As contemporary electronics embrace light weight, increasing efficiency, and high speed, each link of the manufacturing process also conforms to this philosophy, including printed circuit board (PCB) assembly. Soldering has played an essential role in determining the success of electronic products, since electrical connections derive from precise soldering. Compared with hand soldering, automatic soldering has been widely selected due to its merits of high accuracy and speed, and the demands of large volume and high cost-effectiveness. As the leading soldering technologies for assembly, wave soldering and reflow soldering have been most widely applied to high-quality assembly; however, the differences between the two technologies continue to confuse many, and when each should be used is also vague.

Figure 1. The differences among soldering, welding, and brazing.

Prior to formal comparison between wave soldering and reflow soldering, it's of significant necessity to understand differences among soldering, welding, and brazing (Figure 1). Briefly speaking, welding refers to the process in which two similar metals are melted to be bonded together. Brazing refers to the process in which two pieces of metal are bonded together by heating and melting filler, or alloy, at a high temperature. Soldering is actually a low-temperature brazing, and its filler is called solder.

When it comes to PCB assembly, soldering is applied through solder paste. Soldering with solder paste that contains hazardous substances such as lead, mercury, etc. is called lead soldering, while soldering with solder paste without hazardous substances is called lead-free soldering. Lead or lead-free soldering should be chosen according to specific demands of products for which assembled PCBs will be designed to work.

Wave Soldering

As its name implies, wave soldering is used to combine PCBs and parts through a liquid “wave” formed as the result of motor agitation. The liquid is actually dissolved tin. It is carried out in a wave soldering machine (Figure 2).

The wave soldering process is composed of four steps: flux spraying, preheating, wave soldering, and cooling.

  1. Flux Spraying. Cleanliness of metal surfaces is the basic element ensuring soldering performance, depending on functions of solder flux. Solder flux plays a crucial role in smooth implementation of soldering. Primary functions of solder flux include eliminating oxide from the metal surface of boards and component pins; protecting circuit boards from secondary oxidation during the thermal process; reducing surface tension of solder paste; and transmitting heat.

  2. Pre-Heating. In a pallet along a chain similar to a conveyor belt, circuit boards travel through a heat tunnel to carry out preheating and activate flux.

  3. Wave Soldering. As temperature constantly rises, solder paste becomes liquid with a wave formed from the edge boards that travel above. Components can be solidly bonded on boards.

  4. Cooling. Wave soldering profile conforms to a temperature curve. As temperature reaches the peak in the wave soldering stage, it is reduced, which is called a cooling zone. After being cooled to room temperature, the board will be successfully assembled.

Figure 2. A sample wave soldering machine.

As circuit boards are placed on a pallet ready to go through wave soldering, time and temperature are closely associated with soldering performance. As far as time and temperature are concerned, a professional wave soldering machine is necessary, while the PCB assembler's expertise and experience are seldom easy to obtain since they depend on application of up-to-date technologies and business focus.

If temperature is set too low, flux won't be melted properly, reducing the ability to react and dissolve oxide and dirt on the surface of the metal. In addition, the alloy won't be generated by flux and metal if the temperature is not sufficiently high. Other factors such as speed of the band carrier, wave contact time, etc. should be taken into consideration.

Generally speaking, even though the same wave soldering equipment is used, different assemblers offer differing manufacturing efficiency due to operation methods and the extent of knowledge about how to operate the machine.

Reflow Soldering

Reflow soldering permanently glues components that are first temporarily stuck to their pads on circuit boards using solder paste that will be melted through hot air or other thermal radiation conduction. Reflow soldering is implemented in a machine called a reflow soldering oven (Figure 3). As its definition implies, electrical components are temporarily attached to contact pads prior to soldering using solder paste.

This process primarily contains two steps. First, solder paste is accurately placed on each pad through a solder paste stencil. Then, components are placed on pads by a pick-and-place machine. Real reflow soldering won't start until those preparations have been made.

  • Pre-Heating. This step serves two purposes during reflow soldering. First, it allows boards to be assembled to consistently reach the required temperature to fully comply with thermal profiling. Second, it is responsible for expelling volatile solvents contained in solder paste. Otherwise, soldering quality will be compromised.

  • Thermal Soak. Similar to wave soldering, reflow soldering also depends on flux that has been contained in solder paste. Accordingly, temperature has to reach a level at which flux can be activated, or the flux fails to play its role in the soldering process.

  • Reflow Soldering. This phase occurs when the peak temperature is achieved, enabling the solder paste to be melted and reflowed. Temperature control plays a crucial role in the reflow soldering process. Too low a temperature stops the solder paste from sufficiently reflowing; too high a temperature may cause damage on surface mount technology (SMT) components or boards. For example, a ball grid array (BGA) package contains multiple solder balls that will be melted during reflow soldering. If soldering temperature doesn't reach the optimal level, those balls may be melted unevenly, and BGA soldering may suffer due to rework.

  • Cooling. Temperature will go down soon after the top temperature is achieved. Cooling leads solder paste to solidify, permanently fixing parts on contact pads on boards.

Figure 3. Reflow soldering is performed in a reflow soldering oven.

Reflow soldering can be applied in both SMT and through-hole technology (THT) assembly, but is used primarily in the former. When it comes to application of reflow soldering on THT assembly, pin-in-paste (PIP) is usually relied upon. First, solder paste fills in holes on the boards. Then, component pins are plugged into the holes, with some solder paste coming out on the other side of the board. Finally, reflow soldering is implemented to complete soldering.