Effect of (Au, Ni)Sn$_{4}$ Evolution on Sn-37Pb/ENIG Solder Joint Reliability Under Isothermal and Temperature-Cycled Conditions

Abstract

The ternary intermetallic compound Au <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> forms at the Sn-37Pb/ENIG solder interface during aging and temperature cycling, leading to increased interfacial cracking and a corresponding decrease in solder joint reliability for 15 mm ball grid array (BGA) structures. (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> forms at both the board finish (bottom) and component side (top) of the solder joint for isothermally aged, temperature-cycled, and (aged + cycled) joints. For control specimens (reflow only), no cracks or interfacial Au are observed. For isothermally aged joints (170 and 340 h at 125degC), a broken, discontinuous layer of (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4 </sub> is present, but no cracking. For temperature-cycled joints, lowered reliability and interfacial cracking occurs along a continuous (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> intermetallic layer on the solder side of the interface after ~450 h of cycling. Aging + cycling did little to inhibit cracking or formation of (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> . Development of a continuous (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> film at the interface is the key failure mechanism. At low cycle numbers where high joint reliability is observed, the (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> layer is discontinuous and not fully developed. At higher cycle numbers and longer aging times, the (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> layer becomes continuous and encourages crack growth along the intermetallic interface and consequent lower reliability. The correlation of interfacial smoothness with lowered reliability is consistent with recent work showing that, when intermetallic compounds form smoothly at the solder interface, the mechanical properties are degraded (compared to a rough intermetallic) due to the decreased resistance to shear along the interface