Speaking of Flux

Underside of vacuum tube sockets in the old Eniac-1 computer. The rectangular objects are Mica capacitors whereas the tubular ones are resistors. The color dots and stripes encode the values of the specific parts.

Opening the door to the unventilated warehouse, the acrid stench was unmistakable.

“What’s that smell?” someone asked.

“It’s rosin-flux, the wetting agent used when soldering resistors, capacitors and wires to electronic circuits. The rosin comes from pine trees. When the equipment heats up, ┬áthe smell gets stronger.”

Soldering is done at high temperatures that would normally cause oxidation (rust) and keep the molten metal - the solder* - from reaching the surface of the wires. Flux prevents that oxidation. It also acts as a wetting agent to help assure a better solder joint.

Contemporary electronics may use any of three general types of flux, most commonly the “rosin core” with an extract of pine trees injected into a central tube running through the solder or brushed on from a bottle or fiber pen, or some more recent types of “no need to clean” types. (Plumbers and stain-glass workers use a different flux, one that would be corrosive to electronic circuits.)

Rosin flux has a distinctive smell and when, years ago, our host unlocked and opened that door to the warehouse full of old computers and related hardware that would one day fill the Computer History Museum in Mountain View California, I knew the smell the instant we walked through the door. The whole warehouse, and especially the Eniac-1 computer with its hundreds of vacuum tubes and thousands of components, just plain reeked.

Today’s electronics are miniscule by comparison to the parts used in that old Eniac-1 where the rectangular, color-coded Mica capacitors and resistors are a half inch long. The workers assembling them would’ve used 50 or 75 watt soldering irons with thick wooden handles after crimping the wires with needle-nose pliers.

Today, we use stereo microscopes with 10x or sometimes 20x magnification and the teeniest of soldering iron tips to attach the “0204” parts – that means the part is 0.02" by 0.04" and you need very fine tweezers, that microscope and a very steady hand to nudge it – gently! – into correct position. The solder we use today is also tiny, measuring only 0.015" in diameter. Only the smallest of “zits” of solder are used when attached the end of one of those tiny parts.

And don’t breathe before you solder it down – you’ll blow the tiny part out of position or completely off the board. And if it falls on the carpet, don’t bother looking. You’ll never find it.

While the physical world of electronics assembly has shrunk a hundred fold, the smell of that rosin core solder is exactly the same.

It stinks, and I love it.

Note: It’s pronounced “sod’-er.” The “L” is silent.


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