Soldering: (Definition and Types)

Soldering:

Soldering is defined as a metal joining process wherein coalescence is produced by heating the surfaces to be joined to a suitable temperature and melting the filler metal, which is a fusible alloy called solder (melting point usually less than 427°C), so that it may be distributed between properly fitted surfaces of the joint through the capillary action. Soldering operation is performed by bringing molten solder in contact with the pre-heated surfaces (being joined) and heating the joint area to a good wetting temperature (about 55 to 80°C above the melting point of soldering alloy). The solder is then left to cool and freeze as quickly as possible to avoid development of internal microcracks in the joint. The principle underlying soldering is that when the surfaces to be joined are cleaned off well from oxides, they can be joined together using molten solder that may adhere easily to the workpiece surfaces due to molecular attraction. The molecules of solder entwine with the parent metal molecules and form a strong bond.

METALS JOINED BY SOLDERING

The efficacy of soldering depends on the characteristics of oxides of metal being soldered and mutual solubility of solder and base metal. Therefore, all alloys are not equally wetted by solders

1. All types of carbon steels are soldered easily but the wettability decreases with increase in carbon contents in steels.
2. Cast irons are difficult to solder as graphite carbon flakes resist wetting.
3. Stainless steels are difficult to solder because of the chromium oxide on stainless steels. Rougher the surface, better the steel can be soldered. 
4. Copper and its alloys (brasses, bronzes) are easily soldered.
5. Phosphor bronzes are easily soldered.
6. Aluminium or silcon bronzes are not easy to solder due to the presence of oxide.
7. Aluminium and its alloys are soldered with difficulty.
8. Nickel alloys are easy to solder.

SOLDERING JOINTS

 In soldering, butt joints are avoided and lap joints are preferred. The soldering joints are weaker than brazed joints and hence the former needs to be designed such that in service, the soldered joint does not take load, the base structure should take the load. Joint strength can, however, be enhanced to some extent by designing proper interlocking sy: em at the joint. Different types of solder joint designs are given in Fig 8.1. The simple butt joint should be avoided as it has no strength. A lap joint is most widely used where overlapping may be three. times the thickness of the workpiece metal. Lap joint fits more easily and gets good strength de to extended overlap. The joint gap or clearance is necessary for the capillary action of the molten solder. It is usually kept 0.075 to 0.25 mm

TYPES OF SOLDERS

 Solders are broadly categorized as soft solders and hard solders. Soft solders melt usually below 427 C and hard solders melt at temperatures above 600°C. Soft solders are primarily the alloys of lead and tin. More lead increases melting point of solder, whereas tin increases flowability of solder. Soft solder melts at a temperature which is lower than the melting poing of the alloying elements, for example, lead melts at 327°C, tin at 232°C but their solder melts at 205°C. Sometimes antimony, silver, bismuth and cadmium are added to the soft solders Hard solders are basically the alloys of copper and zinc to which silver or phosphorus is sometimes added. Hard soldering is employed when a stronger joint is required than that is obtained by soft solders. Hard soldered joints can stand higher service temperatures. Hand solders are used in the brazing process wherein the melting point of filler metal is more than 427°C.

Soft solders fall under the following categories:

(i) Tin-lead solders

(ii) Tin-antimony-lead solders

(iii) Tin-zinc solders

(iv) Lead-silver solders

(v) Cadmium-silver solders

(vi) Cadmium-zinc solders)

1. Tin-lead solders: These are commonly used solders for joining most metals. These are cheaper and have good resistance to corrosion. Three typical solders of this category are:

• Plumber solder, having tin 32%, lead 68%, melting point 253°C.
• Tin man’s solder, having tin 62%, lead 38%, melting point 183°C.
• Ordinary solder, having tin 50%, lead 50%, melting point 217°C.

2. Tin-antimony-lead solders: Antimony, when added up to 6% of tin contents, increases mechanical properties of the soldered joint. But antimony solder should be avoided on zinc, cadmium or galvanized metals because this solder gives a brittle joint.

• For soldering iron and steel, a solder with tin 12%, antimony 8% and lead 80% is used.
• For general purpose, a solder with tin 43.5%, antimony 1.5% and lead 55% is used.

3. Tin-zinc solders:These are used for joining aluminium components. A typical composition of this solder may have tin 91%, zinc 9% and melting point 199°C.

4. Lead-silver solders:Lead-silver solders easily wet steel and copper components. Addition of silver up to 2.5% forms a good lead-silver solder which melts at 307°C. 5. Cadmium-silver solders: These are used for joining aluminium to itself or other metals. For high temperature application, a solder with cadmium 95% and silver 5% is used. It melts at 338°C and flows at 393°C.

6. Cadmium-zinc solders: These are used for soldering aluminium. Die-cast (zinc base) products are also soldered efficiently with a solder containing cadmium 82.5% and zinc 17.5%; the solder flows at 265°C.

See More : Soldering and Brazing