**11. Voids**

*Welding - Modern Topics*

**10. Dendrites**

**Figure 14.**

**Figure 15.**

*Photo of the tombstone effect.*

Dendrites grow due to electrochemical migration when metal ions go into the electrolytic solution at the anode, plating out at the cathode and creating needle- or treelike formations on the PCB substrate (see photos in **Figure 16**) [16]. Electrochemical migration can be defined as the movement of a metal ion in an electrolytic solution

The time required to create dendritic bridges, which cause short circuits between two conductive paths, is determined by several factors including relative humidity,

between two neighbouring conductors with different electrical potential.

*Schematic representation of the wetting force (F1); upward push force from solvent vapours (F2).*

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**Figure 16.** *Photo of dendrite.*

Voids are non-conductive cavities within the soldered joint, and their excessive presence poses a significant reliability risk for the manufactured product, especially in power electronics, where higher currents are present [20]. Voids are formed during the soldering process, their presence in the soldered joint causes the displacement of electrical and thermal paths, the resistance is higher and the temperature stress is non-uniform. This subsequently causes cracks and lowers the mechanical shock tolerance of the whole PCB.

With the advent of lead-free soldering (new material base, different temperature profiles, different types of fluxes, higher surface tension of solder, etc.), a higher incidence of voids was detected, leading to the lower reliability of soldered joints. Increased attention is therefore paid to the voiding issue.

Voids can be classified into several categories [21]. There are macro- and microvoids, shrinkage voids, voids in microvias, Kirkendall voids and pinhole voids (see **Figure 17**).

#### **11.1 Macrovoids**

Macrovoids are the most commonly occurring type of voids. Macrovoids are formed by the evaporation of gases from fluxes and soldering pastes during the reflow process. Macrovoids may occur anywhere in the solder joint, and their diameter is around 100–300 μm.

The factors that affect the size of the macrovoids are the solder paste's properties (particle size, composition, melting temperature and oxide content), flux (viscosity,


#### **Figure 17.**

*Soldered joint and the location of different types of voids. Intermetallic layer contains Kirkendall voids; pinholes are located on the boundary between the intermetallic layer and the PCB.*

surface tension, activator, solvent, etc.), components (geometry, shape, terminal oxidation, etc.) and the process (thickness, the shape and the parameters of the solder paste's printed layer, temperature profile) [22–26]. An example of macrovoids in the solder joints of a soldered BGA package on the PCB is given in **Figure 18**.
