A study of several board finish solderability with Sn-Ag-Cu under different atmospheres

The technology of surface finishes for printed circuit boards is seeing a dramatic shift from the hot air solder leveling (HASL) towards alternative finishes like electroless nickel immersion gold (ENIG), immersion silver and organic solder preservatives (OSP). It is expected that soldering in nitrogen atmosphere might overcome some of the technical barriers and provide soldered products comparable to those conventional lead-containing materials processed in air. but quantitative data regarding the soldering behaviour of lead-free solders under various atmospheres are sparse. As part of an ongoing study on the effects of inerting on the solderability of lead-free alloys an examination has been made of solderability, as measured in a wetting balance of three common board finishes using a 95.5/3.8/0.7 SnAgCu solder.The board finishes used were ENIG HASL, immersion silver and a copper OSP. A peak reflow temperature of 250°C and an R flux was used for the ENIG and Ag finishes. The OSP finish performed poorly under these conditions and OSP testing was done using a 235°C peak temperature and a 0.5% activated R flux. During testing the atmospheres were controlled at levels of oxygen of 21% (air) 10000, 1000 and 100 ppm. Although inerting improved the solderability of all three finishes, there were differences between the individual alloys, Aging by multiple reflow cycles adversely affected the solder ability of all finishes but the effects were less for ENIG than the other finishes.