Updated Thursday, 8th March 2018
The workpiece material is not melted as in welding, but metal parts are joined using filler metals that have a melting point below 300°C. The molten filler metal is drawn into the weld gap by capillary attraction, eventually solidifying to form the bond. There are various ways to apply the heat.

Principle of soldering and action of suitable flux

  1. Mating surfaces are degreased and pickled before soldering.
  2. Components are assembled or jigged as necessary.
  3. Surfaces to be soldered are coated with flux to dissolve any remaining oxide films, and to promote wettability.
  4. Copper soldering bit heats the solder and conveys solder to the workpiece. In mass production, various forms of heating are used, including "vapour-phase" soldering.

Images to demonstrate 'Soldering' - see article


  1. Surface preparation  degreasing using organic solvents and/or alkaline solutions. Abrasive cleaning and pickling to remove oxide films.
  2. Assembly/jigging (see Brazing L01) – where possible joints should be self-locating.
  3. Heating  mass soldering uses the molten solder as a heat source to melt the pre-coated flux. Direct coating uses a separate heat-conducting source: soldering iron, hot plate or hot-oil immersion. Non-contact methods use ovens, gas burners and induction heating. Comparison of heating methods:
    • Flame: low cost and output, highly versatile, hand-feed systems
    • Electrical resistance: medium cost and output, poor versatility, preforms required
    • Radio-frequency induction: high cost and output, preforms required
    • Dipping bath: medium cost, low output and versatility
    • Soldering iron: hand-feeding gives low cost and output, but high versatility
  4. Finishing – flux and residual solder are removed before inspection.


  1. Metals that do not readily oxidise (Sn, Cu, mild steel, Ni Au and Ag) are most easily soldered.
  2. Solders usually contain tin to promote surface wetting. Special solders with melting points up to 365˚C are available.
Grade % Tin % Lead % Antimony Melting range (˚C)
A 64 36 - 183 - 185
K 60 40 - 183 - 188
F 50 50 - 183 - 212
R 45 55 - 183 - 224
G 40 60 - 183 - 234
H 35 65 - 183 - 244
J 30 70 - 183 - 255
V 20 80 - 183 - 276
W 15 85 - 227 - 288
B 50 47 3 185 - 204
M 45 52.3 2.7 185 - 215
C 40 57.6 2.4 185 - 227
L 32 66.1 1.9 185 - 243
D 30 68.2 1.8 185 - 248
N 18 80.9 1.1 185 - 275
Uses BS 219 grades

General electrical soldering (by hand)

Printed-circuit mass-soldering

Electric-lamp bases

Electric cable conductors

Wiped joints on lead cable-sheath

K, F

A, K

V, W

H, J

L, D

General Engineering

General sheet-metal work (steel, copper, tinplate)

General sheet-metalwork (brass, galvanised sheet)

K, F, R, G, B, M, C

K, F


Heat-exchangers, automotive radiators, refrigerators

Auto-body patching and filling

G, J C



Capillary plumbing joints

Dip-soldering (non-electrical)

Coating (pre-tinning)

Food-can soldering

K, F, G

C, D, N

K, G

K, F, G


  1. Optimum gaps are 0.08–0.18 mm. Below 0.08 mm vapour may be trapped, over 0.18 mm no capillary action.
  2. Lap joints are essential.
  3. Additional mechanical strength is possible.
  4. Solder preforms (BS 1723) affect joint shape.
  5. Designs for soldering components onto printed circuit boards, depend on the strengths required.

Images to demonstrate 'Soldering' - see article

Images to demonstrate 'Soldering' - see article

See Also: Adhesive bonding, Brazing and Fasteners

This article is a part of Manupedia, a collection of information about some of the processes used to convert materials into useful objects.


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