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

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

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

**Figure 5.**

*Schematic representation of the warpage effect.*

**Figure 6.** *Head-in-pillow effect in connection with the temperature profile.*

**Figure 7.** *Photo of a microsection of the head-in-pillow effect.*

the reflowed solder alloy on the solder pad no longer joins with the reflowed BGA solder alloy ball. The Head-in-Pillow effect in connection with the temperature profile is shown in **Figure 6**, together with a photo of a microsection in **Figure 7**.
