Klaus-Jürgen Wolter

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.

Embedded passive components for MCM

MCMs often have a large number of passive components connected to a small number of active devices. Integration of passive components into the MCM substrate improves electrical properties and reliability, and also reduces the cost, size and weight of electronic systems. The embedded components are mostly used in MCM-D (thin film) and MCM-C (thick film) modules. The use of embedded elements for MCM-L is at the early stage of development. The basic information on embedded passives as well as the activity of Dresden University of Technology and Wroclaw University of Technology in the area of LTCC buried passives is presented.

Surface crack detection in ferritic and austenitic steel components using inductive heated thermography

Active dynamic thermography using inductive heating was used for investigations on different steel components with a high allotment of carbon. The set of test specimens are semi-finished products from a continuous casting process. The cross sectional area of the products is 50x50 mm to 250x250 mm. Defects resulting from the production process have to be detected to avouch the desired quality to the customers. The probed surface is rudely brushed to lift off large oxide parts. Anomalies in the surface temperature during and after inductive heating correspond to inhomogeneities in the material. The defects are perpendicular and slanted surface cracks. The penetration depth of the induced eddy current depends on the induction frequency and on the material properties for example the relative permeability value. In the case of magnetic materials, like ferritic steel and high excitation frequency, the penetration depth of the current is very small in comparison to austenitic steel. Thus, the eddy current is much closer to the 0.1-1mm deep surface cracks. By contrast, the eddy current in austenitic steel at the same excitation frequency has a much larger penetration depth and therefore leads to a different behavior around a crack of similar depth. Thus, lower temperatures around the crack are observed. It is shown, that depending on the material properties, the excitation frequency and the duration of the heating pulse higher or lower temperatures around the surface crack are achieved. To homogenize the emissivity of the oxidized surface, a very thin water film superimposed on the surface is tested

The Influence of Size and Composition on the Creep of SnAgCu Solder Joints

The paper presents creep data, that was gained on non eutectic SnAgCu-solder specimens with a variety of compositions. The non eutectic SnAgCu-alloys were tested in different specimen sizes: bulk specimens, FBGA solder balls, flip chip solder joints. The results of the creep experiments show that both solder alloy composition and solder joint size have a significant influence of the creep properties of the solder material. Bulk solder specimens have a rectangular cross section of 4mm times 3mm and contained the following alloys: Sn98Ag2, Sn97Ag3, Sn96Ag4, Sn97.5Ag2Cu0.5, Sn97.1Ag2Cu0.9, Sn98.8Ag2Cu1.2, Sn96.5Ag3Cu0.5, Sn96.1Ag3Cu0.9, Sn95.8Ag3Cul.2, Sn96.9Ag3Au0.1. FBGA solder balls contained three noneutectic alloys with a content of 0.5% < Ag < 4% and 0.2% < Cu < 0.8%. Flip chip solder joints contained eutectic Sn96.5Ag3.5 alloy and an non-eutectic SnAg alloy with Ag < 3%. Creep experiments have been carried out in a temperature range between T = 5degC ... 150degC. The microstructures of the various solder specimens have been analysed to understand their differences in creep behavior

Geometrical and electrical properties of LTCC and thick‐film microresistors

Purpose – This paper aims to present systematic studies of a wide spectrum of geometrical and electrical properties of thick‐film and LTCC microresistors (with designed dimensions between 50 × 50 μm2 and 800 × 200 μm2).Design/methodology/approach – The geometrical parameters (average length, width and thickness, relations between designed and real dimensions, distribution of planar dimensions) are correlated with basic electrical properties of resistors (sheet resistance and its distribution, hot temperature coefficient of resistance and its distribution distribution) as well as long term thermal stability and durability of microresistors to short electrical pulses.Findings – Fodel process gives better resolution than standard screen‐printing and leads to smaller dimensions than designed, smaller absolute error and better uniformity of planar sizes. Microresistors made in full Fodel process show much weaker dimensional effect and exhibit noticeably smaller distribution of basic electrical properties.Origi...

Fodel microresistors––processing and basic electrical properties ☆

Abstract The dimensions of discrete passives, passive integrated components (arrays, networks) and embedded integral ones should be significantly reduced in the nearest future. For example, passive components with 01005 package size will be commercially available in 2010. This means that the effective length of resistor in/on such package should be approximately two times smaller than the package length. This paper presents fabrication and preliminary electrical characterisation of Fodel microresistors fabricated from EP E-93350-153 (1 kΩ/□) photopatternable resistor ink and Ag-based DP 6453 Fodel conductors. Two versions of microresistors––screen-printed and made in full Fodel process––were made and compared. The effective resistor dimensions varied from 50×50 to 800×200 μm 2 . The processing achievements and problems are presented and discussed. The relation between designed and fabricated patterns is compared for both technological versions. The sheet resistance, hot temperature coefficient of resistance and long-term stability at elevated temperature are characterised as a function of resistor geometry and related to length and width of microresistors. Also normalised temperature dependencies of resistance in a wide temperature range (between −170 and 130 °C) are presented and analysed.

Reliability of copper-ribbons in photovoltaic modules under thermo-mechanical loading

Every temperature change induces stress between the module components due to the thermo-mechanical mismatch, which results in a displacement of solar cells in the module and therefore loads the solar cell interconnectors in between. As a result a limiting factor of solar module lifetime is the fatigue behaviour of these electrical cell interconnectors: the copper-ribbons. The purpose of this work is a quantitative estimation of the thermo-mechanical induced strain in the ribbons during service. For this purpose specially prepared solar cells were laminated to a solar module in an industrial process line and filmed during thermal cycling tests. Finally, the loading conditions on the ribbon were assessed by a previously developed lifetime model based on mechanical fatigue testing procedures.

Creep Measurements of 200 μm - 400 μm Solder Joints

For the last decades, many mechanical measurements on solder alloys were carried out. As a matter of fact, the microstructure of the solder materials is affected by their compositions. In addition, external variables like the reflow cooling rate, solder volume, thermal mass of the package and pad metallization may have an influence. For those reasons the discrepancies of creep measurements on solder contact specimen are larger than on tensile samples. A motivation for the creep measurement activities is the lifetime prediction of electronic components, which have solder joints for electric-mechanical connection on their interposer or printed circuit board. Structure-mechanic simulation tools like the FEM can calculate the mechanical interactions between the assembled materials of such complete packages. Often, the solder joints are the weakest participants in the whole assembly and determine the total lifetime. Nevertheless, every simulation is highly dependent on the material laws. Therefore, the FEM needs an accurate fatigue model and a precise material model for the lifetime prediction of this solder. The paper presents a new experimental design for measuring the creep behaviour of area arrayed solder bumps in different sizes of various packages. It focuses on the feasibility of the measurement of industrial manufactured FC, CSP or BGA packages. First measurements were accomplished on solder bumps with 200 μm and 400 μm diameter. The test setup works by cyclic reversible shear force initiation into solder joints. It operates in the temperature range between T = [20...125]°C. High-resolution force adjustment and displacement measurement enables a steady state strain rate measurement range of [10-2...10-8] 1/sec. Industrial demands for introducing the new SnAgCu base solders required a concentration on various high Sn-based alloys.

Extraction of material parameters for creep experiments on real solder-joints by FE analysis

A modern approach to determine material data of solder alloys such as SnAg and SnAgCu is to measure the mechanical behaviour direct on a CSP/BGA solder connection. Advantages of that technique of measurement on industrial manufactured solder bumps are the considerations of miniaturized volumes and the material diffusion from connection pad into the alloy during reflow soldering process. Compared to the tensile test the shear experiment differs in the way of initiation the force load into the solder alloy. The shear force load inducts a multiaxial state of stress. This is the reason for the confrontation with a higher effort into the conversion procedure to determine specific coefficients for the material law. In several publications creep data were published based on shear force load measurements and applied cylinder model to convert primary data into equivalent values. In practice, the specimen bumps may have been different in their shape, depending on pad geometry, solder volume and weight of electronic component. How does the shape of solder joints influence the creep behaviour? A form parameter has been introduced to be able to describe a wide range of solder bump shapes. Every bump shape from barrel to hyperbolic can now be regarded. The form parameter also takes place in the conversion of experimental data into equivalent data. The determined creep material laws, based on the improved analytic model, describe the deformation behaviour of solder joints more accurately, than the commonly assigned creep laws using the pure cylinder model. The shape effect is shown on a FEM analysis of the experimental setup of creep measurements on shape varied Sn96.5Ag3.5 solder bumps. In general, during FEM based material modelling the coefficients of the material laws need to be stepwise changed until the right behaviour occurs. These iterations can stretch over a long time. The improved analytical model shows the potential to shorten the coefficient determination of material laws