We normally make boards from FR4, specifically the Isola 370HR or equivalent. It's usually fine but is lossy for fast stuff, like 100 ps edges on long runs, "long" being maybe 1 inch.
We've used teflon/Duroid boards, which are more like leather than fiberglas. They are expensive and pad adhesion is terrible.
So, has anyone used something in-between? Rigid like FR4, lower dielectric constant, lower losses, and less of the black-oxide effect that adds skin losses. But we still want decent pad adhesion.
I may have asked this before, but it's become serious lately on two or three projects.
What did you use in that board that warped into the shape of a candy dish? (You'll use it on both sides this time I'm sure.) ;)
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
Rogers 4003 or its flame retardent variations. It has an expansion TC much like FR4, so you can use it just for the top layer in a multi layer board.
I currently use Rogers TMM6. That is electrically fine, Er=6, but mechanically like cold candle wax. I broke a 25 mil board accidentally when fastening a SMA connector with fingers only and not much force. But the board house had TMM6. They warned us that a run of 50 of our small boards might take some months because of delays in getting the raw sheets.
OTOH a different customer made some boards for pipeline pigs and we knew that FR-4 swells in hydrocarbon athmosphere, cracking the vias in thick multilayers. And we had lots of layers for fat FPGAs and for passive cooling, transporting the heat with wedge locks to the torpedo-shaped chassis. The mechanics was a little wonder by itself: large parts milled from huge alu bars and soldered together in a salt bath.
But I digress. We choose boards made from Kapton, but the layers delaminated. It turned out that the Kapton prepregs have a very short shelf life, and it is not necessarily good if the board house can deliver too quickly just because they have material left from a project 2 months ago.
Last month, the board house made me a 4L-multilayer TMM6 + FR4 + FR4. That seems to work. The TMM6 is probably too cheesy to introduce warping.
The material is some organic stuff mixed with ceramic powder. I had no adhesion problems, but could peel off the copper easily around that ring resonator I showed last week.
A highly polished egg in a teflon pan has no adhesion, there are not many parameters to change that.
I recently looked into adhesives for gluing silicone and polypropylene. For both the answer is usually chemically or thermally(*) wrecking a thin layer on the surface, which can then be wet by or otherwise accept an adhesive.
I'd think something similar should work with teflon too.
(*) (e.g. flame or plasma treatment to singe the surface)
I used this ten years ago to stabilze temperature coeffiicent (compared to FR4). The 3003 had the same general processing characteristics as FR4, but far better controlled characeteristics, and lower tempco. We did have to change trace widths to preserve characteristic impedances.
Our board houses are suggesting Rogers RO4350B or the apparent equivalent Isola MT40. Both have DK about 3.5 and lower loss than FR4. I'll get some samples and hack some microstrips and compare them to FR4.
A board I'm doing now has 3" traces and I'd like to go as fast as I can with essentially CML logic signals. I'm seeing about a 60 ps (corrected) rise time on a wide 3" trace on 62-mil FR4.
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That's my MMIC-based signal detector on the TX+ line.
--
John Larkin Highland Technology, Inc
Science teaches us to doubt.
Claude Bernard
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Our board houses are suggesting the Rogers or Isola materials, as being something they can get and usually work with. They claim they can do just the top layer of a 4-layer board with the good stuff and keep it flat.
Here is our fidget-spinner board fabbed that way.
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--
John Larkin Highland Technology, Inc
Science teaches us to doubt.
Claude Bernard
The 2011 one? I know 2020 has lasted at least a decade so far, but you seem to be going the other direction. (Good on ya if you can make it stick.) ;)
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
I was going by what someone else wrote that I have but cannot quote. The mo re recent event may not be a tsunami even though that is what was written. I do recall supply issues due to a natural event much more recent than 2011 , but I wasn't paying much attention. Perhaps it was this--I don't know:
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