What are multilayers?
Multilayers, or multilayer PCBs, are circuit boards
made up of several (more than two) electrical layers
(copper layers) superimposed on one another. The
copper layers are bonded together by resin layers
(prepreg).
What manufacturing processes are available?
Multilayers are produced by bonding together inner
layers and outer layers with prepreg. Prepreg is
fiber-glass fabric impregnated with partially
hardened resin. The individual layers must be
arranged in a pressing tool to prevent misalignment
of the layers. Following bonding, the bonded layers
are further processed as double-sided throughplated
circuit boards. However, additional hole wall
cleaning is required before through-hole plating can
take place. The outer layers may consist of eithercopper foil and prepreg or of single-sided or doublesided
copper-clad laminates. The inner layers
consist of double-sided copper-clad, etched
(structured conductor tracks created) and throughplated
board material. The surface area of the
copper must be increased by micro-etching and
subsequent oxidation to improve bonding between
the inner prepreg and copper layers. Immersion
tinning may be used as an alternative to oxidation.
However, the impedance values of the additional
metal layer formed are different from those of
copper inner layers formed by standard oxidation.
How does bonding take place?
In principle there are three methods of forming
multilayers. These are described below using a 4-
layer multilayer as our example:
· Two double-sided copper-clad laminates
(cores) with prepreg in between. One doublesided
copper-clad inner layer, at least two
prepregs and two single-sided copper-clad
laminates or laminates coated on both sides
where one copper side is fully etched away.
· One double-sided copper-clad inner layer with
at least two prepregs and copper foil.
Construction using prepregs and copper foil to
create the outer layers (the copper foil technique or
MassLam technique) is the preferred construction
method because of its lower cost.
Bonding may be performed in a hydraulic press or
in an overpressure chamber (autoclave). In the
case of hydraulic presses, the prepared material
(press stack) is placed in the cold or preheated
press (170 to 180°C for material with a high glass
transition point). The glass transition temperature is
the temperature at which the amorphous polymers
(resins) or the amorphous regions of a partially
crystalline polymer change from a hard and
relatively brittle state to a viscous, rubbery state.The bonding pressure is 150-300N/cm2. The curing
temperature and time must be selected according
to the type of prepreg used, the number of layers
and the thickness of the press stack.
In the case of overpressure chambers (autoclaves),
gas or oil is used to convey the compression force
and heat to the press stack. The press stacks are
placed on platforms in the tiered stand that has a
vacuum connection and are vacuum-sealed in
temperature and pressure-resistant foil. Once the
pressing chamber is loaded, it is closed and the
inert gas or oil is introduced into the chamber. The
isostatic pressure (pressure exerted evenly in all
directions) for bonding is 80 to 200 Newton/cm2.
In contrast to hydraulic press, different press sizes
may be bonding simultaneously in an overpressure
chamber. The advantages of this method of
bonding are improved heat transmission and a
more favorable thermal time gradient. The all-round
application of pressure has a particularly positive
effect on the multilayer stack. It prevents resin flow
– the main cause of stresses in the fiberglass
fabric. Dimensional stability, torsion/warping and
thickness tolerance are significantly improved if
stresses of this nature are not generated.
Furthermore, no resin deficiencies will be found
within the board. A lower bonding pressure is
required for vacuum bonding (vacuum chamber
press, vacuum frame or vacuum autoclave). Fewer
stresses are generated in the multilayer with the
lower bonding pressure. This gives considerably
better dimensional stability of the inner layers,
improved thickness tolerance and reduced inner
layer misalignment. Because the melting point is
lowered in a vacuum, volatile components, including
air and moisture, are removed from the press stack
and virtually void-free multilayers can be achieved. |
