Content area
Full text
Introduction
The global power electronics market is growing due to expansion of the automotive industries both in hybrid and electric vehicles (EV/HEV) (Blaabjerg et al., 2020). In response to this growth, semiconductor industries have strengthened the solder joint quality at the interconnects for better mechanical reliability, heat dissipation and electrical power conductivity by reducing the solder void size from the original requirement of 10%–15% of total void over die size to that of 5% or below. Solder void size reduction helps to create a low and homogeneous thermal resistance at the solder joints for uniform heat dissipation from the die towards the base lead frame (substrate) and thereafter to the printed circuit board. The effective area of coverage by the solder between the die and lead frame package is reduced when solder voids are present. A larger size solder void could lead to the displacement of electrical and thermal paths (Lin, 2007; Singh et al., 2017). Such phenomena result in the accumulation of heat surrounding the solder void, thus generating hot spots (Diehm, 2012; Biswal et al., 2005). The presence of hot spots, in turn, weakens the solder joint and this can lead to serious reliability issues (Katsis and VanWyk, 2006). Furthermore, reduction of the solder area under a die caused by the presence of solder voids would result in solder cracking and delamination, leading to a significant decrease in the shear strength of the solder layer, due to the thermal expansion coefficient mismatch between materials in the packages during the temperature swings in the temperature cycle and power cycle tests (Coyle et al., 2019; Jiang et al., 2018; Manson, 1966; Norris and Landzberg, 1969; VanWyk and Lee, 1999; Ye et al., 2002; Liu et al., 2008; Lutz et al., 2011). In addition, the presence of solder voids could cause die cracking for die with a thickness of 50 µm or below, as is commonly observed during subsequent processing such as wire bonding and moulding if the solder void size is large, due to the weaker tensile strength of the ultra-thin dies.
In general, solder voids are caused by outgassing of the entrapped flux during soldering (Lee and Wanda, 1993). Typically, a semiconductor die is placed...