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34 SMT Magazine • June 2015 lions of x-y coordinate locations, and receives the echoes from any material interfaces that the pulse encounters at those locations. The trans- ducer carries out this function at several thou- sand x-y locations per second while it is scan- ning. The first material interface encountered by the pulse is the couplant-to-package-surface interface. This is the only interface where de- fects (other than flatness variations) do not oc- cur. Internal defects occur at deeper interfaces, such as the mold compound-to-die interface, the mold compound-to-substrate interface, and the die-to-substrate interface. The thinness of the BGA package and the acoustic velocity of the materials in the BGA package (mold compound around 3300 m/s, silicon around 8300 m/s) mean that a pulse launched by the acoustic micro imaging tool's transducer arrives back as an echo in a few mi- croseconds. And the lateral speed of the moving transducer is high: >1 m/s. This is why the trans- ducer can collect echoes from several thousand x-y coordinates per second while scanning. At each coordinate it can collect echoes from vari- ous depths within the BGA package, because those echoes will arrive at the transducer at dif- ferent times. As Figure 1 shows, Echo 1, the echo from the couplant-to-mold compound interface at the top of the BGA package, is the first echo to arrive back at the transducer. The elapsed time since the launch of the pulse is used to measure the distance from the transducer to the pack- age surface at every x-y location. If the pack- age surface is truly flat, the elapsed time and the distance will be the same for each of the mil- lions of x-y coordinates over the entire package surface. When the surface is mapped by assign- ing a pixel color to various distance ranges, the truly flat surface will have the same color across its entire area. In practice, of course, such per- fect flatness is rare or non-existent. The colors used in the color map make distance variations across the package surface more visible. Echo 2 and Echo 3 (and deeper echoes) are from material interfaces below the surface of the BGA package. Their elapsed times are used to verify the depth of an interface. The ampli- tude of the echo is measured and is critical in differentiating between good interface bonding and non-bonds, delaminations, voids or other gap defects. In a PoP, if underfill is not used, much of the space between the two packages is filled with air, which an ultrasonic pulse does ACOuSTIC SuRFACe FLATneSS OF COMPOnenTS AnD BOARDS continues Feature figure 1: echo 1 is from the bGA package surface; the other echoes from internal interfaces.