The most common issue preventing good solder wetting on a substrate is oxidation. All metals are prone to some level of surface oxide formation when exposed to ambient conditions. Solders cannot wet substrates when they or the substrate they are soldering to is oxidized.
How can I be sure that oxidation is my problem? Well, is your solder balling up on the surface like the water droplet shown in this image? If so, you have an oxidation problem.
How do I remove this oxidation? Indium has over fifty fluxes formulated for removing oxides. Every Indium technical staff member is trained to identify the appropriate flux to suit your application and will be happy to do so. Feel free to contact us here.
Can't use a flux? Other oxide removal techniques include chemical etches, plasma cleaning, ultrasonic soldering, or use of a reducing atmosphere.
image courtesy of www.malawicichlidhomepage.com