SMT007 Magazine

SMT-June2015

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36 SMT Magazine • June 2015 acoustic image, as shown in Figure 4. This is a different BGA package from the one shown in Figures 2 and 3, although the color map used was similar. The highest elevation at upper left extends into the white portion of the color bar. The total vertical extent of the warpage extends from 0 to slightly more than 0.4 mm. ASF may also reveal phenomena in the PCB itself. Ultrasound does not penetrate deeply into the complex structure of a PCB, but ASF can image the surface changes caused by phe- nomena within the board. Figure 5 is the ASF image of the back side of a populated board. This board is not warped in the generally symmetrical fashion seen in BGAs, but has more irregular contours. The red regions at left have the lowest elevations, while the highest elevations are dark blue and, just above the white area, a small blue region that has the maximum elevations. The white area is actually a much smaller board attached to the larger board on the back side. The elevation of the top surface of this smaller board is so much higher than the elevations of contours on the larger board that it is white. But this smaller board has several holes drilled through it. Along the top edge of the smaller board, the outlines of the holes (i.e., vertical walls that reflect little ultrasound) are visible. The blue distortion of the larger board in this region was likely caused by the attachment of this smaller board. ASF can also image the populated side of the board, although the top surfaces of the com- ponents will be out of focus. In the areas not covered by components, ASF can map the to- pography of the top surface of a board having surface deformations, and it may be possible to estimate the risk of solder joint cracking or cracking within a component. A printed circuit board may also have surface deformation caused by anomalies within the board. Generally this occurs when an anomaly in the board is very thick, or very close to the surface. ASF can map the local elevation change caused by these conditions, but if the anomaly is deep within the circuit board and very thin, the elevation changes on the board surface may be too slight to be mapped. SMT ACOuSTIC SuRFACe FLATneSS OF COMPOnenTS AnD BOARDS continues Feature figure 5: Acoustic image of a pcb whose surface is asymmetrically deformed, and which has a smaller board (white) attached. Tom Adams is a consultant for Sonoscan.

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