yeah that's a tricky one especially if you have little or no pad
sticking out underneath
which is often the case. and very soon you will start loosing pads too.
what cable are you using? i normally use single core thin wire wrap
wire to patch these
kind of things. i did patch one 0.5 mm pitch tqfp recently but that
was tricky enough and
that was just 2 wires - qfn with same pitch is of course much worse.
anyway here is my 2 cents of hints and experience in the field of fine
pitch patching:
one trick is to use hot glue to hold the cables in place before or
after soldering - with a rigid wire
like the wire wrap wire the hot glue could hold the wire at some
distance just to keep the direction
towards the pad and then you could just fix it to the pad with solder.
normally with a small pad to
solder to, just the weight of the wire would break off the solder
unless you fix it somehow.
if you have a flex pcb cable with 0.5mm pitch(like these ones
http://www.farnell.com/datasheets/5359.pdf
which you might be able to find in some consumer electronics device if
you're lucky) it could work
to solder a bunch of pins next to each other with the flex cable
aligned but still it's very tricky to solder. but
even just getting a few more mm to work with could make the difference.
if you're desperate and have hot air soldering available you could
even tomb-stone
the qfn on one edge - there will be many wires to solder but you would
at least have full access
to pads on both pcb and chip. in all cases being able to lift the chip
with hot air would make
all options simpler in my experience. then you could even put wires
between chip and pad if
you can lift it.
another option would be to manufacture a small pcb that goes between
pcb and qfn
and rearranges the footprint - also needs lifting the chip of course.
i guess the board
would cost about a 100 euros to manufacture with solder mask, without
solder mask
a developer board with next day delivery would be ca 60 euro here -
could probably
even get a couple of them for that price.
also digmata, the company in stockholm that mounts many of my pcb's
specialize in
patching stuff like this - they even patch things under bga's putting
wires between the
balls and such - no clue what they cost though for this kind of work -
but judging from
the stuff they showed me i guess it would be a simple job for them. if
you want to i can
ask them for a price?
i guess it's all comes down to how eager you are to test it before the
next run.
/b
On 30 jun 2010, at 21.09, Sébastien Bourdeauducq wrote:
On Wednesday 30 June 2010 05:44:10 Adam Wang wrote:
Directly solder wires from P[15:8] to P[7:0] on pins should be ok.
Well that's harder than I thought and I've been quite unsuccessful
at it :(
Soldering wires to QFPs is not like soldering QFPs on PCB pads...
short
circuits you struggle to remove, solder joints that don't stick,
solder that
melts releasing the wire when you heat the pin close to it, etc.
Do you think you could do the rework on your second board and then
ship it to
me?
Meanwhile I will continue working on the other parts of the board -
DMX, MIDI,
IR and USB.
Sébastien
_______________________________________________
http://lists.milkymist.org/listinfo.cgi/devel-milkymist.org
IRC: #milkym...@freenode
Webchat: www.milkymist.org/irc.html
Wiki: www.milkymist.org/wiki
_______________________________________________
http://lists.milkymist.org/listinfo.cgi/devel-milkymist.org
IRC: #milkym...@freenode
Webchat: www.milkymist.org/irc.html
Wiki: www.milkymist.org/wiki
