FIG. 2 illustrates a schematic top view of portions of bottom package 100, wherein openings 158 in polymer layer 152 (and tape 154, if any, FIG. 1) are illustrated. Openings 158 include elongated openings 158A and non-elongated openings 158B. In FIGS. 2 through 9, circles are used to schematically represent non-elongated openings 158B, and ovals are used to schematically represent elongated openings 158A. FIGS. 10A and 10B illustrate a top view of some exemplary elongated opening 158A. Elongated opening 158A has length L1 measured in lengthwise direction X and width W1 measured in widthwise direction Y, which is perpendicular to lengthwise direction X. Length L1 is greater than width W1. Elongated opening 158A includes longer axis 134 in lengthwise direction X and shorter axis 136 in widthwise direction Y. Throughout the description, an elongated opening 158A is an opening whose length-to-width ratio L1/W1 is greater than threshold ratio Lt that is greater than 1. Threshold ratio Lt may be greater than about 1.2, 1.6, or 2.0.

Elongated opening 158A may have various shapes including, but not limited to, an oval shape, a rectangular shape, an elongated octagonal shape, or the like. For example, FIG. 10A illustrates an exemplary elongated opening 158A, which includes two half circles 130 connected to opposite edges of rectangle 128. FIG. 10B illustrates an elongated hexagon. It is appreciated that other elongated shapes other than what have been discussed may also be used.