just connecting the rigid boards. Bends
must be precisely designed so boards
line up where they are intended to
mount, while not putting stress on the
connection points. Up until recently,
engineers actually used “paper doll”
models to simulate the PCB assembly.
Now, design tools are available that
provide 3D modelling of the rigid-flex
assembly, allowing quicker design and
much greater accuracy.
Small Products and Dense Circuitry
By definition, wearable products must be
small and unobtrusive. In the past, a medical “wearable,” such as a Holter monitor,
included a fairly large external device
with a neck strap or belt mount. The new
wearables are small and attach directly to
the patient with no or few external wires.
Their embedded microcontrollers enable
them to collect a variety of data and even
perform some local analysis.
An unobtrusive device attaching
directly to the patient dictates flex
circuitry and very dense layouts. In
addition, the board shapes are often
circular or even more unusual shapes,
calling for clever placement and routing. For such small and densely-packed
boards, a PCB tool that is optimized for
rigid-flex designs makes handling odd
shapes much easier.
Careful with the Bend
The point of using flexible circuitry
is to be able to shape the final assem-
bly by bending the flex. This presents
a number of problems that are not
encountered on rigid boards. Bending
produces stresses that do not occur
with rigid boards. Most PCB tools
have tools that allow you to optimize
the flex circuitry. To avoid problems
with bending forces, here are four tips
when design flex:
Don’t use 90° bends on traces: The
corners of traces endure more bending
stress than straight paths. To avoid
delamination problems over time, use
straight paths wherever possible. And
in cases where traces must change direction, use curves rather than anything
Stagger traces on double-sided flex:
when traces are run on top of each oth-
er on double-sided flex circuits causes
uneven distribution of the tension. In-
stead, traces should be staggered. This
also improves the flexibility.
Use teardrops to improve strength
and yield: The flexibility of the
substrate can lead to delamination
over time if not controlled. Instead of
circular pads, teardrop pads (Figure 2)
can be used to add additional material,
providing greater strength to the pad
to prevent delamination. The teardrops
also provide greater tolerance for
Support your pads: The copper on
a flexible substrate is more likely to
detach than on a rigid board because of
the bending. In addition, the adhesion
of copper to the substrate is not as
good as on an FR4 PCB. Fabricators
suggest through-hole plating and
anchor stubs for SMT mounting pads.
They also suggest reducing coverlay
openings as much as possible.
Stackup Design is Critical
The stackup — the map of the PCB
layers — is critical when using rigid-flex
techniques. Ideally, your PCB design
software has the capability to design
your stackup including both the rigid and
flexible parts of the assembly. As mentioned earlier, the layout of the bending
area should be designed to minimize the
stresses on the traces and pads.
Figure 3 shows a stackup illustration
with both rigid and flexible sections.
The number of layers and different materials used adds to the complexity of
the design. Therefore, it is important to
Figure 2: Use teardrops to increase the
strength and improve the yield of rigid
Figure 3: This stackup design shows rigid PCBs on either end, connected by a two-layer flexible circuit.