Flexible Circuit Design – Concept to Connection (Part 2)

							Flexible Circuit Design – Concept to Connection Part #2
by Mark Finstad
June 1, 2007

The first article in this series covered the steps involved in generating the
mechanical layout of a flex circuit. With the mechanical layout completed, it is
time to start routing the conductors.

Because a flexible printed circuit is as much a mechanical device as it is an
electrical device, conductors have to be laid out so that the circuit will function
reliably in its final configuration. Unlike a rigid PCB, a flexible circuit will be
bent, flexed, twisted, and otherwise contorted to fit in the final assembly. These
bending, flexing, and twisting operations can have a severe, detrimental impact
on the internal conductors of the circuit if the conductors are not properly
routed. This segment will cover what to do and, perhaps more importantly, what
not to do during the conductor routing process to ensure a reliable design.

Before getting into the rules of engagement for routing a flex circuit, it is
important to understand the dynamics and material interactions that occur
when a circuit is bent or flexed. Anytime a circuit is bent or flexed, the materials
toward the outside of the bend are subjected to tension (stretching) forces and
the materials toward the inside of the bend are subject to compression forces.
Somewhere in the middle of the material stack, there will be a layer called the
neutral bend axis, which will see very little tension or compression. The neutral
bend axis is defined as a planar region, with no thickness, which does not
experience stretching or compressing forces during bending or flexing.

During a flexing operation, any material that falls to the outside of the neutral
bend axis will experience tension, and any material that falls to the inside of the
neutral axis will experience compression. The further a material is from the
neutral bend axis, the more severe and damaging these forces will be. These
forces are present in any material that is bent. And as the material thickness
increases, the severity of these forces will also increase.

         Figure 1 shows a cross section of a typical single-sided circuit showing
the different forces and the neutral bend axis (dashed line). The neutral bend
axis will not always fall in the exact center of the material stack. The neutral axis
will tend to shift toward materials that offer a greater resistance to stretching
and compressing than other materials in the stack. Copper plane layers, heavy
copper conductors, and thick (> 0.003 in) polyimide layers are all examples of
materials that will offer a greater resistance to stretching and compressing. This
can have a profound impact on the way a circuit will behave during a bending
operation.

For example, consider two double-sided flex circuits of identical size and shape.
One of the flex circuits has 0.007 in conductor traces on both layers and 0.001 in
polyimide film for the base dielectric and for the cover material. The other circuit
has 0.007 in lines on one layer and a heavy ground plane on the other layer. The
first circuit has a balanced (symmetrical) construction, so the neutral bend axis
will land very close to the middle of the stack. This circuit will behave the same
way regardless of which way it is bent. On the second circuit, the neutral bend
axis will shift toward the heavy copper plane because the copper plane will offer
greater resistance to stretching and compressing than the other materials in the
stack. This will result in the 0.007 in conductors experiencing exaggerated
stretching and compressing forces when it is bent. Also, the second circuit
would be able to tolerate a tighter bend radius—when the plane is to the outside
of the bend—than it would if the plane were to the inside of the bend.

The conductor routing guidelines that follow have been created to take into
account the unique mechanical characteristics of flexible circuitry. Deviating
from these guidelines does not mean that the design will fail during installation
or service. However, if there is significant deviation from these guidelines, and
the guidelines of IPC-2223, the circuit design may be questionable.



Whenever possible, vias should be placed in areas of the circuit that will not be
bent or flexed. There are several reasons to keep vias away from bend areas.
Depending on where a plated through-hole falls in relation to the center of the
bend, it could experience shear forces due to the differential
tension/compression between the inside and outside of the bend. Also, a plated
hole (or any hole for that matter) represents a mechanical discontinuity in the
circuit that is very prone to originating a crack on the outer cover material. If a
crack forms in the outer cover, it will almost surely propagate with time and
cause the plated hole to crack and fail as well.

Flex circuits with multiple layers will be thicker and the plated barrels will be
deeper. The combination of deeper plated barrels and the increase in stretching
and compressing forces due to the added thickness will only aggravate the
problem. It is best to place vias in areas that will see limited or no bending
during installation or service. When the design does not allow sufficient space
in non-flexing areas for all plated holes, they should be placed in areas that will
experience the least amount of bending.


It is good practice to place fillets on all termination pads. While not every design
requires fillets to function reliably, they almost never cause a detrimental effect
on the circuit. When fillets are used around termination pads, they can eliminate
stress concentration points that would otherwise cause cracks where the
conductor enters the pad.



Copper Plane Placement


When possible, place copper planes on or near the center layer of the circuit. As
mentioned earlier, copper planes will resist stretching and compressing and can
cause a positional shift in the neutral band axis. By placing the plane or planes
near the center layer, the associated shifting effects on the neutral bend axis
will be minimized. In some instances, the design will not allow the planes to be
positioned near the center of the stack. When this is the case, plane layers
should be balanced as much as possible on opposing sides of the neutral axis.
The effects from the planes will tend to cancel each other out, resulting in the
least amount of neutral axis shift. If the design has a single plane that cannot be
placed near the center layer, it should be positioned such that it is to the
outside of the neutral bend axis on the majority of the bends. Copper
conductors can tolerate compressing better than stretching. By placing the
plane to the outside of all or a majority of the bends, the conductors in those
areas will be more likely to be compressed rather than stretched.


Another trick available to flex designers to reduce the stresses caused by
planes is to either cross-hatch the copper plane or use a conductive coating
such as silver epoxy instead of copper for the plane. Cross-hatching the copper
plane will reduce the amount the plane will resist stretching and compressing
and will cause it to act similar to a standard conductor layer. Silver epoxy is the
most common conductive coating used in the flex circuit industry. The electrical
performance of a silver epoxy plane is similar to a solid copper plane, but it is
much more flexible.


Each of these stress-reducing options has drawbacks. Cross-hatching a plane
will have a significant impact on the impedance of any conductors using it as a
return path. Conductor widths and dielectric thickness will need to be adjusted
to maintain the required impedance. The drawback to using a silver epoxy plane
layer is limited to cost. Incorporating a silver epoxy layer into a flex design will
add extra steps to the manufacturer, and those costs will be passed on to the
end user.



Tracks Crossing Bending Areas


Conductors that cross a bending area should do so perpendicular to the bend.
Running the conductors at a right angle to the bend will minimize stress on
those conductors when the circuit is bent. If the design will not allow the
conductors to run perpendicular to the bend, they should be routed as close as
possible to perpendicular.


Anytime multiple conductors are tracing similar paths from one termination area
to another on multiple layers, these conductors should not be stacked on top of
one another. When conductors are stacked on several layers, they create an I-
beam effect. This practice will effectively increase the overall thickness of the
circuit and will significantly increase the stretching and compressing forces on
the stacked conductors. Many designers like to run signal and return traces on
top of each other to reduce electromagnetic interference. If this method is used
to reduce noise and cross-talk, the stacked pairs should be staggered to
minimize the I-beam effect.


Miscellaneous Considerations


Wider conductors (>0.010 in) are more robust and will tolerate bending better
than smaller conductors. If a bend is pushing the minimum bend ratio limits, it
is a good idea to widen small conductors in the bend area. Because forces from
a bend can radiate out beyond the bend zone, the widening should be gradual
and the conductor should reach its maximum width at least 0.10 in prior to
entering the bend area and should not begin tapering until at least 0.10 in after
exiting the bend area.


The final segment of this series will cover the steps required to obtain prototype
or production circuits. Beginning with generating a complete drawing and data
package, to transferring data and getting quotes from flex vendors, that article
will walk the designer through the procurement stage