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From: Leon on 27 Dec 2009 16:50
On 27 Dec, 19:12, krw <k...(a)att.bizzzzzzzzzzz> wrote:
> On Sun, 27 Dec 2009 13:01:00 -0600, "Jon Slaughter"
>
> <Jon_Slaugh...(a)Hotmail.com> wrote:
> >Any have any tips for QFN packages by hand without a stencil? The main
> >problem is that there seems to be no easy way to check/fix to make sure the
> >leads are soldered since they are completely under the package. I was
> >thinking of using puting solder paste on the pads by hand and attempting to
> >properly align the IC. The problem I see is that most likely the solder
> >paste will end up between the pads rather than on them and I'll probably end
> >up with shorted pads. Does the solder on the solder mask tend to "run" to
> >the copper? (after all, isn't that what the solder resist is for?)
>
> The problem with QFNs is the center pad and getting just the right
> amount of solder on it.  There isn't a lot of room for error here. Too
> much and the part will "float" on the center pad so it won't align
> properly.
>
> >If I have the right amount of paste in the right places and the ic is
> >somewhat closely aligned will it align itself?
>
> It should, though it's a lot harder to get right than a BGA, for
> instance.  The center pad works against you, where in a BGA all the
> balls work in your favor.  Since the pad is huge, in proportion to the
> pins, it can get dicey.  Even with automatic PnP, we have far more
> problems with QFNs than BGAs.  Of course the crappy RoHS processes
> make things far worse.

I don't have any problems hand-soldering QFN chips. I extend the pads
a little and drag-solder the chip connections. I use a large via for
the central pad and feed solder in with the soldering iron tip. A good
soldering station like the Metcal system I use helps a lot.

Leon
From: Leon on 27 Dec 2009 16:52
On 27 Dec, 20:04, "Jon Slaughter" <Jon_Slaugh...(a)Hotmail.com> wrote:
> krw wrote:
> > On Sun, 27 Dec 2009 13:01:00 -0600, "Jon Slaughter"
> > <Jon_Slaugh...(a)Hotmail.com> wrote:
>
> >> Any have any tips for QFN packages by hand without a stencil? The
> >> main problem is that there seems to be no easy way to check/fix to
> >> make sure the leads are soldered since they are completely under the
> >> package. I was thinking of using puting solder paste on the pads by
> >> hand and attempting to properly align the IC. The problem I see is
> >> that most likely the solder paste will end up between the pads
> >> rather than on them and I'll probably end up with shorted pads. Does
> >> the solder on the solder mask tend to "run" to the copper? (after
> >> all, isn't that what the solder resist is for?)
>
> > The problem with QFNs is the center pad and getting just the right
> > amount of solder on it.  There isn't a lot of room for error here. Too
> > much and the part will "float" on the center pad so it won't align
> > properly.
>
> >> If I have the right amount of paste in the right places and the ic is
> >> somewhat closely aligned will it align itself?
>
> > It should, though it's a lot harder to get right than a BGA, for
> > instance.  The center pad works against you, where in a BGA all the
> > balls work in your favor.  Since the pad is huge, in proportion to the
> > pins, it can get dicey.  Even with automatic PnP, we have far more
> > problems with QFNs than BGAs.  Of course the crappy RoHS processes
> > make things far worse.
>
> IF the pad is the problem I can just leave it off then. At this point it is
> not an issue(chips have thermal shutdown and I don't plan on running them
> hot). I have vias that I can inject some solder or thermal paste if I do
> have some issues with heat.

The pad should always be soldered. If you don't, and you drop the
board, the chip is likely to shear off, taking the pads with it.

Leon
From: DJ Delorie on 27 Dec 2009 17:11

"Jon Slaughter" <Jon_Slaughter(a)Hotmail.com> writes:
> IF the pad is the problem I can just leave it off then.

For hand-soldering, put a really big untented via under it, so you can
solder it from the other side later.
From: Falk Willberg on 27 Dec 2009 17:43
Jon Slaughter schrieb:
> Any have any tips for QFN packages by hand without a stencil?

Try to avoid that. Once I had to do 20+. Not fun.

> The main
> problem is that there seems to be no easy way to check/fix to make sure
> the leads are soldered since they are completely under the package.

Mine were of that type: http://www.image.micros.com.pl/_icon_auto/qfn-48.jpg

I took a picture from each side with a quite simple digicam with a good
macro-setting to check soldering.

> I
> was thinking of using puting solder paste on the pads by hand and
> attempting to properly align the IC. The problem I see is that most
> likely the solder paste will end up between the pads rather than on them
> and I'll probably end up with shorted pads.

I found that paste between pads caused the most trouble. It can be
removed using a sewing needle.

> Does the solder on the
> solder mask tend to "run" to the copper? (after all, isn't that what the
> solder resist is for?)
>
> If I have the right amount of paste in the right places and the ic is
> somewhat closely aligned will it align itself?

Yes, it will.

If you do not plan to show it to others, dead bug "design" might be an
alternative.

All hand soldered and working: http://falk-willberg.de/loetkunst.html

Falk
From: qrk on 27 Dec 2009 18:28
On Sun, 27 Dec 2009 13:01:00 -0600, "Jon Slaughter"
<Jon_Slaughter(a)Hotmail.com> wrote:

>Any have any tips for QFN packages by hand without a stencil? The main
>problem is that there seems to be no easy way to check/fix to make sure the
>leads are soldered since they are completely under the package. I was
>thinking of using puting solder paste on the pads by hand and attempting to
>properly align the IC. The problem I see is that most likely the solder
>paste will end up between the pads rather than on them and I'll probably end
>up with shorted pads. Does the solder on the solder mask tend to "run" to
>the copper? (after all, isn't that what the solder resist is for?)
>
>If I have the right amount of paste in the right places and the ic is
>somewhat closely aligned will it align itself?

For non-production type work (e.g. repair and prototype), I tin each
PCB pad so there's a small bump of solder. I make sure the bumps are
approximately the same height. If you have a thermal pad in the
center, only apply enough solder to raise the part slightly. Too much
solder on the center thermal pad will cause problems. I use leaded
solder since it's easier to handle and has a lower melting point than
unleaded solder.

If you're reworking a board that used unleaded solder, I flood the
pads with leaded solder which dilutes the unleaded solder enough where
it isn't a problem. If you have leisured time, you can remove the
unleaded solder with solder wick or a vacuum desoldering station.

Next step, coat the bottom of the IC with flux.

Place the part.

Preheat the board to 150 deg C. I use the Madell QK853 hot air plate
to preheat boards.

Once the flux has thickened enough (I use RMA flux) use hot air
soldering station to solder the part. You will see the part self-align
when the solder melts. If you use water-based fluxes, be sure to wash
the board as soon as possible as water based fluxes are highly
corrosive. Water based fluxes are also conductive.

Hot air systems can be had for under $200 for cheap Chinese equipment
(See Madell QK857D). They are fine for occasional repair work or
prototyping.

Other ways of soldering are toaster ovens and, my favorite, a waffle
iron with flippable plates (waffle or flat).

I have used this technique down to 8-pin parts in 1.6 x 1.6 mm
packages.

--
Mark
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