Screen Printing - April/May 2017 - 31
Schematic processes for FIM with
(bottom) and without (top) integrated
electronics. Courtesy of DuPont.
Images of molded FIM part with
attached LED chips. Courtesy of
complexity is offset by eliminating the
need for a separate rigid printed circuit board (PCB) and other layers. This
reduces the overall thickness of the part
from 25 millimeters to 2 to 4 millimeters,
which conveys a significant advantage for
Incorporating printed electronics
also removes the need to cut holes in the
formed part to allow for switches and dials,
eliminating a process step. These functions are instead integrated into the single
molded part. The process flow for FIM with
electronics follows a path similar to that of
standard FIM, as shown in Figure 4.
In contrast, traditional designs require
many more layers. The FIM layer includes
only the decorative printing. A rigid PCB
needs to be mounted on a backing plate,
and designs with lighting require light pipes
overlaid to allow the light from LEDs to
shine through in the correct location. Capacitive touch features require still more layers.
Figure 5 shows an example of a light dial
with and without integrated electronics.
The same challenges that FIM encounters, such as ink cracking and washout, are
also present when electronics are added
to the process. A decade ago, the biggest
challenge may have been finding suitable
inks that retain their conductivity during
the forming and molding process. Cracking
affects not only the look of the product, but
also its ability to function. Inks, therefore,
need to be extremely flexible. It's also important to optimize their drying rate. Today,
multiple ink formulations are available with
properties that are compatible with FIM processes, so that is no longer the bottleneck.
Component mounting is probably the
next big challenge for FIM with electronics.
The method to add lighting, for example,
typically involves attaching LED chips directly to the formed film (see Figure 6). Such
chips are very small and thin, but attaching
them reliably is where the challenges come
in. Adhesives need to be able to withstand
the heat and pressure of injection molding.
Not only is the film heated to an elevated
temperature inside the mold, but there is
also a significant temperature difference
between the mold and the injected liquid
thermoplastic. The greater this difference,
the greater the likelihood of warping and
blistering. Such defects can be fatal to
electronic components, causing them to
detach from the film. Choices in component location and temperature profile can
minimize this risk. It's important to conduct
thorough testing to identify any defects that
may affect the function of the part.
Washout can be especially detrimental
to FIM parts with printed electronics. When
thermoplastic is injected into the mold
cavity, any spots that experience localized
concentration of heat or pressure can experience voids in the film and defects in the
inks. For inks that are purely decorative, it
may be possible to ignore defects that are
too small to see with the naked eye. But the
pressure of injection molding may affect
the distribution of metallic particles within
a conductive ink, leading to defects that
affect performance, if not appearance, and
render the part unusable.
Adhesives for attaching components
may be cured before the molding step using
either UV radiation or heat. Alternatively, a
correctly chosen epoxy adhesive could be
cured during injection molding, streamlining production. This approach can, however, be risky, since the pressure of injection
molding may dislodge discrete components
if the adhesive is not sufficiently strong.
One approach to solving the challenges
inherent in securely attaching discrete
components is to avoid using them. Instead
of attaching LED chips, it is possible to
print light-generating sources on the same
layer as conductive traces. Inventors from
startup Rohinni, which makes printable ink
infused with microscopic LED particles,
have been granted a patent for their process
of incorporating these particles into FIM.
Despite potential challenges, integrating
electronics into FIM promises significant
advantages. Whether in vehicles, home
appliances, or medical instruments and
devices, the lure of doing away with mechanical buttons and switches and creating
one seamless, aesthetically appealing user
interface is too compelling to ignore.
Julia Goldstein is a Seattle-based freelance writer specializing in technical, scientific, and business
subjects. She holds a Ph.D. in materials science from the University of California, Berkeley.