Secondary Reflow of lead-free joints is not an uncommon defect found in manufacture. X-ray inspection of a lifted J leads (figure 1), was part of an investigation to pin point the possible cause of open joints. In this case a known amount (9 to 10 %) of lead was present on the lead plating in these lead-free joints.
Control of board temperature is very important during wave soldering to avoid secondary reflow on the topside of the board. This can occur when a board passes over the solder wave and the heat is transferred through the bulk of the board by conduction. Correct topside profiling can eliminate this issue by use of a process control tool to monitor changes in contact time and temperature.
It is understood that some companies have experienced problems over the last couple of years with secondary reflow of lead-free joints during wave soldering. This has led to separation of the termination from the bulk of the joint. In the past the same phenomenon has occurred with traditional tin/lead alloys when the top surface of the board reached or exceeded 180 °C. Research work at NPL has examined the level of lead contamination and its impact on reliability. It has demonstrated that small amounts of lead will lower the reflow temperature of lead-free alloys (217 to 221 °C) leading to secondary reflow at tin/lead reflow temperatures 183 to 184 °C.
It’s very important to check components for lead contamination to reduce the possibility of secondary reflow due to a low temperature phase and to meet the requirements of RoHS. The NPL database is a resource for readers to search or submit problems online for possible solutions at
http://defectsdatabase.npl.co.uk/
EPP Europe 404
