to prop it up slightly (for example with some scrap PCB) to keep it flat and horizontal. Get a soldering iron ready; something like a 3mm ellipse shaped tip is ideal, because a lot of heat is needed. Also, regular solder wire is needed.
Soldering the Pad
Apply the solder and the iron to the PCB, so that the copper heats up and heat travels down the vias in the pad. For my 3mm soldering tip, about 10 seconds is needed with a 50W iron set to 330 degrees C; adjust accordingly to suit your iron and tip size. If the time is too short there really is no issue, because you can lift the tape from one end and see if the integrated circuit lifts up with it or not. If it lifts, then just put the tape down again and reapply the heat. Kapton tape is not stretchy and therefore it acts as a hinge when you lift it from one end, and when the tape end is reapplied the part should go down onto the PCB in the same precise position as before, but you can confirm with the magnifier again.
The photo below shows what the pads look like from the underside after the heat has been applied. Once you're happy, flip the board over and remove the tape. Now the device is firmly attached by the center pad!
The remainder pins can be soldered by hand using thin solder and a smaller tipped soldering iron.
After you're done, clean the board and check the pins really are soldered down, and examine with a magnifier to confirm there are no shorts. If there are shorts, desoldering braid can be used to correct it.
I hope the (small bit of) information was useful!