Indium Blog

Bi/Sn/Ag: The Perfect Tabbing Ribbon Coating Solder Alloy?

Let’s quickly review the advantages of Bi/Sn/Ag alloy tabbing ribbon coating before we discuss them further:

  1. Low temperature melting point
  2. Pb-free, by definition
  3. Beneficial mechanical properties


The particular bismuth-based solder alloy that is most commonly used for c-Si cell tabbing consists of 57% bismuth, 42% tin, and 1% silver. This alloy melts at 139-140C, which is considerably lower than the Pb/Sn, Pb/Sn/Ag, and Sn/Ag alloys (179-221C) that are common for tabbing ribbon. This low-temperature melting point allows lower assembly (soldering) temperatures. Lower assembly temperatures are incredibly important when dealing with silicon solar cells, since there is a large coefficient of thermal expansion (CTE) mismatch between the cell and the copper core of tabbing ribbon – which are rigidly attached during soldering.

The second point is very simple: Bi/Sn/Ag is made without Pb and Cd, and conforms to Pb-free and Cd-free mandates.

Points #1 and #2 are fairly straightforward, but beneficial mechanical properties will need some explanation. Here I will rely on testing to tell the story. Seen here is a chart that shows the incredible increase in bond strength between traditional Sn/Pb/Ag (which still retains ~200% peel test criteria) and Bi/Sn/Ag solder alloy.

I’d be happy to discuss the reasons for this increase in bond strength with you offline. You can email me at  

Authored by previous Indium Application Manager Jim Hisert