Andy,

Thanks for you response.  It didn't sound flippant at all, I appreciate the
time and thought you put into the message.

In answer to some of your questions:

1.  I'm in the manufacturing group and I was looking for help so I do not
have to design land-patterns from scratch for every new TQFP device that
gets designed in.  I was hoping there were IPC guidelines (on TQFP's) that I
did not know about to give our CAD guys a place to start so that I only need
to review and approve TQFP land-patterns instead of designing them from
scratch.  We have found  IPC-SM-782 to be an excellent guideline and
starting point for landpattern design.  We do reference the component
manufacturers recommended footprint, but have found that they are better at
designing and manufacturing components than they are at designing
landpatterns and typically the parts are multi-sourced with slightly
different dimensions from each manufacturer.  The landpattern calculator on
the IPC website should help me (thank you to those who responded with that
info.)

2.  This is not our first venture into 20 mil pitch (TQFP devices are not
all 20 mil pitch, we use some that are >30 mil).

3.  We have chosen immersion gold as the finish for all production volume
pcb's with 20 mil pitch or boards with lots of 25 mil pitch devices.

4.  We clean bad prints by hand right now.  Causes a mess, so we are very
careful to avoid bad prints.  We have a stencil cleaner due in house in a
couple weeks and will clean bad prints in it using a new cleaning agent
called Zestron.  (I would like some feedback on Zestron if anybody has had
any experiences good or bad, our tests were positive but have not tried it
in production yet).

5.  We reduce 20 mil stencil apertures 10% in length & width.  We use a 6
mil mono-thickness stencil.

6.  Our solderpaste mesh is -325/+500.
7.  Through experimentation at a previous employer, we have found that
band-etch gives equally good results as laser etch at half the cost.  (We
use chem-etch for standard pitch stencils).

8.  We require soldermask between the pads.  Most fabricators balk at it on
20 mil devices and say they cannot do it, but they usually do it anyway.

9.  We are using LPI soldermask.
10.  We rework 20 mil pitch using our A.P.E. chipmaster (hot air) and Metcal
irons.
11. Our designs are tight so sometimes other components (typically chip
caps) must be removed prior to QFP removal and re-installed after
replacement.  This hasn't proven to be too much of a headache because the
number of FP components that we rework is small and the time to remove and
replace a chip cap is very small compared to the FP device and the component
cost is also relatively small.  Not desirable, but we live with it when real
estate requires it.

This is more info. than most people care to know about us, but I often find
it interesting to see what others are doing in the industry so I thought I'd
share...

Steve


From: Electr1998 <[log in to unmask]>