Investigation of diffusion and electromigration parameters for Cu–Sn intermetallic compounds in Pb-free solders using simulated annealing

We review five solder joint reactions in 3D IC packaging technology which are of wide interest: (1) Scallop-type growth of Cu6Sn5 in solid-liquid interdiffusion reaction, (2) Whisker-type growth of Sn crystals at room temperature, (3) Layer-type intermetallic compound (IMC) growth in solid state aging, (4) Porous-type growth of Cu3Sn in μ-bumps, and (5) Pillar-type growth of Cu/Sn IMC down to 1 μm in diameter. The first two have been well covered in books and reviews on solder joint technology, so only certain specific comments will be given here. On the other three, the layer-type IMC growth has been a long standing kinetic problem due to the extremely small concentration gradient across a stoichiometric IMC, but it has been resolved now, following Wagner’s approach. The porous-type Cu3Sn was found in 2014. Kinetically, it is a complete cellular precipitation, containing a set of lamellar pores. It is rare because up to now all cellular precipitations are incomplete. The pillar-type Cu/Sn reactions down to 1 μm in diameter were carried out in 2016. Owing to a large surface/volume ratio, the reaction is controlled by surface diffusion, accompanied by interstitial diffusion of Cu in Sn.

Recent advances on kinetic analysis of solder joint reactions in 3D IC packaging technology

The growth behavior of intermetallic compounds (IMCs) at the liquid-solid interfaces in Cu/Sn/Cu interconnects during reflow at 250 °C and 280 °C on a hot plate was investigated. Being different from the symmetrical growth during isothermal aging, the interfacial IMCs showed clearly asymmetrical growth during reflow, i.e., the growth of Cu6Sn5 IMC at the cold end was significantly enhanced while that of Cu3Sn IMC was hindered especially at the hot end. It was found that the temperature gradient had caused the mass migration of Cu atoms from the hot end toward the cold end, resulting in sufficient Cu atomic flux for interfacial reaction at the cold end while inadequate Cu atomic flux at the hot end. The growth mechanism was considered as reaction/thermomigration-controlled at the cold end and grain boundary diffusion/thermomigration-controlled at the hot end. A growth model was established to explain the growth kinetics of the Cu6Sn5 IMC at both cold and hot ends. The molar heat of transport of Cu atoms in molten Sn was calculated as + 11.12 kJ/mol at 250 °C and + 14.65 kJ/mol at 280 °C. The corresponding driving force of thermomigration in molten Sn was estimated as 4.82 × 10−19 N and 6.80 × 10−19 N.

/pdf/growth-kinetics-of-cu6sn5-intermetallic-compound-at-liquid-50aj6qqlqx.pdf

Growth kinetics of Cu6Sn5 intermetallic compound at liquid-solid interfaces in Cu/Sn/Cu interconnects under temperature gradient

Evolution of CuSn intermetallics between molten SnAgCu solder and Cu substrate

Phase-field simulations of intermetallic compound growth in Cu/Sn/Cu sandwich structure under transient liquid phase bonding conditions

The role of oxide layers in solder joints

The solder reaction of Cu and molten pure Sn is studied by scanning and transmission electron microscopy. Similar as reported for SnPb solder, the intermetallic Cu6Sn5 product is formed in a scallop-like morphology and a growth kinetic proportional to the cube root of time is found. Size distributions and shape of the scallops are determined experimentally. Comparing the binary reaction couple Cu/Sn with the technical combination Cu/SnPb, a significant difference in the length to width aspect ratio of individual scallops is noticed. We demonstrate by transmission electron microscopy that neighbored scallops are not separated by channels of solder, but by grain boundaries. Thus, grain boundary transport is the rate controlling step and the observed variation in scallop shape is due to differences in the interface tensions of the two reaction couples.

On the mechanism of the binary Cu/Sn solder reaction


 Despite the higher thermal conductivity and the higher lifetime offered by silver sintering technologies, most packaged GaN devices are attached using solders due to technological difficulties in the sintering process. In this work, a silver sintering process for a packaged GaN power transistor on a printed circuit board (PCB) was successfully developed. Different sintering paste types were examined regarding their suitability for this application. Electrical measurements, shear tests, and metallographic cross sections were used for the evaluation. Numerical analyses were used to study the internal stress distribution in the GaN package after sintering depending on the paste structure. In the final sintering process, a shear strength of 20 MPa for sintering at 15 MPa and 240°C, for 300 s with electrical functional devices could be obtained by using nanoscale sintering paste. The authors contribute this to the high initial stiffness of the silver layer, which is obtained much earlier in the sintering process compared with the stiffness of a microscale silver paste. This high initial stiffness counteracts the semiconductor device deflection from the applied sintering pressure and reduces the stresses inside the semiconductor.

Silver Sintering of Packaged GaN-Devices on Printed Circuit Board

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Papers

The role of oxide layers in solder joints

On the mechanism of the binary Cu/Sn solder reaction

Silver Sintering of Packaged GaN-Devices on Printed Circuit Board