Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration IZM

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  • Publication
    Alternative solders for flip chip applications in the automotive environment
    ( 1998)
    Jung, E.
    ;
    Heinricht, K.
    ;
    Klöser, J.
    ;
    Aschenbrenner, R.
    ;
    Reichl, H.
    In addition to Pb toxicity, there are other problems with SnPb solders. In automotive applications, where solder joints are subjected to thermal cycles, severe vibrations, sustained temperatures up to 150 degrees C and peak temperatures of 180 degrees C, the critical failure mode of eutectic SnPb solder in assemblies is bump fatigue. For flip chip technology, induced thermal stresses and strains in solder joints are very hazardous. This paper presents a flip chip process based on electroless Ni/Au bumping and stencil printing of solder paste on wafers. Chemical nickel plating combined with solder printing is a very flexible and cost effective bumping method. The basic process steps and key aspects of this technology are described in detail. Experimental results for an ultra fine pitch printing technique on wafers are shown, and reflowed solder bumps are characterized for uniformity and strength. In comparison to eutectic SnPb, SnBiCu, SnAg, SnCu, and AuSn solder alloys are selected and investigated. The alloys are compared for flip chip technology applicability, microstructure and phase compositions are presented. Microstructure coarsening and phase growth after thermal aging are also investigated. In order to investigate substrate material CTE effects on reliability, flip chip assembly was performed on low temperature cofired ceramic (LTCC) and FR-4 substrates. The flip chip joint quality was investigated by metallurgical cross sections and electrical and mechanical measurements. Finally, the reliability results of these joints after thermal cycling with and without underfill on both types of substrate materials are presented.
  • Publication
    Low cost bumping by stencil printing. Process qualification for 200 mu m pitch
    ( 1998)
    Klöser, J.
    ;
    Heinricht, K.
    ;
    Jung, E.
    ;
    Lauter, L.
    ;
    Ostmann, A.
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    Aschenbrenner, R.
    ;
    Reichl, H.
    A key issue for the introduction of flip chip technology for automotive, telecommunication and consumer applications is the implementation of low cost bumping processes, since the established methods need expensive equipment for metal sputtering and photolithography. At present, there are several methods for creating bumps on the die. One new method that has the potential to be much less expensive than current technologies is stencil printing. In this paper, the stencil printing method for wafer solder bumping is described using electroless nickel as a layer between the IC bond pad and the solder. Stencil printing for SMT and fine pitch BGA structures is established as a low cost standard process. Using the same equipment with modified printing parameters and materials, a low cost wafer bumping process has been transferred to serial production. This paper presents the results of ultra fine pitch stencil printing of solder paste on wafers (down to 200 mu m and 150 mu m pitch) discussing quality and yield. A software tool for stencil layout design was developed and predicted bump heights are compared to experimental results. In the first part of the paper, the process flow of this economical bumping method for flip chip technology is described in detail. The key aspects of solder paste printing with optimized aperture size and shapes are outlined and the printing results are presented. In the second part of the paper, a comparison of measured standard deviations of bump heights and the quality demands for ultra fine pitch flip chip assembly are also shown.
  • Publication
    Implementation of flip chip technology into volume manufacturing demonstration of processes
    ( 1998)
    Jung, E.
    ;
    Heinricht, K.
    ;
    Kutzner, K.
    ;
    Klöser, J.
    ;
    Aschenbrenner, R.
    ;
    Reichl, H.
    ;
    Brommelhaus, A.
    Until now, use of flip chip technology has not been widespread in volume manufacturing, although it provides a number of significant advantages over standard surface mount devices. In particular, the cost aspect of using flip chip technology in a large scale manufacturing environment is expected to overcome the remaining issues. In order to illustrate the equipment and processes involved for incorporation of flip chip attachment into a production process, a demonstration center was established at FhG-IZM. Critical process steps were identified and addressed. This article gives a detailed insight into the actual implementation of a flip chip process into a SMD compatible production line, highlighting aspects of bump provision, cleaning, chip placement and underfilling. Bumping covers low cost processes such as electroless nickel as under-bump metallization (UBM) and stencil printed solder deposits using advanced printing technology. The necessity of cleaning and the key aspects for achievement of high reliability with industrial applied processes is highlighted. Required features of the placement machine and related critical issues are presented. Underfill application is also covered, focussing on important aspects for process optimization.
  • Publication
    Fluxless flip chip bonding on flexible substrates
    ( 1995)
    Zakel, E.
    ;
    Aschenbrenner, R.
    ;
    Gwiasda, J.
    ;
    Azdasht, G.
    ;
    Ostmann, A.
    ;
    Eldring, J.
    ;
    Reichl, H.
    ;
    Klöser, J.
    Flip chip (FC)-technology on flexible circuits is of increasing interest for application in consumer oriented products. In particular, fluxless processes are in demand for compatibility with underfill materials and for improved reliability performance. This paper presents two approaches to a fluxless process based on soldering techniques using Au-Sn metallurgy and on adhesive joining techniques using gold and nickel gold bumps. Soldering is performed with a thermode and with a laser based system. For these FC joining processes, alternative bump metallurgies based on electroplated gold, electroplated gold-tin, mechanical gold and electroless nickel-gold bumps are applied.