I received a request from customer to assemble ceramic substrate (alumina oxide)that printed with a low temperature Copper thick film conductor (the overglaze layer is polymer). The Copper pads are exposed without any coating. This ceramic substrate went through double sided reflow in N2 environment(around 100units of SMT components on each side), followed by the third time hot-bar reflow for lead-frame pins assembly at the 2-edges of the ceramic. The Proto-run result showed that the exposed Copper pads were partially oxidized after 2nd time reflow. Hence, the 3rd time reflow couldn't form acceptable joints. I'm thinking of using OSP technology to prevent/minimize the oxidation from happening especially during 1st & 2nd reflow. Would appreciate any suggestion/comment to prevent/minimize oxidation on bare copper pads.