Reflow small parts help

[scott216]

I had a PCB made (by oshpark) that uses a boost converter (TI P/N TPS61090RSAR) which comes in a 16-QFN package.  I have a toaster oven converted to be uses as a reflow oven.  When I pulled my reflowed PCB out of the oven the TPS61090 had moved and wasn't aligned on the pads correctly.  I tried to fix it with hot air gun, but it didn't work.  I'm looking for any tips on how to successfully reflow small parts like this.

[Mattylad]

I have never tried that method myself but I'd try a weight on top or some glue underneath.

[Precipice]

I'd guess you've got too much solder paste on the belly pad, so the chip floated on that, and the real pins didn't make enough contact to let the solder's surface tension centre the chip.
You do have to place the part very accurately, though - checking and nudging under a microscope before reflow is a good plan.

Did you stencil the solder on? If so, did you reduce the size of the aperture on the belly pad?

You should be able to fix it with hot air and great care (and liquid flux, and maybe more solder paste, and some swearing and a magnifier and lots of light)

[ovnr]

What Precipice said. I'd wager that the center pad had way too much solder.

Try pulling the chip off completely with hot air, clean up the center pad so there's no solder at all on it, slather the whole mess in flux and re-solder it with hot air. Do not add any more solder.

[scott216]

I did not have a stencil, I just used the syringe. I'll clean it up and try again with less solder.  I don't know if I can do it with hot air, the air wants to move the part and it's hard to hold it still.  I'll give it a try and see what happens.  If it doesn't work, I'll try the reflow oven again.

[Precipice]

You shouldn't even (need to) try to hold it still! Hands have no place anywhere near it, you just can't be still enough. Line it up and let surface tension drag it into place. Sometimes a nudge is called for, to hop it one pad sideways, but that's brave.
It sounds like you're using too much air, from too close. (note that it may be impossible to reflow without a preheater on the back of the board, depends on all sorts of things).

Also, I sense frustration. Step away from the board, do something else for a while. Sometimes the little bastards just don't want to be soldered, and you have to respect that

[chicken]

I had good success reflowing/fixing QFN20 parts by heating the PCB (2-layer) with the hot air gun from the bottom, especially if there are vias in the ground pad.

[RiverTown]