Improving the power cycling performance of IGBT modules by plating the emitter contact

Highlights

• Reliability improvement of power modules in terms of power cycling performance.

• Investigation of electroless and electro-plating as a tool to improve reliability of power modules.

• Description of a novel method to manufacture Cu-clad-Al joints, which profits from the benefits of both materials.

• Experimental cycling and evaluating of wire and ribbon-bond layouts.

• Improvement of the cycling capability of power modules by a factor of 4.

Abstract

In terms of power cycling reliability one of the most critical joints of a typical power module is the emitter contact. A typical emitter contact of the IGBTs or diodes consists of a front metallization and bond wires connecting the chips to the emitter lead. In this study, a novel method was implemented to the entire emitter contact to support and reinforce the mechanical as well as electrical durability and reliability. These modules were characterized in terms of the power cycling performance. The emitter contact of the standard wire bonded as well as heavy ribbon bonded HiPak modules were reinforced by Cu electroplating. The two groups of modules were subjected to power cycling tests. The results of the cycling tests proved the method as effective and the power cycling performance was improved up to 4 times compared to the standard wire bonded modules. This study describes a novel method for designing high-reliability emitter contacts for IGBT modules with improved power cycling performance.

Introduction

The power modules are more and more implemented in numerous applications such as traction converters, industrial drives and renewable energy. The HiPak power module platform includes both high voltage and high current modules up to 6.5 kV and 3.6 kA, respectively. The high power density is achieved by parallelizing of up to 24 IGBTs (insulated gate bipolar transistor) and 12 diodes. The chips are protected by a package, which provides mechanical protection, electrical isolation as well as an effective cooling of the chips. To fulfil all these requirements efficiently, components made of various materials are joined together to build up a functional package. Typically for multi-material packages, reliability issues arise when the modules are subjected to thermal load profiles during application. The high reliability requirements of several specific applications is the main driving force for developing high-reliability power modules.

The reliability of power modules are typically determined by well-defined tests, e.g. repetitive thermal cycling tests including power cycling tests. Specific testing strategies are applied to individually stress different joints of the module. Power cycling with short cycling periods like 1 s is an appropriate accelerated method to measure and quantify the cycling reliability performance of the emitter contact [1], [2], which is typically the front metallization of the chips and bondwires connecting the chips to the emitter lead on the ceramic substrates. The heat is generated at the chips due to electrical losses and is removed from the module through a heat sink. Subjected to repetitive temperature profiles during power cycling, the wirebond joint is exposed to cyclic thermo-mechanical stress, which degrades and damages the wirebonds and results in a detachment of the bondwires from the chips. The failure criteria for this specific test can be defined by electrical degradation, which is a 5% increase in V_CE (collector emitter voltage), or by thermal degradation, which is a 20% increase in either the R_th (thermal resistance) or the ΔT_j (junction temperature swing), or a malfunction of the device such as short between the gate and emitter (V_GE, gate emitter voltage). The expected lifetime of the HiPak modules is comprehensively described for each joint individually under numerous conditions in an ABB application note [3].

The emitter contact is one of the most stressed joints for the fast evolving applications such as automotive and renewable energy, which have ambitious reliability requirements throughout the whole life cycle of the power modules. Thus, improving the power cycling performance of the emitter contact of the modules is currently a hot topic where several different approaches are being investigated and implemented by several research groups as well as by the semiconductor industry. Considering that a conventional emitter contact comprises the front metallization of the chips and the bondwires connecting the chip to the emitter lead, a straight forward approach is to optimize this system without changing the main material system. Here, either the front metallization [2], [4], [5] of the wirebond layout [2], [6], [7] was optimized in order to achieve a higher power cycling capability. The optimization is usually limited by economical factors and manufacturing feasibility. Moreover, the improvement of the power cycling performance is limited, as well. One step further is to adapt the material system, for example the implementation of novel Al alloy bondwires [2], [8], [9]. A further approach is to reinforce the emitter contact by introducing novel materials and processes. One example is to replace the Al-based material of the emitter contact by Cu, which means a Cu-based front metallization and Cu bondwires [10], [11], even planar Cu interconnects [12] or Cu pins [13]. A similar approach is to replace the bondwires by on chip sintered flexible printboards [14] or to sinter a buffer plate on the chip which reinforces the front metallization so that it can be connected without being damaged [15], [16]. A further quite smart approach is to use Al-clad-Cu bondwires or ribbons to benefit from the conventional processing techniques of Al and the material properties of Cu [17]. Here, the idea is to bond the Al surface of the wire with a Cu core to an Al-based front metallization in order to realize an Al-Al joint without damaging the chip.

In this study, a novel method for combining the benefits of conventional Al-processing with the properties of Cu is described [18]. However, in this case the core is the soft Al and the metal jacket is the harder Cu, simply a Cu-clad-Al material. The method is actually is a post-treatment of semi-finished products, which are an assembly of wirebonded semiconductor chips soldered on metallized ceramic substrates. Conventionally these semi-finished products are soldered on baseplates, the housings are mounted and they are casted with silicone gel to completely manufacture the power modules. This method introduces a plating step before soldering to the base plate. The emitter path of the semi-finished products are selectively electroplated with Cu in order to achieve a Cu reinforced front metallization as well as bondwires. Also a thin electroless-Ni plating was deposited as an interlayer between the Al and the Cu plating. As a result, the post treatment provided that the chips are connected with Cu-clad-Al bondwires or ribbons without changing any of the manufacturing processes of neither the back-end nor the front-end production.

This method was implemented to manufacture two different groups of modules having either a wirebonded or ribbon bonded emitter layout. Investigating such two considerably different layouts allowed us a better understanding as well as an evaluation of the benefits of the introduced method. Besides the generic validity of the method was proven by testing two different system. The chips (IGBT and diodes) of the first group were connected by Al-bondwires and the second group was bonded with heavy Al-ribbons. Several samples were plated as explained above. Afterwards all samples with and without additional Cu-plating were subjected to power cycling tests until end of life.