TLPB in Cu/In-48Sn/Cu bonds as alternative Pb-free interconnection technology improving service temperature

S. SOMMADOSSI

Brno

Encuentro; Final Meeting of the European COST Project “HISOLD” -Advanced Solder Materials for High Temperature Application-; 2011

European COST ACTION / Institute of Physics of Materials, ASCR, Brno, Czech Republic / Masaryk University Brno, Czech Republic

An alternative lead-free solder alloy In–48 at%Sn with a melting point of 120 °C and its implementation to bond Cu substrates in a diffusion soldering joining method, which below to the Transient Liquid Phase Bonding (TLPB) family, are presented. This method consists in the complete transformation of the original soft solder in intermetallic phases by the solid-liquid interaction with the substrates. According to the EPMA, TEM/EDX and electron diffraction analyses, two different behaviours were observed in the interconnection zone depending on the temperature range: (i) below 200°C a single layer consisting of η phase; (ii) above 200°C a Cu-poor region consisting of η phase and a Cu-rich layer formed by a mixture of thin alternate regions of ζ-Cu10Sn3 and δ-Cu7In3 phases growing perpendicular to the bond plane. The η layer shows two morphologies: large grains and fine grains at the η/In–48Sn (liquid) and at the η/Cu-rich interfaces, respectively. Additionally, the η region shows a gradual change in composition, suggesting a change from the η-Cu6Sn5 to the η-Cu2In structures. Regarding the reliability properties, kinetic, mechanical, electrical, thermal and corrosion behaviour were investigated: - Thermal stability tests indicate that the thermal resistance of the bonds is about 750°C, offering high service temperatures, about 200-400°C above the production temperature. - The thickness of the Cu-rich layer shows a constant growth rate (linear growth) between 290 to 400°C, involving a faster intermetallic growth kinetic than in Cu/In and Cu/Sn interactions. The temperature dependence of the growth rate constant of the Cu-rich layer is described by an Arrhenius relationship with activation energy of 121 kJ/mol and a pre-exponential factor of about 57 m/s. - The joints show a very good mechanical behaviour, especially when the interconnection zone was formed only by the Cu-rich layer, reaching values of 150 MPa for ultimate tensile and shear strength. The η phase has two crystallization morphologies, involving a weak interface between the sublayers composed of small grains and large grains, respectively, shows only 50 MPa. The nano-hardness measured by a Berkovich nano-indenter showed average values of 9.5, 6.7 and 2.3 GPa for Cu-rich, Cu-poor and Cu regions, respectively. - The ρ as function of the temperature was determined by the Four Probe Method applying 20 and 200 mA between 20 and 170ºC. The ρ of the Cu-rich layer shows linear behaviour with the temperature and its average value was ρ25ºC = 8,97 μΩcm, lower than the ones corresponding to the In-Sn y Pb-Sn solder alloys. - The corrosion velocity (VC) was obtained by immersion tests in 1M NaCl, 1M NaOH and 0.5M H2SO4 and weight loss measurements according to ASTM G31-72. The intermetallic surfaces exposed to the electrolytes presented average values of VC of 77 mdd, also lower than the ones reported for In-Sn y Pb-Sn solder alloys.