Maik Mueller

The effect of downscaling the dimensions of solder interconnects on their creep properties

The creep behaviour of solders is an important input for accurate material models for FE-analysis of electronic assemblies. Usually the mechanical behaviour of solders, is been determined by tensile tests on bulk solder specimens. Although performing these tests is not complicated and the results are easy to interpret, one of the key problems lies in the fact that solder joints are very small and therefore cannot be represented by large tensile specimens. The paper describes the attempts to gain deformation data on ultra small solder joints. It compares creep data that was experimentally gained on bulky samples and on small solder joints.

Effect of Composition and Cooling Rate on the Microstructure of SnAgCu-Solder Joints

In this study the solder alloys SnAg3.5, SnAg3.0Cu0.5, SnAg3.8Cu0.7 and SnAg2.7Cu0.4Ni0.05 have been analysed in order to determine variations in microstructure caused by cooling rate, solder composition and ball diameter. Solder spheres with a diameter of approx. Oslash 1100 mum, Oslash 590 mum, Oslash 270 mum and Oslash 130 mum were solidified with cooling rates of 0.14 K/s, 1.1 K/s and 10.9 K/s. Cross sections of these specimens were analysed by optical light microscopy. Interpretations of the analysed microstructure allow a description of the solidification process, which takes place in a solder ball. It could be concluded that this process is divided in three stages: the formation of primary intermetallics, the formation of fine structured regions and the formation of coarse dendritic areas.

The Dependence of Composition, Cooling Rate and Size on the Solidification Behaviour of SnAgCu Solders

The scope of this study is to investigate the influences on the solidification of the micro structure of SnAgCu solders. It will be shown that the solidification process depends on solder composition, specimen size and manufacturing conditions. The influence of solder composition has been investigated on bulk solder ingots by varying the Ag content from 3.0 wt% to 3.8 wt% and the Cu content from 0.4 wt% to 1.5 wt%. The influence of an additional Au content was investigated on a SnAg3.0Cu0.5Au0.14 solder. Solidification experiments with different cooling rates from 0.006 K/s to 0.6 K/s have been carried on bulk solder ingots (length 23 mm; Oslash 7 mm) of the alloys SnAg3.0Cu0.5, SnAg3.8Cu0.7 and SnAg3.5Cu0.4. The results will point out the influence of cooling gradients on the microstructure. In order to compare the microstructure of these large specimen with real solder joints, solidification experiments with cooling rates from 0.33 K/s to 10.9 K/s have been carried out on SnAg3.0Cu0.5 solder balls with four different sizes (Oslash 130 mum, Oslash 270 mum, Oslash 590 mum and 0 1100 mum). Phase sizes and shapes as well as grain orientations have been investigated and compared. Experiments on directed solidification have been carried out in order to investigate the dendritic growth of beta-Sn dendrites. For that purpose a temperature gradient was generated inside the solder during solidification.

The Effect of Downscaling the Dimensions of Solder Interconnects on their Creep Properties

The creep behaviour of solders is an important input for accurate material models for FE-analysis of electronic assemblies. Usually the mechanical behaviour of solders, is been determined by tensile tests on bulk solder specimens. Although performing these tests is not complicated and the results are easy to interpret, one of the key problems lies in the fact that solder joints are very small and therefore cannot be represented by large tensile specimens. The paper describes the attempts to gain deformation data on ultra small solder joints. It compares creep data that was experimentally gained on bulky samples and on small solder joints.

The Size Effect on the Creep Properties of SnAgCu-Solder Alloys

The paper describes the strategies of characterizing the creep behaviour of SnAg-and SnAgCu-solders through experiments on bulk specimens and on very tiny solder joints. The characteristic behaviour of these different volumes will be explained and rationalized by metallurgical considerations about the specifics of solidification in very small volumes.

Failure mechanism of solder interconnections under thermal cycling conditions

Increasing miniaturization, power densities and internal heat dissipation of novel electronic packages have made their solder interconnections more vulnerable to failures. To improve the reliability of electronic devices the underlying physical failure mechanisms of solder interconnections must be clarified in detail in order to find means to control, or even prevent, the development of failures. Therefore, the evolution of microstructures and the development of failures in Snrich lead-free solder interconnections were investigated by employing methods of orientation imaging microscopy: cross-polarized light imaging and electron backscatter diffraction. The as-solidified microstructures of the SnAgCu solder interconnections (composed of a few large Sn colonies) were observed to undergo a notable change of microstructures at the strain/stress concentration regions before cracking. The investigations of microstructures indicate that the change of microstructures take place at two different stages in the course of thermal cycling: 1.) a gradual formation of low angle tilt grain boundaries caused by a rotations of small volumes of the as-solidified microstructures around the [100] and [110] axes. It is suggested that these boundaries are formed by recovery, i.e. the boundaries are a consequence of the rearrangement of dislocations by polygonization. 2.) In subsequent stages the microstructures in the strain concentration regions transformed into a more or less equiaxed grain structure by recrystallization. It is evident that cracking of solder interconnections under thermomechanical loadings is enhanced by the recrystallization, because the network of high-angle grain boundaries extending through the interconnections provide favorable paths for cracks to propagate intergranularly.

The twinning phenomenon in SnAgCu solder balls

The study focuses on a particular solidification phenomenon found in SnAg3.0Cu0.5 solder balls. Investigations on the microstructure of as cast solder spheres showed, that there is an abrupt change in grain structure during solidification. The microstructure of these specimen consists of areas with small grains that are surrounded by large areas with major grain orientations. In order to characterize this phenomenon Orientation Imaging Microscopy (OIM) done by Electron Backscatter Diffraction (EBSD) has been carried out. It will be shown that the fine grains as well as the large grains show three major, distinct orientations. These measurements indicate the existence of a phenomenon, which is supposed to be caused by a twinning mechanism that creates a misorientation angle of approx. 60° between adjoining grains. In further investigations multilayer cross-sections have been carried out, in order to get a 3-dimensional overview of this phenomenon.

Effects of bonding pressure on quality of SLID interconnects

The investigation of the bonding pressure change on the different quality aspects of the solid-liquid interdiffusion (SLID) interconnects is presented. The stacks were produced by a flux-assisted bonding of two Si dies with an area array of square Cu/SnAg bumps on the bottom die and Cu bumps on the top die at approx. 250 °C. The bonding pressure was varied between 0 MPa, 0.35 MPa, 0.69 MPa, 1.04 MPa, 1.38 MPa, 1.73 MPa, 2.08 MPa, 2.42 MPa. Cross-sections of the stacks were analyzed by optical microscopy and scanning electron microscopy (SEM). Tilt, standoff height (SOH) variation, void fraction, interlayer thickness and Cu3Sn thickness were measured. It will be shown that increase of the bonding pressure can reduce the void fraction from 35.1 % (0 MPa) to 10.7 % (2.42 MPa) and decrease the interlayer thickness at the same time. Decrease of the interlayer thickness is accompanied by solder squeeze and increase of Cu3Sn thickness. Shear tests revealed an average shear strength of (81.3 ± 21.5) MPa for the produced samples. The analysis of the fracture surfaces with SEM revealed that the weakest interface is located between Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs) close to the initial Cu bump.

Metallographic preparation of the SnAgCu solders for optical microscopy and EBSD Investigations

In order to investigate the microstructure of lead-free SnAgCu solders by optical microscopy and Electron Backscatter Diffraction it is necessary to provide a qualitative metallographic preparation. The difficulties of a preparation of these solders are connected with a high softness and presence of hard intermetallic compounds. They are discussed in this study and the appropriate recipe is proposed. The samples were manufactured of SnAg3Cu0.7 (wt. %) solder in a form of solder spheres with a diameter of 270 μm, cooled at the rate of 1.1 K/s. The metallographic preparation was carried out on a TegraSystem from Struers. The quality of the prepared samples was checked by image quality maps, created by the Orientation Imaging Microscopy software.

Microstructure Characterization Of Lead‐Free Solders Depending On Alloy Composition

Fatigue and crack nucleation in solder joints is basically associated with changes in the microstructure. Therefore the microstructure evolution of SnAgCu solder joints during solidification and subsequent application is an important subject for reliability investigations and physics of failure analysis. The scope of this study is a systematic overview of the as‐cast microstructures in small sized lead‐free SnAgCu solder spheres after solidification. A total of 32 alloy compositions have been investigated with varying Ag content from 0 to 5 wt.% and varying Cu content from 0 to 1.2 wt.%. The solder spheres had a diameter of approx. 270 μm and were all manufactured under the similar conditions. Subsequent cross‐sectioning was carried out in order to analyze the microstructure by optical and electron microscopy as well as Electron Backscatter Diffraction and Energy Dispersive X‐ray Spectroscopy. The results allow a comprehensive overview of the dependence of the as‐cast microstructure on the solder composit...