you might be watching this video on a TV
or a tablet a phone or a laptop but
regardless of the device the display
itself is almost certainly an LCD or an
OLED
that LCD is basically a piece of fancy
glass with a lot of electronics printed
on the surface and it's attached to the
rest of the device with a flexible cable
but making the electrical connection
between the cable and the glass is
surprisingly tricky because you can't
just solder things to Glass very easily
the solution to this is both incredibly
simple and also something of a
technological Marvel and that solution
is tape or more specifically a special
kind of conductive tape that allows
electricity to flow in only one
dimension electricity will flow through
the thickness of the tape that is in the
z-axis but will not flow through the x
or the y-axis
this is pretty wild if you think about
it like how is it even possible to
restrict the flow of electricity like
that
so I've got some of this tape and we're
going to look at it today under the
microscope to see how it works I also
cut up some actual OLED devices so we
can see the tape being used in real
applications
so here's the problem in more detail we
have this flexible ribbon cable which is
attached to our Electronics board with
conventional solder the cable has I
don't know a few dozen individual copper
traces which need to be connected to the
matching set of traces on the glass but
unlike the ribbon cable the traces on
the glass are extremely thin films of
metal we're talking like a couple
hundred nanometers at the most
so you can't just solder to this
directly like you would on a normal
circuit board it's simply too thin
and it's also usually a metal like
aluminum which is hard to solder too
anyway
you could use something like conductive
silver paste or epoxy but this is tricky
because the traces themselves are very
narrow and the gaps between them are
even smaller
on this cheap low resolution LED for
example the pads on the ribbon cable are
just 50 micrometers wide which is a
little smaller than a human hair but as
we'll see in a minute there's also a
silicon microchip that's been bonded to
the glass and the chip has electrical
pads which are only 30 micrometers wide
and the gap between them is even smaller
about 15 microns
so accurately dispensing an epoxy paste
at these tiny scales is well it's hard
like I'm sure it's doable but you have
to remember that these devices are
consumer electronics produced in massive
quantities manufacturers want a simple
easy and reliable method to make all
these electrical connections not
something that's fiddly and error prone
like dispensing tiny drops of silver
epoxy so the solution is to use
conductive tape but not just any tape we
need anisotropic conducting tape or film
regular conductive tape is nothing
special at all to be honest this is a
roll of copper conductive tape and it's
basically a thin sheet of copper with a
conductive adhesive applied underneath
you can touch anywhere on the copper and
it will conduct to the underlying
surface in this case a piece of metal
but importantly any two locations on the
tape will also conduct it's effectively
conductive in all three dimensions in
the X and Y planes across the surface of
the tape as well as through the z-axis
kind of through the thickness of the
tape and so this just won't work for our
LCDs because the tape will connect all
the traces together which is obviously
not what we want so we need to introduce
some kind of directionality into the
tape and isotropic conductive tape uses
small metal coated microspheres embedded
in the insulating adhesive the
microscope images of this are really
cool and we'll get to that in a second
but I think it would be helpful to take
a look at a demonstration model first to
understand how this works at a bigger
scale
so in this model the bottom section
represents the glass that we're trying
to make connections to these thin little
pads are the metal traces on the LCD
screen
and this upper section represents the
ribbon cable and you can see the pads
are a little bit thicker we'll fill the
cavity with some of this aloe gel
representing the insulating adhesive on
the tape it's just a thick gel that I
had laying around and it will be used to
hold small metal ball bearings now all
we need to do is press the two sides
together you can notice how the ball
bearings get stuck between the pads and
form basically like a conductive bridge
between the two sides
but in the gaps between the pads the
balls don't make connection with
anything and they're just floating in
the gel and even on the right hand side
where there were probably too many ball
bearings added they still don't manage
to bridge all the way from one pad to
another because they still remain
isolated in the gaps
and that's it that is an anisotropic
conducting film
the suspended ball bearings allow
electricity to conduct in the Z
Direction between the two pads
but everywhere else the bearings are not
touching and won't conduct in any of the
other directions
so real ACF uses plastic microspheres
which are coated in a thin film of metal
my tape has spheres which are about
three to five micrometers wide and are
coated in gold and nickel
you can see the Spheres are randomly
distributed throughout the film and are
actually pretty sparse which surprised
me to a degree
but it makes sense because we want just
enough microspheres for them to bridge
the pads but not so many that we
accidentally build a bridge between the
two adjacent pads
so let's take a look at this in a real
world product this little OLED has two
strips of ACF there's one piece that
connects the flexible ribbon cable to
the back of the glass and there's
another piece which connects a small
silicon microchip to the glass as well
if we look at this under a higher
powered microscope we can actually see
the microspheres pretty easily there are
all these little dots that are kind of
scattered around everywhere
I thought it would be neat to see this
as a cross-sectional view so I embedded
the OLED in some epoxy and then sanded
it flat and put it inside of my scanning
electron microscope
the flexible ribbon cable is on top here
and the glass is on the bottom we can
see all the hollow spheres scattered
around the sides and the crushed spheres
underneath the copper pad
it's those crushed spheres that are
actually making the connection
if we turn on the element detector on my
microscope we can see a few neat
features here
so the copper pad is actually coated in
a thin layer of nickel and phosphorus
which I wasn't expecting at all it
probably means it was coated with an
electrolyless nickel plating solution
I'm not sure why to be honest but I'm
thinking it's maybe for Corrosion
Protection
the pads on the glass are very very thin
but we can see that it's made out of
aluminum
if we look very closely we can see that
the microspheres themselves are made
from nickel
another important aspect is that the
microsphere is crushed or flattened
unlike my demonstration which used solid
ball bearings ACF spheres are designed
to get crushed this provides more
electrical contact to bridge the
connection between the two pads and it
makes it a more reliable process
I also made another cross section a
little further up cutting through the
Silicon microchip and here it's a
similar story although with much tighter
tolerances
the Silicon chip is on top here and the
glass is on the bottom
the pads and the gaps between the pads
are much much smaller and the gap
between the glass and the Silicon itself
is also very small
from the optical microscope we can see
that there are only a few microspheres
that bridge each connection to the pad
which is actually pretty remarkable to
be honest and all the rest of them that
are look perfectly circular are isolated
and non-conductant
going back to the element detector on
the sem we can see that the pad is made
from gold this time instead of copper
and there's a thin titanium and tungsten
layer right underneath the gold
titanium tungsten is often used in an
adhesion layer to help other metals
stick better and it can also act as a
diffusion barrier to keep metals from
migrating around the microspheres are
pretty hard to see from just a regular
photo but are easy to spot when we look
at the nickel image
you can also see some of the circuitry
of the chip itself so there are silicon
features aluminum interconnects and
tungsten Vias
a lot of these pads seem to be making
contact without the help of the
anisotropic tape they're just physically
contacting both sides so in this case
I'm assuming the tape is more of an
insurance policy to ensure that all the
pads are actually connecting in case
there's a specific location that didn't
happen to get pressed down all the way
it also provides the adhesion to
actually Bond the chip to the glass and
if you're going to be bonding it with
some type of adhesive layer you might as
well use the ACF one of the interesting
and honestly sometimes challenging
aspects of running a channel like mine
is the diversity of hats that you need
to wear obviously there's the project
that's in the video but less obvious are
the other things going on behind the
scenes filming editing lighting script
writing accounting and finance graphic
design in the videos I've done green
screen work object replacement CGI
hand-drawn animations and for whatever
silly reason I've recently decided to
start a Science magazine a YouTube
channel is basically like running a
small business and at least for my
channel I'm the one that wears all of
those hats which means I need to know
how to do all those things and that's
why a platform like skillshare is
absolutely invaluable to me skillshare
has thousands of courses across
basically all the topics I mentioned
filming editing writing yeah got you
covered accounting marketing graphic
design no problem at all for this
Science magazine project I'm desperately
trying to learn how to do magazine
layouts and cover design it's a field
that I just don't know anything about
right now and for the channel itself I
I'd really like to start doing some
interviews in lab tours but I've
literally never interviewed anyone
before and I'll be honest I'm a little
nervous about it so I have a long list
of courses queued up that I'll
personally be working through to help
get some of these ideas off the ground
the courses on skillshare are high
quality and top by subject matter
experts and it's a great way to
bootstrap a new skill quickly there is a
new creative or career focused skill
you've been wanting to learn the first
thousand people to use my sign up link
will get a one month free trial at
skillshare the sign up link is down in
the video description well I think
that's all I've got for you today I hope
that was interesting and I'll see you
next time