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Alternative solders for flip chip applications in the automotive environment

 

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Zakel, E. ; IEEE Components, Packaging, and Manufacturing Technology Society:
Electronics manufacturing and development for automotives. Twenty Second IEEE/CPMT International Electronics Manufacturing Technology Symposium
Piscataway, NJ: IEEE, 1998
ISBN: 0-7803-4520-7
ISBN: 0-7803-4521-5
ISBN: 0-7803-4522-3
S.82-91
International Electronics Manufacturing Technology Symposium (IEMT) <22, 1998, Berlin>
Englisch
Konferenzbeitrag
Fraunhofer IZM ()
ageing; automotive electronics; bismuth alloys; copper alloys; crystal microstructure; electroless deposition; encapsulation; environmental degradation; environmental factors; fatigue; fine-pitch technology; flip-chip devices; gold alloys; integrated circuit packaging; integrated circuit reliability; integrated circuit testing; mechanical strength; microassembling; reflow soldering; silver alloys; thermal stress; tin alloys; vibration

Abstract
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.

: http://publica.fraunhofer.de/dokumente/PX-3651.html