SELECTIVE VS WAVE SOLDERING VS REFLOW SOLDERING: What is the Difference?

Soldering is putting metal parts together with melted solder, which has a lower melting point than other metals. It is a process essential to the electronics industry and the primary way to connect electrical components. Soldering printed circuit boards are also referred to as PCB soldering. One of the most fundamental skills for everyone who wants to deal with electronics and electrical circuits is this kind of soldering. The most basic definition of the soldering process is that it is a technique of attaching two little components on the surface of the PCB, which is an abbreviation for Printed Circuit Board. There are many various ways you can finish the soldering process. To join two or more separate electrical components on your circuit board, in other words, soldering, is a technique. Types of PCB Soldering: Wave Soldering, Reflow Soldering, and Selective Soldering.

Table of Contents

1- What is Selective Soldering?

Selective soldering is one of the soldering processes that gives the user full control over soldering. In this type of soldering, flux is jetted on the specific portion that needs to be soldered. Then on the component pins, a fountain of molten solder is applied. Now, you can either use drag soldering to solder the entire row or solder individual pins.

Selective soldering has two additional stages that are specifically for preheating and soldering. Each of these stages is programmed to solder different areas of the board. Here, each solder pot is automatically refilled as per need. 

The Ursa selective soldering system helps in the rapid changeover that is built for high mix manufacturing. The soldering of different components at three different stages depends on the engineer. The use of different size nozzles for different size leads is one of the factors that engineers take into consideration.

Selective Soldering

Process Characteristics of Selective Soldering

Selective soldering is not very difficult to understand. We can simply say that selective soldering is slower than wave soldering. The components in selective soldering are sequentially soldered by a local wave individually. It is somewhat opposite to a full wave that hits all of the joints of the solder at once.

If you want to understand the process characteristics of selective soldering fully, you need to compare it with wave soldering. We know that the lower part or portion of the PCB is fully immersed in the liquid solder in wave soldering. But in selective soldering, only a part or portion of PCB comes in contact with the solder wave. 

PCB is not a good medium for heat transfer and fails to heat the solder joints of the PCB during soldering. Hence, flux is needed to be coated on the portion before soldering that part. But in the case of wave soldering, the entire PCB needs to be coated. However, selective soldering is mostly used for soldering interposing components on the PCB.

Selective Soldering Process

The process of selective soldering mainly consists of four processes.

  • Flux Coating Process
  • Preheating Process
  • Drag Soldering
  • Dip Soldering

Each of these processes is significant and contributes to the selective soldering process. Let us get into the details of these processes and see how it works.

1- Flux Coating Process

Flux coating is the first and most crucial step of selective soldering. It is essential to make the flux sufficiently active as it will prevent oxidation and bridging of the PCB, mainly at the end of solder heating and soldering. The flux is usually sprayed by the robot. This robot holds the PCB and takes it through the flux nozzle, and then the flux is sprayed on the PCB to be soldered. Now, there are different types of flux sprays available. These are single-nozzle spray, micro-hole spray, simultaneous multi-point spray, and pattern spray.

The spraying phenomenon of the flux should be accurate during the microwave peak welding after the reflow process. Micro-hole spray is the most accurate and used the most because it never contaminates the area outside the solder joint.

2- Preheating Process

Preheating in selective soldering is mainly to remove the flux from the parts where it is not needed, and the solder will have a proper viscosity before going into the solder wave. Keep in mind that it is not to reduce the thermal stress on the PCB. The settings of preheating temperature depend on the thickness, device package size, and flux of the PCB. 

Preheating can be done in different combinations. Some people think that flux should be sprayed after the preheating process. On the other hand, some people think that there is no requirement of soldering before preheating.

3- Drag Soldering

Drag soldering is mainly done using small single nozzle waves. It can reach tight areas on the board. Heat transfer in drag soldering is inherently better than dip soldering due to the motion of the board and solder. It eliminates bridges from the PCB because it washes off the oxides. It also increases the robustness and reliability of the drag soldering.

In drag soldering, the lead lengths are limited. Another drawback of drag soldering is extended cycle time. 

Drag soldering does not contain certain capabilities like dip soldering. However, the results achieved from both drag and dip soldering are different due to heat distribution characteristics and thermal transfer characteristics of different types of boards.

4- Dip Soldering

Dip soldering involves a process of dipping the PCB on a custom nozzle plate that solders all the joints in one go. Multiple connections are soldered at once, but different tooling plates are required for each type of PCB. Dip soldering has different capabilities than drag soldering; they constitute a robust process with the least cycle times when used together.

Advantages and Disadvantages of Selective Soldering

Advantages:

  • Selective soldering gives the soldering operator the flexibility to utilize the soldering variables so that greater solder joints can be achieved in a short time and with less heat.
  • The amount of solder and the temperature of soldering depends on the operator.
  • The operator has the freedom to specify how the nozzle should move, quickly or slowly, to give enough time for the through-hole to be filled with solder.
  • The solder joints after selective soldering are much more reliable.
  • The operator programs the exact place where the molten solder nozzle should go for soldering or solder application.
  • These authorities allow the operator to be more precise with the soldering process without the need for a steady hand to handle and control the soldering iron.
  • Before selective soldering, expert technicians were required to hand-solder the through holes. But now, this soldering process is done quickly without needing a real hand.
  • When you are using selective soldering, there is no need to use expensive aperture wave solder pallets.
  • Selective soldering allows the operator to customize the board to be able to accommodate different parameters of different components.
  • Selective soldering can lead to cost optimization.
  • The operator can also limit the areas of the board to be masked.
  • Selective soldering works very well for special THT cases.
  • Moreover, selective soldering doesn’t need the application of excess heat.
  • Lastly, there is no requirement or such need for glue for SMDs.

Disadvantages:

  • The setup of selective soldering is very complex.
  • Selective soldering takes much more time than selective wave soldering.
  • Much exposure to heat can cause thermal issues to the PCB, its solder joints, and the components as well.
  • Requires post-assembly cleaning
  • Selective soldering is not suitable for mass production.

When to Choose Selective Welding?

There are a few cases in which hand-soldering is inefficient, and wave soldering cannot be used for different reasons. In such cases, selective soldering is the only solution you’re left with. There are many scenarios in which you need to choose selective soldering; these are:

1- When you use tall components. Tall components restrict the heat of wave soldering to reach the board, and it doesn’t solder the tall components. Here, selective soldering comes in place and makes the soldering possible.

2- Some boards are quite thick or have thick copper layers for both ground and power planes; these boards are difficult to solder by hand. It is nearly impossible for a single soldering iron to effectively solder the through holes to form a satisfactory solder joint. Although all of the metal in the board is thermally connected, it is still difficult to solder it by hand.

3- When the thru-hole components are tightly or closely placed together with SMT components, there is not enough space or room to place a protective fixture that will allow an effective wave soldering. Due to this reason, selective soldering will work here.

4- Whenever large connectors are used with hundreds of pins, it is very difficult for a single soldering iron to solder all of these pins. Due to the dense concentration of thru-hole pins, selective soldering is preferred.

5- Moreover, selective soldering allows the operator to program and handle every pin configuration. Along with this, selective soldering can use a wider nozzle to solder one or two rows of different connector pins in a single attempt.

6- When you need consistency, you can go for selective soldering. Hand soldering will have variations as operators do it by hand. Precision does not come with handheld things. Selective soldering will give you the same result every time.

2- What is Wave Soldering?

Wave Soldering

Definition

As the name suggests, in wave soldering, PCB passes over a wave of solder. The solder is applied to all of the components simultaneously. However, double-sided PCBs may require a second pass. Besides having an improved speed, there are other advantages of wave soldering as well.

Wave soldering is also known as flow soldering. This type of soldering is performed in a protected environment because it uses nitrogen gas. But surprisingly, the use of nitrogen gas significantly reduces the possibility of solder defects.

Wave Soldering Process

The wave soldering process involves simple steps.

1- It is important to spray a layer of flux to clean the PCB and prepare the components for assembly. In case of any impurity, it can severely affect the soldering process.

2- After spraying the flux, it is important to preheat it as well. Preheating ensures that PCB doesn’t suffer from any kind of thermal shock.

3- When all the things are done and the PCB is ready, it is passed through a molten solder. PCB moves over the wave, and a kind of electrical connection is generated between the leads of the components, PCB pads, and also the solder.

4- Cleaning is a must in wave soldering. It means dusting the residues of flux. Clean and then wash the PCB with deionized water and solvents.

Advantages and Disadvantages of Wave Soldering

Advantages:

  • It has a significantly lower cost than other types of soldering.
  • Quick and tedious process.
  • Simple and easy setup.
  • Mostly used for large unit volume production.

Disadvantages:

  • It requires additional masking of different sensitive points on PCB.
  • It Consumes a high amount of flux.
  • Consumes a high amount of solder
  • It requires a high amount of nitrogen.
  • Consumes higher electricity
  • Cleaning is needed after soldering.
  • Increase in rework post-soldering to ensure good solder joints.

3- The Difference Between Selective Soldering and Wave Soldering

The Difference Between Selective Soldering and Wave Soldering

In selective soldering, flux is only applied to the components that need soldering. The board is then preheated to prevent thermal shock, and then a bubble of solder is used for soldering the components specifically. Whereas in wave soldering, flux is sprayed over the whole board. The purpose of this spray is to clean the board and make it ready for the components for soldering. Then the board is preheated to prevent thermal shock. Lastly, it goes through an oven of a molten wave of solder where the board gets soldered.

In previous times, wave soldering was the primary soldering technique that people used for soldering. It is because PCBs were large and had big size components. It was an ideal method to solder for different projects that had conventional thru-hole components and large SMT components. All of these components were spread out on the board, and it was easy to solder it with a wave of molten solder.

But now, as time and technology are advancing, we require small and compact boards with the dense placement of components. Here, wave soldering is not suitable and does not work perfectly. A densely populated board needs individual soldering that varies from component to component. 

With selective soldering, each area or component can be controlled to allow for the adjustments of different parameters. But here, you will need to program every board uniquely to solder it to perfection.

All in all, both of the techniques are ideal and have their pros and cons. Selective soldering is best for a simple board that is densely populated and allows some parameters to be altered. In contrast, wave soldering is best for large PCBs with large SMT components and thru-hole components. It works very well for the mass production of PCBs.

4- Which Soldering Method is More Suitable for Your PCB Assembly?

As stated above, there are different types of soldering methods that you can use for your PCB assembly. It all depends on the size and circuitry of your PCB. 

If your PCB is large, has large SMT and thru-hole components, and is not densely constructed, then you can use wave soldering. But if your board is complex and there are multiple small components, all closely linked with one another, then you can use selective soldering.

The soldering method depends on the assembly and construction of your PCB. You need to see whether the components are small or big and there is enough space for wave soldering or not. Different factors need consideration before making a final decision.

5- What is Reflow Soldering?

Definition

Reflow soldering is a little like wave soldering, except for some of the facts. Before reflow soldering, you need to make your PCB ready. 

To make PCB ready for reflow soldering, you need to perform two tasks. The first thing you need to do is to preheat the board so that the temperature of the board increases. Secondly, the board needs to be thermal soaked so that the flux in the solder paste gets activated for soldering.

First, a sticky paste is applied to the board. This paste consists of both solder and flux. The paste is spread on the board with the help of a solder stencil. After that, components are placed on the board. The paste is sticky, so the components on the board do not fall. Then the board is passed through a reflow soldering oven. A conveyor takes the PCB inside the oven, where the solder melts and the components are fixed and soldered to the board. Finally, the board is cooled down to harden the solder.

Reflow Soldering

Advantages:

  • Reflow soldering is easy to control and monitor.
  • The solder paste only needs to be applied to the area where soldering is needed, with less consumption of solder paste.
  • Reflow soldering is very effective when it comes to different SMT types in a single process.
  • The operator can limit the soldering to prevent bridging.
  • Reflow soldering does not have impurities like others.
  • Thermal shock is very less in reflow soldering because the components are not immersed in molten solder.

Disadvantages:

  • The profile of the reflow soldering must be matched with the specific requirements of solder paste and the thermal thresholds.
  • Lead-free solder reduces the size of the thermal process window, but it does not change the principles of reflow soldering.
  • Due to improper printing parameters, solder defects arise in reflow soldering. More reasons for solder defects include incorrect or damaged design stencils.

6- Wave Soldering vs Reflow Soldering

Although wave soldering is more complex than reflow soldering, it is much quicker and used for mass production. Reflow soldering is simpler but requires time as solder paste is applied with the help of a stencil, and then PCB goes into the reflow oven. 

But when it comes to costs, wave soldering is cheaper as it does not require a stencil and reflow oven. Wave soldering machine is much cheaper than a reflow oven. This comparison has already been discussed in detail. Refer to the article, Difference Between Reflow Soldering and Wave Soldering, to get in-depth knowledge.

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