Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration IZM

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  • Publication
    Thermo-mechanical Design of Resilient Contact Systems for Wafer Level Packaging
    ( 2006)
    Dudek, R.
    ;
    Walter, H.
    ;
    Döring, R.
    ;
    Michel, B.
    ;
    Meyer, T.
    ;
    Zapf, J.
    ;
    Hedler, H.
    Wafer Level Packaging (WLP) technologies are cost effective packaging solutions which are used increasingly. Second level reliability, i.e. mainly the thermomechanical reliability during thermal cycling, is a major concern of WLP. To avoid excessive solder straining, solder balls have been replaced by resilient interconnects, which can adopt the main part of the thermal mismatch deformation. One solution combining an increased reliability on module level with advantages in processing and the capability of full wafer level test and burn-in is ELASTec® (ELASTec Elastic-bump on Silicon Technology), particularily developed for memory products. The new failure risks are mainly related to fatigue of the metallic redistribution layer (RDL). Parametric studies using finite element analyses (FEA) were performed to avoid excessive straining of the metal lines. A balance of metal straining and solder straining had to be achieved. Comparisons were made for different soft bump layouts and RDL patterns. Optimal solutions figured out by FEA were also investigated experimentally by thermal cycle tests. However, the thermo-mechanical characteristics like stress-strain behaviour and fatigue resistance of the metallic films are the most important parameters for reliability predictions. In particular, the elastic-plastic properties of thin metallic Cu and Ni films are shown to depend on features like film thickness, grain size and orientation, resulting in a thin film strength exceeding the bulk strength of the same metal by several hundred percent.
  • Publication
    Fatigue Life Prediction and Analysis of Wafer Level Packages with SnAgCu Solder Balls
    ( 2006)
    Dudek, R.
    ;
    Rzepka, S.
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    Dobritz, S.
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    Döring, R.
    ;
    Kreißig, K.
    ;
    Wiese, S.
    ;
    Michel, B.
  • Publication
    Thermo-mechanical reliability analysis on solder joints of ceramic components
    ( 2005)
    Dudek, R.
    ;
    Döring, R.
    ;
    Michel, B.
    ;
    Picault, A.
    ;
    Autissier, J.-F.
  • Publication
  • Publication
    Investigations on the reliability of lead-free CSP subjected to harsh environments
    ( 2004)
    Dudek, R.
    ;
    Döring, R.
    ;
    Michel, B.
    A study of the performance of Flip-Chip Chip-Scale Packages (FC-CSPs) with lead-free solder interconnects was undertaken. The parametric studies on the CSPs were performed considering a wide variation of geometric and material parameters. Two geometrical versions on organic interposer with different die sizes were investigated theoretically and experimentally by thermal cycling tests -40 °C to 150 °C. In the FE-analyses, several additional parameters were examined including BT-interposer thickness, standoff, perimeter vs. full array, and solder-mask defined vs. non-solder-mask defined (NSMD) balls. Underfilling of the CSPs was an additional option. In the finite element analyses (FEA) both SnAg and SnAgCu solders were considered. For the latter a newly developed combined primary-secondary creep law was applied in the calculations. Both the inelastic strain (creep strain) and dissipated strain energy density represent suitable indicators to evaluate cyclic damage. It is demonstrated that for a thermal test cycle both measures result in similar critical cycle numbers. The calculations show that the creep strain always concentrates at the interfaces of the balls to the package. Maximum straining typically occurs at the inner ball row. Major effects on ball fatigue life are shown to be standoff height, ball geometry on both sides non solder mask defined (NSMD), and a stiff underfill. It is also shown that the CSP reliability using a soft underfill with high CTE or a similar "avarage soft underfill layer", composed of the soldermask layers and the underfill itself, can be worse than for a non-underfilled CSP. Testing results are compared to theoretical predictions. In many cases they agree reasonably well. Finally, differences between simulation and testing results are discussed.
  • Publication
    Thermal fatigue modelling for SnAgCu and SnPb solder joints
    ( 2004)
    Dudek, R.
    ;
    Walter, H.
    ;
    Döring, R.
    ;
    Michel, B.
    Thermal fatigue of solder joints is investigated by means of the finite element method (FEM). During the usual thermal cycle regime with relatively slow temperature ramping rates, solder constitutive response is dominated by secondary creep. In the paper secondary creep laws are given for SnPb and SnAgCu solders, based on our own measurements and literature data. These secondary creep behaviours are compared to other data recently published. Additional primary creep terms are introduced in the creep description. To relate calculation results to fatigue life, criteria of Manson-Coffin type are proposed. These empirical laws are based on either the cyclic inelastic strain or dissipated energy. Effects of the choice of different creep laws on the calculated creep strains and energy densities are studied for two standard components on FR-4 board, a ceramic chip resistor (CC) of size 0805 and a plastic ball grid array with 225 I/O (PBGA 225). It is shown that the choice of the creep law, e.g. the inclusion of primary creep, does significantly affect only the results for the PBGA solder balls.
  • Publication
    FE-analysis of leaded and lead-free solder joints under thermal fatigue loading
    ( 2004)
    Dudek, R.
    ;
    Walter, H.
    ;
    Döring, R.
    ;
    Michel, B.
  • Publication
    Reliability prediction of area array solder joints
    ( 2003)
    Dudek, R.
    ;
    Döring, R.
    ;
    Michel, B.
    Packages for high pin counts using the ball grid array technology or its miniaturized version, the chip scale package, inherently require reliability concepts as an integral part of their development. This is especially true for the latter packages, if they are combined with the flip chip technology. Accordingly, thermal fatigue of the solder balls is frequently investigated by means of the finite element method. Various modeling assumptions and simplifications are common to restrict the calculation effort. Some of them, like geometric modeling assumptions, assumptions concerning the homogeneity of the cyclic temperature fields, simplified creep characterization of solder, and the related application of Manson-Coffin failure criteria, are discussed in the paper. The packages chosen for detailed analyses are a PBGA 272 and a FC-CSP 230.
  • Publication
    Thermo-mechanical reliability of lead-free solder interconnects
    ( 2002)
    Schubert, A.
    ;
    Dudek, R.
    ;
    Döring, R.
    ;
    Walter, H.
    ;
    Auerswald, E.
    ;
    Gollhardt, A.
    ;
    Michel, B.
    Lead-free solder for electronic assemblies and systems is fast becoming a reality primarily because of market driven forces. While the industry has identified possible alternatives to SnPb solder, much work still needs to be done, especially in the following areas: solder materials characterization (temperature and stress dependent inelastic behavior creep and stress relaxation, bulk versus joint behavior), failure mechanisms related to the solder joints of the new alloys (will creep deformation still play a dominant role for e.g. thermally induced low cycle fatigue?), temperature cycle data, for instance, on real components (acceleration factors may depend on accelerated test conditions and solder alloys, and field conditions may be much more benign than accelerated test conditions), life prediction models (models have to incorporate time and temperature dependent behavior of solders, implementation of constitutive equation in FEA software is one of the keys, isothermal fatigue data should not be useful for life prediction model development), solder-surface interactions (solder reacts with metallization to form interfacial intermetallics, intermetallics grow with time and temperature, metallization consumption by intermetallic growth, intermetallics within the solder, thermo-mechanical properties of the intermetallics), and assembly process development. The impact of some of these issues on the reliability assessment of lead-free solder interconnects through experiments and FE calculations is discussed in the paper, shown on examples like SMD-components and Flip-Chips.