Investigating the activation energy of intermetallic layer growth in SAC305 and Innolot alloys

In this paper, the activation energy of intermetallic layer growth was investigated for SAC305 (Sn/3Ag/0.5Cu) and Innolot (Sn/3.8Ag/0.7Cu/3Bi/1.5Sb/0.2Ni) lead-free solder alloys. A testboard was designed, which contained 50 pieces of 0603 size (1.5 × 0.75 mm) chip resistors. The solder paste was deposited by stencil printing, and the solder joints were formed with vapour phase soldering. The intermetallic layer growth was accelerated after soldering with HTSL (High Temperature Storage Life) testing at two different elevated temperatures; 100 and 150 °C. The HTSL testing was carried out for up to 1500 hours. In every 500 hours, samples were taken out from the test to measure the thickness of the intermetallic layer by analysing cross-sections with Scanning Electron Microscopy. It was found that the determined value of activation energy is influenced by the length of the HTSL testing. Since the uncertainty of the layer thickness measurements is higher at lower thicknesses (i.e. at the beginning of the HTSL testing), longer duration of testing (1500 hours at minimum) is recommended. Besides, the results showed that higher activation energy belongs to the layer growth in the Innolot alloy compared to the SAC305 alloy.