Soldering qfn problem ; it don’t go flat

[corrize]

Hello, I tryed to solder 3 qfn chips recently, and same problem : I press it hard and the chip stay in the air, there is a quarter of millimeter between board and chip, I don't see the sides pads of chip soldered to board and I guess the chip is not properly soldered (not tested).

I tryed with qfn 20 and 28, the same.

I start to solder with board pads tinned (lead solder wire), I place the chip without flux under but I put a little bit of flux at two sides of the chip to hold it in place.
After solder, I reflow (400 degree) while pressing chip, two times at two places of the chip.

What I'm doing wrong please ?

[corrize]

Re: Soldering qfn problem ; it don't go flat

See the picture, the chip don't touch the board, maybe it's normal ? The pads at left seems to not have enought tin ?

[lotas]

Re: Soldering qfn problem ; it don't go flat

See the picture, the chip don't touch the board, maybe it's normal ? The pads at left seems to not have enought tin ?

No, this is not normal, you can drop a flux and walk with a thin conical tip of a soldering iron.

[R_J]

Re: Soldering qfn problem ; it don't go flat

Are you using a hot are station? you must also melt the solder under the ic. so make sure to add flux to the old pad. If you don't melt the solder under the ic it will not go flat to the board. Also check that you didn't deform the pad during chip removal, the solder pad should be flat.

[corrize]

Re: Soldering qfn problem ; it don't go flat

Yes, I use hot air and I did all that, I don't have binocular microscope, so I can't see exactly if the pads are fully tinned. That could be indeed the solder that not melting at the pad under chip. I putted a low quantity of solder on it, for ensure that not cause issue.
Maybe temperature too low ? I tryed 390 and 400 degree for 14" asus laptop motherboard.
Edit : airflow minimum

[diif]

Re: Soldering qfn problem ; it don't go flat

400 degree f or c ?

[corrize]

Re: Soldering qfn problem ; it don't go flat

Celsius, I'm french

[corrize]

Re: Soldering qfn problem ; it don't go flat

Picture of the chip desoldered, probably not enought tinned, but that can't explain why it don't go flat ?

[diif]

Re: Soldering qfn problem ; it don't go flat

Too much solder What solder are you using ? I expect the centre pad to look smooth and shiny. Perhaps you're not hot enough or haven't given it long enough to melt properly.

[corrize]

Re: Soldering qfn problem ; it don't go flat

I use solder wire Mechanic 63-37. (I have 2UUL solder paste too, melt point : 183 degree C but not yet tested).
Too much on the central pad ? There is less than when I desoldered the original chip.
Not long enough, yes maybe, the blob on the central pad is at the same side where the chip was the highest. I will retry soon (without chip for checking the melting time at first). Thanks for help

[petehall347]

Re: Soldering qfn problem ; it don't go flat

remove solder and sit chip down on the pcb to see if its looking good then .if so use solder paste .

[diif]

Re: Soldering qfn problem ; it don't go flat

I was on my phone earlier, I can see there's not much solder now. I don't think it's melting, it's a ground pad so will require more time to melt than the pads around the outside.

[corrize]

Re: Soldering qfn problem ; it don't go flat

I removed the solder at ground pad and droped solder paste, and I added solder paste (to wire solder) on pins. I don't like this 2UUL solder paste; it's very liquid and can't control where I put it.
With hot air, I used the biggest nozzle, 480 degree C and 45% airflow and the result is good, and noticed no short with meter.

I don't like this "blind" method with solder paste, see this incredible short video : for qfn 48 just put a puddle of solder paste and... heat. Do you use this method ?
https://www.youtube.com/watch?v=i1b3RVcAFwE

What is the best ? Should I learn to use it or solder wire ? Do you use a lower melting point solder wire for qfn ? (I guess my tools are not good enough for that).

[harp]

Re: Soldering qfn problem ; it don't go flat

On NortridgeFix you can see regulary how Alex do a proper way to solder qfn chip, also and Louis in older videos on Louis Rossmann channel, with big blob in the middle

[corrize]

Re: Soldering qfn problem ; it don't go flat

I don’t watch anymore Northridgefix, he became a solder tools saler while Sorin continue to desolder qfn and resolder a new one directly after… without new solder or flux. For same result.

Alex don’t like and don’t use solder paste. So he probably didn’t try to learn to use it. When he did some video on solder paste, some viewers added comments he don’t use it properly. So probably watching Alex for learning to use solder paste is not a good idea.

But with solder iron he’s incredible.

I compared with them, but my solder has not the same behaviour, it is not enough liquid (solder wire). Maybe the Mechanic solder wire is not the best for qfn.
Hard to tin pads (with any iron tip) and need too much heat (hot air) at least 470 degree, 450 is too low for qfn. And I don’t like exceed 450. I guess I should try a best solder wire.
I never use pre heater, maybe I should ?