Earl -
I agree that many shops do not do well with the more exotic laminates, but
we can not let that determine our choices. BT epoxy is better than FR4 in
some ways but it is still not up to performance standards needed to play in
the 1GHz *clock* range which necessitates a sub .5 nanosecond rise time for
signals. The loss tangent is the limiting factor for BT, with a range of
0.015-0.022 it is still to high for these designs. PPO's lower this
somewhat, 0.01-0.015, and are "flatter" in the Er curve with regard to
frequency. Cynate Ester maybe an answer to the problems, but when it comes
to cost vs. functionality the hands down winner is the ceramic-filled, glass
reinforced thermoset plastics (Rogers 4003/4350 and the like). Hopefully
the commercial fabricators will catch up with the prototype houses in
delivering quality product in the more "bleeding" edge materials.
None of this is meant to say that FR4 can not be used in
high-speed/high-frequency design just that it is easier to get the
performance from other products. I have seen/heard of/worked on designs in
the 2+ GHz range on multilayer FR4 laminates. These can/do work but design
becomes super-critical, and it is not a simple place and autoroute routine.
Not all designer or engineers understand the parameters and attention to
detail required to make it work.
Jeffrey
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4-7582
-----Original Message-----
From: Earl Moon [mailto:[log in to unmask]]
Sent: Friday, November 16, 2001 9:04 AM
To: [log in to unmask]
Subject: Re: [TN] PWB MATERIALS, DESIGNS AND TECHNOLOGY
Jeffery,
I certainly apreciate your posting as a reminder to me as well. I would add,
from a DFM point of view concerning materials and their processing:
Getek has good electrical properties, as you say, but processing and some
results associated with it can be dissapointing. This material must be
processed more along the lines of polyimide. It requires a more damanding
press cycle than those for FR4 types using simple straight up processing
from 0 to 90 minutes or so. Therefore, not all shops like or do well with
it.
The Rogers and Arlon (liscensed from Rogers but with slight variations
concerning properties) material is really neat stuff. It has processing
drawbacks as well as being limited in prepreg glass styles and thicknesses
when reinforced. If improperly processed, as with any material type, it has
several failing characteristics. As is widely known, this type material
often is married/laminated to Getek to meet design and performance
requirements. Really low DC and loss tangents.
My money is on BT epoxy blends when and where possible. It has high
performance characteristics and is easily processed. I have designed it in
and used in in very complex, large, high layer count MLB's with great
success. It is/was used in some Cray designs built by IBM, as one example.
Just wanted to add to your good comments,
MoonMan
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