Second Thoughts on Electronics

Is RoHS really better for the environment?

Author: rickj • Category: Rants • Posted on Thu Jul 10, 2014 12:46 pm • Viewed: 21585x • 0 comments

A surprising number of metals are capable of spontaneously growing whiskers. They do it under ordinary room conditions. The whiskers are thin, but up to millimetres long and highly conductive. They are death to electronics. We've known about this since 1946, and found a way to eliminate metallic whiskers, but we are now inviting them back.

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At least aluminum, cadmium, gold, indium, lead, silver, tin, and zinc will grow whiskers. All of these metals have been important in electronics, though the use of cadmium is rare now that we understand its toxicity. Metallic whiskers are typically 0.5um to 10um in width, so they are difficult to see: compare this to human hair, at about 50um diameter. Their length is impressive; a specimen 23 mm long is the current record holder. With a growth rate as fast as 15um/s (that's a millimetre in 6.7 minutes) they can sneak...

All the noise in resistors

Author: rickj • Category: Theory • Posted on Tue Jun 03, 2014 4:31 pm • Viewed: 31431x • 0 comments

Every resistor contibutes some noise to your circuit. The noise is tiny, and often negligible, so why bother about it? Well, RF people have long recognised that it can dominate receiver performance if it gets out of hand in front-ends. Audio amplifier designers can also find it troublesome. Now, as 12-bit Analogue-to-Digital Converters (ADCs) give way to 16-, 20- and even 24-bit types, resistor noise can be important in any device that collects digital information, and that catches lots of applications. I recently built an instrument in which resistor noise reduced my beautiful 20-bit ADC to a mere 14 Effective Number Of Bits (ENOB), and I had to learn a good deal more about noise to get it working properly. This was annoying, as I was under the fond illusion that I'd already done my noise sums properly.

Resistor noise comes in two basic flavours, Thermal (or Johnson) noise, and Flicker (also known as excess, contact, or 1/f) noise. Thermal noise afflicts all resistors equally. Flicker...