Solder Alloy Contribution to Robust Selective Soldering Process

The number of components and functionality on a printed circuit board increases continuously. SMD components become smaller, pitch dimensions shrink, but there are still some through-hole components left on most of the assemblies. All classes from high-reliability to consumer electronics have through-hole connections. For high-reliability products more expensive materials may be used compared to low-cost consumer electronics. This not only counts for board materials and component selection but also for the solder alloys in a selective solder process. Solder alloys are designed for SMD applications. Some of them have multiple elements to increase reliability. Sn3.0Ag0.5Cu has been the standard lead-free alloy for a decade or more.

There are two drawbacks of this alloy:

  1. Not reliable enough for high-reliability applications.
  2. Too expensive for consumer products.

Under the hood and around the engine applications in automobiles require high-reliability electronics. These components have operation temperatures from -40°C to +150°C. A standard Sn3.0Ag0.5Cu is not good enough. Three specially designed high-reliability alloys were used in a point-to-point soldering machine to see how stable their composition is and how robust their solder process.

For consumer electronics there is a different trend with respect to solder alloys. Low temperature solders are of increasing interest for several reasons. Lower solder temperatures allow the use of cheaper materials and are less susceptible to warpage in reflow processes. These solders are SnBi based, and the absence of Ag makes them less expensive. The alloys have a larger melting range. How this affects the selective process is part of the study. Four different test-boards were used to investigate the robustness of the different alloys in a selective soldering process. The goal of these tests was to define the process windows of the alloys. Hole filling, bridging, solder balls, and open joints were quality characteristics for different Design of Experiments. The hole filling of over 18,000 through hole components were measured for high thermal mass boards as well as thin double layer boards. In two of the tested alloys, the composition of the alloy changed over time. Frozen solder and (Cu, Ni)6Sn5 needles were found during the experiments. Cross sections of the through-hole connections were made to measure the intermetallic thickness at different temperatures and solder conditions. The performance of six new lead-free alloys is compared to Sn3.0Ag0.5Cu.