how to solder QFN/DFN chips?

[Michael Noone]

Hi - I'm going to need to solder a DFN chip in a couple of weeks for a
prototype. This is the chip: http://www.kionix.com/Product Sheets/KXP74%
20Series.pdf. I just got them in the mail today and am now a bit frightened
of them... The pads are entirely on the bottom of the chip - thus are not
exposed at all when placed on a board. How does one solder such a chip? I'm
thinking I'll make the pads long enough that they stick out from underneath
the chip (I haven't sent the board off to be made yet) - then I'll place
the chip on the board and touch my iron to each pad on the board
individually. Does this sound like a good plan?

Would a hot air station be helpful in soldering a chip like this? I've been
looking at some of the inexpensive Hakko knockoff hot air stations (ie
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7527217534). Or is it
best just to stick with an iron? (I have a Hakko 936-12 along with a couple
fine tips)

Thanks for your help!

-Michael J. Noone

 

[Leon]

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

 

[PeteS]

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

 

[Rene Tschaggelar]

Hi - I'm going to need to solder a DFN chip in a couple of weeks for a
prototype. This is the chip: http://www.kionix.com/Product Sheets/KXP74%
20Series.pdf. I just got them in the mail today and am now a bit frightened
of them... The pads are entirely on the bottom of the chip - thus are not
exposed at all when placed on a board. How does one solder such a chip? I'm
thinking I'll make the pads long enough that they stick out from underneath
the chip (I haven't sent the board off to be made yet) - then I'll place
the chip on the board and touch my iron to each pad on the board
individually. Does this sound like a good plan?

Would a hot air station be helpful in soldering a chip like this? I've been
looking at some of the inexpensive Hakko knockoff hot air stations (ie
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7527217534). Or is it
best just to stick with an iron? (I have a Hakko 936-12 along with a couple
fine tips)

Thanks for your help!

That is why I built a little setup:
http://www.ibrtses.com/projects/vapourphasesoldering.html
It is untested till now.

Rene

 

[Leon]

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

 

[Al Clark]

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

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.

 

[JeffM]

Leon wrote (without context):

...your proposed technique...is fiddly

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.

 

[Michael Noone]

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

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

 

[JeffM]

That is why I built a little setup:

http://www.ibrtses.com/projects/vapourphasesoldering.html
Rene Tschaggelar

: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.

 

[Spehro Pefhany]

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

 

[Pooh Bear]

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.

A flux pen ? Never come across one of those.

Graham

 

[Spehro Pefhany]

A flux pen ? Never come across one of those.

Graham

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

Eg. Kester 83-1000-0186

<http://www.kester.com/Data Sheets\F...vated (RMA)\186 Flux Pen Global (01Apr05).pdf>

or http://tinyurl.com/bgtkg




Best regards,
Spehro Pefhany

 

[Tom Del Rosso]

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

Eg. Kester 83-1000-0186

<http://www.kester.com/Data Sheets\Fluxes\Rosin Mildly Activated%2
0(RMA)%5C186%20Flux%20Pen%20Global%20(01Apr05).pdf>

or http://tinyurl.com/bgtkg

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

 

[Spehro Pefhany]

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

 

[Wim Lewis]

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.

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!

 

[Terry Given]

When using it, do you just paint the flux across all the pads
yep

, or do bridges in the flux lead to solder bridges?

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

 

[Paul Hovnanian P.E.]

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

 

[JeffM]

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

http://www.ustr.net/smt
http://www.seattlerobotics.org/encoder/200006/oven_art.htm

 

[Tom Del Rosso]

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

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

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.

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]

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

all the PCBs I work with are HASL'd, so yes. Sometimes I add a little
bit of solder to one corner pin, to tack the device in place. In
practice I dont have one of the nice drag-soldering style wide tips, so
solder pins one at a time. Its easy when you can see.....

And just like Spehro said, I use a flux pen.

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.

The swing arm on my microscope is a 2" steel bar about 3' long,
connected to a base so heavy I need both hands to lift it (and I'm a big
guy

OTBS, it helps hold the house down when the winds pick up. A tree
snapped in half yesterday.....

Cheers
Terry