Circuits Assembly - January 2008 - (Page 40)

Selective Soldering Selective Soldering with Pb-Free Alloys The new family of solders flows differently through the nozzles. I Ursula Marquez de Tino is a process and research engineer for Vitronics Soltec (vitronics.com); umarquez@ vitronics-soltec.com. n recent years, several Pb-free experiments joint’s thermal solderability is important, espeon selective soldering applications looked cially for through-hole penetration. Since small into characteristics of the new alloys. Just as nozzles are used, much heat must be transferred in wave soldering, SnAgCu and SnCu are the most through this small amount of solder into the popular alternative alloys for replacing SnPb. board, connector, pad and soldering area to make During different Pb-free builds, it became clear soldering possible. The flux has to support this that Pb-free alloys behave differently from what process and therefore must be able to withstand we have learned from SnPb processes. Compared temperatures of about 280˚C on the wave. In the to SnPb, for example, Pb-free alloys have differexperiment, all connectors soldered at 280˚C ent surface tensions and density, which results exhibited a good fillet solder joint. in different flow behavior of the solder through Pb-free solder temperatures. Wetting balance the nozzles. This flow should be in control; othtests have indicated minor differences in the erwise, this will result in solder defects such as soldering characteristics of Pb-free alloys; for bridging, or it may affect surface mount compoexample, SnCu exhibits poor solderability comnents located close to pared to SnPb at the the soldering area. same temperature. NevBridging. Independertheless, this experient experiments have ment also showed that demonstrated Pb-free a temperature of 280˚C solders tend to flow was high enough for backward, in the same soldering connectors direction the boards are with SnCuNi. Operatmoving. This may result ing with the temperain bridging (Figure 1). ture too high will affect This has been recognot only the performnized in three different ance of the flux, but Pb-free applications also may accelerate the using SN100 (SnCuwear of some parts that Ni), Sn3.0Ag0.5Cu and Figure 1. Pb-free solders tend to flow in the same direction come in contact with of the boards, resulting in bridging. Sn3.8Ag0.7Cu. solder that has a high Because of varying flow speed. If necessary, surface tension and density, the Pb-free solder’s these parts need special coatings. flow makes selective wave soldering more diffiIn conclusion, what have we learned? cult. Special measures must be taken to maintain • Pb-free selective soldering is viable, but control of the solder overflow. Nozzle technology requires special attention to the flow and will become more important, as it is with SnPb behavior of the solder. soldering. • Soldering can be successful only if there is To avoid bridging, other parameters may result enough thermal heat in the soldering area. in a more stable process, but these were not Small nozzles may experience problems with included in this experiment. As in all soldering this heat transfer; therefore, a nozzle diameter processes, the flux and its activity at the higher of at least 6 mm is needed. This may affect solder temperatures (as well as the nitrogen board layout. atmosphere) may provide improvements, since • High solder temperatures require strong fluxthese parameters influence oxidation and solder es. Material wear may increase if solder temsurface tension. peratures exceed 300˚C. Special coatings on Wetting. Wetting – i.e., through-hole penetracritical parts must be considered. tion of the Pb-free solder – is another critical • New nozzle technologies and material treatissue for Pb-free selective soldering. The solder ments are in development. ■ Circuits Assembly JANUARY 2008 circuitsassembly.com 40 http://www.vitronics-soltec.com http://www.vitronics-soltec.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - January 2008

Circuits Assembly - January 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business On the Forefront Screen Printing Better Manufacturing Walking on Water Flux Selection for Lead-Free Wave Soldering An A-to-Z Guide to X-Ray Inspection, Part II Evolution in Action Research Priorities for the Electronics Industry Selective Soldering Process Doctor Pb-Free Lessons Learned Materials World Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - January 2008

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