A Comparison of Reflow Soldering and Wave Soldering
This article explores the differences between two primary forms of soldering — reflow and wave — by providing details around each type, including best uses for each method.
Soldering is a key component of the printed circuit board (PCB) production process. Manufacturers use soldering to securely attach circuits to boards. There are two primary forms of soldering in the PCB industry: reflow soldering and wave soldering.
For companies operating within the PCB industry, it's critical to use the most effective soldering technique for their circuit board type. Engineers need to make an informed decision on which soldering method is most useful for their production requirements. The chosen method will have a significant impact on production timelines, costs, and other core elements of the PCB manufacturing process.
This article explores the key differences between wave and reflow soldering by exploring each type in more detail below.
Reflow soldering is the most prevalent soldering method in the PCB industry. Unless you are soldering through-hole elements, reflow soldering is the most common method for many manufacturers (especially suitable for SMT assembly).
Figure 1. The process of reflow soldering.
The reflow solder process begins by applying flux and solder (also known as solder paste) to the pads. The PCB is placed in a reflow oven and hot air melts the paste to form solder joints. The process occurs by raising the temperature to pre-determined levels. Preheating is implemented so that the PCB doesn't experience thermal shock during the acute soldering process.
To solder small, individual components on a PCB, hot air pencils are more than sufficient.
Pros of Reflow Soldering
- Suitable for SMT assembly
- Trusted by most manufacturers
- Does not require a large amount of monitoring
- Less thermal shock
- Options for limited soldering
- A less wasteful process that can be applied to specific parts of the PCB
Wave soldering utilizes a different soldering method for manufacturing PCBs. It is the best method for engineers who need to solder a large number of PCBs at the same time.
The wave soldering process begins by applying flux to the components that need to be soldered. The flux removes surface oxide and cleans the metal before soldering, which is a crucial step in quality work.
Next, like with reflow soldering, preheating occurs to ensure that thermal shock is avoided during the acute soldering process.
The 'wave' of solder will move over the PCB and begin soldering the various components—electric connections form during this stage. A cooling method then brings the temperature down and permanently bonds the solder in place.
Figure 2. The process of wave soldering.
The internal environment in a wave soldering oven is critical. If temperatures are not maintained correctly, costly issues may arise. Owners of wave soldering ovens can also face multiple challenges if they are unable to control conditions during the soldering process effectively. If temperatures reach levels that are too high, for example, PCBs may develop cracks or issues related to conductivity. If the wave soldering oven is not hot enough, cavities on the PCB may result in conductivity issues and structural weaknesses.
Pros of Wave Soldering
- Suitable for THT assembly
- More time-saving
- Initiates less warpage
- More affordable
- Can provide strong solder joint quality
- Suitable for through-hole soldering
- Produces a large number of PCBs simultaneously
Reflow Soldering vs. Wave Soldering
The primary differences between wave soldering and reflow soldering lie in the core soldering process.
Reflow soldering uses hot air, while wave soldering uses a 'wave' of solder to mass-produce PCBs. This means that the internal environment in a wave solder oven is far more sensitive—a small change in temperature or conditions might result in catastrophic damage to the PCBs.
These differences also have a substantial impact on the affordability and efficiency of each method. While wave soldering is more complex and demands unbroken attention, it tends to be quicker and cheaper. If you don't have time on your side, it might be the only reasonable option for soldering many PCBs simultaneously.
If you're still unsure about which soldering method is most appropriate for your needs, it can be helpful to look at a birds-eye-view comparison of both methods.
Below is a comparison of the critical details of each type of soldering:
|Reflow Soldering||Wave Soldering|
|Requires the use of reflow, which manufacturers can produce using hot air.||Occurs via the use of a wave crest, which is produced using a melted solder.|
|Often considered the simplest method for soldering PCBs.||
More complex because the internal environment is much less stable. Small temperature shifts can destroy the PCB during manufacturing.
Often takes more time and money, especially for large manufacturing projects.
Allows manufacturers to mass-produce PCBs, which makes the process much more affordable and time-effective.
|Other components, such as pad size, pad shape, and board orientation, are not considered essential during the reflow soldering process.||Manufacturers using wave soldering must consider pad size, pad shape, board orientation, and a host of other factors carefully if optimal PCBs are to be produced.|
|The most common form of soldering in the manufacturing industry, especially with SMT assembly.||
Less common, but it is more prevalent with soldering through-hole components.
Both Soldering Methods are Effective for Different PCB Types
Reflow soldering and wave soldering are both effective methods for constructing PCBs. While it is possible to solder PCBs manually, choosing to avoid reflow or wave soldering methods can prove impractical at any commercial level.
While wave soldering is designed for THT assembly and reflow soldering is used for SMT assembly, hardware with SMD and DIP elements may need a combination of both (a hybrid). It is not uncommon for PCBs to be partially wave soldered and reflow soldered during the manufacturing process, for example.
If time and budget are the most critical elements in your soldering method decision, wave soldering has its advantages. The complexity of wave soldering makes it a problematic choice for producers with no previous experience in PCB production, though.
Reflow soldering is an excellent option for manufacturers that don't require the mass production of PCBs.
Both options have advantages and drawbacks. Engineers should carefully consider the design of the PCBs, as well as their individual needs and budget.
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Not mentioned, wave soldering can be used on surface mount parts with the added step of epoxying those parts to the surface of the board being run over the wave. Using this method, combined surface mount and through hole components can be soldered in one step.
Wave soldering has been around for over 50 years. Most assembly houses have perfected the techniques and processes to minimize problems.
Reflow soldering became mainstream in the 1980’s. It is very much the preferred soldering technique for boards that contain only surface mount parts. Most assembly houses have refined their processes to the point that there are few problems. The worst issues I’ve seen were blobs of solder paste that did not melt into a part lead to pad joint, but wandered off. These often show up landing between leads of fine pitch surface mount parts where they cause the board to malfunction. Again, reflow has been around for a while. Most assembly houses inspect and correct issues such as these before releasing the product.
Pad shape matters in both through hole and surface mount soldering. It’s HIGHLY recommended that you communicate with your assembly house before designing a board to ascertain their recommendations for pad shapes on commonly used device packages. This should include any surface mount packages you may want to use as well as provisions for through hole drill sizes and annular ring recommendations.