RE: [SI-LIST] : FR-4 and PCB Tolerances

Dave Hoover (
Mon, 30 Aug 1999 11:58:41 -0700


>I'll try to add a few comments in below where I can offer some
>dialog. By no means, I'm no Xpert on electrical performance
>info.<I'm learning> But I do know PCBs so here's my $.02......

Abe wrote:
...I am very interested in manufacturing tolerances for various
FR-4 characteristics (including losses, thickness and
dielectric constant) at high frequencies particularly the 33 MHz
to 400 MHz range.

>I'm informed....that the main issue is whether the PCB is digital
>or RF. Successful digital PCBs are being made out of FR-4 in the
>up to 9.6 Gbps. (Reference: Technical Paper #H412 DesignCon99
>The Impact of PWB Construction on High-Speed Signals. Chad Morgan
>and Dave Helster - AMP Corp) RF is another beast. It takes much
>TLC to get good results. There is a short haul/long haul transceiver
>Mfr in the Santa Clara area who has FR-4 bing used on MLBs up to
>8 GHz in RF. They are experienced microwave designers that run
>their designs through a diluge of post processing tools.
>check out for more info because
>one of those top designers runs that website.

As stated in previous
communications, SI simulations are often performed for three corners:
Maximum, Nominal and Minimum. These simulation corners are also
referred to as Fast, Typical and Slow. As mentioned in my earlier
email (that you had referred to), one set of values recommended for FR-4
dielectric constant are 4.2, 4.4, and 4.6 for fast, typ and slow corners
respectively. It is possible for this Er' variation to cover a wider
or narrower range (depending on several factors such as frequency,
application and capabilities of PCB manufacturer). I have not seen the
"military specs" for allowed tolerances of FR-4 attributes, but it would
be interesting to know how tight are such military tolerances (of
course, if they are not classified material).

>IPC-4101 /21 and /24 (Commercial Material spec for FR-4 <regular and
> hi-temp> state permittivity @ 1 MHz of 5.4 maximum. Loss Tangent of
>.035 maximum @ 1 MHz.
>Military is the same.



Abe Riazi

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