Question

Based on the Pb-Sn alloy phase diagram, explain why Pb-Sn alloy was widely used as the solder material in the reflow soldering process in the past and it is much less used in the electronic packaging nowadays.

Figure Q1 below shows the equilibrium phase diagram of Lead (Pb)-Tin (Sn) alloy system. 

Figure Q1 The equilibrium phase diagram of Lead (Pb)-Tin (Sn) alloy system

Answer 1

For electronic soldering, the desired properties of the alloy are:

1. Lower soldering temperature (lower melting point of the soldering alloy, lower than the pieces being joined)

2. Better appearance after solder

3. Corrosion and oxidation resistance

4. Good electrical properties (electrical conductivity)

5. Sufficient mechanical strength required for electronic packaging.

Consider the phase diagram below:

For this question, property-1 is most relevant since we are talking about the phase diagram.

Pb/Sn alloy has melting point lower than the parent materials Pb and Sn. The eutectic point (E) in the figure above has the lowest melting temperature in the alloy. Also, at this composition (61.9% Sn), there is a single melting point than a range of melting points that could occur for any other composition. For example, an alloy with 35% Sn melts in a range of 183-250 degree celcius temperature (refer to the figure above for 60%Sn composition). This means, in this range of temperature the alloy is a mixture of solid and liquid phases which is not desired in soldering; because if the surface is disturbed it could create poor electrical connection. Hence, the eutectic alloy is most suitable. Also, the eutectic alloy looks brighter and gives a glossy finish after solidification than alloys with other compositions.