Solderability and reliability of printed electronics

Two types of printed conductors were studied for their effects on the solderability and reliability of printed electronics. The metal particles of the studied printed conductors were copper and silver and the bonder was phenol resin. The test vehicle was a 6 x 25 mm FR4 coupon, where the printed conductors were formed on it. Dip and look test results indicated that printed silver heavily leached and hence had poor solderability. Wetting balance test showed the studied printed copper had poor solderability. Surface roughness measurement and microstructure observation confirmed that the poor solderability of printed copper was due to its surface roughness and heterogeneity (mixed of copper and void). The reliability of printed interconnects were also studied by analyzing their microstructures. The study found that the solder formed the interconnection by bonding the outer metal particles. However, the outer metal particles did not closely packed and hence the bonding was not continuously formed. The discontinued bonding confirms the heterogeneity of the printed conductor surface, which causes the poor solderability of the printed copper and affects the strength of the printed interconnection. The interface intermetallic layers of the aged printed interconnects were found to be thicker than those of the as-soldered printed interconnects.