Circuits Assembly - December 2007 - (Page 36)

Wave Soldering The Glowing Dross Residues Phenomenon When black powdery oxides are present, a slow exothermal reaction takes place. older is a metal alloy comprised of several metals. The main metal content is tin. Because tin is not a noble metal, it will oxidize when exposed to air. The rate and nature of this oxidation depends on temperature and exposure time. Normally, tin has a closed oxide layer that has about the same volume as the underlying metal. That means the oxide layer will normally protect the underlying metal from further oxidation. This behavior will change, however, when the solder becomes molten and begins to be pumped. In a running machine, the static oxide layer mixes with air because of the turbulence created by solder that flows from the wave nozzle. This turbulence affects three mechanisms. The first phenomenon is that it will form new oxides due to the newly exposed unprotected solder surface areas created by the falling solder. Second, it will fold pure solder into oxide layer particles. This will create dross. This dross has a lower density than the solder, so it will normally float on top of the bath. This means the main part of the dross is below the bath surface. If this layer becomes too thick, dross particles will be picked up by the solder flow and can then be carried in the solder wave. (Regular maintenance, such as dross removal, can prevent this.) The third phenomenon is that this turbulence from the falling solder from the wave will create a new type of tin oxide due to the accelerated contact with air (oxygen). This will transform the originally blue-gray-gold primary oxide skin into a fine black powder (Figure 1). A further transformation of this oxide can take place at pump shafts in the absence of a nitrogen S blanket. The pump shaft’s continuous movement will grind the fine black powdery oxide particles into even finer particles – akin to dust. This fine dust will react again with oxygen in an exothermal reaction. During this reaction, the fine black powder oxide particles will begin to glow as a result of the heat emitted during this further oxidation. The residue of this reaction is a fine yellowish powder (Figure 2). This is just a new type of tin oxide having higher oxygen content. During this slow exothermal reaction, heat will be emitted; however, no flames will be present, just a glow. It is a normal phenomenon that can take place when the black powdery oxides are not regularly removed during maintenance. Normally, the pump shaft area can be covered with a nitrogen blanket, which prevents this reaction by eliminating the oxygen responsible for oxide formation. Parts of the fine black dross particles might be found in the pump chamber as well if proper maintenance is not performed. If, in such cases, the pump is removed, it can contain traces of that fine oxide dust that start to glow spontaneously if these parts come in contact with air. This should be managed as a normal, “unavoidable” reaction and is not cause for concern. Correct maintenance and use of a nitrogen blanket around the pump shaft can avoid these reactions. This will prevent the further oxidation of the black powder oxide particles. Today’s pump shafts are generally “sealed” with a nitrogen blanket that will prevent this oxide formation, but when this system is out of order, this exothermal oxidation reaction can be expected. ■ Ursula Marquez de Tino is a process and research engineer for Vitronics Soltec (vitronics.com); umarquez@ vitronics-soltec.com. Figure 1. Remaining solder residues surrounded by black powder oxide particles. Figure 2. Solder residues with a lot of yellowish oxide powder, the result of further oxidation of the black oxide powder. 36 Circuits Assembly DECEMBER 2007 circuitsassembly.com http://www.vitronics-soltec.com/ http://www.vitronics-soltec.com/ http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - December 2007

Circuits Assembly - December 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Global Sourcing Screen Printing Better Manufacturing Metalization Options for COB Assembly A Test Comparison of SAC and Non-SAC Pb-Free Solders An A-to-Z Guide to X-Ray Inspection BEST: A ‘Funky Chicken’ with an EMS Niche Tech Tips Wave Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Eastern Approaches Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - December 2007

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