Indium Blog

Why doesn't my 80Au 20Sn solder look any thing like gold?

If you have ever handled a piece of 80Au 20Sn solder alloy, one of the things that you might have noticed is that it does not look anything like gold (yellow lustrous metal). In fact, it does not look all that different than tin or any other tin based alloy.

And, yes, before we is the same gold (Au) used in jewelry, etc.

To the AuSn newbie, the first shipment of 80Au 20Sn solder may cause a little bit of alarm. "Did they send me the wrong alloy? This doesn't look like it has any gold in it!!!"

To the human mind, when one thinks of something that is comprised of 80% of a material, one naturally assumes that material will dominate the properties of the composite material. And, normally, that would be a valid assumption. However, in the case of a solder alloy, the composition is almost always reported in terms of percent by weight. So, in the case of 80Au 20Sn, the alloy is 80% by weight gold and 20% by weight tin. The "issue" lies in the fact that gold is more than twice as dense as tin; 19.3 g/cc versus 7.3 g/cc. 

So, let's think about this.............

If we have 100 grams of 80Au 20Sn alloy, you have an alloy comprised of 80 grams of gold and 20 grams of tin. But, it terms of volume of gold and tin, you have 4.15 cc of gold and 2.74 cc of tin. So, by volume, the alloy is 60% gold and 40% tin. The 40% (by volume) of tin in the alloy is enough to "dilute" the gold and greatly diminish any "yellowing" that one would expect the gold to impart to the appearance of the alloy. 

If anyone has ever attempted to accurately photograph a shiny metallic surface, one can appreciate the difficulty in so doing. So, the photo shows some 80Au 20Sn solder preform in comparison to a pair of stainless steel tweezers. Visually there is very little, if any difference in appearance.