[SI-LIST] Re: PCB construction materials

>Can anyone kindly provide me details about PCB constructions materials
like various prepreg=20
>and in which application those has to be used. Dependent on speed which
prepreg material
>suits best to which speed n stuff similar to this.
>

The electrical characteristics, the mechanical/thermal properties and
price are=20
among critical considerations when selecting a material for a high-speed
PCB structure.

PCB is typically constructed as stack of alternating layers of prepeg
mats and laminate
sheets.

Cores or laminate sheets are produced by attaching copper foil to one or
both sides of fully
cured prepeg sheets.

The prepeg mats are a weave of glass fiber yarns preimpregnated with
partially cured
resin. This also indicates how the name prepeg comes about.

Several resin types are utilized for prepeg preparation. These resin
systems usually
differ from each other in electrical properties such as dielectric
constant and loss
tangent.  They also differ in mechanical/thermal features including
coefficient of
expansion, glass transition temperature, and rate of moisture
absorption.

One popular resin form is FR4 epoxy resin, but there exist other resin
systems which
can offer superior mechanical/thermal (i.e., lower coefficient of
expansion, higher glass transition
temperature Tg) or electrical properties (i.e., lower dielectric
constant Er and loss tangent).=20
These alternatives include GETEK, NEGTRON, BT, polyamide and cyanate
ester resin system.

Fibers (normally a type of glass) are applied to strengthen the resins.
These include
the electrical grade E glass and structural grade S glass.

S glass is stronger than E glass and has a lower Er but it is more
expensive and less
widely used. Thermount (a nonwoven Aramid fiber) is also sometimes
utilized as a
reinforcement in PCB applications.
Addition of fiber to resin alters the electrical and mechanical=20
characteristics of the composite structure as dictated by the
glass-to-resin ratio.

It is interesting to note that for a multi-layer PCB, the copper
thickness does not need
to be the same for all layers in the stackup. It is also unnecessary for
all the cores
to be of the same laminate.  Hybrid type construction has been gaining
popularity
in the recent years.

Various properties of prepeg and core laminate materials are described
in
numerous publications. Two excellent sources are referenced below:

1. Stephen C. Thierauf, "High-Speed Circuit Board Signal Integrity",
Pages 2 to 9.
2. Howard Johnson and Martin Graham, " High-Speed Signal Propagation
Advanced
Black Magic", Pages 271 to 275.

Kind Regards,

Abe Riazi
ServerWorks




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