Bertech Notes on Solder Bar

BERTECH NOTES ON BAR SOLDER

Solder has been used for thousands of years for good reason. Tin and lead, the metals that are used to make up solder, are abundant and relatively inexpensive. Different combinations (or alloys) of these metals melt at temperatures lower than their respective base metals. By adding combinations of silver and antimony, one can alter the characteristics of the solder. In electronics soldering, most applications demand that the minimal amount of energy be applied to the solder joint for at least two reasons.

One, you don’t want to shock the components with too much heat. Second, the lower the temperature needed to melt the solder, the lower the energy cost of operating the process. In addition to temperature considerations, workability is important. As most solder alloys are heated, they go through a pasty stage before becoming a liquid. In electronics soldering solder takes longer to flow into the joint. Disadvantages are longer soldering times per joint, increased thermal shock, and possibility of coming off the joint too soon, causing cold joints.

For these reasons, the alloy of choice in electronic work is Sn63, a solder containing 63% tin and 37% lead. It melts at the very acceptable temperature of 361°F. But more importantly, it goes from a solid to a liquid once it reaches that temperature, never going through a paste stage. Sn63 flows faster to start and solidifies immediately after the heat is removed. Other alloys are useful if a paste range is required. For example, Sn50, while in the paste stage, can be worked into joints using the soldering iron tip.

Silver is added to solder to reduce silver leaching. There is a tendency for silver molecules to migrate from the joint to the solder and the soldering tip. By adding silver to the solder, the silver in the joint no longer has that attraction and remains on the joint. Antimony is also added in some alloys to raise the temperature characteristics of the joint and provide it with an additional strength. (Note: Solder is used for an electrical conductivity between objects, not structural connection. Never count on it for that.)