Tin (Sn) based solder is still the best material for PCB assembly in the modern industry today. The only difference is the lead (Pb) free from the solder base due to environmental concerns.
The melting point of tin (Sn) as 231.9°C, is hardly accepted by PCB assembly since most electronic components may not sustain such high temperatures. Maybe it shall say there are still lots of electronic components that can’t reach so high-temperature levels today yet. As a result, the solder powder should contain the majority of tin and a few other alloy components, such as copper (Cu), silver (Ag), indium (In), zinc (Zn), antimony (Sb), bismuth (Bi), etc, to reduce the melting point of solder paste and mass-produce the PCB assembly under lower energy.
So, what is the purpose of adding alloy metals (Cu, Ag, Zn, Sb, Bi) to solder paste as an alloy?
Adding trace metals as an alloy to solder paste to reduce melting point is the main purpose. The other aim lies in its function to improve the performance of solder with better strength and toughness so that perfect properties can be obtained in terms of mechanics, electricity, and thermal performance after soldering.
There are some purposes to add other metals into tin base solder to get better solderability, reliability, strength, and toughness. Here we summarize the information for your reference.
Silver (Ag)
Silver is very popular be part of solder paste. Silver can help to enhance the soldering wetting and strength, and improve the fatigue resistance.
Silver can help solder passing temperature high-low cycling test and thermal shock testing since it can enhance the soldering wetting and strength and improve the fatigue resistance. No recommend adding too much silver (usually no more than 4%) to solder paste. Otherwise, the solder point will become fragile.
• Indium (In)
Indium is a known metal that can form an alloy with tin to get the lowest eutectic point. The eutectic point of In52Sn48 alloy is as low as 120°C, and Sn77.2In20Ag2.8 alloy is even low to 114°C. In some situations, solder with low melting point will be a good choice because of its good physical feature and wettability. Nevertheless, indium is so rare around the world that it is very expensive. As a result, indium can hardly be massively applied.
• Zinc (Zn)
Because zinc is quite an ordinary mineral on the earth, it can be bought at a low price that is similar to that of lead. Although the melting point of the zinc-tin alloy (the melting point of Sn91.2Zn8.8 is 200°C) is lower than that of pure tin, the melting point is no much different. Besides, zinc features an obvious disadvantage that it will easily reacts with the oxygen in the air with oxide generated. The oxide will reduce soldering wettability so that much tin splashes will be created or soldering quality will go down.
• Bismuth (Bi)
Bismuth also performs very well in helping to decrease the melting point of the alloy with tin. The Sn42Bi58 alloy features a melting point that is as low as 138°C while Sn64Bi35Ag1 is 178°C. The melting point of tin, zinc, and bismuth alloy can be as low as 96°C. Bismuth performs very well in soldering wettability and physical features. After lead-free soldering becomes popular, the demands on bismuth drastically rise, and bismuth is mainly used for those electric components that can’t sustain high-temperature soldering. The disadvantage of tin-bismuth alloy lies in its low fragility and insufficient intensity of solder balls, which is why a little silver is added to its alloy to increase soldering strength and fatigue resistance.
• Nickel (Ni)
Adding nickel into the solder alloy will increase the melting point that is different from above mentioned other metals. The purpose of adding nickel is to inhibit dissolving from the copper base during soldering, especially for the OSP (Organic Solderability Preservative) finished PCB, so the solder bar containing SnCuNi (SCN) alloy is recommended and applied in the wave soldering process.
• Copper (Cu)
Adding a few coppers to solder paste can improve the rigidity of solder and enhance soldering strength. Besides, a few coppers can reduce the corrosion effect on the soldering iron. The amount of copper added to solder should be less than 0.8% weight and no more than 1.0%. Otherwise, the copper will possibly degrade soldering quality.