Homebrew PCBs

Does anyone here have any pointers on making a board with a QFN packaged 
device and then hand soldering it?  I'm pondering doing something like 
putting a lot of relatively large vias underneath to made a connection 
with the thermal pad on the bottom.  At first I thought that I might get 
away with using heat sink grease, but it looks like that thermal pad is 
the only connection for Vss.

-- 
David Griffith
dgriffi@...

A: Because it fouls the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?

You should put a large thermal pad without solder stop, connected with
lotsa vias.

I use this package a lot, and it is fairly easy to hand-solder with
hot air. If you only have an iron, that works too. solder the thermal
pad from the far side through the PCB.

ST

On Wed, Jul 29, 2009 at 6:43 AM, David Griffith<dgriffi@...> wrote:
>
> Does anyone here have any pointers on making a board with a QFN packaged
> device and then hand soldering it?  I'm pondering doing something like
> putting a lot of relatively large vias underneath to made a connection
> with the thermal pad on the bottom.  At first I thought that I might get
> away with using heat sink grease, but it looks like that thermal pad is
> the only connection for Vss.
>
> --
> David Griffith
> dgriffi@...
>
> A: Because it fouls the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?
>
>

If you really want to use an iron, what I did was put a big via in the
middle of the pad - big enough to solder the pad from the back side
through the hole.  Don't rely on your smallest tip, though - use
something beefy enough to heat up all that copper.  You'll need flux
in there too.

But I find that using a hotplate makes QFNs much easier.

Also, don't skimp on the amount of pad that extends beyond the chip on
the PCB.  You'll need that for soldering the pins.  You'll need flux
there too.

----- Original Message ----- 

From: "David Griffith" <dgriffi@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Wednesday, July 29, 2009 5:43 AM
Subject: [Homebrew_PCBs] Soldering QFNs


> 
> Does anyone here have any pointers on making a board with a QFN packaged 
> device and then hand soldering it?  I'm pondering doing something like 
> putting a lot of relatively large vias underneath to made a connection 
> with the thermal pad on the bottom.  At first I thought that I might get 
> away with using heat sink grease, but it looks like that thermal pad is 
> the only connection for Vss.

That's what I do. I use a single large via.

Leon

David Griffith wrote:
>  
> 
> 
> Does anyone here have any pointers on making a board with a QFN packaged
> device and then hand soldering it? I'm pondering doing something like
> putting a lot of relatively large vias underneath to made a connection
> with the thermal pad on the bottom. At first I thought that I might get
> away with using heat sink grease, but it looks like that thermal pad is
> the only connection for Vss.

The best method I found is to lay the PCB on a electric hotplate until 
it reaches around 120C~150C surface temperature. Use glass cloth under 
the PCB to avoid local hot spots. Then apply a adjustable hot air gun 
directly over the QFN IC will melt the solder quite quickly. Use plenty 
of liquid rosin flux. The hotplate I use is a cheap $30 domestic thing.
Once I was debugging a circuit and removed and re-soldered a 64 pin QFN 
about 5 times using this method.

Adam Seychell <a_seychell@...> writes:
> Use glass cloth under the PCB to avoid local hot spots.

Some other ideas, depending on what you're trying to accomplish:

1. Aluminum plate distributes the heat pretty evenly.

2. Coins.  Put a coin on the hotplate that's about the same size as
   the chip (a dime for 0.5mm QFN-32), and only that part of the board
   heats up.

I've used the coin trick to remove and re-install QFNs.  I think part
of the trick is to put a few dots of solder paste near the chip so you
know when it gets to melting temperature.