Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration IZM

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  • Publication
    Quality and yield of ultra fine pitch stencil printing for flip chip assembly
    ( 1998)
    Heinricht, K.
    ;
    Klöser, J.
    ;
    Lauter, L.
    ;
    Ostmann, A.
    ;
    Reichl, H.
    ;
    Wolter, A.
    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 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) with regard to quality and yield. A software tool for design of the stencil layout was developed and the predicted bump heights were compared to the experimental results. In the first part of the paper, a low cost bumping method for flip chip technology is described in detail. This technology is based on chemical Ni/Au deposition on wafers. For solder bumping on wafers, the key aspects of solder paste printing with optimized printable apertures are described and the printing results are presented. The second part of this paper is a comparison of measured standard deviations of bump heights and the quality demands for ultra fine pitch flip chip assembly.
  • 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.
    ;
    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
    Reliability investigations of Sn/Pb and lead free solders for flip chip technology
    ( 1998)
    Klöser, J.
    ;
    Jung, E.
    ;
    Heinricht, K.
    ;
    Kutzner, K.
    ;
    Ostmann, A.
    ;
    Reichl, H.
    This paper presents a flip chip technique based on electroless Ni-Au bumping and stencil printing of solder paste on wafers. The process steps and key aspects are described in detail. Experimental results for an ultra fine pitch printing technique on wafers are shown and reflowed solder bumps are characterized in terms of uniformity and strength. In comparison to eutectic SnPb solder, five alloys (BiSn, SnBiCu, SnAg, SnCu, AuSn) are investigated. In the second part of the paper, a comparison of solder alloy properties for applicability to flip chip technology, microstructure evaluation and phase compositions are presented. Microstructure coarsening and phase growth after thermal aging are also investigated. In order to assess the effect of substrate CTE on reliability, flip chip assembly was performed on LTTC and FR-4 substrates. Flip chip joint quality was investigated by metallurgical cross sections and electrical and mechanical measurements. Finally, joint reliability results after thermal cycling with and without underfill on both substrate types are presented.
  • Publication
    The influence of NiSn intermetallics on the performance of flip chip contacts using a low cost electroless nickel bumping approach
    ( 1997)
    Jung, E.
    ;
    Kasulke, P.
    ;
    Giebler, R.
    ;
    Klöser, J.
    ;
    Dietrich, L.
    ;
    Ostmann, A.
    ;
    Zakel, E.
    ;
    Reichl, H.
    A low cost approach to obtain bumped dice using electroless nickel under bump metallization and different solder bumping techniques was realized in this work. This paper demonstrated the use of electroless Ni(P)-Au UBM for flip chip interconnections with solder applied by either stencil printing, solder ball placement, or mechanical solder bumping for an automotive application. In order to characterize the performance and reliability of such flip chip interconnections, the influence of the growth of NiSn intermetallics during thermal aging at 150 degrees C on their electrical performance was evaluated in this work. In this respect, two near-eutectic lead-tin alloys with different dopants were used. The results obtained are compared with those of conventional Ti:W-Cu UBM with electroplated eutectic solder bumps. Destructive mechanical test results are presented and compared for both under bump metallizations. This study indicated the suitability of electroless Ni(P)-Au UBM for flip chips even for harsh environments, stimulating the prototyping of a four chip MCM-L for automotive use, which is presented as a case study.
  • Publication
    The influence of NiSn intermetallics on the performance of flip chip contacts using a low cost electroless nickel bumping approach
    ( 1996)
    Jung, E.
    ;
    Giebler, R.
    ;
    Klöser, J.
    ;
    Dietrich, L.
    ;
    Zakel, E.
    ;
    Reichl, H.
    ;
    Kasulke, P.
    ;
    Ostmann, A.
  • Publication
    Flip chip attachment using anisotropic conductive adhesives and electroless nickel bumps
    ( 1996)
    Aschenbrenner, R.
    ;
    Ostmann, A.
    ;
    Motulla, G.
    ;
    Zakel, E.
    ;
    Reichl, H.
    Flip chip attachments provide the highest interconnection density possible, which makes this technology very attractive for use with liquid crystal display (LCD) packaging methods. This technology stimulated the development of new interconnection techniques, such as anisotropic adhesives. However, several factors have hindered the wide use of this technology. These factors include the availability and costs of bumped wafers. IZM and TU-Berlin have addressed both of these concerns by establishing a wafer-bumping facility which uses electroless nickel bumps. The combination of anisotropic adhesives and electroless nickel bumps has the potential for a low-cost chip on glass (COG) and chip on flex (COF) bonding technology. In this paper, two types of anisotropic adhesives were studied with an emphasis on the properties of COG and COF interconnections. The electrical and mechanical performance of the adhesive bonds was studied by evaluating initial contact resistance and mechanical adhesion as a function of temperature and humidity.
  • 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.