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Solder has been used to join copper pipes for plumbing in every modern house and to join copper wires for circuitry connection in every electrical product. Solder joints are ubiquitous. The essential process in solder joining is the chemical reaction between copper and tin to form intermetallic compounds having a strong metallic bonding. After the iron–carbon (Fe-C) binary system, copper–tin (Cu-Sn) may be the second most important metallurgical binary system that has impacted human civilization, as suggested by the bronze (Cu-Sn alloy) age.

Solder reaction is the wetting of a molten solder on a solid Cu surface. Typically, when a small drop of molten solder touches a large Cu surface, it spreads and forms a cap on the Cu surface. The cap has a stable wetting angle, which is defined usually by Young’s equation for a triple point.

On a silicon chip, thin-film under-bump metallization (UBM) is needed to join the solder bump to the Al or Cu wiring on the chip and also to control the size of the solder bump. This is because the oxide on a free Al surface prevents the wetting of molten solder. On the other hand, Cu reacts extremely fast with molten solder, therefore the Cu thin-film wiring cannot be wetted by molten solder.

In Chapters 2 and 3, we discussed solder reactions on bulk and thin-film Cu, respectively. In those reactions, there is only one interface between the solder and the Cu. However, in a solder joint there are two interfaces. Typically a solder joint joins two pieces of Cu tube together as in plumbing, or it joins two metallic contacts as in Si devices. These two interfaces in a fiip chip solder joint are not independent of each other from the point of view of interfacial reactions.

In Chapter 2, we demonstrated that in the wetting reaction of a cap of molten eutectic SnPb solder on a bulk Cu foil, the intermetallic compound formation of CuSn takes a unique morphology of closely distributed scallops. Morphology controls kinetics. The kinetics of growth of the scallops is a supply-controlled reaction, rather than di.usion-controlled or interfacialreaction- controlled.

Whisker growth on beta-tin (β-Sn) is a surface relief phenomenon of creep [1–16]. It is driven by a compressive stress gradient and occurs at room temperature. Spontaneous Sn whiskers are known to grow on matte Sn finish on Cu. Today, due to the wide application of Pb-free solders on Cu conductors used in the packaging of consumer electronic products, Sn whisker growth has become a serious reliability issue because the Sn-based Pb-free solders are very rich in Sn. The matrix of most Sn-based Pb-free solders is almost pure Sn. The well-known phenomena of tin such as tin-cry, tin-pest, and tin-whisker are receiving attention again.

In this chapter we shall discuss solder reaction with Ni, Pd, and Au. These metals and Cu are being used in under-bump metallization (UBM) or in bond pad, yet the role of Cu and Ni differs from that of Pd and Au. For Cu and Ni, the formation of intermetallic compound (IMC) of Cu-Sn or Ni-Sn is chosen so as to achieve metallic bonds in a solder joint. For Pd and Au, they have been used as surface coating to passivate the surface of Cu and Ni as well as to enhance wetting reaction. Typically, the surface of Cu is protected by a thin film of Au and that of Ni is protected by a film of Pd. Often Au is used on Ni too.

An ordinary household extension cord conducts electricity without electromigration in the cord because the electric current density in the cord is low, about 10 A/cm2, and also the ambient temperature is too low for atomic diffusion to occur in copper.

In 1998, Brandenburg and Yeh reported electromigration failure in fiip chip eutectic SnPb solder joints [1]. Using a current density of 8 × 10 A/cm2 at 150°C for about 100 hr, they detected failure by the formation of a pancake type of void across the entire cathode contact to the Si chip. In addition, a significant amount of phase separation in the bulk of the eutectic solder bump was observed; the Pb has moved to the anode side and the Sn to the cathode side.

In fiip chip solder joints, electromigration-induced failure is dominated by current crowding at the cathode area of contact. The high current density in the current crowding region enhances the dissolution of UBM at the cathode and transforms Cu to CuSn and to CuSn and finally leads to void formation and failure; the interaction between electrical force and chemical force is a key factor in the failure. To understand the interaction and to avoid the complication due to current crowding, straight V-groove samples having Cu wires as electrodes were introduced.