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'[PIC] Hand-soldering PIC18LF2520 leadless QFN?'
I have produced a design for a small wireless device that uses an
18LF252 in a 28-pin SO package. I need to reduce the size of the PCB if
possible, and I am thinking about using an 18LF2520 in a 6mm^2 28 pin
leadless QFN package. Hopefully these boards will be machine assembled
eventually, but I will initially need to be able to hand-solder around a
hundred of these boards, and so that leads me to the obvious question:
can these packages be realistically hand-soldered by someone who is
competent with working with SMT devices?
I notice that the QFN package has a large ground pad in the centre. I
guess that it is necessary to solder this area to the PCB for mechanical
strength, is that right? Would it be very difficult to solder reflow if
I provided a few vias underneath the device and used a heatgun, paste
Would I be strongly advised to stick to the larger SO package?
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I've used some QFN packages from Linear Tech, and the
contract manufacturer did hand assembly on them
(amazed me that they did...but it also had 0201 parts
as well) and found about half didn't get soldered
right. With a magnifying scope, my tech was able to
solder these pretty easily.
My take is...if your doing 100 boards, its still
better to get a CM to flow them.
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At 06.30 2004.10.19 -0700, you wrote:
>I've used some QFN packages from Linear Tech, and the
>contract manufacturer did hand assembly on them
>(amazed me that they did...but it also had 0201 parts
>as well) and found about half didn't get soldered
>right. With a magnifying scope, my tech was able to
>solder these pretty easily.
>My take is...if your doing 100 boards, its still
>better to get a CM to flow them.
CM = ?
My guess is CM = Contract Manufacturer
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
On Tue, 19 Oct 2004 16:10:45 +0100, No Religion <infinito.it> wrote: noreligion
> >I've used some QFN packages from Linear Tech, and the
> >contract manufacturer did hand assembly on them
> >(amazed me that they did...but it also had 0201 parts
> >as well) and found about half didn't get soldered
> >right. With a magnifying scope, my tech was able to
> >solder these pretty easily.
> >My take is...if your doing 100 boards, its still
> >better to get a CM to flow them.
> CM = ?
CM = contract manufacturer
|or, my method of using a heat gun might work very nicely if it's a small
board. you want to wave it back and forth to heat things slowly. i'd
think the "hot plate" method might work nicely as well. the key is that
the solder must melt on all the pins at about the same time, that way
surface tension keeps the part in the right place. you might still need
a stencil to deposit the solder paste correctly however.
i have seen one board that had a cutout through the board under the pad
of such a chip, but this is probably why the motor driver failed in the
cdrom drive it was in (i was hoping to fix it, until finding that the
part was both proprietary and similar parts weren't made by the same
chip maker any more). having said that, if the chip doesn't need the
pad to connect to a heatsink on the board, a small hole through the
board could be used as a place to hand solder it down, i.e. a large pad
on the board in the right place with a large center hole for applying
solder and soldering iron if the pad needs to be electrically but not
most of the rework gear i've seen for this type of thing also uses a
heater bellow the board to provide some of the heat and protect the
board from large thermal gradients so a combination of hot plate and
heat gun might be a good idea. not something i'd like to have to work
out during production of the first 100 units, rather something i'd like
to play with well ahead of time. a good contract manufacturer might be
the way to go, though of course it's hard to tell if a company is really
good at this sort of thing if they haven't done it for you before.
i think that even in commercial manufacture these parts and bga's with
all the pads underneath them have caused serious problems, reading the
suggestions for repairing and inspecting bga assemblies seems to suggest
that they are often a problem and very hard to rework, and not trivial
to get correct even with a reflow oven. not to mention that bga
packages usually necessitate micro-vias for routing unless the pin out
is very cleverly done, and this also greatly increases board cost and
reliability issues. on the other hand, i do expect that i'll eventually
have to solder bga parts, i don't expect to like it.
alan smith wrote:
> I've used some QFN packages from Linear Tech, and the
> contract manufacturer did hand assembly on them
> My take is...if your doing 100 boards, its still
> better to get a CM to flow them.
Philip Stortz, mad scientist at large -- "It is sobering to reflect that
one of the best ways to get yourself a reputation as a dangerous citizen
these days is to go about repeating the very phrases which our founding
fathers used in the struggle for independence." -- Charles A. Beard
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