Keyboard Teardown and Alkaline Battery Leakages

Posted under DIY, Educational, Hacks, Repair, Teardown on September 25th, 2022 by

Had an external Logitech Keyboard stored away in a box. Alas, I left the batteries on them! It leaked to spring contacts and then the circuit. It has corroded a whole most of the Carbon and Copper pads on the circuit. It’s beyond repair and it’s officially dead.

Coming to the teardown, the internals of most keyboards (Non-mechanical switch type) will contain multi-layer plastic sheets with printed silver contacts acting as switches. When you press down, the 2 layers come in contact and a press is registered. It’s usually wired in a row/column approach and it’s polled many times a second to register a press. These sheets are press fitted on Carbon contacts on the main PCB. Carbon contacts are a cheap and reliable way to avoid physical connectors. In the image, you will see them as black fingers on the edge of the PCB. These are driven by Nordic’s old nRF31504 2.4GHz Wireless IC. It has a PCB antenna which communicates to your PC via a 2.4GHz dongle. Few PCB traces are all corroded by the battery leakage. The metal can is a 16MHz external crystal to maintain timing for the chip. It’s a shame that I couldn’t get this working.

Let’s talk batteries now. Why do Alkaline batteries leak? Usually, when left on a product for a long time, the battery discharges and a chemical reaction causes the generation of hydrogen gas, which can break the seal even in good brand batteries. Once the seal is broken, the alkaline electrolyte Potassium hydroxide leaks onto the contacts and it reacts with Carbon dioxide in the air to form your white powder (Potassium Carbonate) which you find on the metal spring contacts. KOH is highly corrosive and it eats away your copper traces. So never keep batteries in your product if you are not going to use them.

Pro Tip: If you have a problem like this and the leakage hasn’t affected the circuit, you can use any mild acid like vinegar to clean those metal contacts and neutralise the white powder and get your device working.

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Flipped Capacitors

Posted under Back to Basics, Educational, Product Design, Technology on September 11th, 2022 by
Different Capacitor Shapes

Most electronic design engineers know SMD capacitor parts come in the “standard” sizes of let’s say 1206, 0805, 0603,0402, 0201 etc. The common element of these parts is that their size is longer along terminals(between 2 pads you actually solder) than along the width of the part. Most times width of the part is half the length. There is a different class of capacitors which are wider than long. These are called flipped or reverse geometry capacitors. So the sizes will be 0612, 0508, 0306, 0204 etc. (To be honest, you might not find a flipped capacitor for every “standard” size capacitor though)

Why do we want to use a flipped capacitor? In one of my older posts, I explained that a capacitor in real life is not a capacitor by itself. It has an equivalent series resistance(ESR) and inductance(ESL). For an ideal capacitor, we would want to have ESRs and ESLs to be zero. Since there is an ESL in a capacitor, you usually see a V-shaped impedance curve with frequency in capacitor datasheets. The impedance value keeps getting low till a point and then it increases due to ESL. Here is where flipped capacitors come into the picture. Flipped capacitors are designed specifically to reduce ESL. When you flip the width and length dimensions of a capacitor, it reduces the soldering pad distances(or the length). Lower travel length means lower inductance and lower conductor length(Not to complicate it more, loop area also reduces, which is a good thing). So current needs to travel a shorter distance. A wider flipped capacitor effectively can have an impedance curve which is similar to 3-4 same standard-size capacitors. That means overall lower BOM count and layout space is lesser. For these reasons, these wider capacitors are starting to gain acceptance in the PCB design space.

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