Okay, I have a confession to make: I’ve always had a grudge against bismuth, ever since I started recommending thermal interface materials.It is the polar opposite of my favorite element (indium) – well, as much as a metal can be.These 2 elements (indium or bismuth) are added to almost every solder with a lower solidus temperature than Sn/Pb.The choice for most thermal interface applications that I have dealt with was indium or an indium alloy, but now I am starting to become very fond of my new friend bismuth for solar applications.
Bi/Sn and Bi/Sn/Ag are now available as a solderable coating for our Tabbing and Bus Ribbon.After getting a feel for this material, I must say I find it pretty nice to work with.Both alloys melt at 138-139degC, with the Bi/Sn/Ag having a greater tensile strength (which is not necessarily a good thing for tabbing ribbon).With a little bit of lab time I have isolated an existing flux that works very well with these alloys.So far GS-5454 has formed good solder bonds down to 160degC.This is great news, because it allows you to minimize the reflow temperature (and stresses) of your C-Si/tabbing ribbon interface.
~Jim
