Optimizing Conveyor Angle in Wave Soldering for Board Assembly

Angle in Wave Soldering for Board Assembly

As board assembly have grown more and more complex, a variety of soldering techniques have been developed. In the early days, pure through-hole assemblies were common, but they gradually gave way to hybrid assemblies (with both through-hole and surface mount components) and then to entirely SMD assemblies. In addition to the usual soldering equipment, these newer assemblies required additional hardware. Glue was often used to fasten the components to the board and this added time, expense, and handling. It also introduced new problems such as glue residues on the boards that were difficult to remove after the soldering process was completed.

As the industry moved toward more advanced electronics, many manufacturers turned to wave soldering as a more efficient and effective process. It was typically faster in setup and soldering than hand soldering and it offered better consistency. Despite its shortcomings, such as bridging and other defects, it continues to be a popular choice for the vast majority of production applications.

The basic principle behind wave soldering is to pass the assembly over a flowing pool of molten solder. The optimum way to do this depends on the type of assembly and its configuration. There are some general rules of thumb that work well in most cases, however. For example, long leads and large pads tend to collect excess solder and can be oriented relative to the conveyor so that they are not swept into the next solder wave.

Optimizing Conveyor Angle in Wave Soldering for Board Assembly

In addition, it is important to provide adequate heat to drive off solvents and activate flux (to deoxidize surfaces to be joined) before the assembly enters the solder wave. This is usually done with heaters under the conveyor. For most plated through-hole (PTH) and some SMT assemblies, heaters on top of the assembly are also needed to prevent the solder from freezing before it is completely vertically filled.

A good flux is also critical for minimizing bridging. This is especially true for fine pitch SMDs where a relatively small amount of solder can bridge adjacent pins and cause other defects. It is recommended to use a low silver alloy flux that is activated by ionized oxygen and provides a high level of surface tension for the surface of the solder.

Finally, it is necessary to ensure that the conveyor angle is optimized for the specific assembly in question. This will vary by design, but in most cases it is appropriate to tilt the conveyor at a slight angle relative to the wave height. This can help to limit bridging to only a few trailing pins and to reduce the overall amount of solder required. Fortunately, there are all sorts of tools available to help in this endeavor such as DoE-based optimization methods [1] and self-organizing maps (SOM) [2].

Many electronic components are moisture-sensitive and can absorb moisture from the environment. During the high-temperature soldering process, this moisture can vaporize, causing internal damage or “popcorning” (where the component package bursts). Pre-baking components at controlled temperatures can mitigate this risk. The PCB substrate material itself is affected by temperature. Different materials expand and contract at different rates when exposed to heat, which can lead to issues like: