Soldering

What is soldering

Soldering is the process of a making an electrical and mechanical joint between certain metals by melting and flowing a filler metal into the joint. This filler metal is a low temperature melting point alloy of lead and tin. Most commonly used solder alloy is solder alloy that is 60% tin (Sn) and 40% lead (Pb). Another solder alloy is one that consists of 63% tin (Sn) and 37% lead (Pb).

Soldering is the process widely applied to assembling electronic component to printed circuit boards. Soldering can be divided in two groups:
- industrial soldering

- hand soldering

In addition, soldering can be also divided into two quite different groups:

- soldering of through-hole components

- soldering of surface mount components

History of soldering

It is difficult to say who first discovered the soldering. History of soldering is thousands years long. Soldering was used more than 5,000 years ago in ancient Egypt to make jewelry. Goldsmiths of ancient Troy also knew how to solder. Then the soldering was introduced in the Mediterranean region, and followed in the Roman Empire, Swiss, and Hungaria. However, the high temperatures required for modern soldering processes became possible only with the development of electric power.

Soldering is one of the oldest methods of joining two metals. Even though there are several techniques of connecting two metals together like bolts and nuts, rivets, etc. soldering is still most popular because of two reasons:
- joint is solid and there is not movement in the joint
- there are no interfacing surfaces to oxidize

 

What is Hand soldering

Hand soldering is process of joining two metals together with soldering iron by the use of a solder alloy to form a reliable electrical path. Soldering is not the simple task since it requires experience and knowledge of fundamentals. It is very important to make a proper soldering joint since faulty solder joints are one of the major causes of circuit boards failure.

What happens during soldering process

For beginner soldering looks like the solder simply sticks the metal like some kind of conductive glue. However, what happens during soldering is far different. When hot solder comes into contact with surface of copper on circuit board, a metal solvent action takes place – the solder dissolves and penetrates the surface of copper. Copper and solder blend together and form new metal alloy that one part is copper and the other part is solder (and solder is already metal alloy of tin and lead).

This can occur only if the surface of the copper and solder are hot enough and if the surface of the copper and solder are clean and free of oxide film. This oxide film forms when the metal is exposed to air. Sometimes a copper surface may look clean before soldering but actually it can be a thin film of oxide on top of copper. In that case, when solder is applied it will not stick to copper and no solvent and penetration action takes place because oxide film will prevent interaction of solder and copper. That is not god soldering joint and that solder can be easily scraped off copper surface.

Therefore, for proper soldering bond, oxide film should be removed, and this can be accomplished with the use of fluxes. Fluxes are mixture of natural and synthetic rosins. Some solvents and abrasives could also be used for cleaning of surfaces to be soldered but in most cases they are not sufficient since oxide film forms very quickly on the surface of heated metal. On the other hand, flux removes oxide film and keeps removing it during soldering. Flux is in solid state and it must melt at a temperature lower than solder. Soldering wire usually has flux in the middle of wire but there are different types of cored solder with different solder to flux rate. After soldering all fluxes should be removed with solvent.

Hand soldering with soldering iron

The main requirement for soldering is the use of heat what is accomplished with soldering iron. The iron tip should be cleaned by wiping it on wet sponge before every use. During soldering, solder should never be applied against the iron tip - it should be applied to a clean and properly heated copper surface. That way solder will melt and flow without direct contact with soldering iron and provide a smooth surface. Otherwise, solder will form irregular appearance, built-up, and poor filling. Soldering is not finished at moment when soldering iron is removed from soldering joint because at that point solder is still fluid – components being soldered should be held tightly in place until the temperature decreases to solidify solder.

A hot soldering iron can significantly damage, deform and even lift the pads and conductors on circuit board if it is applied for too long or with to much pressure. Copper foil on circuit board is very thin – average thickness is .0014 – 0028 inches. The iron tip should never be pushed down on the copper surface – apply on iron same force as it would results from weight of soldering pencil resting on its point

After soldering is completed, soldering iron should be kept in a holder with iron tip cleaned and coated with thin layer of solder.

Factors of soldering

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:

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.

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.

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.

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.