Inverted Scan Transducer Mount Technique: A Cost Effective Acoustic Scanning of IGBT Modules

for Failure Analysis

Em Julius De La Cruz1, Sheenel Karl De La Rea1, Stephen McDonough2, SF Chai3 ON Semiconductor Malaysia Sdn. Bhd. 1, OKOS Solutions LCC USA2, QES (Kuala Lumpur) Sdn. Bhd.3

Lot 122, Senawang Industrial Estate, 70450 Seremban, Negeri Sembilan, W. Malaysia Abstract:

Acoustic Scanning for IGBT modules is a critical process to find anomalies that could lead to field failures. However, the cost to build this capability for failure analysis use is relatively expensive. This paper aims at evaluating a cost effective acoustic scanning technique for IGBT modules suitable for failure.

I. INTRODUCTION IGBT (Insulated-Gate Bipolar Transistor) modules

are gaining popularity in the market because they provide high efficiency and fast switching in aircraft, electric cars, trains and other critical environments. Large IGBT modules typically consist of many devices in parallel and can have very high current handling capabilities in the order of hundreds of amperes with blocking voltages of up to 6000V, equating to hundreds of kilowatts.

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.

The most common causes of overheating in IGBT modules are the following:

1. Voids, delaminations or other gaps within or adjacent to a solder thermal interface material. Even if they are very thin, gaps are efficient insulators.

2. Warping or tilting of ceramic element that may cause differential thickness of solder.

Figure 1 shows an example of an IGBT module. An

IGBT module is typically composed of multiple layers. Figure 2 illustrates the different layers of an IGBT module. The base plate is normally a Nickel-plated Copper; where a Die-Bonded Copper (DBC) assembly is mounted by a solder material, followed by another layer of solder die attach to mount the IGBT die. The importance of this solder die attachment layer is one of the most uncontrolled processes in today’s power module fabrication which is practiced by the IGBT modules manufacturers. Typically a significant amount of solder die attached was dispensed on the area to be attached, that brought two surfaces together then reflowed on an oven. The paste is manually dispensed, which results a random thickness layer from one surface to another surface. As a result, the level of voiding on the paste is extremely significant.

Figure 1. An example of IGBT module.

Figure 2. Typical IGBT layers. Solder layers are the region of interest to be checked for voids.

Voids, delaminations and other gaps at the module’s internal interfaces can form even during assembly. Any type of gap in IGBT modules may grow larger as a result of repeated thermal cycling. At some point the gap becomes large enough to overheat the die, and the module fails electrically. Hence, effective screening methods to detect defects during product development stage and during production are needed to screen out possible defects.

X-Ray imaging and acoustic micrography imaging are the most common tools for flaw detection. Usually, X-Ray imaging is not suitable to all types of materials. X-Ray imaging is based on the variation of X-Ray attenuation by a solid body. However, the sensitivity of this technique is dependent on the thickness of a material, which the beam needs to penetrate. Acoustic image scanning uses reflected ultrasonic signals to generate images representative of internal structures of semiconductor devices, metals, plastics and composites. As semiconductor devices become more complex and multi-layered, extraction of different variants of information from reflected ultrasonic signal, to help end-users interpret the data, becomes critical [1].

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However, manufacturers of IGBTs have little interest in conventional scanning method where water comes in contact with the die at the top of the module. For IGBT modules, the usual method used in protecting the critical components from direct water exposure while scanning is to flip the sample so the critical side of the sample is facing down, away from the flow of water but this is critical and can cause contamination from water splash back.

To solve this problem, there is an available tool in the market utilizing inverted transducer that uses a water plume. Water plume uses inverted waterfall/squirted transducer from underneath and the plume of water constantly makes contact while the system scans the sample, the water level stays below the top. This non-immersion technology keeps the top of the module dry and gives good acoustic access to the internal interfaces.

Figure 3. The water plume technique scans IGBT modules from the bottom side, leaving the topside circuitry dry. The right side of Figure 3 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.

Figure 3 illustrates water plume technique mechanism. 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. Figures 4 and 5 are acoustic images taken from water plume technique. 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 [2].

Figure 4. Acoustic image of Base to Solder interface using water plume technique. The image was taken at 50MHz frequency. White areas indicate void/gap.

Figure 5. Acoustic image of solder to ceramic interface using water plume technique at 50MhZ frequency.

The system can do voids calculation by providing

binary images into good and bad pixels and gives percentage of the defect area. The user can also define the accept and reject criteria.

The same water plume system is also capable of thickness measurement. The technique utilizes Time Difference Mode which evaluates the thickness of the solder at every x-y coordinate into which the scanning transducer pulses ultrasound. At each x-y location, the scanning transducer receives two return echo signals, one from the heat sink-to-solder interface and one from the solder-top late interface. The difference between the arrival times of the two echoes is measured and converted into the thickness of the solder at that x-y location. Scanning the entire area of the ceramic plate yields a map of solder thickness [3].

The transducer pulses ultrasound into thousands of coordinates per second. The resulting acoustic image is a detailed map of solder thickness across the IGBT module. Figure 6 is the Time Difference Mode acoustic image of one portion of a large multi-die IGBT module, imaged from the heat sink side. The ceramic pieces and the die are thus below the heat sink; it is the near side of the ceramic pieces that forms the rectangles seen in the image. This image displays solder thickness rather than the acoustic reflectivity of each feature. The thickest areas solder are pink, while the thinnest areas are orange. Most of the numerous irregularly shaped voids in the solder are red, not because of their high reflectivity but because they are very close to the top of the solder and the Time Difference software measures the distance from the top of the solder to the top of the void. Some smaller voids deeper in the solder are blue [4].

128 2014 IEEE 21st International Symposium on the Physical and Failure Analysis of Integrated Circuits (IPFA)

Figure 6. Time Difference Mode image showing thickness vatiations. The solder in the unit at the far left is thicker toward its right edge (pink) and much thinner toward the left edge (orange). The cause of the variation in the thickness of the solder may be a tilted ceramic piece beneath the solder.

The water plume technique is a breakthrough

solution to perform non-destructive imaging and evaluation of IGBT modules. It is also a good screening tool at production line to check the quality of the modules. However, capital to own this capability is relatively expensive, more so if this capability is to be brought in failure analysis. This motivated the authors to evaluate a cost effective acoustic scanning technique that will produce similar imaging quality and capability as the water plume technique.

The Scan Transducer Mount Technique was evaluated as a potential cost-effective method of acoustic imaging for IGBT modules. The objective is to evaluate the image quality of scan transducer mount technique in detecting and calculating voids on IGBT modules and to evaluate the capability to perform contour scanning, measurement and warp compensation.

II. EVALUATION RESULT AND DATA ANALYSIS

Scan transducer mount technique utilizes the concept of bottom scan technique where a frequency transducer is mounted from the bottom of the tank such that scanning can be performed from the bottom, thus keeps the top of the module dry. This is done by modifying the existing SAT machine tank to be deeper and building a U-shaped arm to hold the transducer (refer to Figure 7 and 8). The transducer arm can hold a high frequency transducer.

Figure 7. Illustration of Scan Transducer Mount Technique for IGBT module acoustic scanning.

Figure 8. Actual set-up of Scan Transducer Mount Technique.

The Scanning Acoustic Machine used for this study

utilizes high frequency digital pulse receiver and 12-bit high speed digitizer. The transducer is immersed in water at the bottom of the tank, in which the water was used as a medium for ultrasound signals. The transducer scans the base plate of the module, pulsing ultrasound into its surface. The ultrasound is propagated upward through the base plate and reflected back to the transducer and is turned into a pulsed electronic signal that is used to display the returning signals in a time and amplitude raster scan to produce the image. Ultrasound pulsed signal into the base plate will create an image from the ceramic layer above the base plate up to the solder die attachments on the top module. Refer to Figure 9.

Figure 9. Acoustic scan image of the Base to Solder interface. White areas indicate void/gap.

Figure 10. Acoustic image of solder to ceramic interface using at

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50MhZ frequency. It is necessary to get adequate signal penetration and

layer separation. Based on the evaluation, a 50 MHz non delay line type produced the best images. Scanning images on sample IGBT module showed capability to scan through layers and detect solder voids. Void calculation can be performed using the software built-in to the machine. The machine’s cluster analysis feature provides extensive reporting of individual voids and percentage voids of entire package or specific regions of interest as shown in Figure 10.

Figure 10. Void calculation can be performed using built-in software in the machine and can be processed using excel file or other similar application.

Thickness measurement was performed using the Time of Flight (TOF) concept. The TOF imaging will show the minimum and maximum TOF results. As shown in Figures 11, to do a thickness measurement a surface follower gate and a data gate is needed. The Follower gate is placed on the first interface to be measured and the data gate will be placed on the second or next interface and the live thickness measurement can be obtained .

Figure 11. Thickness measurement using Digital Scope.

Based on the result, thickness measurement is able to

identify warpage or thickness variations of the interface. Contour Scan was performed as shown in Figure 12 using Surface Mapping. As warping is a mechanical problem, the machine maker has designed this feature to compensate by moving Z axis during scan axis movement.

Figure 12. Thickness measurement using Time of Flight (TOF).

III. CONCLUSION Based on evaluation results, the Scan Transducer

Technique was found to be a feasible method for acoustic scanning of IGBT modules. The cost of ownership is relatively cheaper. A complete set of water plume technology machine would cost roughly USD250K. A complete set of Scan Transducer Mount Technique costs around USD150K. The cost will be even lower if there is already an existing system that can be modified by adding a deeper water tank and transducer arm.

The scanning technique provides capability to inspect and aid in failure analysis of IGBT modules, specifically in checking for solder voids. The technique was found to be reliable and repeatable in defect detection and integrity assessment of IGBT modules.

REFERENCES [1] Hari Polu & Steve McDonough, “Scanning Acoustic Microscopy (SAM) Imaging Tecniques” OKOS Solutions, LLC. [2] Tom Adams, “Inverted Acoustic System Cuts IGBT Failures,” Power Electronics Technology Exclusive Insight; 09/02/2011, p1. [3] Tom Adams, “Size Up Component Defects Non-Destructively,” U.S. Tech; August 2013; p2. [4] Tom Adams, “Acoustically Mapping IGBT Module Solder Thickness,” Power Electronics Technology Exclusive Insight; 11/5/2012, p1.

130 2014 IEEE 21st International Symposium on the Physical and Failure Analysis of Integrated Circuits (IPFA)