qfn soldering help

[bigendian]

Hello,

i am trying to solder by hot air, and later with a thick solder tip on the 4 sides, the CPU in the picture.


Since each time i power it up, i don't see the clock (crystal connected), this means i am not soldering it properly.
It is powered 1.1, 1.8, 2.5, 3.3, voltages are all there.

The PCB is a design i did copied from a similar design with same cpu, checked several time, there shouldn't be any design error,
so suspecting really in the soldering process.

If anyone have suggestion for the soldering process, welcome.

Thanks

 

[NorthGuy]

The amount of solder on the center pad is key. You have too much on your picture. Use a solder wick to suck it out before you start the next attempt.

Put the solder on the footprint on PCB - on every pad - do it with a big chisel tip and hold it flat to the PCB. Use lots of flux while doing so (there cannot be too much flux). The center pad should receive less solder (much less solder) than the rest. If it has more solder than needed, it will work as a bubble under the chip and then there's no way all 4 sides get soldered. On the other hand, if you put too little, the center pad will suck the chip down and will prevent self-alignment. Unfortunately, it's hard to describe how much is enough, but you need to find the correct amount.

Then clean the old flux, apply a thin layer of new flux all over the footprint, put the chip on top and blow hot air (300 - 350 C) from the top until you see the chip settling on its place (don't confuse this with a chip being blown away by the air). After this, if you pock the chip ever so slightly it should come back, but this may not happen if there was too little solder on the center pad even though  the ship might be soldered Ok.

[bigendian]

Thanks.

I have seen some video where they don't put any tin in the center, they just solder corners with a specific tip. but this particular chip si really fine pitchm, and has a single ground pad in the center, so i need to solder it by hot air.

[jpanhalt]

1) Follow NorthGuy's advice.
2) I have done only one or two in my life and used a Schmartboard BOB.   In brief, make a PTH in the center of the pad.  Get the peripheral contacts done.  Then solder through that hole to do the pad.  THAT is an experiment.  It is not a proven method.  See: Attachment.

https://schmartboard.com/schmartboard-ez-qfn-12-pins-to-24-pins-5mm-pitch-65mm-pitch-2-x-2-grid-202-0016-01/

You can browse around the site for ideas.

[bigendian]

Really a special thanks to @NorthGuy and all, this is a great forum.

Originally i posted without any faith i could get help on this. I soldered many at least 10, without success (1 partially good).

Actually, seems i soldered it properly, clock 27Mzh is there.

Starting from @NorthGuy suggestion this is what i did:

- used a brand new CPU,
- removed all the tin in the middle pad, with old sock/net trick,
- made all pads flat,
- just put some old brown solder paste i have here and know is very good, on all the pads,
- blocked with a special spring clamp, otherwise my hot air even with air at minimum, moves it away,
- hot hair in the middle until i see soldering shining.

Got my 27 Mhz clock

[Smokey]

Tips for paste reflow (most consistent):
1) Use a stencil.  It's like $7 more for the stencil when you checkout from PCBWay, JLCPCB, etc.  Use that.
2) Use the manufacture recommended solder paste apertures.  Follow the datasheet when making the paste cutouts.
3) Use leaded solder paste.   This stuff works fine.
https://www.digikey.com/en/products/detail/chip-quik-inc/SMD291AX/1160002
4) Use a hot plate for the reflow. 
5) Inspect with a microscope so you can actually see if there are any solder bridges.
6) Clean up any solder bridges with a lot of tacky flux and a small high quality iron tip.

Tips if you have to rework just one chip like this (so no stencil):
1) Start with clean pads.  Get rid of all the old solder with wick.
2) Use leaded solder, and a high quality tacky flux like this:
https://www.digikey.com/en/products/detail/sra-soldering-products/TF5000/10709784
3) Apply a small any even amount of solder directly to the pads on the chip.  Don't add too much to the big thermal pad.
(optional: also add a TINY amount of solder to the pads on the board).
4) Apply a bunch of tacky flux and place the chip on the board.
5) Use Hot Air to reflow the part.  Use tweezers (and a steady hand) to push down on the center of the chip after it reflows to confirm all pads are making contact.
(if a bunch of solder squirts out when you push the chip down, you added too much.)
6) Use paste and iron to clean up any bridges as above.

[Kasper]

1) Follow NorthGuy's advice.
2) I have done only one or two in my life and used a Schmartboard BOB.   In brief, make a PTH in the center of the pad.  Get the peripheral contacts done.  Then solder through that hole to do the pad.  THAT is an experiment.  It is not a proven method.  See: Attachment.

https://schmartboard.com/schmartboard-ez-qfn-12-pins-to-24-pins-5mm-pitch-65mm-pitch-2-x-2-grid-202-0016-01/

You can browse around the site for ideas.

I've done this on an ESP32 module.  Worked well.