Soldering is a process by which two metals are joined by using heat to melt the filler material (solder) at a relatively low temperature that is above melting point of the solder and below the melting point of materials being joined. In soldering operations, parent materials are not fused as explained earlier. Drops of molten solder to its required area between the work pieces is done by capillary force which cools down, solidifies and then produces a joint. Soldering is slightly different from brazing. Their difference is in melting point of the filler alloy. Solders melt at 850°f (450°c) while brazing filler melt above this point.

Comparing soldering and welding processes their difference is more sufficient. Soldering is a light joint that joins to parts without melting them using a solder that has low melting point. Soldering joints has relatively low tensile strength of 10000 psi (70 MPa) but in welding processes parent metals are melt and edges of work pieces are either fused when or without filler metals are included.

At the end of this article, reader are expected to known the followings listed below:

  • Basic concept on soldering
  • Soldering methods
  • Surface treating and soldering fluxes
  • Lead-free solders
  • Tin-lead solders
  • Advantages and Disadvantages of soldering

Soldering methods

Wave soldering

Wave soldering method are used for soldering through-hole parts on a printed circuit boards. It uses a tank brim with a molten solder which is pumped and allows to flow till it forms a wave of a predetermined height. The printed circuit boards pass besides the wave touching each other at lower edges.

Wave soldering process

Reflow soldering

Reflow soldering method is obtained by a mix of solder and a flux particles. This material is placed to the areas needed to be joined and then heated to a temperature above 850°f. This process is conducted in a continuous furnace with a different sections, such as, preheating, reflow, soaking and cooling. The joint is formed when the solder is cool and solidifies in a cooling section of the furnace.

Reflow soldering process

Hand soldering

Hand soldering are used in repairing of works and for low volume production. A iron soldering exploit heat generated by a soldering iron while a torch soldering utilizes a heat of the flames from a torch. It mixes a fuel gas with oxygen or air in proper ratio and flow rate which provide combustion at a required temperature. The torch flame is placed close to the work pieces with a flux applied on its surfaces. When the work piece sense the heat at a specific temperature, solder is then placed to it to obtain a joint when cooled.

Hand Soldering Process

Surface cleaning and soldering fluxes

Before solder is applied on a work piece it is highly advisable to clean its surfaces to obtained a perfect joints. Contaminant on the surfaces to be remove may includes organic soils, polishing and buffing compounds, mineral oils, solid particles, scale, rust, oxides and etc. Capillary effect is accomplished by a proper surface preparation and use of flux for cleaning and wetting the surfaces that requires soldering. Work piece are cleaned by soaking cleaning, chemical cleaning and mechanical methods.

The primary function of a soldering flux is to prevent the surface from oxidizing and to cleans the surface , it also dissolved the metal oxides. Melting point of a soldering flux is beneath the melting point of solder. Soldering flux melt during the preheating and spread all-around the joint area. The surface tension of soldering flux should be low enough for wetting work piece surface and to provide a displacement of flux by the fused solder. Soldering flux is applied on work piece by several ways like, brushing, dipping, and spraying (mostly gas flux.) it is advisable to clean soldering flux away after work because it is acidic and may cause corrosion if not removed.

Soldering Flux

Lead-free solder

Lead-free solder is a tin base alloys of type 96.5Sn-3Ag-0.5Xu, 99.3Ag-0.7Cu and 95Sn-5Sb. The composition of alloy 96.5Sn-3Ag-0.5Cu is very close to eutectic. Its melting point is 423°f (217°c) and its fatigue strength is similar to snpb solder but poor in wettability but when 1-3% of bismuth (Bi) is added to alloy, it improves wettability and reduces its melting point but impairs its fatigue resistance. At this stage, alloy can now be used for hand soldering, wave soldering and reflow. Alloy 99.3Ag-0.7Cu is used for wave soldering. It has melting point of 441°f (227°C ). It is a low cost alternative of silver containing alloy. Alloy 42Sn-58Bi is used for work when a low melting point is required. It has melting point of 280°f (138°c). This alloy also have low ductility, fatigue strength and tensile strength but may increase if some silver (Ag) is added. The least explained is alloy 95Sn-5Sb. It is used when it comes to soldering on plumbing works. It has melting point of 450°f (232°c). The following alloy explained above are Lead-free solder used and suitable for specific works.

Tin-lead solder

Tin-lead solder is used for joining electronic components like computers, telecommunication, television, radio and etc. that requires minimum heat when soldering. Tin-lead is the most popular alloy made of eutectic composition 63Sn-37Pb also known as 63/37. It has the lowest melting point of all Sn-Pb alloys with melting point of 361°f (183°c). Solder such as 5Sn-95Pb (5/95), 15Sn-85Pb(15/85) and 10Sn-90Pb (10/90) are also low tin solders, they are used for sealing radiator and containers, joining and coating metal parts. The alloy 70Sn-30Pb (70/30) is used for coating parts before soldering.

Advantages of tin-lead alloys:

  • Less expensive
  • Simple equipments like soldering iron and torch can be used
  • Low melting point
  • Low skill person can use it

Advantages of soldering

  • Low power is required
  • Low process temperature
  • No thermal distortions and residual stresses in the joint parts
  • Microstructure is not affected by heat
  • Easily automated process
  • Dissimilar materials may be joined
  • High variety of materials may be joined
  • Thin wall parts may be joined
  • Moderate skill of the operator is required

Disadvantages of soldering

  • Careful removal of the flux residuals is required in order to prevent corrosion.
  • Large sections cannot be joined.
  • Fluxes may contain toxic components.
  • Soldering joints can not be used in high temperature applications.
  • Low strength of joints.

I hope you’ve enjoyed reading this soldering article, just as promised we’ve clarified on basic concept of soldering, soldering methods and we also discussed on surface cleaning and soldering fluxes. Tin-lead and free-lead solder has also been examined. I guess you now understand soldering but don’t hesitate to ask questions or drop your experience via the comment box and please share this article on your social media.

if you really want to learn how to solder. just click the video embedded below;