WFigure 3. 3D image of a sample with significant vertical


structures.


XFigure 4. Flip chip


imaged in 3D cutout mode.


it encounters is the interface between the water that couples it to the sample and the sample itself. For Time Differ- ence imaging, the transducer measures the time between this echo and the echo from the first internal interface. Figure 1 is the Time Difference im-


age of the back side of an unmounted BGA device. Black items are the solder bumps. Green identifies the solder mask at the substrate level, but in three re- gions the surface of the solder mask is represented by colors (blue, magenta, white) that indicate a higher elevation. Te image as a whole indicates that the


substrate is flat, and that the three defects, which may be voids, extend upward to var- ious heights. Te reader can envision the three-dimensional structure of the defects, but that structure is not shown directly. In most of the 15 or so acoustic imag-


ing modes, gates are set for the collec- tion of returning echo signals. Te gate dictates that only those echoes arriv- ing between time A and time B (both measured in nanoseconds) will be used to make the acoustic image. In plastic packaged ICs, the echo from the top sur- face of the mold compound is ignored. This sample differs in that there is no gate; echoes are received from the first interface encountered at any depth. The same sample is imaged in Figure


2, but in 3D. The color map begins with green, but then runs in the opposite di- rection from the color map in Figure 1: the solder mask defects rise with altitude to yellow, then to red. Some of the solder bumps within the defect areas also ap- pear taller than bumps in other regions. Figure 3 shows the same form of 3D acoustic imaging applied to a more


complex sample—in this case, a plastic IC package that has suffered a popcorn crack that originated underneath the die and traveled upward. Te IC package was scanned from above, and the results tilted to make all three dimensions visible. Return echoes were collected from


a wide (vertically) gate, extending from just below the top surface of the part to the lead fingers. Within this gate, only the first echo arriving at each x-y loca- tion was used. If a portion of the pulse traveled deeper and generated a second echo, that second echo was ignored. Te color map used to indicate distance


is at left. In the upper part of the image, there are black spaces between lead fingers because no echo was received. Te surfac- es of the lead fingers and the die paddle are red. At the center is the die, whose green top indicates that it lies above the lead fin- gers. Te sides of the die are green because the vertically oriented pulses pick up no information from the sides. Software bor- rows the color of the die top to prevent the sides from appearing empty. Te four upward-reaching cracks sur-


round the die. Two rise no higher than the top of the die. Te crack at right rises higher; its top is blue. The crack at left rises through blue, magenta and white to the top of the color map. The top of this crack abruptly ends in a horizontal profile not because it has reached the top surface of the mold compound, but because it has reached the top of the gate just below the top surface. Echoes from above this depth will be ignored.


C-mode imaging with software stacking There is a third way in which acoustic microimaging can produce a 3D image.


First the transducer uses C-mode imag- ing to scan the entire thickness of the sample in multiple narrow (vertically) gates. Tere may be 20 or so gates. Each gate is saved as a separate image. Software then stacks the gates, thus


reproducing the entire internal structure of the component electronically. It then begins with the top gate and removes the imaging data from an area defined by the user. It does the same for subsequent gates until the desired depth is reached. The results can be seen in Figure 4.


The sample is a flip chip, imaged from the tops of the solder bumps to the substrate below. The top of the image, outside the cutout, represents the top of the underfill. The two yellow arrows point out two white voids at the top the underfill. Within the planar cutout, the yellow


arrow indicates a “white bump,” not con- nected to its pad on the substrate. The green arrow, however, points to an ap- parently isolated dark dot. The vertical structure behind the dot is the acoustic shadow from the solder bump, which reflected ultrasound back to the trans- ducer and thus cast an acoustic shadow downward. Beside the solder bump at the top surface is the barely visible por- tion of a void. The other portion of the void, lying in the now absent cutout, likewise casts an acoustic shadow onto the substrate.


Tom Adams is a consultant at Nordson SONOSCAN. He can be contacted at T100adams@comcast.net


JANUARY 2019 EVALUATIONENGINEERING.COM 17


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32