**5. Corrosion**

Material selections for solder are not normally designed for corrosion resistance. If this material is in contact with a corrosive environment, it could cause failure of the circuit. Once the corrosion has occurred, mechanical and electrical failures make the product stop. The corrosion properties of Sn‐based lead‐free alloys in corrosive environments have not been widely reported, even though it is important in many automotive, aerospace, maritime and defense applications [10]. Corrosion mainly originates either within the circuit (flux corrosion) or from the environment [11, 12].

#### **5.1. Flux corrosion**

Flux corrosion is related to the solder flux residue produced during soldering and remains corrosive even after the soldering operation has completed. Solder flux residue acts as a cor‐ rosion promoter in the presence of ionic substances and a resin component. Aggressive ions, such as Cl<sup>−</sup> or Br<sup>−</sup> in flux, will increase the corrosion activities. Flux residue resin also accumu‐ lates dust during operation and later provides a hydrophilic surface which forms a medium for the ions to react with the solder materials [12, 13].

The failure analysis of flux corrosion has been reported by several authors. Jellesen et al. [12], for instance, used a drop of solution (DI water or flux solution) on a micro tactile switch under DC bias. This reportedly helped to prove that the corrosion originated from flux contamina‐ tion and condensation (**Figure 2**).

**Figure 2.** SEM micrograph of a failed switch due to severe corrosion and migration. (1) Silver, (2, 3) excessive deposits of Sn—major corrosion species, (4) carbon from the plastic housing and (5) flux residues, adapted from Ref. [12].

**Figure 3.** SEM image of a corroded Cu/Sn–9Zn/Cu butt joint after tensile strength measurement, adapted from Ref. [14].

### **5.2. Environment corrosion**

The failure analysis of flux corrosion has been reported by several authors. Jellesen et al. [12], for instance, used a drop of solution (DI water or flux solution) on a micro tactile switch under DC bias. This reportedly helped to prove that the corrosion originated from flux contamina‐

**Figure 2.** SEM micrograph of a failed switch due to severe corrosion and migration. (1) Silver, (2, 3) excessive deposits of Sn—major corrosion species, (4) carbon from the plastic housing and (5) flux residues, adapted from Ref. [12].

**Figure 3.** SEM image of a corroded Cu/Sn–9Zn/Cu butt joint after tensile strength measurement, adapted from Ref. [14].

tion and condensation (**Figure 2**).

4 Recent Progress in Soldering Materials

Air moisture or aggressive mediums contained in natural environments are another source of solder corrosion. The most reported studies involved Cl<sup>−</sup> and OH<sup>−</sup> ions. Many alloy com‐ positions and new elements have been introduced to improve corrosion resistance without reducing other properties.

Research on the corrosion of solders includes various aspects of solder characterizations. Examples include open circuit potential, galvanic cell, polarization, electrochemical imped‐ ance spectroscopy and so on.

One interesting corrosion study is the combined effect of corrosion on mechanical properties. Corrosion‐mechanical studies are varied, e.g., the effect of immersion time of Cu/Sn‐9Zn/Cu [14] (**Figure 3**). The focus here is more on the preferential dissolution of Zn and Sn. The forma‐ tion of corrosion products and grooves proves the cause of joint failure. Later, the formation of cracks is the final stage that causes the mechanical properties of solders.
