What is Eutectic Solder?

Eutectic solder is a type of solder that melts and solidifies at one single temperature. For example, Tin 63% / lead 37 % that melts and freezes at 183°C (see figure below). Most solder starts to soften at a certain temperature and only fully melts at a higher temperature. For instance, the Sn96.5Ag3Cu0.5 solder starts to melt at 219°C (426.2°F), but only reaches its full liquid state at 221°C (429.8°F). 

So any solder alloy that melts and solidifies at one single temperature is called Eutectic solder. When we change the composition from Tin 63% / Lead 37% to Tin 60% / lead 40%, then the melting point becomes a melting range. The solder alloy having a melting range is known as non-eutectic solder.

What is the advantage of Eutectic Solder?

Since Eutectic solder melts and solidifies at a specific temperature, it is better for the solder reflow process than non-eutectic solder. It speeds up the solder reflow process which results in a faster PCB assembly process and a higher throughput which in turn lowers the effective cost of PCBs.

Other advantages include:

• Eutectic solder flows much better than non-eutectic solder.
• Eutectic lead-free solders are made up of alloys that are less expensive to manufacture. 
• When the solder melts and freezes rapidly the quality of the solder joint is good. 
• Eutectic solder has a nice chrome finish compared to non-eutectic solder that usually has a dull and grainy appearance.
• During the solidification process, non-eutectic solder will go through a period of partly solid and partly liquid states. At this time, if any parts shifts or reflow process is interrupted it can cause cracks in the solder joints (hot-cracking). These cracks are difficult to detect during the QC inspection.

A list of a few Eutectic Solder materials can be seen in the table below.

Lead-free Eutectic Solder Composition

How do you select eutectic solder composition?

There are various compositions of eutectic solder available. Choosing the right composition for an application is based on several factors. But, the solder melting point of the process at hand is the main factor to consider.