Petar Ratchev

Thermal cycling reliability of SnAgCu and SnPb solder joints : A comparison for several IC-packages

This paper deals with a comparison study between SnPb and SnAgCu solder joint reliability. The comparison is based on non-linear finite element modellin. Three packages have been selected: silicon CSP, underfilled flip chip and QFN package. Also the effect of thermal cycling conditions has been investigated. Comparing the induced inelastic strains in the solder joint, the leadfree SnAgCu generally scores better thanks to the lower creep strain rate. On the other hand for the CSP and flip chip package, SnAgCu scores worse for the more extreme loading conditions when the inelastic dissipated energy density is selected as damage parameter. The main reason is that due to the lower creep strain rate, the stresses become higher for SnAgCu resulting in higher hysteresis loops with more dissipated energy per cycle. For the QFN package, SnAgCu scores much better.

Mechanical reliability of Au and Cu wire bonds to Al, Ni/Au and Ni/Pd/Au capped Cu bond pads

This work is an assessment of the mechanical reliability of Au and Cu ball bonds to Al, Ni/Au and Ni/Pd/Au surfaces in terms of high temperature storage. All systems show very good shear strength after thermal storage for up to 120 days at 150 °C. The Au ball bonds on Al surface show Kirkendall voiding starting from 60 days. This did not decrease their mechanical strength but it is expected to become a reliability issue in the long run. The Cu wire bonds on Al caps show a higher initial strength, much slower intermetallics formation and no Kirkendall voiding. This makes them a potentially better industrial solution. Excellent bond strength was found for Cu- and Au-bonds on Ni/Au and Ni/Pd/Au caps. No intermetallics formation or other microstructural change have been found on these interfaces up to 120 days at 150 °C, which was related to the full solubility of the materials along these interfaces. This result suggests that they can be a successful industrial solution for the next generation of packages.

Reliability and failure analysis of Sn-Ag-Cu solder interconnections for PSGA packages on Ni/Au surface finish

The reliability and failure modes of eutectic Sn-Ag-Cu solder joints were studied and compared to eutectic Sn-Pb-Ag ones. Two different failure modes occur: brittle fracture and fatigue. The results show that with a Ni/Au surface finish the reliability of Sn-Ag-Cu solder is much better than that of Sn-Pb-Ag solder. First, when the joint is deformed at high thermomechanical strain, the chance of brittle fracture at the Ni-Au interface is significantly reduced when using Sn-Ag-Cu solder. The reason is a reduced formation of the brittle (Au, Ni)Sn/sub 4/ intermetallic at the UBM interface, responsible for the brittle fracture mode. Second, after deformation at low thermomechanical strain, both solders fail due to solder fatigue, but the Sn-Ag-Cu shows a better lifetime. This better reliability of the Sn-Ag-Cu solder is attributed to a new solder fatigue mechanism: the crack propagates through the bulk of the solder in a web-fashion way, linking the Au-containing particles formed in the volume. This is beneficial for the joint reliability as it hinders the crack propagation.

A comparative study of two Al-Mg-Si alloys for automotive applications

In search for lighter cars, the automotive industry shows a considerable interest in the application of aluminium sheets for car body panels. The formability and strength of aluminium alloys is inferior compared with steel. This necessitates to acquire a better understanding of the link between processing, microstructure and mechanical properties of these alloys. The present work is a comparative study of two Al-Mg-Si alloys which differ in the level of Si. The influence of ageing, deformation and a second solution treatment on the mechanical properties has been studied. The associated microstructures, evaluated by means of transmission electron microscopy (TEM) and X-ray microanalysis (EDS), have been used to explain the observed properties.

Study of the temperature dependence of the bainitic transformation rate in a multiphase TRIP-assisted steel

A prerequisite to the development of multiphase TRIP-assisted steels is a good understanding of the bainitic transformation that takes place during the related thermo-mechanical processing. In this framework, the present paper proposes to investigate the formation of bainite when originating from intercritical austenite in a Si bearing steel. The experimental results suggest the contribution of a martensitic type mechanism to the transformation process. Yet, the overall bainitic reaction rates are found to strongly depend on the holding temperature. This original kinetics is correlated with the typical microstructure the steel exhibits after the intercritical annealing stage. To this extent, the crucial role of the adjacent development of bainitic ferrite for the observed temperature dependence is discussed

Direct gold and copper wires bonding on copper

Abstract The key to bonding to copper die is to ensure bond pad cleanliness and minimum oxidation during wire bonding process. This has been achieved by applying a organic coating layer to protect the copper bond pad from oxidation. During the wire bonding process, the organic coating layer is removed and a metal to metal weld is formed. This organic layer is a self-assembled monolayer. Both gold and copper wires have been wire-bonded successfully to the copper die even without prior plasma cleaning. The ball diameter for both wires are 60 μm on a 100 μm fine pitch bond pad. The effectiveness of the protection of the organic coating layer starts from the wafer dicing process up to the wire bonding process and is able to protect the bond pad for an extended period after the first round of wire bond process. In this study, oxidization of copper bond pad at different packaging processing stages, dicing and die attach curing, have been explored. The ball shear strength for both gold and copper ball bonds achieved are 5 and 6 g/mil2 respectively. When subjected to high temperature storage test at 150 °C, the ball bonds formed by both gold and copper wire bond on the organic coated copper bondpad are thermally stable in ball shear strength up to a period of 1440 h. The encapsulated daisy chain test vehicle with both gold and copper wires bonding have passed 1000 cycles of thermal cycling test (−65 to 150 °C). It has been demonstrated that orientation imaging microscopy technique is able to detect early levels of oxidation on the copper bond pad. This is extremely important in characterization of the bondability of the copper bond pad surface.

Mechanical issues of Cu-to-Cu wire bonding

Cu-to-Cu wire bonding provides benefits both from economical and from electrical point of view. However, since Cu is harder than Al or Au, it is expected to induce higher mechanical stresses in the substrate and more bonding problems during and/or after wire bonding. There are three steps during bonding: bond forming, ultrasonic vibration, and cooling down. In order to understand their physical nature, different techniques are applied. The bond forming process is simulated using finite element analysis (FEA) and the stress generated at the last stage (when the wire ball reaches its final shape) is presented. The grain distortion in the Cu wire bond after bonding is studied using scanning electron microscopy and orientation imaging microscopy. After ultrasonic vibration, the whole structure cools down. Due to the difference in coefficient of thermal expansion between Cu and Si, extra stress is built up. The final residual stress is measured by Raman spectroscopy. The result is compared with FEA and an excellent agreement has been achieved. The impact on the substrate by the capillary tool is clearly shown. The result also shows that the maximum tensile stress in the substrate is located near the edge of the pad and depends highly on the positioning of the wire bond.

Effect of preheat temperature on the orientation relationship of (Mn,Fe)Al6 precipitates in an AA 5182 Aluminium—Magnesium alloy

Abstract The precipitation of the (Mn,Fe)Al6 phase during preheating of a commercial AA 5182 Al Mg alloy was studied by means of scanning electron microscopy, transmission electron microscopy and selected area electron diffraction. The existence of two different morphologies with low and high aspect ratio (called here rhomboidal and platelike) was confirmed. The influence of preheating parameters on the precipitation was found to be close to the one known for Al-Mn alloys. It was found that the platelike dispersoids bear orientation relationships with the matrix of type [100]m||[2¯10]pp and (0¯11)m||(001)pp, which are not yet reported in the literature. On the other hand it was shown that rhomboidal precipitates do not follow any orientation relationship with the matrix. The more harmful influence on recrystallization and hot ductility of the rhomboidal precipitates compared to the platelike ones is discussed.

Creep resistant aluminum alloys for use in MEMS

Creep is expected to be a reliability issue in MEMS where high temperatures and stresses are present in the moving part. In this paper, we describe a method of measuring the creep parameters, ΔF and τ, in metal thin films. Substrate curvature measurements were used to study different Al alloys—Al98.3Cu1.7, Al99.7V0.2Pd0.1, Al93.5Cu4.4Mg1.5Mn0.6 and Al99.6Cu0.4 films—during isothermal tensile stress relaxation. We show that there is a direct relation between the measured creep parameters and the coherency, size and spacing of precipitates observed by TEM and SEM in the alloys. Furthermore, we confirm that the plastic deformation is controlled by the motion of dislocations inside grains in the Al alloy films. A strengthening process called precipitation hardening was used to create stronger precipitates within the grains in Al99.6Cu0.4 to hinder the movement of dislocations more effectively and thus to make the alloy more creep resistant.

Hot working of AA1050 - Relating the microstructural and textural developments

Abstract An aluminum alloy AA1050 was deformed in plain strain at different hot working conditions. An increase in temperature or a decrease in strain rate reduced the relative drop in cube {001}〈100〉 and the relative increase in rolling texture components of Cu {112}〈111〉 and S {231}〈346〉, especially apparent at the higher strain. Along with such textural changes, significant differences in hot worked microstructures were observed. The two distinct microstructural features, as observed by polarized light optical microscopy, were grain boundary serrations (GBS) and in-grain inclined lines (IIL), typically observed at an approximate angle of 35° with rolling direction (RD). At higher temperatures and lower strain rates, and correspondingly lower Zener–Holloman factors ( Z ≈10 9 −10 10 s −1 ), coarse but nearly equiaxed grain interior substructures and GBS were observed. Interestingly, orientation imaging microscopy (OIM) clearly showed insignificant/non-noticeable differences between the substructures of different orientation components. An increase in Z aligned the grain-interior low angle boundaries at an angle of approximately 35° with RD and at higher Z ( Z ≈10 12 −10 13 s −1 ) the main microstructural feature was the IILs. Development of in-grain long range misorientation (LRM) was estimated to be the mechanism behind the optical visibility of the IILs. The appearance of IILs had two apparent effects—first the substructures of different orientation components were different, and secondly the stability of cube grains dropped noticeably. Generalizing the IILs or 35° inclined cell walls as plastic instabilities or strain localizations, the observed differences in their relative appearance at different deformation conditions and/or texture components could be explained. When formation of such strain localizations are considered as “necessary” for the reorientation of grain segment(s), the cube stability at low Z deformation could also be understood.