What happens when paste residues and cleaning solvents are incompatible?

As debuted in these pages last month, the National Physical Laboratory’s new interactive assembly and soldering defects database lists problems and solutions. The database permits searching and viewing of countless defects and solutions. Users also can submit defects online for recommendations at http://defectsdatabase.npl.co.uk/.

This month we feature one recent submission: white residue rings on BGA joints. All BGA solder joints featured a white ring around each ball termination. The ring was perfectly formed and not like normal white residue often seen from a standard cleaning operation (Figure 1).


In fact, the ring consisted of solder paste residues found after Pb-free reflow. The sample in this case had also been through a standard immersion cleanliness testing process for ionic residues. Some combinations of paste residue and cleaning solvents are not compatible and leave a white- or brown-like dust on the board surface. In this case, the combination of 75% IPA and 25% water has failed to dissolve or displace the residues from around the base of the ball and pad interface and creates a most interesting image.

This issue was only seen on a few boards intended for a consumer application, and as such, there was no specific corrective action. The paste was a no-clean material; the cleanliness testing was just part of a process evaluation and created these unique rings that did not prove to be harmful to the application.

Dr. Davide Di Maio is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); defectsdatabase@npl.co.uk.