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From: Jon Slaughter on 27 Dec 2009 14:01
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?

From: krw on 27 Dec 2009 14:12
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?)

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.
From: Nico Coesel on 27 Dec 2009 14:49
krw <krw(a)att.bizzzzzzzzzzz> wrote:

>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?)
>
>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.

That is usually solved by putting several uncapped vias underneath the
QFN. The vias also allow to solder the center pad manually. Preheat
the board using a heat-gun and put solder in one of the vias. Wait
until the solder comes up from the other via. Then you'll know the
center pad is soldered.

>>If I have the right amount of paste in the right places and the ic is
>>somewhat closely aligned will it align itself?

Nope. If you make the pads a bit larger you can hand solder the part
with a very fine soldering tip. I usually wrap a piece of thin solid
copper wire around the soldering tip to solder the pins. Copper is
still the best material for soldering tips.

Ofcourse, you'll need flux as well.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico(a)nctdevpuntnl (punt=.)
--------------------------------------------------------------
From: Jon Slaughter on 27 Dec 2009 15:04
krw wrote:
> 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?)
>
> 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.

From: krw on 27 Dec 2009 15:45
On Sun, 27 Dec 2009 14:04:14 -0600, "Jon Slaughter"
<Jon_Slaughter(a)Hotmail.com> wrote:

>krw wrote:
>> 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?)
>>
>> 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.

Be sure to check that the pad isn't (the only) ground. I've seen this
more than once.

QFNs are a great idea but what a PITA. QFNs and interboard connectors
are our two largest problems.
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