mechanical chassis design for rf/microwave module

[marco110]

microwave mechanical aluminium

anyone has any design guidelines on this? about what material to use, plating? the method used to transfer signal to outside world. sealing or no sealing? thermal issues? emi?

 

[VSWR]

microwave module

What is the operating frequency?

 

[lguancho]

For microwave modules, aluminium is a common material used for chassis design. Plating is necessary when you want to reflow your substrate directly onto the chassis body, or you want to reflow your feedthrough capacitor. Gold is commonly used.

There are different grade of aluminium. The 6000 series is commonly used. If you need laser sealing, special care must be taken on the al material. Usually, people use 4000 series for the cover, and 6000 series on the chassis body.

Question of whether sealing is required? Depends on your module design. Do you use bare die/chip, with bond wires, or you use sealed components? Do your module operates in enviroment where water condensation is possible. If yes, most likely you need some kind of sealing.

Method to transfer signal out/into the module? You can use SMA connector up to 20 GHz with no problem. There are different type of SMA connector. Some can be used for laser hermetic sealing, some are for normal application. Check out the website of MA/COM or SouthWest Connectors for more info (Do a google search)

8)

 

[lguancho]

Hi, I attach two articles about hermetic sealing and laser welding. It addresses some of your questions directly. Enjoy.

 

[marco110]

eh can't see the attached articles.

i'm looking at microwave frequency range.
is reflowing of substrate a preferred method? how about interconnecting using high quality semi-flex cables? if this is used does that mean i can do w/o gold plating?

we usually use the 6000 series. any reason why the plate uses the 4000 series?

i've some components that is chip level that requires bonding, and likely to encounter condensation. besides hermetic sealing, is anyone familiar with parylene coating? will there be any impact on microwave components?

 

[lguancho]

I don't know in details about your circuit design. In general, if I am using soft substrate such as roger 5880 series (10 to 15 mil thickness) for the microstrip interconnecring lines, I will reflow them directly to the chassis. For alumina circuits, I will solder them to some kovar carrier and them screw them into the chassis.

Plating is to improve the solderability of bare aluminium. If you do not need to reflow any of your substrate directly to the chassis or any DC feedthrough capacitors, then you don't need any plating.




The reason for different material is stated in the design guide uploaded by me. You mean you can't download it? Just do some google search for laser sealing guides.

I don't think people use conformal coating for microwave circuit. It affects the circuit performance (effective dielectric constant is changed).

Cheaper sealing could be solder, but it is a heat intensive method. Could destroy your circuit. Laser sealing is a localized welding method, most reliable but most costly too.

 

[lguancho]

Hi, you may be interested in the following book:

Microwave component Mechanic Published by Artech House. Could be downloaded at:



What I had discussed so far are mentioned in the book too. 8)