We review five solder joint reactions in 3D IC packaging technology which are of wide interest: (1) Scallop-type growth of Cu 6 Sn 5 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 Cu 3 Sn 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 Cu 3 Sn 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.