I have recently been helping my friend Judy organize and get her Electronics Parts store back on it's feet. This has meant I get to dig through the old stock and reorganize to my heart's content. As I was sorting packages of capacitors yesterday I came across some with and unexplained coil of what looked like solder. The coil was very tightly wound so that it had the appearance of a small spring 1 cm in length. They looked like this:
At first I thought this was a waste as no one would be able to use such short pieces of solder without burning fingers. It didn't make sense to me until I let my mind wander back to the era before there were circuit boards.
Electronic design and assembly have gone through many phases over the years. Right now we seem to be leaving the through the hole era and entering the surface mount era. There have been various schemes for putting electronic components into proper position relative to one another over the years.
To understand these little springs of solder we have to step back all the way to what I call point to point assembly. In this scheme the tube (Valve) sockets were mounted on the chassis of the device and the capacitors and resistors were strung point to point through the air between pins. For nodes where there wasn't a convenient pin terminal strips were mounted to the chassis and served as common connection points. All the valve (tube) sockets, potentiometers, chassis mounted electrolytics and terminal strips were conveniently supplied with large terminals that usually had a hole through which the wire of the component could pass. The recommendation was to make a good mechanical connection before soldering the connection. As I recall from building kits in the 1950s this was a standard admonishment in all the soldering instructions that accompanied the Heath and Knight kits. Making a good mechanical connection meant that the lead would go through the terminal, wind around one time and be pinched tight with a needle nose pliers before solder was applied. This made a very good connection that was made to last hopefully forever.
To illustrate this I have found an old terminal strip and demonstrated the mounting and soldering of a capacitor.
Notice the leads are wound around the terminals.
And finally soldered down. In a real assemble there may be two to many wires on each terminal but for our purposes there will be only one.
The electronic piece of equipment assembled this way would go into service and function very well. Unfortunately nothing lasts forever and eventually something goes wrong and the equipment ends up in the repair shop. For the purpose of our demonstration the capacitor above has failed and needs to be replaced. The repair technician is now faced with the problem of removing the existing capacitor from the terminal strip and installing a new one. The good mechanical connection suddenly is a real problem as the solder must be removed or melted and the wire unwound and removed. This gets very difficult particularly if there are multiple wires in the terminal.
Here is where our little springs of solder come in. They are in fact not just solder but rather a copper core wire surrounded by solder and flux and shaped into a helical coil. I am going to replaced the old bad capacitor with a new one that came in the package with the coils of solder. The first step is to cut the leads of the bad component leaving a half centimeter of lead.
There is no need to try to desolder the original connections.
The coils of solder and wire are in place and the new capacitor is now inserted into the other sides of the coils.
Finally heat from the solder iron tip is applied to the coils until the solder melts and bonds with the wires.
The connection was easy to make and there is minimal disturbance to the rest of the connections in the terminal. Further the coil of internal wire satisfies the perceived need to have a very good mechanical connection. The added bonus for me was that for some reason the solder from 50 years ago smelled better than the current formulation so I was able to enjoy a whiff from the past.