HOW SOLDER WORKS? WETTING AND NON-WETTING
When the liquid comes into contact with the walls of another solid surface it may happen that the particles of the solid attracts the liquid particle more strongly than the liquid itself. This leads to wetting. Where the attraction of the solid particles is less, we get what is Non-wetting
The angle between the surface of the liquid and the surface of the solid is called the contact angle. In a well soldered joint the contact angle is less than 90 degree.
The surface to be soldered must be sufficiently hot to permit wetting by the solder. The board must be suitably designed so that the solder does not cool down before the joint has been effected. The components to be soldered should not be damaged by heating. The metal surface to be wetted must have good wettability. Good solderability results in good wetting. The wetting of the materials must be according to the soldering process employed. NEED FOR FLUX. Flux is used to remove oxides so that the desired cleanliness f the surface can be achieved.
FUNCTIONS OF FLUX:
Remove the oxide film from the surface. Assist the transfer of heat source to the joint area. The reaction products must be removed from the surface to permit the solder to come in touch with the metal surface.
CONDITION FOR SELECTING FLUX:
The ability of the flux to promote solder wetting. The ability of the flux residues of affect the solder products after the soldering process.
The mixture of two pure metals which melts at same temperature is called EUTECTIC COMPOSITION. The temperature is called EUTECHTIC TEMPERATURE. For tin and lead the composition is 63 (per) and 37 (per) respectively.
Joining electronic components SnPb40 ( Tin lead ) Tem=183-189.
Tinning winding wire Tem=183-245.
Soldering metallic components PbSn20Sbl Tem= 183-277
High service temperature PbSn5 Tem=300-315.
SOLDERING WIRE: The solder alloy wire is used with longitudinal cavity containing flux. Wire diameter from 0.25 mm to 1.25.
TYPES OF SOLDERING IRON:
This is essentially a copper tip which is electrically heated by means of a heater coil. This acts as a heat store and a heat conductor. Wattage rating 10w to 250w (Generally 40w for electronics maintenance applications)
Tips are plated with a layer of iron or iron and nickel to increase the life of the tip.
CARE ABOUT FOR THE TIPS:
Never clean a tip using a file of a steel brush.
Never use a tip as a lever.
Don’t use tip to spread solder.
Don’t wipe the surface to be joined with the bit.
Use a wet cellulose sponge when cleaned.
1. IRON STAND.
2. HEAT SINKS.
3. EXHAUST FAN.
BASIC RULES FOR GOOD SOLDER.
- Chose the proper rating of soldering iron.
- Wattage 18w to 50w watts are usually sufficient.
- For connecting large wires to a capacitor terminal a 40w to 50w iron may be used.
- Pick the proper solder wire.
- Select the proper tip.
- Let the flux perform its cleaning iron and the joint till the good reading of the solder has been realized.
- Apply proper quantity of solder.
- Don’t allow the parts to move during solidification.
SOLDERING FOR ELECTROSTATIC SENSITIVE DEVICES:
The most sensitive devices like MOS-ICS,FETs,etc., may be damaged by voltages below 50w.High voltages can be generated due to friction. Component s must be delivered in an antistatic of conductive boxes. The personal connected with the handling must be continuously connected to the earth potential via wrist straps with a 0.5 1 M ohm resistor. All equipments working on the mains must be provided with proper leakage switches.
I cicles, peaks or spikes are also called stalactites and are caused by excess solder. This fault can be only of cosmetic nature but can cause high voltage circuits. It can also cause short circuit if the length is more The problems which are noticed when soldering are:
- De wetting of conductor surfaces
- Non wetting of conductor surfaces
- Blowholes surrounding resistor leads.
- Incomplete flux residue removal.
- In sufficient heat.
- Over heat resign flux residue is charred.
soldering wick and pump
The spring loaded plunger when released creates, a vacuum which sucks the solder into the pump.
Use of solder braid:
This is a braid of twisted copper wire which are available in various widths ranging from 0.035 to 0.220 inches.
This works on the principle that if the braid is placed near the joint.
Using special wells:
Used for multi leaded components. Those can be special wave of static wells with a mechanism to change the solder level. The components to be replaced is centered over the well. The solder rises and melts the joints. The operator can replace components quickly within the time the solder stays up.