PROGRAMUL CEEX CONTRBUTII STIINTIFICE IN TRANSPORTURI UCP AMTRANS - NOIEMBRIE 2007 159 DFM SOLUTIONS FOR TOMBSTONING IN VAPOUR PHASE SOLDERING TECHNOLOGY BASED ON THE RESEARCH PROJECT Contract CEEX-X2C09 I. Plotog*, G. Varzaru**, C. Turcu**, T. C. Cucu*, N. D. Codreanu* *Center for Technological Electronics and Interconnection Techniques (UPB-CETTI), POLITEHNICA University of Bucharest, Spl. Independentei 313, 060042-Bucharest, Romania, Phone: +40 21 3169633 E-mail: norocel.codreanu@cetti.ro ; ** Intrarom S.A, 17 Fabrica de Glucoza St., 020331-Bucharest, Romania, Phone: +40 21 2040702 Abstract The physics and chemistry of lead-free reflow soldering process phenomena’s revealed the inter- dependency between the solder paste, the component, the printed circuit board characteristics and reflow parameters, especially temperature versus time oven profile, going from the ambient value, approximately 25ºC, to over melting points, in order to assure liquid status of the solder paste. The paper presents the applied research performed in order to increase the quality of Lead-Free (LF) reflow/double-reflow VPS, by ruling out especially tombstoning. Key words: VPS, vapour phase technology, lead-free alloy, tombstoning, SMT soldering process 1. INTRODUCTION The goal of this paper is to determine the general requirements for PCBs and stencils design when using the „Vapour Phase Soldering” (VPS) technology in SMT reflow/double reflow process and to optimize VPS process parameters. The first experiments had shown an unacceptably large number of a specific defect, tombstoning (fig. 1), the partial or complete lifting of passive SMT components during reflow, which is the phenomenon whereby small chip components, which are being soldered on SMT pads at opposite or polar ends, suddenly flip up vertically; sometimes the lifting is partial, and sometimes the chips are staying on one end like tombstones in a cemetery, on one of the pads. a. b. Fig. 1a, b. Tombstoning after VPS 2. PHENOMENON ANALYSIS The tombstoning phenomenon is generally determined by an imbalance of the forces which take action over the composition when the solder paste laid-down in the form of paste is in a liquid state. An analysis of the forces (Fig. 2) shows that the surface tension of the solder will produce a turning moment. The condition under which the turning moment will be large enough to lift a component is: F1 + F2 < F3 (1) Where, F1 = vertical component of chip weight; F2 = vertical component of surface tension force acting under the chip; F3 = vertical component of surface tension force acting on the top of chip.