Key words: flip chip bumping, solder balling, WLCSP packaging, stencil printing, solder paste.">

SMTA International Conference Proceedings


Authors: Dionysios Manessis and Lars Böttcher,et al.
Company: Microperipheric Research Center, Technical University Berlin and Fraunhofer Institute for Reliability and Microintegration (IZM) Berlin
Date Published: 10/4/2009   Conference: SMTA International

Abstract: At R&D level, IZM has advanced stencil printing very close to its technological limits at pitches even down to 50µm. Innovative electroformed and laser-cut with nano-treatment stencils have been manufactured with an extreme thinness of 20µm for bumping wafers at Ultra fine pitches (UFP) of 100µm, 80µm and 60µm. Specifically, for 100µm pitch bumping, both type 7 (2-11µm) and type 6 (5-15µm) pastes of eutectic composition Sn63/Pb37 have been successfully employed. Bumping using 25 µm electroformed stencil thickness has yielded bump heights of 42.3±3.8µm and 43.6±3.5µm for type 7 and type 6 pastes, respectively. A newly prototype developed type 8 paste (2-8µm) has been used for the first time to bump chips with peripheral contacts at 80µm and 60µm pitch. Bumping at 80µm pitch with nano-treated laser-cut stencil has yielded bumps of 28µm in height. For bumping at 60µm pitch, a 20µm thick electroformed stencil was used with 35µmx80 µm oblong apertures. Printing at 60µm pitch has yielded very promising results and has proved the capability of electroformed technology to manufacture accurate and robust thin stencils. The bump height at 60µm pitch was measured to be 28 ±3 µm.

Paste-in-Resist technology has been developed as an alternative to stencils in order to overcome the manufacturing difficulties of making extremely small apertures. Paste is printed in resist apertures which have been opened by photolithographic processes. In this way, bumping has been demonstrated up to 50µm pitches. Complimentary to stencil printing processes, IZM has developed balling technologies up to 400µm pitch up to 8” wafers with a thickness of 150µm. Solder balling can be achieved either by “perform ball print” using conventional stencil printers with specially designed stencils or by "ball drop" techniques. Balling technologies have demonstrated the application of 300µm and 250µm Sn-Pb and Pb-free balls at respective area array pitches of 500µm and 400µm, the main I/O pitches for WL-CSP bumping.

Key words: flip chip bumping, solder balling, WLCSP packaging, stencil printing, solder paste.

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