Microstructural evolution of eutectic Au-Sn solder joints

Abstract

Current trends toward miniaturization and the use of lead(Pb)-free solders in electronic packaging present new problems in the reliability of solder joints. This study was performed in order to understand the microstructure and microstructural evolution of small volumes of nominally eutectic Au-Sn solder joints (80Au-20Sn by weight), which gives insight into properties and reliability. The study particularly concentrated on the effects that the joint size and the type of substrate metallization have on both the bulk and interface microstructures of the joints. The systems studied were eutectic Au-Sn on Cu and Cu/electroless Ni/Au and for each system, two sets of sample geometries were used. The solubility characteristics and diffusional behavior of substrate metals into eutectic Au-Sn solder determines the detailed microstructure and microstructural evolution of the ultrafine eutectic Au-Sn joints. Two important things to be noted from the results are as follows: First, the overall microstructures of these joints are very coarse with respect to the size of joint, and hence the properties of the joints cannot be assumed from bulk solder data. Second, despite having a nominally similar starting eutectic Au-Sn composition, the microstructural features of the joints are unique for a given substrate metallization. Both of these results should be take into consideration during the design of reliable eutectic Au-Sn solder joints.

Gold/tin soldering of flexible silicon chips onto polymer tapes

“…The solder material and gold/tin-tonickel interface remained undamaged. In [6], it was found that eutectic gold/tin bumps on an electroless nickel UBM yielded a (Ni, Au) 3 Sn 2 intermetallic layer with an average thickness of 0.7 µm for reflow temperatures similar to temperatures used in this work. According to [6], this intermetallic allows for good adhesion and large fracture strength between the eutectic gold/tin and the nickel layer.…”

“…Before thinning, the test chips were prepared for Daisy chains and Kelvin contact measurements by electroplating of gold wires and two-layered Au/Sn bumps. Gold/tin interconnects are known for their superior properties, as for example low creep, large fracture strength, a eutectic melting temperature of 280 • C (for the eutectic composition Au80Sn20), low resistivity, large corrosion resistance and biocompatibility [5][6][7]. To minimize spreading of the tin along the gold wires, a 2 µm nickel diffusion barrier was also electroplated before the gold/tin layers.…”