Soldering Technique

[yoda_bear]

How should I go about soldering this component back into place with the connections underneath?

Thank you for any advice.

[Benta]

Wow, nasty!
I see hope, though. You'll need to remove the solder from the bottom of the device to get it seated cleanly.

From what I see, based on your photo, the bottom of the IC is just a heat spreader.
I'd do a normal QFP soldering of the pins around the edge of the device, and then use the 3 x 3 vias for soldering the central pad by heating and filling in solder through the vias.

[wraper]

I see hope, though. You'll need to remove the solder from the bottom of the device to get it seated cleanly.

Nope, unless applying new solder paste. Otherwise how is it supposed to to make a solder joint again? The question is how this IC got off the PCB, if it was done by hot air, then this is a way how to place it back.

I'd do a normal QFP soldering of the pins around the edge of the device, and then use the 3 x 3 vias for soldering the central pad by heating and filling in solder through the vias.

I would like to see how you will do this with vias surrounder with solder resist. Not to say they are too small.

[KL27x]

Well, now that you have said it, I'll take that bet. I'll just scrape off some of the soldermask.

I'm afraid hot air (just from the top) will perhaps take too much heat because of the vias. But I dunno. I have never made a board like that. I'd probably tack just one or 2 pins and try to flow it from the bottom with the iron. Then wiggle the chip to make sure it's connected on the center pad. It will be hard to verify after you solder all the legs, and it could take more heat than you think.

[Shock]

Clean up all the pads absolutely flat, apply solder paste sparingly, place the IC on and tack the corners, warm the entire IC and surrounding pcb evenly with hot air then go to the underside and do the same, back and forwards if required, bring the hot air up to reflow temp, give the underside one last blast at reflow temp and then do a full reflow from the top.

If you feel it may easily move when getting to the underside of the pcb use kapton tap to secure it from the top first. Don't be too concerned about perfect reflow of the pins at first as they can be touched up if required later. One disadvantage of not reflowing it all at once though is you will not beable to see the IC align itself automatically due to the meniscus effect of the molten solder. If you use kapton tape obviously you need the area to cool before removing it.

[KL27x]

^That's way too logical. Someone is gonna come along and disagree.

[jmelson]

How should I go about soldering this component back into place with the connections underneath?

Thank you for any advice.

Remove all solder from bottom and lead pads with solder wick.  Apply solder paste to bottom pads, get chip in alignment, and then use IR or hot air to melt bottom pad to board.  This will take a LOT of heat!  When it is in liquidus, check quickly for proper lead alignment.  When all cooled off, solder leads with conventional solder and iron.

I do not envy you, this is a tough assignment!

Jon

[wraper]

Apply solder paste to bottom pads, get chip in alignment, and then use IR or hot air to melt bottom pad to board.

Why everyone wants to align it perfectly and make 100% flat so much? Apply flux gel, put the chip on top and reflow it with hot air. It will align by itself or you can help it a bit with tweezers once solder is melted. With solder paste there is an issue that it's hard to apply a right amount of it. Use too much of it, and it will either squeeze out or chip won't stay flat on board once reflowed.

When all cooled off, solder leads with conventional solder and iron.

There is already right amount of solder on the pads and pins left. There will be perfect solder joints if just reflowed as is.