Introduction
Solder is a metal alloy that contains several metals. The main metal content is tin. Since tin is no noble metal, it will oxidize when exposed to air. The rate and nature of this oxidation is depending on temperature and exposure time. Normally tin has a closed oxide layer that has about the same volume as the metal underneath. That means that the oxide layer will normally protect the underlying metal for further oxidation. This behaviour will however change when the solder becomes molten and will be pumped around.
Behaving of oxides during soldering
In a running machine the static oxide layer is mixed with air due to the turbulence that is created by the solder that flows out from the wave nozzle. This turbulence will affect three mechanisms. The first phenomena are that it will form new oxides due to the newly exposed unprotected solder surface areas that are created by the falling solder. Secondly 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 flow on top of the bath. The third phenomenon is that this turbulence will create a new type of tin oxide due to the intensivated contact with air (oxygen).This will transform the originally blue-grey-goldish primary oxide skin into a fine black powder. A further transformation of this oxide can take place at pump shafts if no nitrogen blanket is used. The continuous movement of the pump shaft will grind the fine black powdery oxide particles and so become finer more dusty particles. This fine dust will next react again with oxygen with an exothermal reaction. During this reaction the fine black powder oxide particles will start to glow as a result of the heat that is emitted during this further oxidation. The residue of this reaction is a fine yellowish powder. This is just a new type of tin oxide having a higher oxygen content. During this slow exothermal reaction heat will be emitted, however no flames will be present but just a glow will be seen. 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 to prevent this reaction by eliminating the oxygen that is responsible for this oxide formation. Parts of the fine black dross particles might be found in the pump chamber as well if maintenance is not done properly. If in such cases the pump is removed it can contain traces of that fine oxide dust that starts to glow spontaneously if these parts come in contact with air. This should in that case be managed as a normal ‘unavoidable' reaction and should cause no worries. Correct maintenance and the use of a nitrogen blanket around the pump shaft can however avoid these reactions. This will prevent the further oxidation of the black powder oxide particles