The Tech Blog

I was doing some high-speed PCB layout for a project and ran across DC Blocking capacitors or AC Coupling capacitors. These 2 names are used interchangeably and, in my opinion, remain the same for high-speed design. These caps play an essential role in maintaining signal integrity while enabling proper interfacing between different circuit sections.

A DC blocking capacitor is simply a capacitor connected in series that passes AC signals while blocking DC components in that line. It is effectively a high-pass filter. It removes the unwanted DC bias in the line. These can happen when there is some sort of encoding of signals in the line and it comes up as a non 50% duty cycle of 1s and 0s. Also when on an actual PCB, there may be 2 different chips communicating via a diff pair and each can be of a different DC operating point. So directly connecting them without a blocking capacitor can cause unwanted current flow or blow up the Tx or Rx drivers.

Since these caps are used in series on differential lines on PCBs with a particular line impedance, it causes a line impedance discontinuity. This can cause reflections for high-speed signals. So ideally we want to minimize the impedance changes. For that, we usually prefer using the smallest-sized resistors like 0201 rather than let’s say 0603 because to add a cap physically, you need to widen the controlled impedance traces to accommodate it. Now another option is to remove the return layers right below the capacitor as a slot. This will increase the impedance at the spot as the return layers are much farther. There are a few papers and a nice Intel app note which sums up the size of the slot. It shows how with a slot you effectively make the capacitor invisible(Check images) in terms of impedance changes. The usual chosen caps are 0.1uF or 0.01uF with low ESR.

In summary, DC-blocking capacitors are essential for interfacing high-speed communication lines like USB 3.0+, PCIe, SATA etc. Do take care on the placement position and PCB considerations when using them in your design.

Recently, while working on a project involving LCD screens, I came across Anisotropic Conductive Films (ACF). Though they’ve been around for decades, with patents dating back to the 80s and 90s, many aren’t aware of how vital they are in miniaturized devices. Some of you may have wondered how you bond, say, metallic flex cables to glass. In LED displays, the row and column wires are extremely thin. The challenge arises when you need to connect these delicate wires to larger driver boards, especially when the connections are on glass. You can’t solder on glass, and alignment at such small scales can be a nightmare.

Enter ACF. It’s a thin adhesive strip filled with tiny conductive particles (typically metal-coated polymer balls). These films allow you to make electrical connections between two layers, like a flex cable and a glass substrate, while ensuring the electrical flow happens only in one direction. This selective conductivity is what makes ACF “anisotropic” – it conducts electricity vertically (through the thickness of the material) but not laterally (across the surface), preventing shorts between adjacent wires.

So how does it do it? Those tiny balls in the adhesive become sandwiched between the 2 layers that you want to electrically connect(Check Images). Now the balls since they are relatively sparse, don’t form a bridge(by balls lining up) between consecutive wires shorting them. Now to bond it, usually, it’s heated/pressed together the conductive particles become trapped between the surfaces, creating electrical pathways only where they’re needed, typically on pads or traces. This allows ACF to be used in applications with very tight spacing, where conventional soldering or connectors would be impractical.

These are useful in display tech like LCDs, OLEDs, Flex PCBs, and Chip on Glass Assemblies where ICs are mounted on glass substrates(These are the small black rectangles you see on flex cables on OLED modules). ACF may not get as much attention as other electronic materials, but they’ve quietly revolutionized how we connect components. Worth learning about!