Microwave printed wiring boards

Updated March 8, 2008

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Click here to go to a new page on hints for RF PWBs (new for March 2008, contributed by Greg!)

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, sorry!

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.

Fabricating PWBs

Here is a list of steps that are done to create a microwave PWB:

1. Mask-making

2. Material selection

3. Cleaning

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

6. Etch

7. Drill

8. De-smear

9. Metalize

10. Solder mask

11. Finish metalization

12. Stencil

Ounces of copper

Copper for PWBs is specified in "ounces". This arcane term actually originates from the ounces per square-foot of copper. The

Some definitions that are related to PWB processing:

dry film photo-resist

negative photo-resist

positive photo-resist

blind via

through hole

Undercutting

Etchfactor

This is the nominal difference between the mask feature and the etched features.

Hey, PWB designers, help us out with some content!