randeel wimalagunarathne <
randeelwrw@...> writes:
> ic is trf7960
Ah, a QFN with a thermal pad. I've done those with the hotplate, but
it's critical to get the right amount of paste down.
> no haven't made a board for it. because we are confused about
> soldering this tiny IC.
Rightly so, the thermal pad is going to be very difficult with an
iron, because you need a LOT of heat to heat it - after all, it's a
THERMAL pad, you need to design the board to suck heat away from the
pad, which makes it hard to solder.
> 3. it's the normal soldering iron with a tip having a pen shape.
Ah, ok.
Depending on how critical the heat issue (thermal pad) is, there are a
couple of options.
If you're doing your own boards, and can't do through-plated vias, and
the heat issue isn't critical, you can just etch a rectangle to solder
to and hope the pcb substrate itself will move enough heat.
Without PTV if heat is an issue, cut a rectangular hole in the pcb the
size of the pad and attach a heat sink with silver thermal grease.
With PTV, put a bunch of small ones in that pad to pull heat to copper
on the other side of the board.
In any case, bring the pads for the QFN out far enough that you have
room to heat the pads with your iron. Mine has a 0.020" tip so I pull
them out 0.020" past the side of the IC package. Even if you use a
hotplate, you'll want this for any repairs.
Now, if there's copper under your thermal pad, you must use a hotplate
or oven to put this part on. I use a hotplate[1]. Put solder paste
on the thermal pad but don't cover 100% else the chip won't seat low
enough to contact all the other pads. The chip's PDF pas a
recommended paste stencil layout. Put paste on the other pads too,
out past the body (you want more paste to ensure a connection, but you
don't want it underneath where it might short). Place the part, but
try to get it right the first time and don't press down hard, either
will smear the paste and you'll have to start again. Heat it. With
the hotplate, you can press down a bit once it starts melting, to make
sure all the pins are contacted. Make sure you don't bump the part
off the solder until the solder cools.
If you cut a hole under the part, you can use your iron. You still
use paste, but only put it on the little pads. Place the chip, and
use the iron to reflow the paste - touch the pad outside the body and
let it heat up the copper until the paste melts. Start with opposite
corners, then do the rest.
To repair, again use paste - you can use an xacto blade to squeeze
paste under the chip, and reflow it with your iron. Copper braid can
be used to remove most shorts.
As for putting the paste itself down, you can either use a stencil (I
etch brass ones myself, but you can have mylar ones laser cut at
www.smtstencil.com) or an xacto blade. A sharpened toothpick is great
for removing paste from between pads.
[1]
http://geda.seul.org/projects/djs_pcbs/ at the bottom (note: I
forgot the "rinse" step with the liquid tin; that's why the boards
look corroded)