[Amps] Re: Using Surface-Mount Devices

Dave Haupt emailw8nf at yahoo.com
Tue Sep 16 00:56:56 EDT 2003

WSPs aren't that bad.

We've done a few solder-bumped WSPs, with 20mil
(0.5mm) center-to-center spacing on the pads.  There
are rumours that industry is going to head towards
0.4mm and even 0.35mm, but they will reach a limit. 
You cannot control PC board etch tolerances well
enough to achieve traces or spaces tighter than 3mil
before you risk shorts and pinholes.  So, at least at
first when they go to the smaller pitches, the balls
will be on the perimeter only, since you can't route
PC traces out from in between them.

For the 0.5mm packages, we get a screen from the same
vendor who makes the PCB - they're good at that.  We
squeegee some solder paste through the screen then
inspect with a 10X microscope to make sure we got it
centered.  If not, clean off with alcohol, clean the
screen, then squeegee again.  Once we're happy with
the solder paste, we press the component into the
paste, then place in an oven at about 50 degrees below
soldering temp.  After 20 minutes, pull out of the
oven, and do a quick spot-heat with a reduced-speed
heat gun, to get that one little area just over
soldering temp.  The paste melts fairly evenly, and
self-centers the IC on the pads.

Homebrewing such a thing is too pricey, though - it's
pretty hard to find a reasonably priced PC board shop
that can etch the fine pitches required, or that
stocks the high Tg materials needed to handle the

0402s, on the other hand, I hand-solder using a 5X
Opti-Visor.  Mind you, my eyesight is bad enough that
there are days I'm not legal to drive, so no
complaining due to age!

By the way, the tight pitch is all about low cost -
high frequency hasn't much to do with it.  We've used
1.27mm c-c spaced ICs operating at 10Gb/s data rates
without any troubles - and they had internal
substrates re-routing signals from package pins to
silicon.  The largest device I've had to test to date
was 1200 pins, on 1mm pitch.


Dave W8NF

Peter wrote:

>They're nothing - just wait until you get WSP (Wafer
Scale Packaging). 
that, you take the IC chip and put a layer of
insulation over the top, 
>via holes to the pads. Then you metalise through the
vias, and put 
>on the insulation to connect to set of solder balls,
spaced 10 thou 
>in US terms, 25micron apart)and about 3 thou in
diameter. That you 
>down like a BGA, and a 40 ball device is about .05
inches square. I 
>think you'll do that with a normal hot air gun, and
it's even closer to 
>totally unrepairable. It's not a bad package from the
viewpoint of RF
parasitics, though.


>Peter G3RZP (Back in Stockholm for the week)

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