An example that comes to mind (and it's really hypothetical, I don't know
how valid it is...): say a fab house uses several different supppliers of
FR-4 material. Maybe they know that vendor A typically has Er=4.3, vendor B
typically has Er=4.1, and vendor C typically has Er=3.9. If you just say you
want 5mil dielectric thickness, they'll probably choose any of those vendors
for you. If you happened to base all your calculations on Er=4.3, and they
give you Er=3.9, you might have a problem. On the other hand, if you
explicitly say you are controlling impedance with an assumed Er=4.3 in your
calculations, they would know to choose vendor A to get what you want.
At 10:12 AM 3/25/99 -0800, Ron Miller wrote:
>I am appaled by this vendors remarks but not surprised.
>We engineers design the boards and if some fab houses do not want to build them
>as directed, or maintain the tolerances required then we will find someone
>The fab houses have no way of knowing that a pad size is designed to provide a
>capacitance that is optimized and simulated to make the entire circuit work as
>desired. They do not make differential impedance measurements for differential
>Basically, what it boils down to is that they have been getting away with
>adjusting their processes and even the artwork to get the impedances that
>because they have not been keeping their materials properties dimensions and
>post process trace widths to the fab drawing requirements.
>It is time for that to change. I for one will not let the PCB fab house
design the board
>Kim Flint wrote:
>> At the PCB West conference yesterday, during the course on Materials for
>> High Speed Design (which was quite interesting, I wasn't familiar with
>> other than FR-4, but that's another topic....) a fellow in the audience
>> happened to be from a major fab house and made some rather interesting
>> comments that seem to pertain to your questions.
>> One was that FR-4 material is made by a variety of vendors, and it is
>> all the same. A fab house will be taking those differences into account
>> their controlled impedance calculations, depending on the material
>> they use. This could account for differences, even if processes are
>> supposedly equivalent otherwise. (In any case, the dielectric constant
>> FR-4 is not very stable and could be a signigicant source of error.)
>> He also said about the same thing Katie said below, that different fab
>> houses tweak their calculations according to empirical data from their
>> particular process.
>> Most interestingly, he said they've lately had a lot of trouble with
>> engineers possibly knowing a little too much for their own good (or not
>> enough :-) and trying to achieve controlled impedance just through
>> specifying trace widths, copper weight and stackup dimensions. Since the
>> real world of PCB fabrication is not that ideal, and all fab houses are
>> equal, this often results in completely wrong results on the board. And
>> fab house can't do anything about it since they don't really have enough
>> info to know how to compensate their process to meet your requirement.
>> request to engineers was to include, in addition to stackup, trace
>> etc., the impedance you are targeting, the calculations you used to
>> determine that, and any assumptions you might have made. (i.e., Er=4.3.)
>> This info should be *on the fab drawing*, as that is the thing that will
>> actually get looked at during fabrication. With this data, the fab house
>> should be able to compensate appropriately for their process, and
>> they might be able to do a better job of getting you what you want.
>> good luck...
>> At 01:51 PM 3/24/99 -0500, Laurence Michaels wrote:
>> >Katie Rothstein wrote:
>> >> Laurence,
>> >> Just a quick thought about your question. Different board
>> >> different processing capabilities (line width, total board thickness,
>> >> size, etc.) This could be why some board shops can meet your required
>> >> impedances and some cannot. I'm not sure exactly what you are asking
>> about the
>> >> different calculations, but many board houses have impedance calculation
>> >> that take empiricle data to help make corrections for their own processes.
>> >> -Katie
>> >It sometimes seems that, even with 'equivalent' processes, allowing the
>> >same board thickness, size, line width, and layer stackup, board vendors
>> >will come up with different values for trace impedance, generally
>> >requiring them to tweak the board stackup. Since we sometimes like to
>> >use different board vendors, even on the same board, it would be nice to
>> >be able to specify FR4 and copper, and use only one layer stackup,
>> >instead of having to keep track of which vendors require which different
>> >dielectric heights. It would be even better to come up with a
>> >theoretical calculation that works in practice, since some people seem
>> >to think that anything should be able to be calculated without having to
>> >add tweak factors for different vendors. I'm beginning to realize that
>> >this is not the case.
>> >What I'd really like is to come up with a set of layer stackups, perhaps
>> >including different stack heights for different vendors, for a few
>> >different controlled impedances and numbers of layers. Doesn't seem too
>> >likely, as even including the true dielectric constant for the specific
>> >vendor's material in the calculation doesn't always come out within 10%
>> >of the measured impedance. Plus I'd rather only have one layer stackup
>> >(including diel. heights) per board, regardless of vendor. Wishful
>> >I'd also like a less expensive way to measure trace impedance than a
>> >TDR, so that we can check the board vendor's claims ourselves. Any
>> >ideas here?
>> >-- Laurence
Kim Flint, MTS 408-752-9284
ATI Research email@example.com
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