What are the different types of solder?
Solder on a PCB can be thought of as connective tissue. It serves as the conductive glue that sticks components to substrate and brings continuity to the circuit board. It’s hard to imagine what modern electronics would be like without this convenient, low-melting point alloy at our disposal.
Not sure which solder to use for your project? In this post we’ll look at the different types of solder that are out there and how you can choose the right solder for your needs.
What Is Solder?
Solder hails from the middle english word soudur, which derives from the latin word solidare, meaning “to make solid.” It’s a fusible alloy (i.e. low melting point) used to bond metal workpieces together. The idea of using a lower melting point alloy to bond two or more metals has been around for thousands of years, but today’s solder is typically a blend of tin, lead, and/or flux.
Meet the Different Types of Solder
There are so many different kinds of solder available on the market today that it can be daunting picking the right one for your project. Fortunately, there are really only three main categories of solder which you can use to narrow down your search:
Lead based solder was what kicked of the electronics revolution. The most common mixture is a 60/40 (tin/lead) blend with a melting point around 180-190°C. Known colloquially as soft solder, tin is selected for its lower melting point while lead is used to inhibit the growth of tin whiskers. The higher the tin concentration, the better the tensile and shear strengths.
Lead free solder started taking off when the EU started restricting the inclusion of lead in consumer electronics. In the US, manufacturers could receive tax benefits for using lead-free solders. Tin whiskers can be mitigated by using newer annealing techniques, incorporating additives such as nickel, and using conformal coatings. Lead free solders generally have a higher melting point than conventional solder.
Flux core solder is sold as a spool of “wire” with a reducing agent at the core. The flux is released during soldering and reduces (reverses oxidation of) metal at the point of contact to give you a cleaner electrical connection. It also improves the wetting properties of the solder. In electronics, flux is usually rosin. Acid cores are for metal mending and plumbing, and should not be used on electronics.
Understanding solder blends
Remember 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.
It’s important to be mindful of potential galvanic corrosion or brittle caused by placing dissimilar metals in contact with one another when choosing solder blends. The wide variety of solder blends can help you find the right combination of properties for your electronic device.
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.).
In summary, there are three main types of solder: lead-based, lead-free, and flux. Lead-based solders are the best understood, are reliable, and preferred in mission critical applications such as aerospace or medical electronics. Lead-free solders are available for electronics that need to meet health and environmental requirements. And flux solders contain a rosin reducing agent at the core that is released during soldering removing oxidation from the bonding site.
For most hobbyist applications your standard Sn60Pb40 solder is fine. If you’re looking to improve workmanship on the manufacturing floor, it may be worth exploring blends that boast improved wetting or convenient eutectic melting points. Picking the right blend is about identifying the requirements you need to meet, finding the right properties to meet those requirements, and optimizing for cost. Pick the best solder for your needs.