[SI-LIST] Re: VLP copper on outer layers
- From: Chuck Corley <corley@xxxxxxxxx>
- To: Hermann.Ruckerbauer@xxxxxxxxxxxxx
- Date: Sun, 20 Mar 2022 09:03:21 -0700
I see the advantage of smooth copper for high speed routing, and the
disadvantage of de-lamination for reliability.
Unlike the inner layers, the outer layer is plated up while the via
holes are being plated, etc.. On many boards the outer layer's
impedance may be marginal or not be reliable enough for high speed
signals because of the plating being less predictable than the fixed
copper used on inner layers. If the outer layer plating thickness is
off of the originally predicted value then the impedance may be
different than what was expected. The trace impedance may also vary
somewhat along trace paths, depending on the plating thickness
variations along the trace path.
For those reasons some people only use the un-plated inner layers for
high speed signals. In this case if you're not using the outer layers
for high speed signals then smooth copper isn't required for the outer
layers for reducing high speed signal loss since there would be no high
speed signals on the outer layers.
There are a number of exceptions. For example PC computer motherboards
may only have 4 or 6 layers and are forced to use the outer layers for
high speed signals. Sometimes in situations like that you just have to
know you're going to have this kind of challenge with those outer layers
and work with what you have.
Chuck
---
Chuck Corley
Speeding Edge
On 2022-03-18 00:40, Hermann Ruckerbauer wrote:
Hello Experts,
maybe I can get some opinions for making decisions...
I see the advantage of smooth copper for high speed routing, and the
disadvantage of de-lamination for reliability.
So far i was only thinking about the de-lamination issue from a
reliability view of e. g. many temperature cycles over time.
But now i got a different aspect: is there a difference of this issue on
outer vs. inner layers ?
On outer layers pads will see additional stress due to soldering and
mechanical stress due to the components.
So I would assume that this is might be even more critical on outer
layers than on inner layers.
The only way around would be to e. g. use different PCB Material /
copper on the outer layers, but I'm not sure if this is worth the effort.
So far I'm no fan of mixed material stackups and it was more driven by
cost saving (only use the improved material where real high speed
routing is used, and maybe not for power layers).
I'm a bit conservative on this one and do not expect, that every PCB
vendor can handle material mixing out of the box...
Any feedback on theses thoughts is highly appreciated!
Hermann
--
EKH - EyeKnowHow
Signal Quality - Made in Bavaria
Hermann Ruckerbauer
www.EyeKnowHow.de [1]
Hermann.Ruckerbauer@xxxxxxxxxxxxx
Itzlinger Strasse 21a
94469 Deggendorf
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