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Manufacturing Processes > Joining processes > Soldering

 

Soldering

 

Process description

Heat is applied to the parts to be joined which melts a manually fed or pre-placed filler solder metal (which has a melting temperature < 450°C) into the joint by capillary action. A flux is usually applied to facilitate ‘wetting’ of the joint, prevent oxidation, remove oxides and reduce fuming.

Materials

  • Most metals and combination of metals can be soldered with the correct selection of filler metal, heating process and flux. Commonly, copper, tin, mild and low alloy steels, nickel and precious metals are soldered. Some ceramics can be soldered.
  • Magnesium, titanium, cast iron and high carbon or alloy steels are not recommended.
Soldering

Process variations

  • Gas soldering: air-fuel flame is used to heat the parts. Can be manually performed with a torch (TS) for small production runs or automated (ATS) with a fixed burner for greater economy.
  • Furnace Soldering (FS): uniform heating takes place in an inert atmosphere or vacuum.
  • Induction Soldering (IS): components are placed in a magnetic field surrounding an inductor carrying a high frequency current giving uniform heating.
  • Resistance Soldering (RS): high electric resistance at joint surfaces causes heating for brazing. Not recommended for brazing dissimilar metals.
  • Dip Soldering (DS): assemblies immersed to a certain depth in bath of molten solder. Can require extensive jigging and fixtures.
  • Wave Soldering (WS): similar to dip soldering, but the solder is raised to the joint area on a wave. Used extensively for soldering electronic components to printed circuit boards.
  • Contact or iron soldering (INS): uses an electrically heated iron or hot plate. Most common soldering process used for general electrical and sheet-steel work.
  • Infra Red Soldering (IRS): heat application through directed spot of infrared radiation. Used or small precision work and difficult to reach joints.
  • Laser beam soldering: provides very precise heat source for precision work, but at high cost.
  • Ultrasonic soldering: uses an ultrasonic probe to provide localized heating through high-frequency oscillations. Eliminates the need for a flux, but requires pre-tinning of surfaces.
  • Filler metal (solder) can be in preforms, wire, foil, coatings, slugs and pastes in a variety of metal alloys, commonly: tin-lead, tin-zinc, lead-silver, zinc-aluminum and cadmium-silver. The selection is based upon the metals to be soldered.
  • Flux types: either corrosive (rosin, muriatic acid, metal chlorides) or non-corrosive (aniline phosphate), in powder, pastes or liquid form.

Economic considerations

  • High production rates possible for WS.
  • Very flexible process.
  • Economical for very low production runs. Can be used for one-offs.
  • Tooling costs low. Little tooling required.
  • Equipment costs vary depending on degree of automation.
  • Direct labor costs low to moderate. Cost of joint preparation can be high.
  • Finishing costs moderate. Cleaning of the parts to remove corrosive flux residues is critical.

Typical applications

  • Electrical connections
  • Printed-circuit boards
  • Light sheet-metal fabrication
  • Pipes and plumbing
  • Automobile radiators
  • Precision joining
  • Jewelery
  • Food handling equipment

Design aspects

  • Design complexity high, but low load capacity joints.
  • Most common joint the lap with large contact areas or a combination of lap and fillet. Fillet joints predominantly used in electrical connections.
  • Can be used to provide electrical or thermal conductivity or provide pressure tight joints.
  • Joints should be designed to operate in shear and not tension. Additional mechanical fastening is recommended on highly stressed joints.
  • Joints should be designed to give a clearance between the mating parts of 0.08–0.15 mm.
  • Joint strength directly affected by clearance. If the clearance is too great the joint will loose a considerable amount of strength.
  • Tolerances on mating parts should maintain the joint clearances recommended.
  • On lap joints the length of lap should be between three and four times that of the thinnest part for optimum strength.
  • Parts in the assembly should be arranged to promote capillary action by gravity.
  • Machine marks should be in line with the flow of solder.
  • Design joints using minimum amount of solder.
  • Jigs and fixtures should be used only on parts where self-locating mechanisms, i.e. seaming, staking, knurls, bending or punch marks not practical.
  • If jigs and fixtures used they should support the joint as far from the joint as possible, have minimum contact with the parts to be soldered and have low thermal mass.
  • Soldered joints in electronic printed circuit boards should be spaced more than 0.8mm apart.
  • Provision for the escape of gases and vapors in the design important with vent-holes.
  • Minimum sheet thickness =0.1 mm.
  • Maximum thickness, commonly =6 mm.
  • Unequal thicknesses possible but may create unequal joint expansion.
  • Dissimilar metals can cause thermal stresses at the joint on cooling due to different expansion coefficients.

Quality issues

  • Virtually stress and distortion free joints can be produced.
  • Solderability improved by coating metals with tin.
  • Coatings should be used on parts to protect the parent metal prior to soldering, classed as: protective, fusible, soluble, non-soluble and stop-off coatings.
  • Control of the time and temperature of the applied heat important.
  • Contamination free environment important for electronics soldering.
  • Subsequent operations should have a lower processing temperature than the solder melting temperature.
  • Heat sinks should be used when soldering heat-sensitive components, especially in electronics manufacture.
  • Jigs and fixtures should be used to maintain joint location during solder cooling for delicate assemblies.
  • Choice of solder important in order to avoid possibility of galvanic corrosion.
  • Surface preparation important to remove any contaminates from the joint area such as oxide layers, paint and thick films of grease and oil and promote wetting. Degreasing and pickling of the parts to be soldered is recommended.
  • Smooth surfaces preferred to rough ones. Abrading the joint area using emery cloth is acceptable.
  • Correct clearance, temperature gradients and use of effective use of gravity promote flow of solder metal through capillary action.
  • Flux residues after the joint has been made must be removed to avoid corrosion.
  • Surface finish of soldered joints excellent.
  • Fabrication tolerances a function of the accuracy of the component parts and the assembly/jigging method.

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