When I was learning about solder, I was very surprised when I received a call inquiring about fluxless soldering. Truly providing a good solder joint without using flux; pretty self explanatory. Ever since that call I have wondered how this is possible.
The traditional soldering procedure consists of three steps:
(1) pre-cleaning and deoxidation of surface oxides.
(2) solder reflow and/or reflow joining.
(3) post-soldering cleaning.
After researching, I found that the first step in fluxless soldering would be to remove the oxide layer. This can be done using chemicals or plasma. Fluxless soldering can also be accomplished by producing the solder materials in a non-oxidizing environment. Gold coatings are widely used to promote solderablity and, when soldering in a vacuum or protective atmosphere, they can eliminate the need for fluxes. You have to be careful if there is tin in the assembly process because the formation of the brittle gold-tin intermetallic is a concern for most applications.
My colleague, Eric Bastow, mentions this phenomena in his blog post entitled Intermetallics. Indium Corporation has an application note on fluxless soldering which can be found here. As the electronics packaging industry continues to face challenges in making faster, smaller, and smarter devices, fluxless soldering will become more desirable.
'Til next time,
-Adam
IMAGE: The copper mirror test evaluates the corrosivity of a solder flux. Many applications can not accept ANY flux corrosivity.