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to Metal joining technologies (welding, brazing, soldering)
Dr. Dmitri Kopeliovich
Soldering is a method of joining two metal work pieces by means of a third metal (solder) at a relatively low temperature, which is above the melting point of the solder but below the melting point of either of the materials being joined.
Flow of the molten solder into the gap between the work pieces is driven by the capillary force. The solder cools down and solidifies forming a joint. The parent materials are not fused in the process.
Soldering is similar to Brazing. The difference is in the melting point of the filler alloy: solders melt at temperatures below 840°F (450°C); brazing filler materials melt at temperatures above this point.
The difference between soldering and welding processes is more sufficient: in the welding processes edges of the work pieces are either fused (with or without a filler metal) or pressed to each other without any filler material; soldering joins two parts without melting them but through a soft low melting point solder.
Soldering joints have relatively low tensile strength of about 10000 psi (70 MPa).
Capillary effect (wettability) is achieved by both: a proper Surface preparation and use of a flux for wetting and cleaning the surfaces to be bonded.
Contaminants to be removed from the part surface are: mineral oils, miscellaneous organic soils, polishing and buffing compounds, miscellaneous solid particles, oxides, scale, smut, rust.
The work pieces are cleaned by means of mechanical methods, soaking cleaning and chemical cleaning (acid etching).
A soldering flux has a melting point below the melting point of the solder, it melts during the preheating stage and spreads over the joint area, wetting it and protecting the surface from oxidation. It also cleans the surface, dissolving the metal oxides.
It is important that the surface tension of the flux is: 1. Low enough for wetting the work piece surface; 2. Higher than the surface tension of the molten solder in order to provide displacement of the flux by the fused solder. The latter eliminates the flux entrapment in the joint.
The flux is applied onto the metal surface by brushing, dipping, spraying, in form of a gas-flux foam or by a flux wave (flowing flux forms a wave and the printed circuit board moves over the apex of the wave).
Flux is acidic therefore its residuals may cause corrosion if not removed.
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Traditional lead containing solders consist of tin (Sn) and lead (Pb).
The most popular alloy in this group is eutectic composition 63Sn-37Pb (commonly called 63/37). The melting point of this alloy is lowest of all Sn-Pb alloys: 361°F (183°C). This solder is used for joining electronic components, to which minimum heat may be applied (computers, telecommunication devices). The 63/37 alloy may be modified by addition of 1.4% of silver (Ag) for improvement of the joint Creep resistance.
Low tin solders such as 5Sn-95Pb (5/95), 10Sn-90Pb (10/90), 15Sn-85Pb (15/85) are used mainly for sealing containers and radiators, joining and coating metal parts working at increased temperatures (above 250°F/121°C).
The alloy 70Sn-30Pb (70/30) is used for coating parts before soldering.
The advantages of tin-lead alloys:
The main disadvantage of these alloys is toxicity of lead.
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Most of lead-free solders are tin base alloys: 96.5Sn-3Ag-0.5Cu, 99.3Ag-0.7Cu, 95Sn-5Sb.
The alloy 96.5Sn-3Ag-0.5Cu has a composition very close to the eutectic. Its melting point is 423°F (217°C). Fatigue strength of the alloy is similar to that of SnPb solders, however its wettability is poorer. Addition of 1-3% of bismuth (Bi) to the alloy improves its wettability and decreases the melting point but the fatigue resistance deteriorates. The alloy is now used for wave soldering, reflow and hand soldering.
The alloy 99.3Ag-0.7Cu with the melting point 441°F (227°C) is low a cost alternative of the silver containing alloy. It is used for wave soldering.
When a low melting point is required, the alloy 42Sn-58Bi is used. Its melting point is 280°F (138°C). Fatigue strength, tensile strength and ductility of the alloy are relatively low but may be improved by some addition of silver (Ag).
The melting point of the alloy 95Sn-5Sb is 450°F (232°C). The solder is used in the plumbing works.
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Iron soldering utilizes a heat generated by a soldering iron.
Torch soldering utilizes a heat of the flame from a torch. The torch mixes a fuel gas with oxygen or air in the proper ratio and flow rate, providing combustion process at a required temperature.
The torch flame is directed to the work pieces with a flux applied on their surfaces. When the work pieces are heated to a required temperature, solder is fed into the joint region. The solder melts and flows to the gap between the joined parts.
Hand soldering is used in repair works and for low volume production.
The method uses a tank full with a molten solder. The solder is pumped, and its flow forms awave of a predetermined height. The printed circuit boards pass over the wave touching it with their lower sides.
The method is used for soldering through-hole components on printed circuit boards.
In this method a solder paste (a mix of solder and flux particles) is applied onto the surface of the parts to be joined and then are heated to a temperature above the melting point of the solder. The process is conducted in a continuous furnace, having different zones: preheating, soaking, reflow and cooling. The joint forms when the solder cools down and solidifies in the cooling zone of the furnace.
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