Comparing Wave Soldering and Reflow Soldering: Making the Right Choice

The electronic industry follows two main types of automated soldering techniques for assembling their PCBs. These are wave soldering and reflow soldering. There are major differences between the two, and the circumstances of their use are vastly different. In this article, we, at Rush PCB Inc, will briefly explain the two technologies, their commonalities and differences, and the various circumstances under which the industry decides their use.

What is Soldering?

Soldering is the technique of anchoring electronic components to the pads of a PCB or printed circuit board. Solder is an alloy that melts at low temperatures and in its molten state, forms a metallurgical bond between the component lead and the copper pad on the PCB. On cooling, it hardens to form a mechanically rigid and electrically conducting bridge between the pad and the component lead. Its mechanical rigidity allows it hold the component in place, while its electrical conductivity allows the component to function as intended in circuit.

How to Solder?

Soldering requires application of heat to melt the solder and make it flow, and subsequent cooling to allow the solder to resolidify. Traditionally, technicians have used a soldering iron to supply the heat, and regular convection for cooling. They apply the hot tip of an electrically heated soldering iron to the pad and the component lead, while simultaneously applying solder and flux to the hot surfaces. Flux acts as a reducing agent here, as most metals generate a non-solderable layer of oxide on their surfaces in the presence of heat and oxygen from the surrounding atmosphere.

The heat melts the solder, and it flows along the hot surfaces, creating a metallurgical bond. As the solder cools, it solidifies and rigidly binds the component to the copper pad.

However, the above process requires heating each joint individually for soldering. The process is slow and laborious. Moreover, the formation of the soldered joint depends on the skill of the operator, and there may be variations in quality between the joints. For high volume production, and to obtain high and consistent quality of solder joints, the electronic industry relies on automated soldering processes, of which there are two main types.

Wave Soldering

A bulk soldering process, wave soldering can solder large number of boards in very short periods. A conveyor carries each assembled circuit board over a pan or tub of molten solder. A pump in the pan or tub drives molten solder through a spout, thereby creating a wave of solder. The solder wave touches the bottom of the board and the component leads as they pass overhead and partly attatches to them.

As the board passes over, it cools naturally, or may be forced to cool down with air blown over it. On cooling, the molten solder hardens and anchors the component in place.

The wave soldering process takes place in a long chamber, typically known as the wave soldering oven, and it has several zones with varying temperatures in each zone. The conveyor carries the PCBs in at one end of the oven with the lowest temperature. As the PCBs progress through the oven to the subsequent zones, the temperature gradually increases. In the final zone, the boards reach the solder tub, and here they face the highest temperature, suitable for the actual soldering process. Once the boards cross this point, the temperature drops quickly to the ambient.

The wave soldering process is susceptible to proper temperatures at specific zones. Failing to control the temperature necessary at the zones can subject the board to large mechanical stresses, leading to cracks and loss of electrical continuity in traces. Insufficient temperature at preheating zones may cause uneven heating in small and large components, thereby compromising the soldering quality. Wrong solder temperature can lead to unequal wetting and improper solder thickness, and this could lead to development of stress in the board.

Reflow Soldering

This is another bulk soldering process, also suitable for soldering large number of boards in short periods. However, this process is suitable only for SMT or surface mount technology components. These components are very small and most do not have leads. The process for mounting SMT components on the PCB also differs.

Rather than using a tub of molten solder, reflow soldering requires application of solder paste on the PCB pads prior to mounting the SMT components. The assembly of SMT components sitting on a thin layer of solder paste will then go for reflow.

The reflow oven also has several zones through which the conveyor carries the PCBs with SMT components on solder paste. As the boards move through the zones, the temperature gradually increases until at the soldering zone, it reaches the melting point of the solder paste.

In this zone, the solder paste melts and binds the component to the PCB pads. The next zone allows cooling, where the molten solder solidifies and anchors the component. Like the wave soldering process, the temperature in each zone of the reflow oven is critical to the quality of soldering on each joint. Operators must create a temperature profile for each board type that undergoes reflow soldering.

Comparing Wave and Reflow Soldering Processes

Both wave and reflow soldering processes mechanically anchor components to the board while providing an electrically conducting medium for them to function correctly. However, while the reflow soldering process can solder only SMT components, the wave soldering process is suitable for both, SMT and THT components, but with some restrictions.

As the actual soldering takes place at the bottom of the board when it is undergoing wave soldering, all SMT components to be soldered must be mounted on the underside of the board. To prevent them from falling off, they need to be glued on before they go into the wave soldering oven. The wave soldering process cannot solder SMT components mounted on the upper side of the board.

Wave soldering is very suitable for soldering THT or through hole technology components. Most of these components are rather large in comparison to SMT types, and have long leads. The board must have holes drilled into it to accommodate the component leads that pass through. Once the board assembly is over, the body of the THT components typically remain on the top side of the board, while their leads pass through and are available on the underside.

When these boards pass over the solder tub in the wave soldering oven, molten solder attatches to the leads and anchors them to the pads. If the board is multilayered, each hole is plated through.

Advantages of Wave Soldering

Wave soldering is an economical process suitable for boards using THT components. As each hole in the board is plated through, it draws in molten solder due to capillary action, providing adequate quantity of solder for anchoring the component. This provides a very strong mechanical anchoring for THT components.

Disadvantages of Wave Soldering

The quantity of solder necessary for each run of a set of boards is rather high. The hot molten solder forms dross that the operator must remove periodically. Of late, newer nozzle designs are available that take care of this problem.

The molten solder touching the component leads returns to the tub. This causes contamination of the entire tub from metals present in the leads. Decontaminating the solder in the tub is a major exercise.

After the wave soldering process completes, the board must undergo a trimming process for pruning the excess leads of the THT components.

Wave soldering is not suitable for SMT components mounted on the top side of a board.

Advantages of Reflow Soldering

The reflow soldering oven only requires infra-red heaters to operate. No solder tub is necessary, as the board and components carry their own solder. This arrangement makes the process technologically simpler and easier compared to wave soldering.

As each SMT component joint carries its own solder paste, the quality of soldering is high and there is very little overflow to cause bridging. This allows designers to achieve very high component density on boards with SMT components.

Disadvantages of Reflow Soldering

Reflow soldering cannot solder THT components. Therefore, for boards that contain a mixture of THT and SMT components, a two-step process is necessary—one for soldering all the SMT components, and the other for the THT components.

Reflow soldering requires an additional step for depositing solder paste on the board. This requires a high-precision stencil through which the operator deposits solder paste.

SMT components are too small to be manually handled. As such, an automated pick-and-place machine is necessary to pick them from their carriers and place them on their proper positions on the board.

Conclusion

PCBs are necessary for various industrial and commercial applications. There are many elements that make the board suitable for individual purposes. The method of soldering is one of the most critical of these elements. With a vast experience of designing, fabricating, and assembling printed circuit boards, Rush PCB Inc has offered a very concise and generic explanation of the process of soldering.