Reliability and microstructural studies of Sn-Ag-Cu lead-free solder joints in pulse-heated reflow soldering

With the advent of miniaturization in the electronics industry, the application of flexible interconnections has become necessary and inevitable for many new designs such as flexible circuits (flex) to printed circuit boards (PCB), flex to flex, and multiwire to PCB. Pulse-heated reflow soldering is demonstrably a reliable and repeatable soldering process for manufacturing such products, especially in the attachment of flex-to-PCB in the electronics industry. This paper reports on the microstructure and reliability of flex-to-PCB solder joints. Flex-to-PCB samples were made using Pulse-heated reflow soldering and conventional reflow oven soldering. To study the reliability of the solder joints, three different environmental tests were conducted including thermal shock, thermal humidity, and thermal aging. Microstructural studies and failure analysis were performed on all samples before and after the reliability tests in order to ascertain the cause of failure in both bonding methods. Additionally, a comparison of both attachment methods comprising pulse-heated reflow soldering and reflow oven is presented demonstrating their applicability in manufacturing flex-to-PCB assemblies.