Effects of Laser Selective Reflow on Solder Joint Microstructure and Reliability

New applications and materials continue to pose challenges and opportunities for automated microelectronics manufacturing. Alternative approaches are becoming available but for most purposes the use of soldering in high volume applications still offer major advantages. However, the inability of many inexpensive materials to survive conventional mass reflow temperatures present a serious obstacle to this. One potential work-around still offering a relatively high throughput would be the use of selective laser reflow. This allows the local heating of solder joints and paste deposits for such a short time that other parts of the assembly are not heated nearly as much. Of course the properties of the resulting solder joints are somewhat different than typically achieved with mass reflow. An example of the use of a commercial infrared (IR) laser reflow instrument and its consequences in terms of solder joint properties was considered. Details including the impact of material properties and sample geometry on thermal profiles will be described for a representative example. The unique solder microstructures achieved because of the high heating and cooling rates were characterized, and associated properties such as creep rates and fatigue life quantified. Results are discussed in terms of consequences for life under realistic long term service conditions.