An Investigation Into Low Silver Lead-Free Alloy Solder Paste For Electronics Manufacturing Applications
Authors: Jasbir Bath, Manabu Itoh, Gordon Clark, Kimiaki Mori, Hajime Takahashi, Kyosuke Yokota and Roberto Garcia Company: Koki Company Limited and Christopher Associates Inc. Date Published: 10/16/2011
Abstract: The electronics industry has widely adopted Sn3Ag0.5Cu solder alloys for lead-free reflow soldering applications. One of the challenges with this alloy is the use of the three percent silver which has caused rapid price increases in lead-free solder alloys because of the limited global availability of silver metal. Low silver containing lead-free alloys have been developed to address the cost issue but they have been mainly found to have reduced thermal cycling reliability performance compared with Sn3Ag0.5Cu. Development was done on a new lead-free solder alloy (Sn0.1Ag0.7Cu0.03Co) as a potential replacement for Sn3Ag0.5Cu. Bulk alloy mechanical testing was done for Sn0.1Ag0.7Cu0.03Co, Sn3Ag0.5Cu, Sn0.3Ag0.7Cu and Sn0.7Cu0.05Ni solder alloys. In additional copper erosion tests were done with the low silver cobalt containing alloy showing reduced copper erosion versus Sn3Ag0.5Cu. Boards were assembled with Sn0.1Ag0.7Cu0.03Co, Sn3Ag0.5Cu, Sn0.3Ag0.7Cu and Sn0.7Cu0.05Ni to understand intermetallic compound (IMC) growth rate after board assembly and thermal ageing as well as wetting/ spreading tests on different board surface finishes. The cobalt containing low silver alloy was found to reduce IMC growth rate after assembly and thermal ageing and had good wetting/spreading performance. Solder joint thermal cycling tests showed equivalent or better reliability for the Sn0.1Ag0.7Cu0.03Co alloy compared with Sn3Ag0.5Cu. Reflow profile development was also done with the 227ºC melting point Sn0.1Ag0.7Cu0.03Co alloy to understand peak temperature and time above liquidus guidelines during board assembly. The results of the tests are reported.