SOLDERING PROCEDURE

The soldering procedure basically involves the preparation of the following three items:

(a) The soldering iron selection;

(b) The component to be soldered; and (c) The circuit board.

5.4.1 Se le ction of Solde ring Iron

The soldering iron selected should be of the precision type, small but powerful enough t o reliably solder components to printed circuit boards. An iron between 25 and 40 watts with a nickel plated tip, or one of the miniature irons capable of a tip temperature o f 400°F, is the most suitable. Hotter temperatures run a real risk of spoiling the adhesive bond that holds the copper foil to the board. Do not use a higher temperature to make up for an improperly-tinned tip.

The ideal tip is a single flat or chisel tip of about 2.5 mm. The old style unplated copper tips are not very suitable, as they wear away very quickly.

The soldering iron should be examined carefully every time it is to be used. The soldering iron tip should be properly connected or screwed into the holder and it should be free from oxides. The shape of the tip must meet the requirements of the task to be performed. If any one of these items is not as good as it should be, the following steps are adopted:

(a) The oxides from the tip surface are removed by using an abrasive cloth or sand paper.

(b) The tip is given its proper shape generally by filing. This is normally done on the unplated copper tip.

(c) The iron is heated to the minimum point at which the solder melts. Before using the iron to make a joint, the tip is coated or tinned lightly by applying a few millimetres of solder.

(d) For keeping the tip clean, after it has been prepared, the heated surface of the tip should be wiped with a wet sponge. This is to remove dirt, grease or flux which, if allowed to remain, can become part of the joint and make the joint dry and defective.

If during soldering, excessive heat is generated at the soldering iron tip and the component gets heated beyond its maximum temperature, the component may be permanently damaged, weakened, or drastically affected in value or characteristics.

Such effects may not be noticed during the assembly or test but may show up later when the equipment is in use.

The tip temperature to be selected must be based on the temperature limitation of the substrate. The circuit boards which have a substrate of fibre-glass epoxy of 280°C should not be heated for more than 5 minutes. Hotter temperatures reduce the time in inverse relationship; the higher the temperature, the less time the boards will stand it before being damaged.

Further, heat transmitted along the leads may cause unequal expansion between leads and packages, resulting in cracked hermetic seals. In general, for hand soldering, the recommended soldering iron wattage is 20-25 W for fine circuit board work, 25-50 W iron for general soldering of terminals and wires and power circuit boards, 100-200 W soldering gun for chassis and large area circuit planes. With a properly sized iron or gun, the task will be fast and will result in little or no damage to the circuit board plastic switch housings, insulation, etc. For iron temperatures ranging between 300°C and 400°C, the tips of the soldering iron should be in contact with the lead for not more than 5 seconds. Particularly, the ICs and transistors should be soldered quickly and cleanly.

5.4.2 Compone nt Pre paration

Cleaning: Before any component lead, wire or terminal is soldered in a circuit, it is essential to clean it with some braided cleaning tool followed by brushing the cleaned surface with a stiff bristle brush dipped in alcohol. The surface is dried with paper or lint-free rag.

It must be noted that the solder will just not take to dirty parts. They should be free from grease, oxidation and other contamination. Old components or copper board can be notoriously difficult to solder because of the layer of oxidation which builds up on the surface of the leads. This repels the molten solder and the solder will form globules,

which will go everywhere except where needed. While the leads of old resistors and capacitors should be cleaned with a small hand-held file or a fine emery paper, the copper printed circuit board needs to be cleaned with an abrasive rubber block or eraser. In either case, the fresh metal underneath needs to be revealed.

Component Forming: In order to ensure that the components fit properly into the circuit, in which they are to be installed, they must be properly formed (Fig. 5.9). Forming of the component has two main functions:

t Fig. 5.9 Forming of component leads for soldering (a) To secure the lead to the circuit; and

(b) To provide proper stress relief. The relief is necessary to prevent rupture of the component lead from the component or in case of a wire, to prevent a stress pull on

the solder joint and rupture of the wire strands.

The following steps need to be taken to properly bend the leads of the components:

(a) The bend should be attempted no closer than 3-5 mm from the component body.

(b) The radius of the bend should be equal to twice the thickness of the lead wire.

(c) Centre the component between its solder connections.

(d) Bend the protruding lead 45° after insertion into the circuit board with the help of a bending tool.

(e) Cut the lead so that no portion, when bent, exceeds the perimeter of the pad. Press the cut lead firmly against the lead.

(f) On a joint, when the lead is not bent, cut the lead to the thickness of SWG 20.

Forming is not necessary in the case of integrated circuits. They are simply soldered to the board without cutting their leads.

Lead bending is the most important factor in forming. A few sharp back and forth bends in a component lead can easily cause it to break or crack. Bending a lead too close to the component encapsulation may result in excessive stress at the lead entrance, and cause cracks in the encapsulation. Such cracks allow moisture to enter inside the component, causing gradual degradation of the component subsequently resulting in its premature failure.

5.4.3 Circuit Board

Although the principle of general cleaning also applies to circuit boards, it is, however, necessary to follow certain precautions. This is because circuit boards may contain some components that may be spoiled if a braided brush is used. In case of circuit boards, a sharply pointed typewriter eraser may be used to remove dirt, contaminants or other foreign substances from the pad to be soldered. It is then cleaned with alcohol and a brush and left to dry.

It may be noted that in the whole process of replacement of components by soldering, the single-most crucial factor is cleanliness, which should be scrupulously followed.

With dirty surfaces, there is a tendency to apply more heat in an attempt to force the

solder to take. This will often do more harm than good because it may not be possible to burn off any contaminants anyway, and the component may become over-heated. In the case of semiconductors, the temperature is quite critical and they get damaged when excessive heat is applied.