PCB Tech - Binary metallographic diagram of PCBA tin-lead solder

Tin Binary Metallographic Diagram

Although lead-free solder paste (solder) has become the mainstream of modern environmental protection electronic technology, based on the consideration of reliability, there are still many products in the automotive industry and military electronics that still use lead-containing solder, because PCBA processing has lead solder The welding strength is much higher than that of lead-free.

The main component of leaded solder paste is tin (Sn) and lead (Pd). Other trace components include metals such as silver, bismuth, and indium. Each has a different melting point (MP). However, this article assumes that these traces of other metals The metal composition does not affect the characteristics of the solder paste, so we can first use the binary phase diagram of tin-lead to explain the characteristics of the solder paste, because the phase diagram of more than ternary is too complicated.

And whether it is solder or IMC, the more its components, the more complex the structure, the less easy to control, and the worse the reliability.

Refer to the tin-lead binary phase diagram. The abscissa represents the weight percentage of tin-lead (Wt%), and the ordinate represents the temperature in degrees Celsius (°C).

The melting point of lead is 327°C, so the upper left corner of the phase diagram starts at 327°C (100% tin, point A). As the tin content of tin-lead weight ratio increases, this Melting point (Liquidus mp)] The temperature of the line is getting lower and lower. When the weight ratio of tin to lead reaches the best Sn63/Pb37 (actually Sn61.9/Pb38.1, because the early measurement was not correct, it caused errors ), its liquefaction melting point also reaches the lowest 183°C. If you continue to increase the ratio of tin, its liquefaction melting point temperature will reverse and rise, reaching 232°C when pure tin.

Except for the weight ratio of 61.9/38.1 for tin-lead alloy solder, which has a unique [common solid point (E point) (Eutectic)] of 183°C, other different weight ratios will have two melting points. The higher temperature is called It is [Liquidus mp], and the lower temperature is called [Solidus mp]. The solder between the two melting points is called "pasty", which is a high-viscosity fluid in which solid and liquid co-exist. The so-called pasty is actually a bit similar to the type of earth-rock flow, because it may be that tin has become liquid but lead is solid (αPb+L), or just the opposite (βSn+L).

As for why we must use the weight ratio of Sn63/Pb37, this is because the melting point of pure tin is as high as 232°C, which is not easy to be used for general PCBA processing and welding, or that current electronic parts cannot reach such high temperatures, so it must be Tin is mainly used, and then other alloy solders are added to lower its melting point, so as to achieve the main purpose of mass production and energy saving. It can also lower the temperature resistance threshold of electronic parts, because most PCBA products are only used and stored in the environment It is only between -40°C and +70°C, so the melting point of 183°C is more than enough; the secondary purpose is to improve the toughness and strength of the solder joints.

General phase diagrams will have symbols such as α, β, and γ to indicate solid solutions in the phase diagram. The tin-lead phase diagram is only binary, so only α and β are used. In this phase diagram, α refers to the solid solution of lead (Pb), and β refers to the solid solution of tin (Sn).

The αPb phase region (CBA) is a lead-rich solid solution, but tin will dissolve in lead and tin becomes a solute. In this phase region, the solubility of tin has its upper limit, starting from point C, as the temperature rises ( When the CB line) reaches 183°C (point B), the solubility of tin also reaches the highest 18.3%. When the temperature continues to rise (line BA), the solubility of tin gradually decreases to zero (point A).

The βSn phase region is a solid solution rich in tin, and the relative lead is dissolved in the tin, and the lead becomes a solute. Starting from point H, as the temperature rises (HG line) to 183°C (point G), the solubility of tin also reaches the highest 2.23% (=100-97.8), when the temperature continues to rise (GF line) However, the solubility of tin gradually decreases to zero (point F).