Solder Blends
The three main materials found in solder: lead, tin, and flux? There are countless varieties of solder available on the market based on the relative ratios of these materials. To complicate matters further, there are also additives and other metals that may be added to give solder certain properties or enhance its conductivity. Here are just a handful of examples of alloy additives and what they do:- Antimony increases mechanical strength without reducing wettability while preventing tin pest.
- Bismuth significantly lowers the melting point and improves wettability. Inhibits growth of tin whiskers.
- Copper lowers the melting point and improves wetting properties in the molten state.
- Indium lower the melting point, improves ductility, and is used for soldering to gold or for cryogenic applications due to its high resistance to temperature swings. Indium alloys are expensive and prone to corrosion.
- Nickel in solder alloy can protect UBM (under bump metallization) layer from dissolution.
- Silver provides mechanical strength, but with lower ductility than lead. It can improve resistance to fatigue from thermal cycles in lead-free solders.
Often it is the specific combination of elements in a solder blend that can be used to get a certain result.
For example, solder normally melts within a certain range, but a eutectic blend such as Sn63Pb37 melts at exactly 183°C. In70Pb30 is compatible with gold contacts (low gold leaching) and has high resistance against fatigue under thermal cycling.
SAC (Sn-Ag-Cu) is a lead-free solder popular in Japan for reflow soldering and wave soldering, a bulk soldering process where the circuit board is passed over a pan of molten solder. Waves wash over the board, soldering the components.
It all depends on what you’re trying to do, which properties you’re looking for, and what standards your electronic product will have to meet (e.g. corrosion resistance, thermal resistance, etc.).