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64 SMT Magazine • June 2014 solder paste is heated up and transferred into a gaseous state and gets entrapped by the solder of the ball. Voids may also originate from the board design, e.g., if any pads feature micro- vias. Size and number of voids depend mainly on the selected solder paste, the flux percentage and the chosen temperature curve of the reflow oven. The following example shows that also the amount of printed paste influences void generation. Typically, during void checking, the void area (not the void volume) is determined. Mainly, the ratio between void area and ball area is calculated and given as a percentage. As- suming that voids take a spherical shape, the void volume can be calculated from the void area. In practice, however, this is rather atypi- cal. The AXI system determines the void area in the centre level of the BGA balls. The image down left illustrates automatic void determina- tion. IPC-A-610E sets the limit value for voiding at 25% of the total solder joint area. As seen in Figure 5, not only can solder balls be evaluated with regard to shape, presence and voids. Even short circuits between solder balls can be detected. Figure 5 also shows a tilted BGA. All balls have electrical contact and received a "pass" from the boundary scan test. The optical evalu- ation, however, unveils the tilting in the 3D X- arTICLe figure 4: Illustration of voids. figure 5: Illustration of voids, short circuits and non-co-planar bGa device. THe TROuBLe WITH BgA SOLDeR JOInTS continues