December, 2016
Post-Reflow Acoustic Micro Imaging
Continued from page 49
compound-to-die interfaces. The first will be bright white in the grayscale acoustic image, while the second will be some shade of gray. By conven-
area surrounding the IC is black, which confirms that the top layer of the board is delaminated. Also, surrounding this compo-
nent are several small components imaged in red that may or may not be defective as the ultrasound needs to be focused at their depth to be sure. When the transducer is scanning a board, the single flat top surface that most components have is lacking. Instead, the transducer travels in its single plane and receives return echo signals from whatever the pulse strikes — the flat top of an IC pack- age, the curved side of a capacitor, or the board texture.
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Page 51 Many of these return echo sig-
nals will not correlate well with the expectation, for example, that a red or yellow feature is definitely a gap-type defect. Red may simply be the soft- ware’s interpretation of echoes from the curved side of an out-of-focus small component. In some cases, however, a small component may deserve a more detailed analysis. The multilayer ceramic chip
Figure 3: Arrows pinpoint cracks in mounted multilayer ceramic chip capacitors.
capacitor shown in Figure 3 was first identified as questionable in a whole- board scan. Ceramic chip capacitors present a special problem because they may contain vertical cracks that will cause failure by creating path- ways between electrodes. Vertical cracks reflect very little ultrasound, so a different method was used for a high-resolution image of this capaci-
Continued on page 53
Figure 1: Surface scan C-SAM image of a portion of a populated board.
tion, published images often use a color map that displays gap-type defects as red.
Image Analysis Figures 1 and 2 are C-SAM
images of one small region of a popu- lated PCB. This small, thin telecom- munications board had over 100 com- ponents of various sizes. In Figure 1, only echoes from the first interface encountered by the pulse are used to make the acoustic image. This sur- face scan shows the surface of each feature, no matter at what depth it lies within the gate selected — the top surface of a large IC package, the top surface of a small capacitor, and the top surface of the PCB. A surface scan such as this is sensitive to the features about 50 µm below the sur- face. The large white area around the IC package reveals that the board surface in this region is significantly brighter than the other areas of the circuit board, a phenomenon that is typical of a multilayer board whose top layer may be delaminated, thus
Figure 2: Subsurface C-SAM image of the same area as
Figure 1. In the large IC pack- age, delaminations (red) of the mold compound from the die paddle nearly surround the die.
returning more of the ultrasound than if it were bonded. Figure 2 is a C-SAM image of
the echoes from material interfaces below the top surface of the highest component, but within the gated depth. The large die is at center. The area of the die paddle surrounding the die shows many red regions, indi- cating that the mold compound is delaminated from the substrate. The risk is that the delaminations will expand under the die and block heat dissipated. In addition, the board
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