Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

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Melting point

#6318

Melting point | 22 August, 2000

Hello, Dr. Ning-Cheng Lee

The melting point of solder PB-Free its to higher do you now the lower temperature of an PB-Free and its constitution.

Its true if the solder paste have in its constitution a Bismut component the melting point its lower?

Thank`s

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Dr. Ning-Cheng Lee

#6319

Re: Melting point | 22 August, 2000

An example of very low melting point Pb-free solder is 58Bi/42Sn. Different versions of this alloys have been tried, such as doping with various additives, in order to enhance the reliability further.

Yes, Bi is one element which promises low mp for alloys containing Bi. Therefore, SnAgBi system usually has a somewhat lower mp than SnAg or SnAgCu systems. The mp can be lowered significantly if a large % of Bi is employed. The tradeoff is a wider pasty range as well as poorer mechanical properties.

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#6320

Re: Melting point | 23 August, 2000

Dr. Lee: Taking a tangent on your response ...

When talking about alternate solders. Bismuth forms an alloy with lead with a melting point of 93�C. Are there special conditions that cause this alloy to form? Or does the alloy form commonly when lead and bismuth are combined? What is the impact on finished assembly operation of using low temperature solders?

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Dr. Ning-Cheng Lee

#6321

Re: Melting point | 23 August, 2000

The ternary SnPbBi phase does form commonly when Pb and Bi are combined. No special condition required. Presence of this phase jeopardizes the reliability for applications involving temperature near 96C.

Using low temperature solders means a lower service temperature. Often, the reliability at ambient temperature is also compromised, due to a low homologous temperature for this solder.

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