Lead-Free solders

Traditional electronic assembly relies on the use of tin-lead based solders with tin to lead compositional ratios of 60:40 or 63:37 that melt at, or around, 183°C. These are ideal for electronics assembly as the soldering temperature is high enough to enable product operation at relatively high temperatures without requiring soldering temperatures that could damage components and circuit boards during assembly. Tin-lead based solders also offer good solderability, reliable joint formation and relatively low costs.

There are many Lead-Free alloys to replace tin-lead and they have a wide range of melting points, some higher and others lower than conventional tin-lead solder. Although electronics production using Lead-Free solders is well established, it is generally accepted that none of the alternatives offer all of the benefits of conventional tin-lead solders. However they can all be used with the appropriate process changes.

Melting temperatures and compositions

The actual choice of solder will depend on several factors and solders with melting points from as low as 138°C to as high as 240°C are being used in current electronics production. Some of the more popular Lead-Free alloys are shown in the table below.

In order to bring some degree of conformity to Lead-Free assembly, various industry organisations have recommended that the choice of solders is minimised and specific alloy types that are suitable for most mainstream applications have now been defined. Solders based on tin-silver-copper alloy compositions are widely used for both reflow and wave soldering and tin-copper alloys are also used for wave soldering.

The tin-silver-copper solders, which are also known as SAC alloys, have compositions with melting ranges between 215°C and 220°C. Some of the specific compositions have been patented by individual companies and the slight variations in composition may have as much to do with patent infringement avoidance as specific performance benefits. The likely composition of tin-silver-copper alloy eutectic (i.e. one that has a pure melting point rather than a melting range) is Sn-3.5Ag-0.9Cu with a melting point of 217°C.

* melting points may vary slightly depending on measurement conditions, data taken from various publications.

Choosing the most suitable Lead-Free solder

The tin-silver-copper alloys have markedly different properties to tin-lead alloys and the solidified alloys have much duller and more granular structures than tin-lead. This is discussed in Lead-Free - Inspection.

The main properties of the different solders are discussed below. It is important to remember these alloys behave differently to tin-lead alloys. They are typically harder, which impacts probe testing, and have different mechanical performances. Generally, Lead-Free solder joints can offer at least as good reliability as tin-lead alloys, especially when performance is measured under relatively mild conditions.

Lead-Free solders are available commercially in a variety of forms with no-clean, water washable or rosin flux systems and can be drawn down to the finest wire diameters.

For most companies switching to Lead-Free solder, the key challenges throughout the whole manufacturing process are focused around the use of different solder compositions and the requirement of significantly higher processing temperatures.

Tin-Copper Eutectic

For wave soldering and some hand soldering operations, the tin-copper eutectic alloy (Sn-0.7Cu) is recommended. This melts at 227°C and, because it doesn't contain any silver, represents one of the lowest cost Lead-Free alloys available. The absence of silver does lead to a considerably higher meting point, which may be problematic for some applications and solder joint reliability may not be as good as with tin-silver-copper alloys.

The tin-copper eutectic alloy can also be improved by the addition of small quantities of nickel. Alloys with added nickel can exhibit fluidity that compares favourably with that of conventional tin-lead eutectic. This means that excess solder drains off the joints more easily, so that problems such as bridging are avoided. Another positive effect of a nickel addition is that the alloy gives smooth, bright, well-formed solder fillets that make inspection easier. Tin-rich alloys also tend to erode assembly and processing equipment more quickly than tin-lead alloys and this can be a major problem with the tin-copper eutectic. The use of nickel in the alloy is reported to reduce this type of erosion, although it still remains greater than for conventional tin-lead alloy.

Although tin-silver-copper and tin-copper alloys will provide a suitable Lead-Free solution for many companies, there are numerous other solders that may be used.

Tin-Silver

Tin Silver eutectic (Sn 96.3 - Ag 3.7) has a melting point of 221°C. This is close to that of the tin-silver-copper alloys and offers good solder joint reliability performance. It is often a good choice for higher temperature applications such as in the soldering of automotive parts. The tin silver eutectic has been used for many years although its wettability may be inferior to tin-silver-copper.

Castin alloy

This is a tin-silver-copper alloy with the addition of a small amount of antimony which aids grain refining and also reduces the melting point. The alloy has a composition of Sn 96.2, Ag 2.5, Cu 0.8 and Sb 0.5 and a melting temperature of 216°C. The addition of antimony also helps to inhibit copper-tin intermettalic growth and helps to improve the thermal fatigue resistance of the alloy.

Bismuth alloys

In the Far East , Lead-Free alloys containing bismuth have been very popular for some time and have been shown to offer good performance. These alloys have melting points lower than the tin-silver-copper alloys and the tin-bismuth eutectic (Sn 42 - Bi 58) melts at only 138°C. Bismuth is also used in tin-silver-bismuth alloys where the melting points are typically in the region of 205°C to 215°C. These alloys offer good solderability, wettability, reliability and joint strength but they have one serious potential disadvantage in that they are seriously compromised if there is any lead present in the joint. Bismuth forms a low melting point alloy with tin and lead that melts at 96°C and thus, if any lead is introduced into a tin-bismuth alloy from, for example, a component, the formation of this low melting point phase can seriously compromise reliability.

Tin-Antimony

Tin - antimony (Sn 95 - Sb 5), has a melting range of between 232°C and 240°C. The alloy is suitable for high temperature soldering and has found use in electronics applications where peak operating temperatures of up to 200°C may be encountered.

Zinc and Indium alloys

Alloys containing zinc or indium are also available but are unlikely to be adopted for mainstream applications. Zinc is a very low cost metal and also forms solder alloys with melting points closer to that of traditional tin-lead (e.g. Sn 91 - Zn 9 which melts at 199°C). Unfortunately, zinc is very reactive with oxygen which leads to excessive drossing during wave soldering and poor wetting properties due to oxidation. Indium is a good metal for reducing the melting points of solder alloys and gives excellent wetting, but its high price is likely to rule out its use in all but specialist applications.

Cost implications

Tin-silver-copper alloys contain appreciable amounts of silver and are more expensive than tin-lead alloy. This can result in significant cost increases, especially in wave soldering and hand soldering, as the cost of the metals represents a significant proportion of the final cost of both solder bar and wire.

The cost of the silver can also have an impact on the price of solder pastes. The Sn96.5/Ag3.0/Cu0.5 alloy is popular because, with only 3% silver, it is lower in cost than some of its more silver-rich alternatives. It is approximately 15% less expensive than alloys containing 4% silver, but is still double the cost of tin-lead. This lower cost is achieved at the expense of a slightly higher melting point.

Summary

• A number of alternative Lead-Free solders exist.
• Each alloy exhibits different characteristics and melting points.
• Due to the range of different materials used costs of Lead-Free solders can be higher than that of traditional tin-lead alloy.
• Careful consideration of a number of factors is needed to choose the most suitable alternative.
• Tin-silver-copper alloys offer a good combination of solderability, reliability, and reasonable melting temperatures making them ideal for general purpose soldering operations.