On 8/18/07, Andrew <
andrewm1973@...> wrote:
>
> If the chips have legs like TQFPs or
> even stubby metal stumps like _some_
> QFNs have then you can drag a pool
> of solder across the pins and the
> surface tension will leave a nice
> clean set of legs.
>
> However the QFNs that you can hardly
> see the pads on the side of there is
> not much choice but to reflow them.
> Also anything with an exposed pad
> you must reflow.
Yes, you must also reflow if there is a thermal pad. Although there is
a method to pre-tin the pad, the reflow it, and then drag solder the
surrounding pads. It is suggested for rework this way by one
manufacturer of QFN cases, i don't remember the name. This will work
well when you don't have any SMD paste, or if you find the one you
have has dried out.
>
> I have a hot air tool and all the
> gear needed to do it properly,
> however when I want to do it quick
> and dirty I just use a butane torch.
> (only on prototypes mind you)
Which hot air tool do you have?
We plan to puchase SMD tools at work, and i am looking for likely candidates.
The only time i tried to tin a QFN part with 60/40 and iron i promptly
ripped out one of the pads of the IC. Suffice to say first thing i did
was order a syringe of SMD paste.
I also use a very large tip on the iron as "pinpoint hot plate" to
replace small 8 pin QFN with thermal pad. Just place it opposite the
IC (it is a thermal design with many VIAs so heats through well). I
have also replaced 32 pin QFN by heating from the top with a large
glob of solder on a large tip. Not good, takes long, but the ICs
survive. Almost anything is better than this method though.
>
> I then place the target PCB on a
> scrap of FR4 and preheat from
> below for a bit. I then move the
> heat to the top side and very
> gently continue the heat. When I
> see the chip move and allign itself
> I give it a bit more heat close up
> and then back off and let it cool
> slowly.
When i reflow PCBs i too put them on a scrap of PCB (actually a
special material that the PCB supplier puts under the boards for
drilling, he puts those scraps in the package when shipping and they
make a great heat-proof surface to solder on)
Then i heat with a temperature adjustable hot air gun, usually only from above.
As you can see we really need to purchase some tools at work, but i'm
unsure what is best. There are hot air pencils, slightly larger hot
air tools but still smaller than a heat gun, then there are preheat
(and possibly reflow) hotplates, and reflow ovens, and infrared
heaters, and lotsa other stuff. I'm wondering what will be most
useful.
> The fine needle tips I have
> (on a Hakko) just don't get the heat
> down the business end fast enough.
>
That is true, i have few uses for the very sharp long tip and when i
use it i must increase the temp. setting to get anything molten. I
don't even attempt leadfree with that tip.
> I get much better results dragging
> the solder away with a tip that is
> over a mm than one that is 0.7mm.
Same here.
> Also touching new solder to the pool
> very regularly to keep flux in the
> game helps me out.
I found that SMD soldering paste has more/better flux. So often i will
apply some of that, and then drag-solder over it.
Stupidly i just bought 1.5kg of solder that doesn't have very good
flux, or possibly too little. I especially ordered the more expensive
solder with 5 flux cores (Edsyn), expecting to get something good,
that it will wet well, but it turns out the opposite is true. The
total percentage of flux is much lower than with the old solder. It
will take years to use it all up.....
ST