— Soldering is a common process for joining steel, copper and other materials at a low temperature.
— Soldering is defined as a group of joining processes wherein coalescence is produced by heating to a suitable temperature and by using a filler metal having a liquidus not exceeding 800°F (427°C) and below the solidus of the base metals. The filler metal (i.e., the solder) is usually distributed between the properly fitted surfaces of the joint by capillary attraction.
Stages of soldering process
- Selection of base metal (s) to be soldered.
- Joint design for proper bond formation.
- Precleaning of workpieces to help wetting of base metal by the solder.
- Fluxing, in order to remove all traces of surface tarnish and to prevent formation of new surface film before and during the soldering operation.
- Selection solder (i.e., filler metal) to obtain the desired properties in the finished product.
- Heating method to provide proper soldering temperature, etc. The various heating methods are:
(i) Using a soldering iron.
(ii) Torch soldering.
(iii) Dip soldering.
(iv) Oven soldering.
(v) Spray gun soldering.
(vi) Resistance soldering, etc.
- Alignment of workpieces (to be soldered) so that movement does not take place during heating and cooling. Fixtures, etc., may be used for the purpose.
- Post cleaning of the soldered workpieces in order to remove all flux residues.SOLDERING ALLOYS (OR SOLDERS): They are1. Tin-lead solders— They constitute the largest portion of all the solders in use.
— Tin-lead solders are employed to join most metals.
— They possess good corrosion resistance to most media.
|and ends at 183°C|
2. Tin-antimony-lead solders
— Addition of antimony up to 60% of the tin content increase the mechanical properties of solder but with slight impairment of the soldering characteristics.
|Sn%||Sb%||Pb%||Solidification range,0C||Types and uses|
|31||2||67||235-188||Plumber’s solder for wiping joints|
|43.5||1.5||55||220-188||General purpose solder|
|12||8||80||250-243||For soldering iron and steel|
3. Tin zinc solders
— Tin-zinc solders are used for joining aluminium.
4. Lead-silver solders
— Addition of silver to lead results in an alloy which will more readily wet steel and copper. Flow characteristics however are very poor.
— Pb-Ag solders are susceptible to humid atmosphere corrosion in storage and may become unusable as solders.
— Addition of 1% tin increases their wetting and flow characteristics and in addition reduces their susceptibility to humid atmospheric corrosion.
|Pb%||Ag%||Sn%||Solidification range,0C||Types and uses|
|97.5||2.5||-||Solidifies at 3050C||For soldering copper and its alloys|
5. Cadmium-silver solders
— Cadmium-silver solders are used for joining aluminium to itself or to other metals.
Improper use of this solder however may lead to health hazards; this is because of cadmium content.
6. Cadmium-zinc solders
— They are used for soldering aluminium
7. Zinc-aluminium solders
— Zn-Al solders are used for joining aluminium.
— Zn-Al solder develops joints with high strength and good corrosion resistance.
— The heated aluminium surface is stroked with the solder stick and wetting is accomplished without a flux.
8. Indium-tin solder
— 50% Indium and 50% Tin solder is used for glass to metal and glass to glass soldering.
9. Fusible alloys
— Fusible alloys are preferred where a soldering temperature below 183°C is required; for example for soldering heat treated surfaces where high soldering temperature would soften the part
2 METALLURGICAL ASPECTS OF SOLDERING
(i) wets the base metal;
(ii) flows freely over the surfaces, i.e., spreads and makes contact with the adjoining opening
(iii) is drawn into the joint by capillary action;
(iv) solidifies as a sound, firmly adhering film;
(v) has adequate mechanical strength, e.g., 4.5 to 6.0 kg/mm2.
Wetting and Spreading
— Liquid solder will wet and spread freely if the surface energy of the parent surfaces is high relative to the combined liquid and liquid/solid interface energies.
— Refer to Fig. 50.10.
(i) Complete wetting, wherein the droplet of solder has spread (feathered) over the surface to give the desired effect
(ii) Partial wetting, which is the condition normally encountered in soldering, since there is seldom time to let the process go to completion, and the solder joint is usually frozen before it ever reaches equilibrium. Thus the partial wetting condition is an indication of equality. Should the contact angle be smaller than 60-70°, the solder joint is usually considered acceptable.
(iii) Total non-wetting, wherein the droplet of solder, due to gravity, sits on the surface without making the intimate contact required.
— The condition of the surfaces to be joined is also important.
In general, liquid solders do not wet clean unfilmed stolid metals: for example lead-tin solders have a contact angle 6 between 25° and 70° with a steel surface, depending upon solder composition (Fig. 50.10).
However, tin is capable of alloying with iron, and if a film of tin is in some way deposited on and alloyed with the iron surface, then tin-lead solder will wet it.
Thus it is clear that alloying is essential to wetting and hence to successful soldering.
Take another example, lead, which does not alloy with iron, will not wet a steel surface.
Alloying also helps spreading the liquid solder, since, if the liquid metal dissolves in the solid, it can diffuse beneath small areas of oxide and detach them and will guide the flow of bulk liquid over the whole surface.
— In general a solder will wet a metal surface provided that either
(i) It forms an intermetallic compound with solid, or (ii) The solid metal can take the solder into solution.
— The character and degree of spread depends upon
(i) The nature of two metals.
(iii) Presence or absence of flux.
(iv) Roughness of solid metal surface.
(v) Degree of oxidation of solid surface.
— Since oxide layers interfere with wetting and spreading, a flux may be required to remove oxide film and expose the underlying metal.
Typical fluxes include zinc chloride, mixtures of solid zinc and ammonium chloride, wood or gum resin, olive oil, etc.
Filling the Joint
— The capillary action influences the joint filling capacity and the effectiveness of filling surface imperfections. The filling is alsoaffected by ,
(i) the heating rate and uniformity of heating,
(ii) flux used,
(iii) temperature, and
(i v) distance be*ween the faces to be joined.
— Uneven heating results in irregular filling.
— Fluxed joints almost invariably contain flux inclusions.
— The majority of solders have a narrow freezing range and are therefore close to an eutectic composition.
Solder Bond Formation .
— The solder bond occurs on a molecular level and as a result of the surface energies of the materials involved. These energies are the sum total of the interacting forces between adjacent molecules in a material which consist mainly of van der Waals forces.
— The bond between solder and base metal is, therefore, more than adhesion or physical attachment.
The essential feature of the soldered joint is that a metallurgical bond is produced by a metal solvent action. The solder dissolves (not melts) a small amount of base metal to form a layer of an intermetallic compound. Upon solidification, the joint is held together by the same attraction between adjacent atoms that hold a piece of solid metal together. The ease of wetting is related to the ease with which this solvent action occurs.
Note. Bonding is affected by the formation of an intermetallic layer between the bonding and the base metal. When solder is applied to Copper, layers of Cu3Sn and Cu6Sn5are formed.