Inverted Acoustic System Cuts IGBT Failures

Many of the IGBT modules that arrive at Sonoscan's headquarters applications laboratory are there because they have failed either in service, in reliability tests or during process control monitoring. They will be imaged by acoustic microscopes to gain a non-destructive view of the layered structures between the die and the heat sink in an effort to pin down the root cause of the electrical failure. Some failed IGBT modules show little external evidence of electrical failure, but on others the plastic encapsulant is distorted or burned.

Because they are high-voltage and high-power switches, IGBT modules generate a great deal of heat that must be dissipated at a rate sufficient to avoid over-heating. Some heat escapes upward from the top of the die and through the plastic encapsulant, but much more heat travels downward through a pathway designed to provide an adequate rate of dissipation for that particular module. Multiple die are typically attached to a ceramic substrate by the die attach material, which also serves as a type of Thermal Interface Material (TIM). The bottom surface of the ceramic is in turn bonded to a metal heat sink by a second TIM, usually solder. Note that each TIM has interfaces above and below, making a total of four internal interfaces that are important in interface-sensitive acoustic imaging.

The purpose of the IGBT module's layered design is to create an efficient low thermal resistance pathway from the circuitry atop the die to the bottom side of the heat sink where the heat is carried away. Including the die, heat in this typical design will travel through five layers of material. The ceramic substrate and the heat sink are chosen in part for their low resistance to the propagation of thermal energy, as are the TIMs.

But IGBTs can still overheat and fail in service. Two of the failure mechanisms seen in Sonoscan's lab are:

1. Voids, delaminations or other gaps within or adjacent to a TIM. Even if they are very thin, gaps are efficient insulators. Thermal energy is transmitted downward from the die by conduction. Little thermal energy crosses a gap; instead, it is redirected back toward the die (Fig. 1). In gap-free regions, conduction and radiation continues across the material interfaces until, at the bottom of the heat sink, the heat is carried away from the module. The greater the collective x-y area of the gaps, the more thermal energy is aimed back at the die, and the greater the risk of IGBT failure.
2. Occasionally a ceramic element is warped or tilted; the die above may be tilted as well. The warping or tilting may cause the solder TIM between the ceramic and the heat sink to have differential thickness which in turn may create a local hot spot that can cause the die to overheat or crack.

Voids, delaminations and other gaps at the moduleís internal interfaces can form during assembly. A void is typically simply a trapped air bubble within the TIM material, while a delamination, generally thought of as being thinner than a void, may result from contamination on a surface that prevents bonding. Any type of gap in IGBT modules and other electronic assemblies may grow larger, usually in its x and y dimensions, as a result of repeated thermal cycling. In some modules other factors such as shock and vibration can create gaps. As a gap becomes progressively wider (but not necessarily thicker) its ability to block thermal transmission increases. At some point the gap becomes large enough to overheat the die, and the module fails electrically.

Acoustic microscopes image IGBTs and other samples by using a transducer that pulses high-frequency ultrasound into the sample while scanning the sampleís surface. The job of the moving transducer is to send a pulse of ultrasound into the sample and to receive the return echoes from various depths a few millionths of a second later. In one second, the transducer can collect echoes from thousands of x-y coordinates, picking up data that will create thousands of pixels. Ultrasound is "interface-sensitive" because it is reflected only from the interfaces between both solid materials and gaps but not from the bulk of homogeneous materials. The ultrasonic frequencies used for imaging IGBTs are typically from 30 MHz to 50 MHz.

Because ultrasound will not travel through air, it must be coupled to the sample surface by water or another fluid. Makers and users of IGBTs understandably have little interest in water coming in contact with the die at the top of the module. To solve this problem, Sonoscan has invented an inverted transducer (patent pending) that uses a water plume that pulses ultrasound into the bottom side of the heat sink. This new technology keeps the top of the module dry and gives good acoustic access to the internal interfaces.

The right side of Fig. 2 demonstrates what happens when the pulse encounters a gap, even a gap as thin as 1 micron. Because solder and air have profoundly different acoustic properties, the solder-to-air interface at the bottom of the void or other gap reflects virtually all of the pulse back to the transducer. Practically speaking, no ultrasonic energy remains to cross to the top of the gap, where it would encounter another air-to-solder interface. The near-total reflection by a gap has two outcomes:

• Echoes from gaps have far higher amplitude than echoes from bonded interfaces, and are bright white in an acoustic image. Bonded interfaces are some shade of gray.
• Features directly above a gap cannot be imaged because no ultrasonic energy reaches them.

Fig. 3 shows how the system operates. The transducer scans the bottom of the heat sink, pulsing ultrasound into its surface. Ultrasound is propagated upward through the heat sink, and sends back echoes from both the bottom and top of the solder layer. At each of these interfaces, a portion of the ultrasound is reflected back to the transducer, and another portion travels on. The pulse then reaches the die attach and sends back echoes from the top and bottom interfaces of the die attach. Blue arrows at the left in Fig. 2 are the pulse; purple arrows are the return echoes from each of the four interfaces.

Fig. 3 is the acoustic image of the solder layer between the heat sink and the ceramic substrate of the three die in an IGBT module. To make this image, only echoes whose arrival time at the transducer indicated that they were from a depth from just below the solder layer to a depth just above the solder layer - that is, that they included the bulk solder and both of its interfaces.

The gray regions in Fig. 3 represent the partly transmissive good bonds between the solder and the adjacent solids (ceramic and heat sink). The white areas, however, are voids in the solder that are reflecting much more of the pulse energy. The total area of the voids in this image is small and would be unlikely to cause overheating unless the size of the voids grew after repeated thermal cycling.

Fig. 4 is the acoustic image of the solder die attach layer under all six die on a larger IGBT module. There are a few very small voids here, and one large void (arrow) that could conceivably grow large enough to cause a failure.

Fig. 5 is the acoustic image of the die attach layer in another IGBT module. The large but faint dark circular feature was formed when the deposited die attach material cured slightly before it was spread out by the placement of the die. The white features are voids in the die attach. They are numerous enough and large enough to impede thermal transmission, especially if they grew in area. Some of them are close enough to coalesce if they expand.

Because IGBT modules provide high efficiency and fast switching in aircraft, electric cars, trains and other critical environments, they are increasingly being imaged acoustically before installation in order to find anomalies that could lead to field failures. Such imaging is performed during product development and also during production, the latter because both processes and materials may change in subtle ways that can lead to internal defects. IGBT modules from which obstacles in the thermal dissipation path have been designed out are likely to have long and trouble-free lifetimes.