This is to all the PCB Experts (sorry, it is a bit off topic):
I have a customer requiring a minimum dielectric thickness between all metal
layers in a Backplane to be always thicker than 0.4mm and at the same time, the
overall thickness is restricted. There is no way to lower the layer count, so I
started questioning the 0.4mm rule. It turned out that it is based on an old
Railway Specification from 1995 where it is stated that:
“only a minimum dielectric thickness of >400µm can guarantee to avoid shorts
during pressing and to maintain the isolation resistance”
The spec doesn’t specify any minimum isolation resistance, so I can’t do any
functional proof of thinner dielectric. I did a calculation of the dielectric
strength and isolation resistance of the used material compared to “old FR-4
Minimum performance specifications” which helped in the discussion, but the
customer is still not convinced that anything below 0.4mm will prevent shorts
during pressing even though there are 3 Prepregs in between.
To give you some numbers: our stack will have 0.39mm thickness by design, thus
0.35mm at worst case theoretical minimum tolerance on 1oc copper with 85%
copper fill, so technically this is absolutely save (Max 120V on L2, 48V
between L15 and 16, all other Layers GND and Differential signaling). They
don’t even allow to just lower the dielectric thickness on the pure ethernet
signaling layers.
Long story, now the question is: Does anyone of you know of any old Study
showing the risk of shorts between layers during pressing? Or maybe some notes
about discussions of that time in IPC or anywhere? I’d really like to find out
where this rule is technically coming from to be able to convince the customer
that this rule is no longer valid.
BR
Gert
----------------------------------------
HARTING Stiftung & Co. KG | Postfach 11 33, 32325 Espelkamp | www.HARTING.com
Persönlich haftende Gesellschafterin:
HARTING Führungsstiftung | Amtsgericht München | HRA 108479 | München
Vorstand: Dipl.-Kfm. Philip F. W. Harting (Vorsitzender), Dipl.-Kffr. Maresa W.
M. Harting-Hertz, Dipl.-Kfm. Dr.-Ing. E. h. Dietmar Harting,
Dipl.-Hdl. Margrit Harting, Dr.-Ing. Kurt D. Bettenhausen, Dipl.-Ing. (FH)
Dipl.-Wirtsch.-Ing. (FH) Andreas Conrad, Dr. iur. Michael Pütz
Sitz der Gesellschaft: Espelkamp | Amtsgericht Bad Oeynhausen | HRA 9021 |
UST-ld Nr. DE812136745
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