Karsten Meier

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

Life time prediction for lead-free solder joints under vibration loads

In this work the life time of solder joints of SMD components is studied under vibration loading. This kind of purely mechanical load is one of the main failure causes in automotive electronics. A test vehicle has been designed to enable vibration testing on SMD capacitors (size 0805). The test vehicle is distinguished with a line clamping and stripe shape which both leads to a concerted loading of the component solder joints. The test vehicles were analysed for their resonant frequency prior test and loaded with a sinusoidal vibration close to the resonant frequency for certain numbers of cycles. The loading of the individual components was measured using a vibrometer. Components were stressed with wave amplitudes from 0.3 mm up to 0.8 mm and normalised cycle counts from 1 up to 12. All tests were accomplished at room temperature. Cross sectioning was performed to evaluate damage location within and cracking of the solder joints. The observed damage location clearly differs to results from temperature cycling tests. Cracking of the intermetallic interface was not observed. Cracks propagated in the solder bulk. This enables a solder fatigue model since no interface effect is involved in the damage process.

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.

Characterization methods for determination of temperature depended electrical, thermal, mechanical and fatigue properties of SnAg3.5 solder

The application of solder joints for electrical and mechanical interconnections between two functional layers in electronic packages will be present in future devices, even in 3D-electronic devices. Very different approaches are investigated such as copper pillar, full inter-metallic joints or Au-Sn nanosponge. But the interconnection realization by μ-solder bumps as well as Flip-Chip solder joints will be one high potential connection method. The fundamentals about the solder alloy properties are necessary for material selection at product planning phase and further for the reliability estimations. Modern simulation tools are used for weak point estimations as well as for lifetime estimations (e.g. FEM). Any simulation software demands martial properties. The paper presents an excerpt of characterization techniques and results for the following physical data of SnAg3.5 solder: • Elastic modulus • Poisson ratio • Creep • Lin. thermal expansion (CTE) • Thermal conductivity • Electrical conductivity • Fatigue.

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

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.

Creep measurements on SnAgCu solder joints in different compositions and after mechanical and thermal treatment

Solder joints are still the main conductor element between electronic components and carrier substrates. The visco-plastic behaviour of the particle strengthened SnAgCu solder alloys depends strongly on its composition. Additionally, the mechanical behaviour is influenced by solder condition after the soldering reflow process. Further influencing factors are dissolution of metal from the connection pad area, cooling rate, volume, thermal or mechanical load. An in-house developed creep measurement unit allows the observation of real package solder joints in sizes Oslash = 200hellip400 mum within a temperature range of T = 20hellip125degC. The resulting creep measurements were fitted by mathematical description, either by sinh- or by power-law. Finite-Element-Method calculations were used to determinate the right material coefficients (activation energy, stress exponent, etc.) and complete data sets are provided related to solder alloys under certain micro-structural conditions. The paper presents creep measurement results of SnAgCu-Alloys to determine effects on solder behaviour influenced by: different silver content mAg = 0hellip3,5 %, different pad metallization (Cu, Sn, Ni, Ag), supply of solder (paste, ball, preform) and time above liquidus- temperature during reflow soldering (tL = 45 s/ 180 s). Further, the paper describes measurement results of isotherm mechanically and thermally aged solder joints. Thermal aging causes changes in the microstructure of solder joints. The measurements show the effect of thermal aging over time ranges of t= 168 h/500 h at temperatures T = 150degC. Mechanical load leads to changes in the microstructure. In situ observations of material behaviour were done by creep measurements over isothermal creep cycles up to 100 (epsivpiast = max. 1,5 %).

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

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.

Characterisation of the mechanical behaviour of SAC solder 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.

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

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.

Nanowire filled polymer films for 3D system integration

In the present paper, the use of high aspect ratio metallic nanowires (NWs) as functional interconnects between three-dimensionally stacked chips is proposed. First practical preparation steps, as there are the preparation of templates and the deposition of Ag-NWs, are presented. Later on, the applicational technology for vertically aligned NWs is proposed as their embedment into a polymer matrix resulting in an anisotropically conductive composite film. This is discussed in both technological and functional aspects.