printed wiring boards
here to go to our main page on microwave packaging
to go to our page on microwave circuit card assemblies
here to go to a new page on hints for RF PWBs (new for March
2008, contributed by Greg!)
here to go to our page on FR-4
here to learn how to extract DK and DF from transmission line
measurements (new for August 2010!)
It's about time we added some
content this important topic. This is gonna take some time, but
it will go a lot faster if we get some audience participation like
we did from Mike and Greg!
Here's a tip from Microwaves101
reader Mike on how to fix a board with a short circuit:
I recently ran across a
problem and solution that you may find valuable enough to post
on you great page. My company which I will keep anonymous recently
had to do a commutator board. When the boards were received from
the manufacturer, one of them had a slight problem. One of the
RF traces of the switch output was grounding out. A call to the
manufacturer lead us to this diagnosis: when the board was processed,
a sliver of metal from the ground plane was left still in contact
with the RF trace. The company told us a quick fix for this, without
having to have them build a new board for us was to ground the
ground plane and send a high amount of current through the RF
trace. This current would cause the sliver that is grounding out
to burn up and there would be no more problems with the trace.
Low and behold it worked and we didn't have to wait for a new
board and then have to mount all new components to it! From now
on, we are spending the extra cash and having all the boards tested
by the company before they are shipped to us.
This is still a jumble of topics,
PWBs for microwaves are fabricated
similar to PWBs for digital and "low frequency" applications.
However, in microwaves we often are restricted to two-sided boards
instead of multilayer boards. Instead of ceramic substrates with
gold metalization, lower-cost "soft" substrates with copper
metalization are process.
Here is a list of steps that
are done to create a microwave PWB:
2. Material selection
Thorough cleaning is essential
to the etching process. Cleaning is accomplished in a combination
of solvent bath, rinsing with deionized water and or alcohol, blow
drying and baking.
4. Apply photo-resist
5. Imaging: here the mask is
used to expose the photo-resist
10. Solder mask
11. Finish metalization
Copper for PWBs is often specified
in "ounces". This arcane term actually originates from
how many ounces of copper would be rolled out per square-foot. Half-ounce
copper is 0.7 mils (18 microns), one ounce copper is 1.4 mils (36
microns), and two once copper is 2.8 mils (71 microns) thick. Thanks
to Ricardo for correcting our math in converting from mils to microns!
Some definitions that are related
to PWB processing:
dry film photo-resist
This is the nominal difference
between the mask feature and the etched features.
Hey, PWB designers, help us out
with some content!