Need to solder a 4x4mm QFN by hand

Thread Starter

cmartinez

Joined Jan 17, 2007
7,703
It's a PIC controller. The thing is tiny, minuscule ... almost microscopic ... it only measures 4x4mm, and has extremely close leads, only 0.65mm (0.026") spacing between them.

1643774331191.png

Other than good luck, any advice as to how it would be best to proceed? I'd like to use a soldering iron and avoid a heat gun. My main concern is not shorting the leads, but rather burning the thing while working on it.
 

Ian0

Joined Aug 7, 2020
5,147
I can't see how you can manage it without the heat-gun, as there is no way you can solder the centre earth pad with a soldering iron, and it is generally the IC's only connection to 0V, so very important that it is soldered.
These are easy to solder with a heat-gun as they tend to align themselves to the pads when the solder melts, and it is usually possible to remove any solder bridges with the tip of the soldering iron.
 

MrChips

Joined Oct 2, 2009
26,112
I do this by hand and a fine soldering iron tip.
The tricky part is getting the pins lined up on all four sides.

If there is a centre pad, I have to layout the PCB with a wide hole under the chip so that I can apply solder from the reverse side.
 

Ian0

Joined Aug 7, 2020
5,147
The tricky part is getting the pins lined up on all four sides.
Surface tension is your friend, but it does rely on having all the solder melted at the same time.

If I have the IC in the right place but the “pins” are not soldered, I solder it with a big “screwdriver” tip, at least 3mm, one that is flat across the top. Just add a little flux and drag it along the side.
 

Ya’akov

Joined Jan 27, 2019
5,663
Aside from the underside pad, drag soldering is very effective for packages like that. I found the hardest part to start was scaling down my physical expectations to the very small sizes involved. But, with my microscope, good solder, and a good iron (as you say, the screwdriver tip) it went surprisingly well.

The flux I use is the sticky kind, and it helps to hold the part initially. I have been using Amtech NC-559-Y2-TF with good success. You have to kind of slop it on, then clean up afterwards.
 

Ian0

Joined Aug 7, 2020
5,147
I find the leadless packages easier to solder than the leaded TQFPs with 0.5mm spacing.
Good old (and smelly) brown gel flux! It’s OK on the leadless parts, but if I need it on the TQFPs, it gets trapped underneath. If ever I remove a TQFP that has needed flux, then there is flux underneath, and I don’t really like leaving flux on the boards.

If I get solder bridges, then using the large tip, I can generally move all the surplus solder to be end of the row, where I can remove it with a solder-sucker. I do have solder braid, but it seems such a waste to use it unless there is no other way.
 

Ya’akov

Joined Jan 27, 2019
5,663
I find the leadless packages easier to solder than the leaded TQFPs with 0.5mm spacing.
Good old (and smelly) brown gel flux! It’s OK on the leadless parts, but if I need it on the TQFPs, it gets trapped underneath. If ever I remove a TQFP that has needed flux, then there is flux underneath, and I don’t really like leaving flux on the boards.
The Amtech flux is supposed to be OK to leave, though I wouldn’t leave it the way lighter no-clean flux is left, it’s a mess. But from what I understand, despite aesthetic objections, there is no harm in the residue.

Apparently, some use ultrasonic cleaners which might be enough to remove the residue from underneath. I don’t have one, so I’ve never tried it.
 

Ian0

Joined Aug 7, 2020
5,147
The Amtech flux is supposed to be OK to leave, though I wouldn’t leave it the way lighter no-clean flux is left, it’s a mess. But from what I understand, despite aesthetic objections, there is no harm in the residue.
Probably I should buy some, instead of using this jar of brown stuff I rescued from the bin because someone that didn’t know what it was threw it away.
And I am sure it’s not Marmite.
 

tindel

Joined Sep 16, 2012
876
These parts are massive compared to some of the parts I'm learning to solder.

Take your time, use solder paste and a heat gun. It will take you a bit to learn how to do it, but it's totally manageable by hand.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,703
These parts are massive compared to some of the parts I'm learning to solder.

Take your time, use solder paste and a heat gun. It will take you a bit to learn how to do it, but it's totally manageable by hand.
Thanks for your advice and encouragement, Tindel. I would've preferred using of an iron. But after reading these posts, more and more I'm being convinced of simply making a stencil and use soldering paste and a gun.

I hadn't thought about the center pad (centre pad?) until Ian mentioned it. The chip does have a ground pin, but I'm not sure if it's connected to said pad.

On the other hand, I have used a heat gun before (I own one) and I damaged half the chips that I worked with ... guess I'm gonna have to be real careful and patient with this one...
 

tindel

Joined Sep 16, 2012
876
The exposed pad (EP) on the bottom almost always is connected to the substrate, which is tied to ground. There are a few exceptions so read the datasheet to be sure.

Note that I said nothing about a stencil... put a little dab of paste on the EP, a solid line of paste on each pin area, use a heat gun, watch for solder flow, and you'll likely be successful. Use an iron to clean up, if required.
 

Ian0

Joined Aug 7, 2020
5,147
You only tend to damage chips with a heat gun if they are powered up at the time! (I know from experience)
If you don’t have paste and a stencil, tin the pads first with a soldering iron, place the IC on top and heat with the heatgun. You might not get quite the right amount of solder at the first attempt. If you have too much on the centre pad, press the IC down to squidge it out.
I use this heat gun
https://cpc.farnell.com/duratool/d03323/2-in-1-smd-rework-station/dp/SD02291
though I can solder complete boards with the paint-stripping type. Place the board on an upside down steel baking tray. The conduction of heat through the metal preheats the areas you have not soldered yet.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,703
Well, things (apparently) worked out.

I gave in and decided to make a stencil instead of risking soldering this thing by hand. I chose that go down that venue when I finally took first hand look at the chip, and sure enough, it looked so tiny that I got scared as hell to even try.

A year ago I decided to buy a small B&D electric mini oven to try it out in case an event such as this presented itself. And it was today when I finally plugged it in and tried it out.

463bde75-11da-45fe-b1ef-d21189357545.jpg

I first tested the oven without the circuit in it, to see how its temperature profile behaved. And I quickly learned how to adjust it so as to get a temp curve roughly similar to the one recommended by Microchip.

In the end, what happened is that most of the SMT parts were neatly soldered after the process. Except for the tiny QFN SOB thingy ... its leads had a little too much paste on them because I overestimated the necessary width of the stencil's traces. And so the pins were shorted in three places. BUT! ... a lightbulb moment came to me, and I took my soldering iron, heated it to about 500°F and touched the PCB traces a couple of mm away from the chip's leads and, sure enough, the heat traveled through the trace and melted the solder when it reached it, also attracting the solder towards the tip and therefore breaking the short being caused by excessive paste.

20220204_185705.jpg

I never, never ever touched the chip with the soldering iron, and I don't think I placed its tip too close for it to overheat the chip and burn it. SO I'M HOPING that it was not damaged in the process.

crossing fingers....


@Wolframore , I think you might find this thread to be of your interest.
 
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