**4. Wrong solder mask design**

Another problem that can occur with the solder mask is due to wrong PCB design, especially if the solder mask is used to cover pads for unused BGA package leads (see **Figures 3** and **4**). In this case the BGA balls become deformed and the risk of bridging is increased.

**123**

**Figure 5.**

*Schematic representation of the warpage effect.*

terminal is not used.

**Figure 3.**

**Figure 4.**

*design.*

**5. Warpage effect: head-in-pillow effect**

"Head-in-Pillow" effect, is shown in **Figure 5**.

*Overview of Selected Issues Related to Soldering DOI: http://dx.doi.org/10.5772/intechopen.91023*

*Section—deformation of BGA balls due to inappropriate solder mask design.*

This is mainly caused by the designer's unfamiliarity with correct PCB design. A soldering pad on the PCB must be present for each BGA terminal, even if the

*Detail of the sections from Figure 3, showing the deformation of BGA balls due to inappropriate solder mask* 

In the case of differing thermal expansion, coefficients between the PCB and the component package, during heating/cooling, there is mutual deflection/ deformation called the "warpage effect". This effect may result in correctable errors (bridges, open joints), but also in unrepairable errors—cracks (on components, inside the PCB) [2]. This effect, along with some other defects, including the

The Head-in-Pillow effect is a characteristic in the case of soldering of BGA components. During heating the BGA terminal/balls lift up from the soldering pad coated with solder paste. In the reflow zone, the soldering paste and the BGA solder alloy balls reflow independently. During cooling this sag is counteracted, nevertheless *Overview of Selected Issues Related to Soldering DOI: http://dx.doi.org/10.5772/intechopen.91023*

**Figure 3.**

*Welding - Modern Topics*

**3. Dewetting**

*Problems with the non-wettability of soldering pads.*

**Figure 1.**

Dewetting usually happens in the case of a prolonged heating, leading to the formation of an intermetallic that results in a change in the composition of the solder alloy as is the case, for example, when soldering a tin-lead solder on copper substrate, where the tin is sucked out of the solder alloy, which is involved in the formation of an intermetallic layer, thus increasing the proportion of lead in the solder alloy, which has poor wetting. Dewetting may also arise due to the dissolution of the precious metals in the solder alloy or the influence of a poor soldering surface

under the surface finish. Photos of dewetted surfaces are shown in **Figure 2**.

Another problem that can occur with the solder mask is due to wrong PCB design, especially if the solder mask is used to cover pads for unused BGA package leads (see **Figures 3** and **4**). In this case the BGA balls become deformed and the

**122**

**Figure 2.**

**4. Wrong solder mask design**

*Photos of soldering pads dewetting.*

risk of bridging is increased.

*Section—deformation of BGA balls due to inappropriate solder mask design.*

#### **Figure 4.**

*Detail of the sections from Figure 3, showing the deformation of BGA balls due to inappropriate solder mask design.*

This is mainly caused by the designer's unfamiliarity with correct PCB design. A soldering pad on the PCB must be present for each BGA terminal, even if the terminal is not used.
