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Reducing Dross and Solder Balls in Selective Soldering

By Bob Klenke, Technical Consultant, Nordson SELECT T

in oxide, commonly referred to as dross, is created when solder is heated to its molten state and exposed to oxygen during the wave

soldering or selective soldering process. Tin oxide has been found to distort the wave shape at the tip of selective solder nozzles, which disturbs the uni- form flow of solder and adversely affects contact with the PCB. Since oxygen is needed to form tin oxide, virtually all selective soldering systems have nitrogen continuously flowing around the sol- der nozzle to minimize the exposure of molten sol- der to oxygen.

Dross Formation Tin oxide has a box-like crystalline molecular

structure that tends to bond to itself and develop sheets as it forms from elemental tin in the presence of oxygen that can be minimized by removing oxy- gen. Sheets of tin oxide have been observed on selec- tive solder nozzles and are likened to “scaffolds,” as they build up from the bottom of the nozzle toward the tip. Allowing a free flow of any tin oxide that forms away from the nozzle tip is important. Any collection point will allow the tin oxide structure to form and work its way up the nozzle to the tip where it can restrict the flow of molten solder. Most solder alloys contain varying amounts of

tin. It is this tin that combines with oxygen to form dross, such as with tin-lead (SnPb) solder. Commonly used lead-free solder alloys including tin-silver-copper (SAC305) and tin-nickel-copper (SN100C) have a much higher tin content than tin- lead solder and generally produce greater amounts of tin oxide. In general, SN100C lead-free alloy has been

found to have better flow characteristics than SAC305 solder alloy. This is due to SAC305 having

higher surface tension, since SAC305 was initially formulated for surface mount soldering applications. Because of this, SAC305 is generally not the recom- mended solder alloy for selective soldering, due to its higher surface tension. In addition, SN100C has a small quantity of geranium added to the alloy, which raises the melting point and ensures proper solder flow. This germanium also appears to reduce bridg- ing, due to its lower surface tension.

cules to reach elemental tin causing further oxida- tion. Research further indicates this is a molecule- by-molecule process and the presence of any oxygen will react when in contact with molten tin at elevat- ed temperatures, quickly forming tin oxide. Oxygen mapping testing conducted by

Nordson SELECT has verified the presence of insignificant oxygen levels around the solder noz- zle as a function of the nitrogen cap height sur- rounding the solder nozzle. The oxygen levels immediately adjacent to the solder nozzle tip do not appear to be a dominant issue in dross reduc- tion providing they are within the desirable range. Oxygen analyzer measurements at accurate x

and y-axis positions surrounding the top of a selec- tive soldering nozzle indicate repeatable readings between 90 and 200 ppm. This is well below 500 ppm, which is considered an acceptable level for selective soldering since this nitrogen environment is out in the open rather than in a contained atmosphere.

Solder Ball Creation Solder balls sometimes appear on printed cir-

Nordson SELECT Cerno 103IL inline selective soldering system.

Oxygen Mapping and Nitrogen Flow Research shows that tin oxides form immedi-

ately in molten tin whenever oxygen molecules are present. Undisturbed dross on the top surface of molten solder protects the molten solder below from forming further tin oxide, unless the surface oxide layer is broken by agitation allowing oxygen mole-

cuit boards when the dome of molten solder moves between adjacent though-hole solder joints. This is caused by a “snapping” action as the solder nozzle moves along the pins commonly seen with multi- row connectors or similar type components. Solder balls should be minimized as much as possible to ensure defect-free results. Solder balls also form inside a diffuser or

riser chimney as surface tension energy is released by solder splashing from solder agitation or turbu- lence. These solder balls often adhere to colder sur- faces, such as the chimney or riser, as well as to

Continued on page 56

July, 2020

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