Hi Brian,

The best solution is to use the standard build from the manufacturer. This
will vary between manufacturers depending on their methods.

A 4-layer construction is commonly built using a double sided laminate core
for the inner layers. Pre-preg (resin impregnated glass cloth) is then used
to cover both sides of the panel and topped off with copper foil. A thicker
inner layer panel does not increase cost significantly but using more
pre-preg does so we try to minimise the amount of pre-preg used. If the
inner layer is 2 oz, more resin is needed for encapsulation so an extra
layer of pre-preg might be used both sides.

We use a standard construction of 1.00mm (0.040") inner layer panel with 2
layers of pre-preg both sides to make up to 1.6mm (0.062" thickness). Copper
foil on inner layers must be High Tensile Elongation (HTE) foil to withstand
the elongation in thermal cycles due to Z-axis expansion.

If the layer spacings are design critical for impedance reasons, then the
manufacturer will use materials to meet the specification. This could mean
ordering the inner layer laminate to get the right thickness so there could
be an extended delivery and, if more pre-preg is needed, then there will
also be a material cost increase over the standard build.

Once you have a constuction which you are using for a particular part
number, then stick with that if you change supplier because it might alter
the performance of the circuit if the spacings were to alter significantly.

In short, go with the manufacturers' standard build and only specify layer
spacings if necessary.

Hope this helps

Paul Gould
Teknacron Circuits Ltd
www.teknacron.com

 -----Original Message-----
From: Brian Gaynor <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 07 October 1998 11:27
Subject: [TN] (fab) Specify Multilayer Build


>     Hi technet readers,
>
>     What details are required when specifying the inner layer build of a
>     typical 4 layer multi (0.062" thick)?
>     If you state  "make me a 4 layer board" what would you expect to
>     receive?
>
>     Best regards,
>     Brian Gaynor
>     Ireland
>
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