How to solder fine pitch QFN parts?

dl324

Joined Mar 30, 2015
16,845
The parts should align themselves while the solder is molten. You could use a drop of adhesive to hold the package in place, but it shouldn't be necessary unless you're doing boards with parts on both sides.

Couldn't look at the link. My browser complained that it couldn't provide a safe connection...
 

Sensacell

Joined Jun 19, 2012
3,432
Here is my method for proto / rework, it takes skill, but once you get it, it's fairly foolproof.

Normally you would use solder paste, but without a stencil, it's hard to 'meter out' and position the right amount.

1) Using COPIOUS flux, I tin the PCB pads and chip pins with solder, just enough to create a clean clear bump on both.
If the chip has a thermal pad, just apply the tiniest amount here, it's easy to over-do these pads, due to the large surface area.
I set my iron tip to about 640F for this.

2) With the board and chip prepped, place the chip on the pads, again with COPIOUS flux. Use hot air to heat the board and chip to melting temp. You will find that the chip will tend to "snap" into position when the solder melts, have a tool ready to prod it gently into position if it's not aligning correctly.

Apply the heat more to the board than the chip, it has much more thermal mass.

The point is to have both the chip and board prepped with fresh, shiny, oxide free solder bumps, these will merge effortlessly once they melt.
You MUST do this prep to both surfaces, any oxidized old solder will inhibit the blobs from merging, resulting in a bad joint.
Use tons of flux, you cannot have too much flux!
 

Thread Starter

Fred4ka

Joined Apr 27, 2018
5
The parts should align themselves while the solder is molten. You could use a drop of adhesive to hold the package in place, but it shouldn't be necessary unless you're doing boards with parts on both sides.

Couldn't look at the link. My browser complained that it couldn't provide a safe connection...
Thanks for the tip, boards do have these parts on both sides.
 

Thread Starter

Fred4ka

Joined Apr 27, 2018
5
Here is my method for proto / rework, it takes skill, but once you get it, it's fairly foolproof.

Normally you would use solder paste, but without a stencil, it's hard to 'meter out' and position the right amount.

1) Using COPIOUS flux, I tin the PCB pads and chip pins with solder, just enough to create a clean clear bump on both.
If the chip has a thermal pad, just apply the tiniest amount here, it's easy to over-do these pads, due to the large surface area.
I set my iron tip to about 640F for this.

2) With the board and chip prepped, place the chip on the pads, again with COPIOUS flux. Use hot air to heat the board and chip to melting temp. You will find that the chip will tend to "snap" into position when the solder melts, have a tool ready to prod it gently into position if it's not aligning correctly.

Apply the heat more to the board than the chip, it has much more thermal mass.

The point is to have both the chip and board prepped with fresh, shiny, oxide free solder bumps, these will merge effortlessly once they melt.
You MUST do this prep to both surfaces, any oxidized old solder will inhibit the blobs from merging, resulting in a bad joint.
Use tons of flux, you cannot have too much flux!
Thanks for the suggestions. I do remember that "flux is your friend" from back in the day and use plenty of it. Are you able to have a consistent amount of solder at the center ground pad? My concern is too much can cause excessive standoff from the board and signal pads may not connect or be too thin. On the flip side, not enough solder may cause poor heat dissipation and overheat the part. do you have to hold the part down using your method?
 

dl324

Joined Mar 30, 2015
16,845
Are you able to have a consistent amount of solder at the center ground pad? My concern is too much can cause excessive standoff from the board and signal pads may not connect or be too thin. On the flip side, not enough solder may cause poor heat dissipation and overheat the part.
You use a stencil if you want to apply a uniform layer of paste.
do you have to hold the part down using your method?
When I use a hot air tool, I don't have to hold the part if the air flow and temp are set properly.
 

Sensacell

Joined Jun 19, 2012
3,432
If you have to hold the part down, something is wrong.
The surface tension of molten solder is very high, it will hold small parts firmly, in fact, holding the parts manually can prevent the pins from self-aligning.

It is hard to get the thermal pads just right, just tiny bit of solder.
 
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