Selective tinning of soldering lugs on printed circuits - where solder paste contg. flux is applied by screen printing

摘要

Prodn. of dosed amt. of tin in the form of a paste is applied to each individual soldering- lug or -eye, pref. by screen printing, and is then melted. The paste pref. contains a flux in addn. to tin or a tin alloy. The pref. paste contains by wt. 80-95% Sn or an Sn alloy, plus 5-20% flux and solvent. The pref. tin alloy powder contains 55-70% Sn, 30-45% Pb, and 0-10% Ag, esp. 63% Sn, 27% Pb. The flux is pref. based on colophony, with activators consisting of organic halogen cpds., or organic carboxylic acids. A bright copper conductor pattern is selectively covered with a polymer layer forming a mask and leaving the soldering lugs or eyes bare. The lugs or eyes are then covered with the paste described above. The lugs and eyes remain solderable even after long storage of the printed circuits. The solder layer obtd. has a uniform thickness, and does not result in oval or uneven holes.