Influence of Reflow Temperature Profile on the Intermetallic Layers Thickness at Different Surface Finishes

It is well known that the reflow temperature profile has an influence on the thickness of intermetallic layers. The higher the thermal treatment, especially above the melting temperature of solder alloy, the higher the thickness of intermetallic layers. The heating factor (time-temperature integral of temperature profile above the melting temperature) is commonly used for comparison of the heat treatment. This paper deals with a comparison of three different reflow temperature profiles (with three different heating factors: 242, 1061 and 1652 s.K), three surface finishes (Organic Solderability Preservative - OSP, Hot Air Solder Leveling – HASL and Electroless Nickel Immersion Gold – ENIG) of copper soldering pads and two solder pastes with the same composition of solder alloy (Sn96,5/Ag3,0/Cu0,5) but with different flux type (less aggressive - ROL0 and more aggressive - ROL1). The cross-sections of the samples were prepared after the reflow soldering process, followed by analysis by scanning electron microscope and measurement of intermetallic layers thicknesses. Results confirmed that the heating factor significantly influences the intermetallic layers. The highest thickness of intermetallic layers was achieved with the highest value of the heating factor. The surface finish HASL had the highest values of intermetallic layer thicknesses from all used surface finishes, whereas ENIG had the lowest. The higher values of intermetallic layer thickness were observed for the solder paste with a more aggressive flux, ROL1, than for ROL0.