What are the particular features of the
production of inner layers and the use of
copper-clad outer layers?
The copper surface area of the inner layers must be
increased by microetching and subsequent
oxidation to improve bonding between the prepreg
and the inner copper layers.
A registration system is required to achieve precise
alignment of the several copper layers bearing a
layout in a multilayer during bonding. This
registration is done using locator holes drilled in the
production board or in the individual layers. An
exception to this is the floating„ bonding process
used for 4-layer multilayers. This involves bonding
the inner layer with prepregs and copper foil in the
same way as an outer layer. The locator holes for
drilling the multilayer are then obtained by milling
and drilling the targets (registration marks) on the
inner layer.
How can typical faults be avoided or reduced?
In general, the warp direction of the fabric material
used for the laminate (e.g. fibre-glass fabric) should
run parallel to the longer side of the laminate
because the warp direction is subject to definite
shrinkage after bonding. This distorts the layout
(and is also characterized as variable or low
dimensional stability).
However, warping and torsion of the multilayer can
be minimized by improving the design. Torsion and
warping are reduced by even distribution of copper
over the entire layer and by ensuring symmetrical
construction of the multilayer (i.e. the same order
and thickness of prepreg, copper and laminate
layers should be present from the center of the
multilayer layers to both outer layers). The
prescribed minimum distance (dielectric thickness) between two copper layers is = 0.089 mm. The rule
of thumb for calculating the minimum distance
states that the minimum thickness of the prepregs
after bonding must be at least twice the thickness of
the copper being embedded. In other words, where
you have two adjacent copper layers, each of which
is 30 μm thick, a minimum prepreg thickness of 2 x
(2 x 30μm) = 120μm is required, which can be
achieved by using two 1080 prepregs (1080 is the
type of fiber-glass fabric). The cooling rate for
bonding multilayers must be as slow as possible as
too great a temperature gradient within the press
stacks gives rise to varying rates of shrinkage
between the outermost and innermost layers in the
press stack, thus causing distortion in the
multilayers. In extreme circumstances, the press's
cooling system may be switched off so that the multilayers take 12 or more hours to cool down (e.g.
metal-core multilayers). Before the drilled
multilayers can be through-hole plated, hole wall
cleaning must be performed as the action of drilling
can heat the resin to above the glass transition
temperature, allowing the resin to soften and be
smeared over the end face of the inner layer copper
by the drill bit. This smear layer must be removed
so that copper is present not only on the wall faces
and so that contact between the inner layers is not
impeded in any way. This thickness of the smear is
generally 2-6 μm; however, it may be as thick as 12
μm if the drilling parameters are not selected
properly. Chemical processes or plasma desmearing
may be used to perform hole wall
cleaning. Three-stage cleaning with permanganate
is the most suitable and widespread of the chemical
processes available.
Processing microwave substrates in
multilayers
Prepreg materials with low relative dielectric
constants are suitable for use in radio frequency
and microwave engineering. The low dielectric
constant gives rise to a low signal delay at radio and
microwave frequencies. Electrical losses are
minimized by low loss factors in the substrates.
Prepreg RO® 4403 is a new material produced by
ROGERS CORPORATION (http://www.rogerscorp.
com/mwu/index.html). This material is
compatible with other substrates (such as RO 4003
or RO 4350, used for microwave boards) used in
the construction of standard multilayers (FR-4
material).
Which production method can be used for
prototyping?
Floating bonding in a hydraulic press is ideal as a
cost-efficient process for prototyping multilayers (4-
layer) and does not require expensive equipment.
The inner layer carrying the layout and registration
marks is bonded with prepregs and copper foil as
an outer layer. The locator holes for drilling the
multilayer and the layout for the outer layers are
obtained by milling and drilling the registration
marks (inclusion of registration and locator holes to ensure perfect alignment of the inner and outer
layers, including the holes for through-hole plating).
This is followed by the creation of the layout for the
outer layers, drilling and through-hole plating of the
multilayer. Computer-controlled systems, such as
the LPKF MultiPress II, enable highest-quality
floating bonding of 6-layer multilayers to be carried
out within your own laboratory, due in no small part
to the fact that the cooling phase can be monitored
and performed under pressure. |
