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What is Alloy Metal Added to Solder Paste for?

Up to now, tin is still regarded as the best material for soldering. Even lead-free solder paste is also mainly made of tin and the only difference lies in the absence of lead.

The melting temperature of pure tin (Sn) is as high as 231.9°C that is actually hardly accepted by soldering in PCB (Printed Circuit Board) assembly since some electronic components can’t stand such a high temperature. As a result, alloy solder should be added to tin powder that contributes to the majority of solder paste, such as silver (Ag), indium (In), zinc (Zn), antimony (Sb), copper (Cu), bismuth (Bi) etc. With the trace metals added to tin powder, the melting point of solder paste can be reduced so that PCB assembly can be implemented in quantity with energy saved.

The other aim of adding trace metals to solder paste lies in its function to improve the performance of solder balls such as their toughness or strength so that perfect properties can be obtained in terms of mechanics, electricity and thermal performance after soldering.

Now, it’s not difficult to understand why Sn63Pd37 accounts for the majority of solder paste with lead. Such solder paste can get a melting temperature as high as 183°C that is much lower than that of pure tin. When it comes to lead-free solder paste, if a little SAC305 is added, the melting temperature can be lowered to 217°C while if a little SCN is added, the melting temperature can be lowered to 227°C Either 217°C or 227°C is lower than the melting temperature of pure tin, that is, 231.9°C.

After two types of metals with high melting temperatures are proportionally mixed together, the melting temperature of the composite goes down. Isn’t that surprising? In fact, the reason for that results from some chemical features and will not be discussed in this article.

There are some types of metals that can be added to solder paste to make soldering smoothly carried out. The features and functions of those metals will be introduced in the following article.

• Silver (Ag)

Generally speaking, adding silver to solder paste aims to improve wettability of soldering and strengthen soldering strength and fatigue resistance. Solder paste is able to pass cold-hot recycling test. However, adding too much silver (usually more than 4%), solder balls will become fragile instead.

• Indium (In)

Indium is possibly a type of metal that can be mixed with tin to become alloy metal with the lowest melting temperature. The lowest melting point of 52In48Sn can be as low as 120°C while that of 77.2Sn/20In/2.8Ag can be as low as 114°C. In some situations, solder with low melting temperature 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 ordinary, it can be bought in a low price that is similar to that of lead. Although melting point of zinc tin alloy is lower than that of pure silver, there’s no difference. 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 temperature of alloy. The alloy Sn42Bi58 features a melting temperature that is as low as 138°C while that of Sn64Bi35Ag1 is only 178°C The melting temperature of alloy of tin, zinc and bismuth can be as low as 96°C Bismuth performs very well in wettability and physical features. After lead-free soldering becomes popular, demands on bismuth drastically rise and bismuth is mainly used for products that can’t stand high-temperature soldering. The biggest 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 increase strength and fatigue resistance.

• Nickel (Ni)

Adding nickel to solder is not to decrease the melting temperature. After all, compared with nickel tin alloy, pure tin features a lower melting temperature. Adding a little nickel is just to stop copper substrate from dissolving during soldering. Nickel is especially used in wave soldering to stop OSP (Organic Solderability Preservative) board from “biting” copper, so solder bar containing SnCuNi (SCN) alloy is applied in wave soldering.

• Copper (Cu)

Adding a little copper to solder paste is capable of increasing the rigidity of solder so that the intensity of solder balls can be increased. In addition, a little copper is capable of reducing corrosion effect caused by solder. The amount of copper added to solder should be less than 1% since more than 1% copper will possibly decrease soldering quality.

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