Optimization of Lead-Free SMT Reflow & Rework Process Window (APEX)

Abstract

Elevated SMT reflow temperatures for Pb-free soldering are placing excessive thermal demands on certain families of electronic components. The HDPUG Consortium conducted an extensive study on optimizing the time / temperature profile for reflow to reduce temperature variations from high to low thermal mass components and to identify the minimum peak temperatures that will produce acceptable solder joints. This paper details the preliminary findings of profile characterization work for a range of PCB design types, assessing the impact of adjusting both time and temperatures. Development of thermal profiles for rework of SMT components is also included. Results of metallurgical analysis are given to support provisional recommendations on minimum peak temperature requirements. These were subsequently used for the build of the GPLF (General Purpose Lead Free) test vehicle to assess reliability of Sn/Ag/Cu solder joints produced at the limits of the process window.

Much of the Pb-free assembly and evaluations done to date have been in support of early adopters whose products are typically characterized as being simpler in complexity from a thermal mass, component mix or reliability perspective. One of the big challenges facing many of the member companies within the HDPUG consortia is that their products are larger, thicker, use a broad variety of component packaging styles and have long field life/reliability requirements. This pushes the limits of current assembly technology from a soldering materials, equipment and components perspective; both for primary attach and rework. This project is part of the broader HDPUG GPLF (General Purpose Lead Free) initiative looking at several of the remaining challenges preventing further industry transition to Pb-free for the manufacture of printed circuit boards in the high complexity/high reliability product space.

• presents results assessing the minimum temperature required to produce mechanically sound Sn/Ag/Cu solder joints from a metallurgical perspective.
• reviews the design features incorporated into the GPLF test vehicle used in the assembly, build, rework and subsequent ATC reliability testing.
• describes the DOE performed to intentionally create solder joints at the extremes of the Pb-free process window. These assemblies were then thermal cycled to assess whether the temperature affected solder joint reliability and provided a reference library of grain structures under the various assembly conditions and component finish combinations.
• looks at factors to help reduce peak reflow temperature variation across an assembly