New die attach technologies are necessary to meet the demand for faster and more reliable power electronic devices.Technologies based on sintering such as silver sintering and silver sinter adhesives are currently in the focus of technology development because of their high strength and very high thermal performance. To ensure the reliability of such a die attach reliable, fast and non-destructive failure analysis (FA) are needed. Transient thermal methods (TTM) provide techniques for detection of different types of defects such as delaminations of interfaces, cracks , electrical short circuits and voids in die attach layer [1]–[3]. In this work, pulse thermography (PT), one of the TTMs, was successfully used for nondestructive testing of the die attach layers under industrial conditions. We have studied the physical principles and presented fundamental possibilities and limitations given by formulas, nomograms and parameter sensitivity studies. Finally, we demonstrate the PT as accompanying FA to process qualification of a sintered silver adhesive process.
This paper deals with the development of an innovative test stand for the measurement of thermal and electrical conductivity of metals, semiconductors, highly conductive die attaches and substrates using the steady state technique for thermal characterization and four-terminal sensing with pulse delta technique for electrical characterization. We present a systematic study of sintered silver in order to investigate the influence of sintering conditions on thermal and electrical properties. Several
sintered silver samples have been prepared under sintering temperature variation between 200°C and 270°C and sintering
pressure variation between 5 MPa and 25 MPa. The characterization results diversify up to 200% for thermal conductivity and up to 330% for electrical conductivity. The Wiedemann-France law describes the relationship between electrical and thermal conductivities for bulk metals. We proved that this law applies not only for bulk metal but also for porous sintered metal.
Thermal greases allow a low stress bond at low bond line thicknesses (BLT) at medium thermal conductivities and simple application, all of which make it an alternative to solders, thermal adhesives or pads. It is widely used in power and microprocessor applications, most of which involve large areas to be used for heat transfer. However, for years thermal overload failure of power modules and chips has been a pressing problem due to pump-out of thermal grease as a die or module thermal interface material (TIM): Most thermal greases are Bingham fluids and thus not solids, so they can be squeezed out from in between the gap, driven by thermo-mechanical action of the adjacent layers as e.g. DCB substrate or silicon chip with the heat sink. Today, thermal greases have to be qualified in lengthy stress tests in a product relevant environment which consumes substantial resources as often a system test is required. Therefore, a fast test is necessary which accelerates testing and thus allows a fast screening of commercial greases on one hand, and guidelines for material development on the other. For that purpose this paper addresses this topic in a combined simulative and experimental way, where at the same time a novel test procedure is proposed for accelerated grease pump-out testing (GPOT) in the framework of a completely new approach, combining loading with in-situ failure analytical techniques and decoupling thermal from mechanical loading. This allows for the first time a realistic loading of greases during accelerated testing with testing times below one hour. The method is demonstrated on various commercial and custom greases, varying their composition and structure, and benchmarked against industry standard thermal cycling tests. Further, two fundamental failure mechanisms have been identified being at work simultaneously, notably fluid transport (which constitutes actually a pump-in phenomenon) and air entrapment. We were able to identify key properties of the materials and loading variables, on which their intensity depends.