Factors of soldering operation

Soldering is very complex process with many different factors influencing it. Before we start any soldering operation we should take into consideration the following factors:

• 1) Thermal mass of the soldering joint

Single pad on single-sided printed circuit board has small mass, so it heats up quickly. Double-sided printed circuit board with plated-through holes has two times more the mass. Multi-layer printed circuit board has even more mass. The mass of any component lead should be added to total mass of the pad. Some components have the leads which are many times longer or larger than others so thermal mass of lead itself can significantly vary for different leads. Thermal mass will be further increased with one or more copper conductors on the circuit board which are connected to the solder joint (the thermal mass depends of the length and width of the conductor as well as of the thickness of cooper foil).
Therefore, each soldering joint has its own thermal mass and it determines the time and temperature rise during soldering process. If we have a small work mass and a big iron tip, temperature rise will be rapid. However, if we have a large work mass and a small iron tip, temperature rise will be slow. It is very important to choose appropriate soldering iron for each soldering work. Ideally, the joint should be brought to desired temperature rapidly and held there for a short period of time.
There are many different sizes and shapes of iron tips. It is very easy to replace iron tip on soldering iron. Of course, tip should be replaced before soldering while the soldering iron is still cold. For small soldering work, which requires only small flow of heat, we should use a conical tip. On the other hand, for large soldering area where a lot of heat is needed, we should use a large iron tip. In addition, if we need a lot of heat at a particular joint, beside the appropriate iron tip and precisely controlled temperature we also need a soldering iron that can recover fast enough. The iron’s recovery rate is determined with heater block size and wattage of soldering iron.

• 2) Clean surface

Even though the soldering iron is proper wattage, and iron tip is right size, we may not be able to deliver enough heat to the solder joint to melt the solder when the surface of solder joint is dirty. Before any soldering work we must clean properly the surface. The surface of copper pad should be wiped with a solvent such as isopropyl alcohol to remove any grease and if needed with abrasive stick. Then, some flux should be applied. You can make a good solder joint only on a clean soldering surface.

• 3) Thermal linkage

Thermal linkage is the area of contact between the iron tip and surface. The contact area between the iron tip and surface is usually very small, and it is straight line along iron tip. This contact area can be significantly increased by applying a small amount of solder to the line of contact between iron tip and surface. Molten solder forms a heat bridge between the tip and the joint. This solder bridge provides the thermal linkage and the quick transfer of heat into the solder joint. Solder always flows from the cooler area towards hotter area.

• 4) Time

Heat should be applied to soldering joint for less than 2 seconds. If we keep applying heat longer than 2 seconds, this can damage the pads or connections on printed circuit board or temperature-sensitive components. Sometimes is good practice to hold lead of component with tweezers during soldering. That way the tweezers will take away some heat from lead and prevent component under soldering from damaging. Caution should be taken when soldering components onto printed circuit board due to the heat sensitivity of components and circuit board.