Solder bumps. Scanning electron micrograph (SEM) of rows of solder bumps on a substrate. Solder is a metal alloy that is melted to join metallic surfaces. The process is known as soldering. The type of alloy used depends on the metals that are to be joined. Alloys with low melting temperatures are known as soft solders, those with high melting temperatures are known as hard solders.
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