K.-J. Wolter

Electro-optical printed circuit board (EOPCB)

Recently attention has been given to optical waveguides in order to elude the interconnect bottleneck caused by insufficient discrete copper wiring at the board-level. But the cost increase, due to additional manufacturing steps and equipment, are justifiable only for those applications where electronic interconnections are unable to perform satisfactorily. Thats why the future high-performance board will most likely be a hybrid carrier which combines electrical and optical circuitry. This paper will introduce a suitable technology to generate multimode waveguides on standard base materials like FR4. Great importance has been attached to designing a manufacturing process for the optical layer, which is essentially compatible with conventional PCB production. The results of early characterizations encouraged us to pursue this technology. Our layout contains bent waveguides, optical bifurcations and even crosses, the latter being unthinkable in electrical wiring layout. We will also address concerns like sidewall roughness of the waveguide structures, propagation loss, thermal and environmental stability, etc. which all are significant to the high performance EOPCB.

Scaling effects on grain size and texture of lead free interconnects — investigations by electron backscatter diffraction and nanoindentation

In this paper, the application of electron backscatter diffraction (EBSD) methods to solder interconnects is demonstrated, with particular emphasis on the effect of solder material volume on grain orientation, grain size and grain distribution in SnAg3.0Cu0.5 material, as well as with respect to the analysis of intermetallic compounds (IMC) formed in solder to board metallization interfaces. Investigations were carried out on solder spheres and solder interconnects between Cu/Sn or NiAu metallizations. It is shown, that the EBSD results derived allow to study the correlation between mechanical deformation properties, e.g. as characterized by nanoindentation testing, and microstructure in solder material in more detail which is of considerable significance for future improved reliability investigations.

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.

Creep of eutectic SnAgCu in thermally treated solder joints

The creep behaviour of Sn96.5Ag3.5 and Sn95.5Ag3.8Cu0.7 solder was studied specifically for its dependence on technological and environmental factors. The technological factors considered were typical cooling rates and pad metallizations for soldering joints in electronic packaging. The environmental factors included microstructural changes as a result of thermal aging of solder joints. Creep experiments were conducted on three types of specimens

Mechanical behaviour of typical lead-free solders at high strain rate conditions

flip-chip joints, PCB solder joints and bulk specimens. Flip-chip specimens were altered through the selection of various under bump metallizations (Cu vs. NiAu), cooling rates (40 K/min vs. 120 K/min), and thermal storage (24h, 168h, and 1176h at 125/spl deg/C). PCB solder joints were studied by using a copper pin soldered into a thru-hole connection on a printed circuit board having a NiAu metallization. Bulk specimens contained the pure alloys. The creep behaviour of the SnAg and SnAgCu solders varied in dependence of specimen type, pad metallization and aging condition. Constitutive models for SnAg and SnAgCu solders as they depend on the reviewed factors are provided.

Influence of artificial body fluids and medical sterilization procedures on chemical stability of Parylene C

3D packaging is one of the main emerging markets especially for mobile application within the last view years. Mobile devices are exposed and hence need to be reliable under to vibration or mechanical shock conditions. Therefore, material properties covering strain rate dependency have to be at hand within reliability studies. Miniature bulk specimens were utilized to gain stress and strain data at strain rates from 20 s−1 to 600 s−1. Specimens consist of SnAg1.3Cu0.5Ni, SnAg3.0Cu0.5 and SnAg3.5 solder compositions. Ultimate strain, ultimate transversal contraction and fracture surfaces were analysed to determine the deformation and fracture behaviour. The strain rate dependent yield stress was determined and introduced into a material description feasible for finite element analysis (FEA). Finite element simulations were conducted for all solders at strain rates of 100 s−1 to 600 s−1. Results on fracture behaviour agreed well to the experiments. All studied solder compositions revealed ductile deformation behaviour changing only little with strain rate. A higher silver content as well as copper presence lead to higher yield stress level. The sensitivity to the strain rate alters with the copper presence only.

Combining experimental and simulation methods for the mechanical characterisation of lead free solder alloys under high strain rate loads

Among materials suitable for flexible encapsulation poly-para-xylylene (Parylene C), often chosen as protective coating for biomedical devices and variety of anticorrosion applications due to its favorable chemical and biological resistance, high thermal stability, low water vapor absorption, permeability, high biocompatibility as well as excellent dielectric and mechanical properties, is one of the most promising. In spite of a wide use only few systematic studies on biological and chemical stability of Parylene C have been carried out. In this work the influence of autoclave, electron beam (e-beam), gamma, ethylene oxide (EtO) and H2O2-plasma sterilization procedures as well as influence of in-vitro dynamic loading with artificial blood plasma (ABP) and cerebrospinal fluid (ASCF) and 0.9 % NaCl on chemical resistance and crystallinity of Parylene C were studied by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The samples, treated with e-beam sterilization, show substantial changes in their chemical content comparing to the untreated state. The XPS-analysis revealed the intrusion of CH- and CN-groups into the polymer structure and formation of inorganic chlorides on the surface of Parylene C. The second effect was also present in the samples after EtO-treatment. No detectable changes in chemical content of polymer films were observed after gamma, plasma and autoclave procedures. The reduction of Cl-concentration in Parylene C, resulted from damaging of its structure, was found in all the samples loaded with fluids. Additionally, after the fluid influence, the already mentioned implantation of nitrogen and formation of inorganic chlorides have been observed. According to the XRD-results, the autoclave sterilization and fluidic treatments caused the significant ride of Parylene C crystallinity grade. Low crystallinity increase was detected after EtO and plasma procedures, while the both irradiation treatments leads to stronger marked amorphism of the studied structures.

Characterisation of the mechanical behaviour of SAC solder at high strain rates

The scope of the paper is to present the result of combining experimental and simulation methodologies to determine mechanical material properties at high strain rates. Miniature bulk specimens which have a diameter comparable to BGA joints are used in the experiments. Stress-strain data recorded at high strain rates proof the high resolution capability of a novel tensile setup. A FEM model of the experiment is designed and used for evaluation of the experimental results. Explicit solver technology enables the analysis of this kind of highly dynamic scenario. Finally, conclusions for the mechanical behaviour of the used lead free solder alloy under high strain rate loads will be drawn.

Experimental Determination of Time-Independent Elastic-Plastic Behaviour of Solder Joints at High Strain Rates

The scope of the paper is to present the result of the determination of mechanical material properties of a SAC based solder at high strain rates. Miniature bulk specimens which have a diameter comparable to BGA joints are used to investigate the solder behaviour experimentally. Stress and strain data are recorded at high strain rates using a high resolution tensile test setup. Explicit FEM methodology enables the FEM analysis of this kind of highly dynamic scenario and is therefore used for the evaluation of the recorded data. The material parameters of a material model covering the strain rate dependency of the solder behaviour are derived. Finally conclusions for the mechanical behaviour of the used SAC solder alloy under high strain rate loads will be drawn.

Rate dependent mechanical behaviour of SAC solder in fast tensile experiments

In order to investigate the time-independent behaviour of solder joints at high strain rates, a test setup was created capable of measuring the force vs. time reaction of small solder joint in shear loading at strain rates between 10 -1 - 104 [1/s]. The paper describes the design of this high strain rate tester. Details providing for the fast and high resolution force measurement are given. The results of analytic and FEM analyses applied to optimize the force sensor are presented. The paper also points out the challenges of high strain rate measurements of small solder joints. The validation tests applied alumina chips with five SnAgCu solder joints, which had barrel shape with about 200 mum diameter and stand-off height. The test results show good reproducibility of the force vs. time reactions. They match the benchmark results obtained at moderate shear speeds with existing equipment. In addition, fractographic analysis clearly found the two failure modes, bulk and brittle, that had been expected. Hence, the new test setup seems well suited for the tasks it has been designed for.