This work is a contribution to the evaluation of the power cycling reliability of different packaging and interconnection solutions at high temperature swings. It provides the designer of power circuits data for module lifetime prediction especially at high operational temperatures. Failure analysis with the different microscopic techniques provide cognitions about the failure mechanisms and eventual weak points of the power devices at high thermal stresses. / Diese Arbeit liefert einen Beitrag zur Qualifizierung der Lastwechselfestigkeit von modernen Aufbau- und Verbindungstechniken bei hohen Temperaturhüben. Dadurch wird den Designern von Leistungsschaltkreisen Daten zur Abschätzung der Lebensdauer ihrer Komponente besonders unter höheren Umgebungstemperaturen zur Verfügung gestellt. Eine Ausfallanalyse mit dem Rasterelektronenmikroskop (REM) und der Ultraschallmikroskopie liefert Erkenntnisse über die zu erwartende Ausfallmechanismen und die eventuellen Schwachpunkte der Bauelemente bei hohen Temperaturen

This work is a contribution to the evaluation of the power cycling reliability of different packaging and interconnection solutions at high temperature swings. It provides the designer of power circuits data for module lifetime prediction especially at high operational temperatures. Failure analysis with the different microscopic techniques provide cognitions about the failure mechanisms and eventual weak points of the power devices at high thermal stresses. / Diese Arbeit liefert einen Beitrag zur Qualifizierung der Lastwechselfestigkeit von modernen Aufbau- und Verbindungstechniken bei hohen Temperaturhüben. Dadurch wird den Designern von Leistungsschaltkreisen Daten zur Abschätzung der Lebensdauer ihrer Komponente besonders unter höheren Umgebungstemperaturen zur Verfügung gestellt. Eine Ausfallanalyse mit dem Rasterelektronenmikroskop (REM) und der Ultraschallmikroskopie liefert Erkenntnisse über die zu erwartende Ausfallmechanismen und die eventuellen Schwachpunkte der Bauelemente bei hohen Temperaturen

The low-temperature joining technique (LTJT) by silver sintering is being implemented by major manufacturers of power electronics devices and modules for bonding power semiconductor chips. A common die-attach material used with LTJT is a silver paste consisting of silver powder (micron- or nano-size particles) mixed in organic solvent and binder formulation. It is believed that the drying of the paste during the bonding process plays a critical role in determining the quality of the sintered bond-line. In this study, a model based on the diffusion of solvent molecules and viscous mechanics of the paste was introduced to determine the stress and strain states of the silver bond-line. A numerical simulation algorithm of the model was developed and coded in the C++ programming language. The numerical simulation allows determination of the time-dependent physical properties of the silver bond-line as the paste is being dried with a heating profile. The properties studied were solvent concentration, weight loss, shrinkage, stress, and strain. The stress is the cause of cracks in the bond-line and bond-line delamination. The simulated results were verified by complementary experiments in which the formation of cracks in bond-line and interface delamination was observed during the pressure-free drying of a die-attach nanosilver paste. Furthermore, the important drying parameters, such as drying pressure, low temperature drying time and temperature ramp rate of nanosilver LTJT process, are experimentally studied and analyzed with the numerical simulation. The simulated results were consistent with the experimental findings that the quality of sintered silver bond-line increases with increasing external drying pressure, with increasing low temperature drying time, and with decreasing temperature ramp rate. The insight offered by this modeling study can be used to optimize the process profile that enable pressure-free, low-temperature sintering of the die-attach material to significantly lower the cost of implementing the LTJT in manufacturing. / Ph. D.

As the first level interconnection in electronic packages, chip attachment plays a key role in the total packaging process. Sintered nanosilver paste may be used as a lead-free alternative to solder for die-attachment at sintering temperature below 300 °C without applying any pressure. Typically, the substrate, such as direct bond copper (DBC) substrates, has surface metallization such as silver or gold to protect the copper surface from oxidation during the sintering process. This study focused on developing techniques for die-attachment on pure copper surface by low-temperature sintering of nanosilver paste. One of the difficulties lies in the need for oxygen to burn off the organics in the paste during sintering. However, the copper surface would oxidize, preventing the formation of a strong bond between sintered silver and copper substrate. Two approaches were investigated to develop a feasible technique for attachment. The first approach was to reduce air pressure as a means of varying the oxygen partial pressure and the second approach was to introduce inert gas to control the sintering atmosphere. For the first method, die-shear tests showed that increasing the oxygen partial pressure (PO₂ from 0.04 atm to 0.14 atm caused the bonding strength to increase but eventually decline at higher partial pressure. Scanning electron microscopy (SEM) imaging and energy dispersive spectroscopy (EDS) analysis showed that there was insufficient oxygen for complete organics burnout at low PO₂ condition, while the copper surface was heavily oxidized at high PO₂ levels, thus preventing strong bonding. A maximum bonding strength of about average 8 MPa was attained at about PO₂ = 0.08 atm. With the second method, the die-shear strength showed a significant increase to about 24 MPa by adjusting the oxygen exposure temperature and time during sintering. The processing conditions necessary for bonding large-area chips (6 mm à 6 mm) directly on pure copper surface by sintering nanosilver paste was also investigated. A double-print process with an applied sintering pressure of less than 5 MPa was developed. Die-shear test of the attached chips showed an average bonding strength of over 40 MPa at applied pressure of 3 MPa and over 77 MPa under 12 MPa sintering pressure. SEM imaging of the failure surface showed a much denser microstructure of sintered silver layer when pressure was applied. X-ray imaging showed a bond layer almost free of voids. Because the samples were sintered in air, the DBC surface showed some oxidation. Wirebondability test of the oxidized surface was performed with 250 μm-diameter aluminum wires wedge-bonded at different locations on the oxidized surface. Pull test results of the bonded wires showed a minimum pull-strength of 400 gram-force, exceeding the minimum of 100-gf required by the IPC-TM-650 test standard. / Master of Science

Quenching and partitioning (Q&P) steels show a good balance between strength and ductility due to a special heat treatment that allows to adjust a microstructure of martensite with a fraction of stabilized retained austenite. The final heat treatment step is performed at low temperatures. Therefore, joining of Q&P steels is a big challenge. On the one hand, a low joining temperature is necessary in order not to influence the adjusted microstructure; on the other hand, high joint strengths are required. In this study, joining of Q&P steels with Ag nanoparticles is investigated. Due to the nano-effect, high-strength and temperature-resistant joints can be produced at low temperatures with nanoparticles, which meets the contradictory requirements for joining of Q&P steels. In addition to the Ag nanoparticles, activating materials (SnAg and Sn) are used at the interface to achieve an improved bonding to the steel substrate. The results show that the activating materials play an important role in the successful formation of joints. Only with the activating materials, can joints be produced. Due to the low joining temperature (max. 237 °C), the microstructure of the Q&P steel is hardly influenced.