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74 SMT Magazine • November 2014 (GaN) die that, because of the high power, must dissipate a great deal of heat. There are many types of high-power IBGT modules, but they generally have a three-lay- er structure, with the die resting on a ceramic raft, which in turn rests on a metal heat sink. Additional layers of material include the adhe- sive bonding the die to the raft, and the solder or other adhesive bonding the raft to the heat sink. These assemblies are imaged acoustically before encapsulation in a plastic package that will cover everything except the heat sink. The heat generated by the die must flow down- ward, through the raft and the heat sink. Much less powerful assemblies can fail if they overheat; the bonding of the elements in the module must be very good. The typical defects are voids or oth- er gaps in the adhesive materials, or the tilting of the ceramic raft. Finding gap-type defects here before encapsulation of the module is critical be- cause the module can still be reworked. Until recently, though, high-power IGBT modules generally could not be imaged by acoustic micro imaging tools. The problem was the die's sensitivity to contamination. The transducer of an acoustic system must be coupled during scanning to the surface of the part by a small column of water. But the power used in IGBT modules is so high that even the faintest residue left by the momentary passage of a water column could later become a leak- age path. (Note that there are also lower-power IGBT modules, used for example in audio and automotive applications, where these restric- tions do not apply.) To make high-power IGBT modules inspect- able, Sonoscan designed an acoustic micro im- aging tool whose transducer points upward and is accompanied by a small water jet to create an upward-flowing column to couple the trans- ducer's ultrasound to the metal heat sink at the bottom of the IGBT module. The module (or modules; the system can have two transducers) rests in a cutout in a plate above the transducer. There is no path by which water can reach the module's die, and ultrasound pulsed into the heat sink can image features up to and includ- ing the die attach. Figure 2 is the acoustic image showing the solder layer between the heat sink and one ce- ramic raft on a high-power IGBT module. The solder (or adhesive) layer is of great interest be- cause defects here can block the dissipation of heat from the die. The irregularly shaped features of various colors are voids (air bubbles) in the solder. They vary in color because Sonoscan's Time Differ- ence software is being used to display the dis- tance of a feature from the transducer, instead of the amplitude of the signal. The red voids are nearest to the transducer (i.e., near or in con- tact with the heat sink). Other voids are deeper within the solder. But the color also measures the depth of the raft's surface, and thus the thickness of the solder. The solder is thickest in the dark blue regions at left, and thin or lack- ing at the upper right. Conclusion: the raft is tilted, and voids are present—both conditions that will impair heat dissipation. Integral mode imaging offers advantages in situations where it is desirable to more ac- figure 2: acoustic image of the solder layer between the heat sink and the raft in an igBT module. irregularly shaped features are voids in the solder; colors indicate distance to the raft, and thus the thickness of the solder. arTiCle aDvances in acoUstic imaging oF meDicaL eLectronics continues