PCB material selection for antenna 1

[BasketballJones]

I am working on a 3GHz PCB-based antenna. Does anyone have any ideas on which PCB material is best for the cost? My short list is FR4, FR408, Rogers 4003, Nelco 40013. Any other materials that people use? I understand that FR4 is lossy at those frequencies.

Many thanks.

 

[logbook]

The Rogers site has a useful comparison graph of loss on various laminates, found when searching on RO4003. At 3GHz the worst loss (on the worst material) is only 0.2dB/inch in a 50 ohm system. A 3 inch antenna wouldn’t give 0.6dB loss though because the current drops off with distance along the antenna.

I found another microwave laminate comparison chart from Metclad; I was searching on Arlon Diclad at the time. This is a PTFE material with a lower dielectric constant and dissipation factor. You would have t pick the laminate before designing the antenna because the size would change according to the dielectric constant. FR4 is perhaps a bit dubious at 3GHz in terms of the guaranteed repeatability.

What I would mention is that the PTFE materials have a lower dielectric constant but are not as mechanically rigid as the FR4 types. FR408 seems to be a higher operating temperature version of FR4 with a slightly reduced dissipation factor as well.

Cost is something you would have to find out by quotations. Is 0.3dB less loss worth paying an extra 10% on the price of the pcb? This is a very specific question which only you can answer.

 

[BasketballJones]

Many thanks logbook. Is there a price list anywhere, so that I can make that x.xdB loss vs y.y% price comparison? Calling all these people will take a lot of time (sounds like I am lazy, but if someone has already done this why reinvent the wheel). Also, is there a rule of thumb for how much cheaper PCB material gets with volume? Any experience with Chinese PCB houses/manufacturers?

Thanks again.

ps. I know I am asking for A LOT.

 

[logbook]

Sorry, can’t help. I would buy pcb material through a pcb supplier rather than directly. If you are going to spend a lot of money with a pcb vendor they should do the work for you, ie you get them to quote for the finished pcb in FR4, Diclad, Rogers, etc. Each pcb vendor will no doubt have their own preferred substrate vendors. Again the price of the basic material is not the whole story, there is also the price of processing; it seems that some materials may be harder to process than others.

Suppose the project is just an antenna on a double sided pcb with a couple of pads to the transmitter. On something like this the cost of the finished board will consist of the cost to process your paperwork, ship the boards, produce photo-tools, mill the profile, apply solder resist (if desirable), hot air solder level, ….

The raw material cost might be less than 10% of the total bill. I don’t think you can "work it out". You just have to try different variants and see what they cost. Typically a pcb vendor will charge much more for a material supplier they don’t ordinarily use. I would get on the phone and talk to some pcb vendors. Ask them what material they would recommend and they will tell you a vendor they use.

Even if there were a price list of the various materials, and I don’t know if such a thing exists, it would not really help that much given the other factors listed above. The last time we enquired after exotic materials it was a lot of effort and we eventually stuck with plain FR4. However, this particular vendor processed it such that the dielectric constant was lower than normal (4 rather than 5). I was more concerned with dielectric absorption that dielectric loss because all the projects were below 1GHz.

Big boards (>8 inches on each side) in PTFE based material were generally unsuitable because the material rigidity is not nearly as good as FR4. We then started looking at hybrid materials with layers of PTFE and FR4 combined for optimum strength and dielectric properties.

I think you would have to be making thousands to tens of thousands of units per year to justify spending too many days and weeks trying to get the "best possible" solution. I would tend to think you should just get an acceptable solution and go with it.

As for cheaper materials, I have been burned on this relatively recently. Remember I didn’t pick the material, I picked the pcb vendor. We got some cut price boards made because they were simple little boards, 4 layer FR4, two inches on a side. They were fine when hand soldered but when they were infra red re-flowed (surface mount) they partially de-laminated and had to be binned. The cost of the pcbs being binned was negligible compared to the cost of getting new pcbs done on short turn around!

By all means use a cheap offshore vendor, but do it cautiously in order to not have batch to batch variations. The cheapest solution has to involve the cost of sorting out problems, and all vendors have some sort of difficulties that have to be sorted out.