Conformal coat CC2530?

John,

With respect to the thermal question, the thermal pad on the CC2530 package is only used for the good RF ground connection it provides.  I doubt that you could ever overheat this part without some external influence or unless the conformal coating has an extremely high thermal impedance and even then it would take quite a bit of time.  Having it connected to the ground plane certainly mitigates this even more.  In general, I wouldn't worry about power dissipation here but it's always a good idea to setup a thermal budget to be sure.  The conformal coating manufacturer should be able to give you the thermal characteristic data for the coating.

For the RF path, this can have some significant effects depending on the electrical properties of the coating.  Generally any material located in close proximity to a conductor will increase the velocity of the electromagnetic wave as the dielectric constant will be higher than the surrounding air.  This means the electromagnetic wave will travel down the conductor faster changing the effectivel electrical length of the conductor.  If your RF path has portions with specified impedances then the coating will most likely change those impedances.  The amount will depend upon the coatings electrical properties and the manufacture should be able to provide you the information there as well.

If you are going to be using a PCB or chip antenna which will also be covered by the coating, you will need to adjust the antenna layout to accomodate these effects as well.

Your best bet in minimizing the number of iterations you will make on your board will be to get a 3D EM modeler and simulate the system.  Of course it depends upon the needed efficiency of you system just how far you need to go to optimize it.

Jim

Thanks Jim, exactly what I needed.  We are using an RF consultant who should be able to help with the RF considerations.

John

I recommend to avoid conformal coating the RF area; use a water-tight enclosure instead. Conformal coating can wreak havoc on RF path, especially since there is variation in compounds, thickness, etc. If necessary move RF stuff to a small PCB that you can water-tight pretty well.

--Derek

Plastic NEMA 4X enclosures are great. The commercial telco guys use them all the time. Conformal coating is a mess. Coatings will effect your match and microstrip and re-work is virtually impossible. For both solutions, NEMA 4x enclosures, or coatings, put the material in the microwave oven for a few seconds, if it gets hot don't use it with RF. Heat dissipation is not a problem with CC2530, remember it's a "low-power" chip.

RatFink,

Just curious about your microwave oven test. What is it telling you if the enclosure or coating gets hot after placing them a few seconds in the microwave?

Thanks,

Rafael. 

Heat indicates the material is absorbing energy in the 2.4 GHz band. If you use this material as an enclosure, or coating, this energy is coming out of your link budget and attenuating your signals. Plastics can have conductive or E&M resonate ingredients, or usually metallic paint, that effect E&M permeability. The microwave test is a quick and dirty way to spot the really bad offenders. In the more subtle cases the permeability of plastic enclosures can effect the complex impedance of enclosed antennas, this is why you may have to re-tune an antenna match when you put it in a box.

Just coat the whole thing with Parylene; I found out it's used on another CC2530 application with no problems.

Google "parylene" and you'll find companies that do this.

--Derek