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June 2015 • SMT Magazine 35 ACOuSTIC SuRFACe FLATneSS OF COMPOnenTS AnD BOARDS continues not penetrate. Where the two packages in a PoP are bonded, acoustic imaging of the bonding may be possible. The ASF data from the top surface of the BGA is collected during the same transducer scan as data from deeper interfaces, and thus creates a separate surface flatness image without requir- ing additional scan time. The amplitude of the ASF echo is seldom of interest because the two materials involved—couplant and mold com- pound—do not vary. Figure 2 is the ASF image showing in color the surface topography of a plastic BGA pack- age that is the top member of a PoP assembly. The surface resembles a very shallow bowl: the deepest part is at the center (dark red in the col- or map at left). From the deepest part the sur- face moves upward to the dark blue regions at the corners of the package. The degree of warp- ing is not consistent. For example, the large yel- low region in the lower left quadrant indicates slightly less upward curl. The difference in el- evation between the lowest and highest points is 0.4 mm (400 microns). The degree of warping is thus of concern because it is four times the al- lowable departure from planarity permitted by the JEDEC design guide for PoPs. The data can also be viewed in other ways. Figure 3 is the exaggerated profile of a single scan horizontally from left to right across the center of the BGA package in Figure 2. The somewhat irregular extent of warping can be seen clearly in this profile. Top-to-bottom profiles can also be produced. The surface of a warped BGA package can also be represented as a three-dimensional Feature Figure 3: Vertically exaggerated profile of a left-to-right section through the center of the same BgA. figure 2: The surface of this bGA is lowest (red) at center and highest (blue) at the corners. Figure 4: 3D acoustic image of a similarly warped bGA.