Back to Basics: Color E-Papers and its Working

Posted under Back to Basics, Educational, Technology on June 16th, 2024 by

E-papers/E-ink displays are something most of you would be familiar with. These ultra-low power displays mimic the appearance of ink on paper. They reflect light like paper and get better for reading when there is more surrounding light, unlike any other electronic screens. They don’t need power to retain the image on a screen and have a 180-degree viewing angle. So how do they work?

Think of these displays as small wells with transparent electrodes containing a transparent viscous fluid with charged particles of 2 colors, white(-ve) and black(+ve). Now if you apply a voltage to this well with the top being -ve and the bottom +ve, the black particles get attracted to the top & white goes to the bottom. Now remove the power, the black stays on top with no power needed as the suspended liquid is viscous. This is a classical 2 color e-paper that you see in Amazon Kindles.

Now the original manufacturers of E-ink launched something amazing very recently. A multicolor display that can display up to 60,000 colors. It’s called Spectra 6 and Samsung is launching a whole line up of massive-sized screens for outdoor ad display screens.



So how do these work? It’s an estimated guess as I can’t find much literature on it. They contain Red(+), Blue(+), Yellow(-), White(-) particles. Now how can it display any color on screen with these particles? These particles are of different sizes and quantities in a single well. Now when a variable voltage is given, it takes more time for a larger particle to come up on top than the smaller ones, That’s the secret sauce. Now apply different sequences of voltage to mix and match particles to display true colors, all without continuous power.

The main drawback is slow refresh rates; full-color models take up to 12s for image changes, making them ideal for static content. Think about the use cases. Refreshable Mall Displays where you currently print and paste. Grocery store price tags which change with time. Bus stop signs that run on solar with these low-power displays. Truly displays of the future!

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BackToBasics: Wire Bonding in PCBs

Posted under Back to Basics, Product Design, Technology on June 2nd, 2024 by

You would have seen those black blobs in PCBs in those cheap mass-manufactured toys. It’s called a Chip on Board(COB).

COBs are placed on the PCBs via Wire Bonding. Wire bonding is a solid-phase welding process that joins a thin wire and pad surface. Now a COB is usually a chip with just the bare silicon die with exposed metal pads. This means that the plastic packaging that you find in most ICs which makes it a packaging style of QFN, TFQP, etc is missing. The metal pads on the die are connected via wire bonding directly to the PCB pads. Pour some black epoxy for strength and there you have a full COB on a PCB. It always need not be black epoxy, for SMD RGB LEDs if you look carefully you will see wire bonding used to connect the die to the package, and then the clear epoxy is poured to act as a lens to focus light.

Wire bonding can be done using Gold, Copper, or Aluminium wires based on requirements with Gold having good conductivity, resistance to oxidation, and good malleability to create strong bonds. But they are expensive. Copper or Aluminium is used when price is a concern. The wires are extremely thin with diameters in 10s of micrometers. To bond the wire on the pad you have different techniques like thermocompression bonding(using high heat and pressure to bond), ultrasonic bonding(Ultrasonic vibration to excite the spot to bond), or thermosonic(which is a combination of the above two methods).

Now Why wirebond on PCBs? Why not just use the chip as it is with the plastic package and solder it? The answer lies in the cost. For extremely large volume manufacturing, you can get the manufacturer to just give you the bare dies at a much cheaper cost than that with a package. Now wirebonding + die is cheaper than having a package. This is the reason why you see them a lot in dirt-cheap products. Another reason is that the actual die is usually much smaller than the plastic package, so for extremely small products it makes sense to do die + wirebonding to save a few mm of space in the PCB area.

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