Author Topic: best way to hand solder a QFN?  (Read 1784 times)

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Offline FrankBuss

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best way to hand solder a QFN?
« on: March 20, 2017, 01:27:24 pm »
I plan to solder this chip:

http://www.abracon.com/Support/AppsManuals/Precisiontiming/AB18XX-Application-Manual.pdf

3 x 3 mm QFN, with 0.5 mm pitch and a package thermal pad which must be connected to Vss (I'm using the SPI version of the chip, so I'll add a via for this on the board, no way I can route this out between the pads). My idea is to use some solder paste and a heat gun. I don't want to buy a solder stencil just for one chip. I can design the board in a way to make it easier to solder it, if this helps. I guess a thin syringe would be useful, to place some solder on each pad? Any product recommendation for suitable solder paste and syringe, which I can get from Digikey, or is there an easier way?
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Offline Koen

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Re: best way to hand solder a QFN?
« Reply #1 on: March 20, 2017, 01:45:20 pm »
Flux it and use a regular soldering iron to place regular solder on the pads. Just a bit on the center pad, don't make a dome. Clean it all. Flux it all. Place the chip. Hot air.
 

Offline mikeselectricstuff

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Re: best way to hand solder a QFN?
« Reply #2 on: March 20, 2017, 01:56:19 pm »
Yes - no need to do each pad seperately - just flux, and wipe over with a fairly large, loaded tip.
You don't want too much on teh centre pad so braid off as necessary.
The more flux, place part and hot air - it will pull on to the pads.
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Offline capt bullshot

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Re: best way to hand solder a QFN?
« Reply #3 on: March 20, 2017, 01:58:16 pm »
Basically, same as Koen said.
The thermal pad is best done by putting a bit of normal solder (don't use lead free) on the PCB pad, grossly put flux on it and use a small heat gun (best a special hot air rework station). Air flow from a home improvement heat gun will just blow away the chip. Set temperatur to 400 ... 450 deg C, soldering may take some 10s of seconds. The pins can be done by using a soldering tip for SMT (these dished ones) using the same technique as TQFP chips.
I've done this successfully a few times.
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Offline wraper

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Re: best way to hand solder a QFN?
« Reply #4 on: March 20, 2017, 02:07:55 pm »
400 ... 450 deg C
If your intention is to scotch the board, sure. If using hot air gun, there is no need to solder the pads afterwards. Apply solder to the pads, add some flux and solder all of that with a hot air.
 

Offline NivagSwerdna

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Re: best way to hand solder a QFN?
« Reply #5 on: March 20, 2017, 02:25:37 pm »
Flux it and use a regular soldering iron to place regular solder on the pads. Just a bit on the center pad, don't make a dome. Clean it all. Flux it all. Place the chip. Hot air.
Indeed. build a small puddle on the centre pad, hot air it and squish. Then work around the edges.
 

Offline FrankBuss

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Re: best way to hand solder a QFN?
« Reply #6 on: March 20, 2017, 02:26:05 pm »
Thanks, will try it.
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Offline mbless

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Re: best way to hand solder a QFN?
« Reply #7 on: March 20, 2017, 05:39:13 pm »
I've done it 2 ways so far.

1. Pre-tin the pads. I like to use tacky flux with this method since it's hard to get all the pre-tinned pads to have the same solder height. Tacky flux will hold it in place well.
2. Solder paste. I haven't used a stencil, so it's always been a best guess knowing how thick a stencil is. I like this method better because it's easier to tell when the solder melts.

Both ways I set the hot air to 260C (I only use leaded solder) and low flow for a small chip like that. Once it appears to have melted all of solder, I give the chip a gentle tap with tweezers to see if the chip pulls back to the pads; that tells me all of the solder has taken.
 

Offline james_s

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Re: best way to hand solder a QFN?
« Reply #8 on: March 20, 2017, 08:13:01 pm »
I use a hotplate to reflow. Use a toothpick to put a dab of solder on each pad, place the IC, cook it. Lacking solder paste you can tin the pads, add some flux and do the same process. You have to be careful to keep the IC from sliding around that way though as the solidified solder doesn't hold it the way paste does.
 

Offline JoeN

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Re: best way to hand solder a QFN?
« Reply #9 on: March 20, 2017, 08:16:39 pm »
I do it with hot air too.  It helps to inspect with a microscope afterwards (clean it first), you can usually easily see if there is a solder connection between the QFN pads and the PCB pads.
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