Lead-Free Solders

Lead-Free Solders:
From Materials Science and Engineering By William D. Callister, Jr. (Very good Book)

Solders are metal alloys that are used to bond or join two or more components (usually other metal alloys). They are used extensively in the electronics industry to physically hold assemblies together; furthermore, they must allow expansion and contraction of the various components, must transmit electrical signals, and also dissipate any heat that is generated. The bonding action is accomplished by melting the solder material, allowing it to flow among and make contact with the components to be joined (which do not melt), and, finally, upon solidification, forming a physical bond with all of these components.

In the past, the vast majority of solders have been lead-tin alloys. These materials are reliable, inexpensive, and have relatively low melting temperatures. The most common lead–tin solder has a composition of 63 wt% Sn–37 wt% Pb.

According to the lead–tin phase diagram, Figure 9.8, this composition is near the eutectic and has melting temperature of about 180⁰ C, the lowest temperature possible with the existence of a liquid phase (at equilibrium) for the lead–tin system. It follows that this alloy is often called a “eutectic lead-tin solder.”

Unfortunately, lead is a mildly toxic metal, and there is serious concern about the environmental impact of discarded lead-containing products that can leach into groundwater from landfills or pollute the air if incinerated. Consequently, in some countries legislation has been enacted that bans the use of lead-containing solders. This has forced the development of lead-free solders that, among other things, must have relatively low melting temperatures (or temperature ranges). Some of these are ternary alloys (i.e., composed of three metals), to include tin–silver–copper and tin–silver–bismuth solders. The compositions of several lead-free solders are listed in Table 1(given below).

Compositions, Solidus Temperatures, and Liquidus Temperatures for Five Lead-Free Solders
Composition(wt%)	SolidusTemp (0C)	Liquidus Tempreture (0C)
52 In/48 Sn*	118	118
57 Bi/43 Sn*	139	139
91.8 Sn/3.4 Ag/4.8 Bi	211	213
95.5 Sn/3.8 Ag/0.7 Cu*	217	217
99.3 Sn/0.7 Cu*	227	227

*The compositions of these alloys are eutectic compositions; therefore, their solidus and liquidus temperatures are identical.Adapted from E. Bastow, “Solder Families and How They Work,”Advanced Materials & Processes,

Vol. 161, No. 12, M.W. Hunt

Of course, melting temperatures (or temperature ranges) are important in the development and selection of these new solder alloys, information that is available from phase diagrams. For example, the tin-bismuth phase diagram is presented in Figure 9.10. Here it may be noted that a eutectic exists at 57 wt% Bi and 139⁰C which are indeed the composition and melting temperature of the Bi–Sn soldier in Table 1.