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Selective Soldering vs. Wave Soldering: Manufacturing Concerns

Building of PCBs requires two major stages – fabrication and assembly. Fabrication has to do with constructing the board architecture physically. Assembly is where the addition of components takes place and the completion of the PCBA takes place.

The major activity that takes place during the assembly stage is attaching the components to the board by soldering. This soldering depends on the components used. For instance, if you are making use of through hole technology, manufacturers should take note of how selective vs. wave soldering can have an impact on the costs and time of the board build.

Selective Soldering: What does this mean?

SMT Selective Soldering

A system for selective soldering is a machine, which will solder thru-hole leads on a panel or board. It makes use of a pump system and solder reservoir to move molten solder to pass through a nozzle, then to the leads coming through the circuit board’s bottom. The mechanism of this solder is built on an assembly moving in multiple directions. This movement works under a program’s control. This program is developed by the operators for the panel or board being assembled.

Asides from the movement, the programming also allows the operator to have precise control or the time duration of the application of the solder, and the solder’s temperature. This is to make sure the best solder joints results.

You can also develop this programming on a computer having solder system software in it. This is based on the Gerber data used for the board fabrication. Selective solder helps to fill the gap between manual hand soldering and automated wave solder. For through hole component soldering, wave solder is your first choice. This is because it takes less programming and preparation, and it is quick.

However, selective wave soldering comes with its limitations. Their boards have dense parts placement on the board’s back. Some boards need a custom pallet. This will mask off components of the surface mount on the bottom side, giving way for the solder wave to only contact the through hole pins.

For cases where parts of the surface mounts are too close and you cannot use a masking pallet, the other alternative for you is to solder the through hole leads manually. Doing this manually is time consuming and could cause human error. However, with the process of selective soldering, you can get rid of the possible issues associated with manual soldering.

Process Involved in Selective Soldering

Compared to wave soldering, the process involved in selecting soldering is a slower one. The components are sequentially soldered individually by a local wave. This opposes a full wave, which hits all the joints of the solder at once.

However, there are several benefits that have led selective soldering to be chosen as the preferred method in so many cases. Below are some of the great benefits offered by the selective soldering process. Following these benefits, are a few pitfalls you need to take note of.

Selective Soldering: Benefits

One disadvantage of doing hand soldering manually is the time taken required. It also requires the circuit board to undergo serious heating, in order to achieve a great solder joint.

When much heat is applied, it can lead to thermal issues for the board, its components, as well as other already-placed solder joints. Selective soldering allows the operator to have the required flexibility to manipulate the soldering variables to get great solder joints within a short period and with less heat overall. While the selective soldering process is on, the operator:

  • Specifies how the nozzle should move quickly to give enough heating time for the through hole to be filled with the solder
  • Programs the amount of solder to be utilized and the temperature it should be
  • Programs exactly the place where the molten solder nozzle is meant to go for the solder application

With this control, the operator is given a better soldering precision without the need of a steady hand to hold the soldering iron. This used to require some technicians, hand-soldering of the thru-hole leads, which couldn’t pass through the wave. However, this process can be handled faster utilizing a selective solder system

Other Benefits of Selective Soldering

  • There’s no need for glue for SMDs
  • Repeatability
  • It can be customized to accommodate different parameters of the components like pitch
  • Makes use of less flux and solder
  • You can limit the masking to the areas of the board that requires soldering
  • Great for special THT cases whereby size prevents selective wave soldering use
  • This doesn’t need the application of excess heat to board areas. This may be very sensitive to temperatures that are high.

Disadvantages of Selective Soldering

  • Not efficient enough for large-scale production
  • The setup is complex
  • Compared to selective wave soldering, it consumes more time

Selective soldering has what it takes to provide a beneficial process for different scenarios, most especially for the densely packed and small boards of today.

Process Involved in Wave Soldering

Wave soldering can be described as the automated PCBA soldering process. From the name, the printed circuit board passes over a solder wave. PCBAs with two sides that have components attached on both the bottom and top surfaces may need a second pass. With this process, all the components will be bonded simultaneously. Asides the improved speed, the process of wave soldering offers other great advantages.

Benefits of Wave Soldering

  • Setting it up is easy
  • Low costs involved
  • Quick process

However, anytime you are deciding to choose this process, you should also consider some of its disadvantages.

Disadvantages of Wave Soldering

  • Some of its components require more masking
  • Rework is usually more common; this is to ensure that the quality of the solder joint is great
  • Needs more flux, power, nitrogen, and solder
  • It usually leads to increased cleaning after the assembly

As the size of boards decreases and smaller SMDs proliferate, producers will need a soldering process that is more precise to make sure that the solder joints of these compliments are of top quality. The answer is selective soldering.

Comparing Both Selective and Wave Soldering

Applying selective soldering vs. wave soldering reveals a process choice between flexibility and efficiency, so far your CM has what it takes to institute both. We will be comparing these types of soldering using some attributes.

Set up: Wave soldering has a simple setup, while the set up of selective soldering is complex

Cost: Wave soldering is less expensive compared to selective soldering, which is more expensive

Cleaning: Additional cleaning is usually required for wave soldering, while less cleaning is required for selective soldering

Need for rework: For wave soldering, you are more likely to need rework, while for selective soldering, you are less likely to.

Material requirements: Wave soldering needs more material requirements, while the material requirements of selective soldering are less.

Repeatability: No repeatability for wave soldering, while for selective soldering, there is repeatability

Speed: wave soldering is faster, compared to selective soldering, which is slower

From the above comparison, we can see that each of the two methods come with its benefits. For example, manufacturers can see great cost savings, when they use wave soldering for producing high volume. Alternatively, for low-volume manufacturing and prototyping of complex boards, you can get a better cost-efficient process by selective soldering.

How Selective Soldering Can Assist your CM in Assembling PCBs

Just sit and consider a backplane having connectors on the two sides. The connectors’ tall profile makes the creation of custom pallets for wave solders a very difficult process. There was a time when the only thing which could work fine on these boards was the handheld soldering iron tip.

Now, rather than rely on the manual process of soldering by hand each connector’s hundreds of pins, the selective solder nozzle’s automated tip could handle the job at a lesser time frame and deliver even better results.

Usually, your manufacturer will go for wave soldering for assembly speed. If just a little extra soldering is needed after wave soldering, then hand soldering done manually can be done in a shorter time compared to running boards via selective solder. However, if your board needs something better than just a touch-up soldering, then the system for selective soldering will be the most preferred  choice for accuracy and speed, while still reducing the board’s thermal abuse.

Conclusion

By now, you should have gained vast knowledge about selective soldering. The major activity that takes place during the assembly stage is attaching the components to the board by soldering. This soldering depends on the components used. A selective soldering machine will solder thru-hole leads on a panel or board. It makes use of a pump system and solder reservoir to move molten solder to pass through a nozzle, then to the leads coming through the circuit board’s bottom. We have compared selective soldering and wave soldering, as well as outlined their benefits and pitfalls. Choose whichever suits your needs.

Looking back at the development process of the electronics industry in recent years, we can notice that a very obvious trend is reflow soldering technology. In principle, conventional inserts can also be reflow soldered, which is commonly referred to as through-hole reflow soldering. The advantage is that it is possible to complete all solder joints at the same time, minimizing production costs. However, temperature sensitive components limit the application of reflow soldering, whether it is an interposer or an SMD. Then people turned their attention to the choice of welding. In most applications, selective soldering can be used after PCB reflow soldering. This will be a cost-effective and efficient way to complete the remaining inserts and is fully compatible with future lead-free soldering.

PCBA

Process characteristics of selective welding

The process characteristics of selective soldering can be understood by comparison with wave soldering. The most obvious difference between the two is that the lower portion of the PCB in the wave soldering is completely immersed in the liquid solder, while in selective soldering, only a certain portion of the area is in contact with the solder wave. Since the PCB itself is a poor heat transfer medium, it does not heat the solder joints that melt adjacent components and PCB areas during soldering. The flux must also be pre-coated before soldering. Compared to wave soldering, the flux is only applied to the part of the PCB to be soldered, not the entire PCB. In addition, selective soldering is only suitable for soldering of the interposing components. Selective soldering is a completely new approach to thoroughly understand that selective soldering processes and equipment are necessary for successful soldering.

Selective welding process

Typical selective soldering processes include: flux coating, PCB preheating, dip soldering, and drag soldering.

Flux coating process

In the selective soldering, the flux coating process plays an important role. At the end of solder heating and soldering, the flux should be sufficiently active to prevent bridging and prevent oxidation of the PCB. The flux is sprayed by the X/Y robot carrying the PCB through the flux nozzle and the flux is sprayed onto the PCB to be soldered. Fluxes are available in single-nozzle spray, micro-hole spray, and simultaneous multi-point/pattern spray. The most important thing is the accurate spraying of the flux during the microwave peak welding after the reflow process. The micro-hole spray type will never contaminate the area outside the solder joint. The minimum flux point pattern diameter of micro-spraying is greater than 2mm, so the positional accuracy of the solder deposited on the PCB is ±0.5mm, so that the flux can always be covered on the part to be welded. The tolerance of the sprayed soldering dose is provided by the supplier. The technical specification should be The flux usage is specified and a 100% safety tolerance range is usually recommended.

Preheating process

The main purpose of preheating in the selective soldering process is not to reduce the thermal stress, but to remove the solvent pre-drying flux, so that the solder has the correct viscosity before entering the solder wave. During soldering, the effect of preheating the heat on the soldering quality is not a critical factor. PCB thickness, device package size, and flux type determine the preheating temperature setting. In selective soldering, there are different theoretical explanations for preheating: some process engineers believe that the PCB should be preheated before the flux is sprayed; another point of view is that soldering is not required without preheating. The user can arrange the selective soldering process according to the specific situation.

Welding process

The selective soldering process has two different processes: a drag welding process and a dip soldering process.

The selective drag welding process is performed on a single small tip solder wave. The drag welding process is suitable for welding on very tight spaces on the PCB. For example: individual solder joints or pins, single row pins can be dragged. The PCB moves at different speeds and angles on the solder wave of the tip to achieve the best solder quality. In order to ensure the stability of the welding process, the inner diameter of the tip is less than 6mm. After the flow direction of the solder solution is determined, the tip is installed and optimized in different directions for different soldering needs. The robot can approach the solder wave from different angles, that is, from 0° to 12°, so the user can solder various components on the electronic components. For most devices, the tilt angle is recommended to be 10°.

Compared with the dip soldering process, the soldering solution of the soldering process and the movement of the PCB board make the heat conversion efficiency during soldering better than the dip soldering process. However, the heat required to form the weld joint is transmitted by the solder wave, but the solder wave quality of the single tip is small, and only the temperature of the solder wave is relatively high, so that the requirements of the drag welding process can be achieved.

For example: solder temperature is 275 ° C ~ 300 ° C, drag speed 10mm / s ~ 25mm / s is generally acceptable. Nitrogen is supplied to the soldering area to prevent solder wave oxidation, and the solder wave eliminates oxidation, so that the drag welding process avoids the occurrence of bridging defects, which increases the stability and reliability of the drag welding process.

The machine is characterized by high precision and flexibility. The modular design system can be customized according to the customer’s special production requirements and can be upgraded to meet the needs of future production development. The robot’s radius of motion covers the flux nozzles, preheating and soldering tips, so the same equipment can be used for different welding processes. The machine-specific synchronization process can greatly shorten the board process cycle. The ability of the robot to make this selective weld has the characteristics of high precision and high quality welding. The first is the highly stable and precise positioning capability of the robot (±0.05mm), which ensures that the parameters of each board are highly repeatable. Secondly, the 5-dimensional movement of the robot enables the PCB to contact the tin surface at any optimized angle and orientation. Good welding quality. The tin wave height stylus mounted on the manipulator splint device is made of titanium alloy. The height of the tin wave can be measured periodically under program control. The height of the tin wave can be controlled by adjusting the speed of the tin pump to ensure process stability.

Despite these advantages, the single-nozzle solder wave dragging process is also inadequate: the soldering time is the longest in the three processes of flux spraying, preheating and soldering. And because the solder joints are one by one, the soldering time will increase greatly as the number of solder joints increases, and the soldering efficiency cannot be compared with the conventional wave soldering process. However, the situation is changing, and the multi-tip design maximizes throughput. For example, double-welded nozzles can double the throughput and flux can be designed as dual nozzles.

The immersion selective welding system has multiple soldering nozzles and is designed one-to-one with the PCB to be soldered. Although the flexibility is not as good as the robot type, the output is equivalent to the traditional wave soldering equipment, and the equipment cost is lower than that of the robot. Depending on the size of the PCB, single or multiple boards can be transported in parallel, and all solder joints will be soldered, preheated and soldered in parallel at the same time. However, due to the different distribution of solder joints on different PCBs, special solder nozzles are required for different PCBs. The size of the tip is as large as possible to ensure the stability of the soldering process and does not affect the surrounding adjacent devices on the PCB. This is important and difficult for the design engineer because the stability of the process may depend on it.

The immersion soldering process can be used to solder solder joints of 0.7mm to 10mm. The soldering process of short pins and small pads is more stable and the possibility of bridging is small. The distance between adjacent solder joints, devices and soldering tips should be More than 5mm.