how to solder QFN/DFN chips?

I've managed to solder a similar device using your proposed technique. It is fiddly, though, and I needed to feed some solder in to get a decent joint. Now I have a stereo microscope it should be a lot easier. Hot air and solder paste is another way that should work.

Leon

I see (in common with a lot of these) that the device has a pad underneath, although there is no guidance on where that's connected.

Anyhow, with such a device, hot air makes the most sense. One guy I have do a lot of work for me uses a simple hot air gun (you know, the industrial type) for these things. Does the job and a whole lot cheaper than a pro hot air station.

Get some solder on the pads (if you have paste that's even better) and then heat it gently (but not TOO slowly) from an angle of about 45 degrees, moving around the device. It should get hot and reflow in a very short time.

Cheers

PeteS

For prototyping the pad can be soldered by putting a via in the centre and heating it up with the soldering iron tip whilst feeding solder into it.

Leon

That is why I built a little setup:

Rene

"Leon" wrote in news:1121337560.969907.54040 @f14g2000cwb.googlegroups.com:

QFNs can usually be soldered just like leaded SMT parts if you extend the pads outside the body of the part. We use lots of flux, a microscope and a soldering iron. If the part has a large center pad, we have used solder and a heatgun effectively for this connection. We also started placing a large thru hole in the center of the pad so that we can access the pad from the bottom.

Leon wrote (without context):

Leon, FYI: It's easy to get context when posting from Google Groups: Instead of clicking the Reply link that is in plain sight, click the **show options** link, then click THAT Reply link.

I hope PeteS read this; he's dong it too.

:Some fearlessness from electricity is required at this point. :I might have to look for some high temperature insulation at a later stage. : Heh...or new employees. . . Proofreading: Beside that no overheating occures at any locations. Besides that, no overheating occurs at any locations.

"Leon" wrote in news:1121316804.180551.91280 @g49g2000cwa.googlegroups.com:

How does one solder a part with hot air and solder paste? I've always thought that it seemed like the hot air would just blow the part away... And how/where does one apply solder paste?

thanks,

Michael

I've done it by preheating the board to ~100°C, then melting the solder with hot air, putting the chip on there (no preheat on the chip, hey, it's just a prototype), and waiting until the chip settles into place with the pads all nice and watery. A proper layout should draw the chip into position from surface tension. You might want to practice a bit first. A flux pen doesn't hurt either.

Best regards, Spehro Pefhany

They look like a Sharpie felt marker, but are filled with flux. Just a few dollars each.

Eg. Kester 83-1000-0186

or

Best regards, Spehro Pefhany

A flux pen ? Never come across one of those.

Graham

When using it, do you just paint the flux across all the pads, or do bridges in the flux lead to solder bridges?

Just paint across all the pads.

Best regards, Spehro Pefhany

I've "hot air reflowed" SOICs over a small paraffin lamp, mostly to see if it would work. It did work, though I had to touch up a couple of pins with an iron (not enough paste on those pads). Single-sided boards only, of course!

I seem to recall an article (in Circuit Cellar) a year or so ago about using a toaster oven to do hot air soldering.

-- Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com

------------------------------------------------------------------ David Bradley (IBM programmer who created the Ctrl-Alt-Del reboot keyboard sequence), "I may have invented it, but Bill made it famous."

yep

quite the opposite - excess flux prevents bridging. Wide wedge-shaped tips can be used to solder all the legs on, say, a tqfp by:

- applying lots of flux

- lying a piece of thin solder across all the pins

- applying wide iron tip to all pins simultaneously, melting solder then dragging tip away from chip

voila, all pins soldered.

Soldering station salesmen show similar setups at trade shows. They say the iron/tips are the magic part, but the copious quantities of flux are what really helps, along with lots of practice.

then you need to clean off the flux.

IMO a (binocular) microscope + soldering iron with reasonably fine tip + thin solder is all thats required. At a minimum you need to be able to see between pads, to find shorts.

Cheers Terry

So is pre-tinning the pads or pins typically done before using hot air, but not before using an iron?

I have a (monocular) microscope and I've used it to repair PQFP pins, but I could only use it near the board edge. I'd love to have a swing arm if I can find a cheap one.

Terry Given wrote in news:BvVCe.1485$ snipped-for-privacy@news.xtra.co.nz:

I have never used a flux pen. I use a small bottle to squirt flux on the pads. You can't use too much flux but it important to wash your boards after soldering. If you need to finish your board at a later time (perhaps the next day), take the time to wash the boards before quitting.

I have seen many approaches to soldering QFPs and other fine pitch parts. We use a small chisel tip. Apply solder to the tip and let it wick into your pad & pin. I know this is contrary to the way you solder larger thru hole connections.

You NEED good vision. I solder using a 7x setting on a stereo microscope. With a little practice, you will always know where the soldering iron tip is even when you can't see it. I don't think I have burned a finger in 3-

I also apply a little solder in one corner to tack it down. Then I rotate and tack down the opposite corner. Make sure alignment is absolutely perfect before soldering more pins. I like to either use a very small sharp probe from my multimeter or an xacto knife to verify that all the pins are secure after soldering.

Fresh solder is also helpful. Solder ages (or at least the flux inside them). I like Kester water soluble .020 63/37. I don't know what we will use when we move to lead free. I expect that a great flux will be the most important element to good soldering.