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Flexible Circuits

Metalizing Difficult-to-Plate Substrates Introduction

 Metalizing Difficult-to-Plate Substrates
Metalizing materials such as polyimide used
for flexible circuitry provides a significant chal-
lenge for process engineers. Conventional elec-
troless copper systems often required pre-treat-
ments with hazardous chemicals or have a small
process window to achieve a uniform coverage
without blistering. It all boils down to enhanc-
ing the adhesion of the thin film of electroless
copper to these smooth surfaces. In addition, in-
ternal stress in the copper deposit is a significant
factor with respect to adhesion of the plated
metal to the substrate. This process of plating on
polyimide flexible materials is very much POP
(plating on plastics). Electroless copper has been
adapted for metalization of difficult to plate sub-
strates and materials such as polyimide, POP and
molded interconnect devices (MID). The author
will present MID in a future column.
The Challenge of Metalizing
Smooth Surfaces
One significant concern with metaliizing
thin films over substrates is the concept of
deposit stress. Hydrogen gas is a by-product
of the electroless copper deposition process.
Consequently, hydrogen may be incorporated
into the thin deposit with a negative influence
on internal stress. The hydrogen gas issue has
been attributed to blister formation in the cop-
per deposit. In addition, a challenge typically
encountered on smooth surfaces such as poly-
imide-based flexible circuit materials is ensur-
ing good adhesion of the electroless copper to
the substrate. Adhesion of a thin film deposit
to a substrate that lacks sufficient microrough-
ness is compromised. To mitigate this issue, one
solution is to reduce the internal stress of the
copper deposit as much as possible.
Now, one remedy to minimize hydrogen
gas effects on the deposit is to lower the sur-
face tension of the electroless copper electro-
lyte. By lowering surface tension using specific
wetting agents, the hydrogen gas bubbles are
less likely to remain on the plated surface. The
hydrogen gas issue not withstanding, one must
also be concerned with electroless copper de-
posit stress. Excessive deposit stress will cause
the copper plated deposit to blister or pull away
from the substrate. Essentially this is a stress re-
lief phenomenon.
It is well known that polyimide materials for
flexible circuit fabrication have a rather smooth
texture even after plasma desmear. Unlike most
of the epoxy-based resin systems, polyimide for
flex remains void of the micro-roughened sur-
face texture generally experienced with alkaline
permanganate chemical desmear. With these
“anchoring” sites (Figure 1), one can see that
there is ample surface area for which the thin
film of electroless copper can adhere to epoxy-
based resin systems.
Another limiting factor with respect to ad-
hesion of copper to flexible polyimide is the
low surface energy of polyimide films. Surfaces
with low surface energy tend to repel chemical
interactions, making it more difficult for pro-
cess chemistry to effect good adhesion proper-
ties. In addition, flexible circuits are by design
flexed and bent during their useful service life.
Therefore, creating excellent adhesion of the
plated copper to the substrate is critical to the
functioning of the circuit. Certainly, plasma
treatment of the polyimide material provides
an improvement to the low surface energy of
polyimide. However, such treatments often do
not go far enough to insure long-term adhesion
by relieving internal stress.
The Influence of Stress
Stress in the electroless copper deposit can
be either compressive or tensile. The copper
deposit can be thought of as a spring that is
either under tension (stretched), which is ten-
sile stress, or compressed, which is compressive
stress. Compressively stressed copper deposits
lift or blister off smooth surfaces that lack an-
choring sites such as adhesive-less polyimide
flexible materials.
The spiral contractometer as shown in Figure
3 utilizes a strip of metal wound into a coil or
helix. The unit is attached to the plating cell and
the metal is then deposited on one side of the
coil only. Depending on the type and extent of
the internal stress, the coil will either expand or
contract. If the deposit is compressively stressed,
the coil will tighten. If the stress is tensile, the
coil will expand. The contractometer is equipped
with a gauge to measure quantitatively whether
the stress is compressive or tensile.
So it is somewhat obvious that excellent plat-
ing adhesion to the substrate is necessary for a
circuit that will experience potentially many flex-
ures over the course of its life. In addition, low
to no stress in the copper deposit is preferred. It
is important to keep in mind that adhesion of a
thin film to a substrate is a complex mechanism.
Internal deposit stress is just one factor. Addi-
tional factors affecting adhesion are:
• Ionic bonding between two surfaces
• Adsorption: adhesion is based on
interatomic and intermolecular
interactions such as van der Waals and
perhaps Lewis Acid interactions
• Mechanical interlocking
The last bullet point depends on the contri-
bution from a roughened surface. In the case of
flexible polyimide, a roughened topography is
not achievable.
With respect to flexible polyimide, opti-
mum adhesion of the electroless copper deposit
is heavily dependent on internal deposit stress
and adsorption of the palladium catalyst to a
plasma desmear treated surface. Please keep
these factors in mind when troubleshooting an
adhesion issue. PCB
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