Do you want to solder electronic components on your printed circuit board quickly? Do you find it time-consuming to solder them using the soldering irons manually? It is also unsafe to inhale smoke for a long time. So, who would not want to look for an alternate way of soldering? You just got lucky as there is another standard way of soldering and is super quick. Can you guess what we are talking about? It is wave soldering!
This process of soldering will allow you to make several printed circuit boards in a short time. So, this article is all about wave soldering. Stay put.
1.1 What is Wave Soldering?
Back to the days when surface mount technology was not entirely developed, “wave soldering” was a very famous soldering technique. Nearly every PCB used wave soldering for placement of electronic components. Wave soldering is a mass soldering procedure that will permit you to make many PCBs in a short time.
You will need to pass every PCB over a pan of liquefied solder. There, a pump will create a surge of solder that will resemble a standing "wave." This standing wave showers over the printed circuit board and the electronic components will get soldered to the PCB. So, the contact between the solder and the wave does the magic.
After that, the printed circuit board gets blowing air or water spray for safe cooling. This cooling process will secure the components in their place. Furthermore, wave soldering is usually done in a shielding gas environment as nitrogen usage is helpful in mitigating solder defects. Figure 1 shows electronic components placed on PCB and all ready to go under the soldering machine.
Image 1: Soldering Process
1.2. Technical Details about Wave Soldering
Technically, this soldering process utilizes a whole container of tin for performing welding. Tin goes through high temperatures, which melt its bar and molten tin is formed. The liquefied tin is taken as "lake water." It is called "leveling wave” when the lake is static and horizontal. And, It is called “spoiler wave” when there are waves in the lake.
The printed circuit board can be considered as a boat. It will float over the rough or calm lake, permitting the tin to attach electronic parts to the board. Then after the tin bath, you will see that it will cool rapidly and the solder will do its job. And, what is that? Of course, it will solder the electronic components to the PCB.
Moreover, you should make sure that temperatures are adequate during this process. If the temperature control is not enough, the circuit board can experience mechanical stress. That, in turn, will lead to conductivity loss and cracks. And, a low soldering temperature can cause improper solder thickness that can further lead to board stress.
1.3 When to use Wave Soldering?
Luckily, wave soldering can be used for both surface mount and through-hole printed circuit boards assemblies. In surface mount, you will need to glue electronic components onto the PCB surface using placement equipment. After that, it will be ready to pass via the liquefied solder wave.
In general, wave soldering is mostly used for welding of through-hole electronic parts. Hence, in many large-scale applications, where surface mount components are mostly used, you can use reflow soldering instead of wave soldering. Now you must be wondering what reflow soldering is? Don't worry; we will get to it shortly.
However, you can always use wave soldering for the applications which broadly makes use of through-hole components. We hope that by now you know the basic concept of wave soldering. In the next chapter, we have described the wave soldering process in detail.
2、Wave Soldering Process
2.1 Wave Soldering Machine
In the market, you will encounter many kinds of wave soldering machines. You can buy lead wave solder machines or lead-free wave solder machines. It is all up to you. However, the main principles and fundamental parts of all of these machines are alike. A conveyor is a fundamental part that is used during this process. It takes printed circuit boards via various zones.
Next, you will see a pan of solder and pump that is responsible for generating the main wave. Furthermore, you will also get a flux sprayer and a preheating pad. Hence, these four main parts make up a soldering machine. The solder in wave soldering machines is mostly made up of metals’ mixture.
If the machine has leaded solder, it will contain 49.5% lead, 50% tin, and 0.5% antimony. However, in the latest machines, lead-free models are available due to health concerns. So, tin-copper-nickel and tin-silver-copper alloys are frequently used. Figure 2 illustrates a wave soldering machine.
Image 2: Wave Soldering Machine
2.2 Wave Soldering Temperature
Currently, tin alloys for soldering are regularly using Sn 60/Pb40 and Sn 63/Pb37. Therefore, it is recommended that you should make sure that the operating temperature stays around 260° ± 5°C. Nonetheless, you should also take into account the overall weight of PCB and the parts.
Practically, heavy components can be heated up to 280°C. And, lightweight components, which are heat-sensitive, can be heated at a temperature as low as 230 °C. Further, you should also consider preheating and the conveying speed. Figure 3 shows close up of melting tin.
Image 3: Wave Soldering
However, it is best if you change the conveying speed rather than tin’s temperature, because, the temperature change will damage the solder joints' quality by affecting the fluidity of the liquefied tin. At high welding temperatures, copper will start to dissolve, and this will ruin the quality control of overall soldering.
During the wave soldering, you should apply liquid flux on the printed circuit board surface. You will observe that fluxing will enhance the soldering quality of the electronic components. These components, printed circuit boards and also the tin liquid, when stored, are exposed to the atmosphere. This exposure can make them oxidized and thus affects the soldering quality.
Flux mainly removes dirt and oxides on the surface of the metal. Further, it also creates a film to stop the air from reacting with the metal surface during high-temperature setup. Thus, solder cannot get oxidized easily. Nonetheless, you should use the liquefied tin for soldering during the wave soldering process.
Currently, the melting point of SAC305 lead-free solder is around 217 °C. And, the flux cannot be exposed to such a high temperature for long. Hence, if you wish to use flux, you should add it before printed circuit board goes through the tin solution.
Normally, flux can be applied by two means. First, you can use a foaming flux and second, you can add it by spraying. In foaming flux, the flux gets attached to the circuit board, which passes through it. The major downside of this way is that you may observe that flux is not uniformly applied. Thus, poor soldering can happen in the areas where flux doesn’t exist.
In spraying method, the flux is sprayed through the nozzle as the circuit board passes. The drawback of this method is that the flux can be easily passed through the board gaps. And, flux can also directly pollute the electronic components of the circuit board’s front. Further, if the flux is not processed and only dropped straightly on the board, you can also observe corrosion of board.
Normally, Preheating before the main wave welding process begins. It can increase the upper plate's temperature to between 65 and 121°C with heating rate lying between 2 °C /s and 40 °C /s. You will not be able to get the best soldering results if preheating is insufficient. It is because flux may not be able to reach every part of PCB. On the other hand, if you set a very high temperature for preheating, "no-clean" flux can suffer. Now, if you are wondering what exactly a “no-clean” flux is, we have explained it in the next sub-section.
Cleaning process washes a PCB with deionized water or with solvents to get rid of flux remains. However, there exists a kind of flux which doesn't need cleaning. Can you guess which one? Yes, of course, the "no-clean" fluxes, their remains after the soldering process are benign.
But, you need to be careful; there are some applications which do not want “no-clean” fluxes. It is simply because “no-clean” fluxes can be very sensitive to the conditions of the process. Now that you know all about the wave soldering process. The coming chapter will relate wave soldering with other types of soldering.
3、Types of Soldering
3.1 Dip Soldering vs. Wave Soldering
In simplest terms, dip soldering is a soldering process with limited scope. Just like wave soldering, it can be used both for surface mount and through-hole circuit board assemblies. Further, the solder rains over the bare metallic areas of the printed circuit board. Thus, you will observe a reliable electrical and mechanical connection. Finally, dip soldering is the manual version of the automatic soldering process.
3.2 Reflow Soldering vs. Wave Soldering
Reflow soldering is the most famous way of fixing surface mount components onto the printed circuit board. You will need to create a solder paste out of flux and solder powder. And then, you will use that paste to fix electronic components onto the contact pads. You will further heat the whole package under an infrared lamp or in a reflow oven. The solder will then liquefy and make connections between the joints.
On the other side, you can also solder different joints with a hot air pencil. Figure 4 shows the printed circuit board assembling moving into the reflow oven machine.
Image 4: Wave Soldering
Now, you must be wondering which technique should be used and when? Generally, wave soldering is more complicated than reflow soldering. In wave soldering, the time for which the PCB stays in the solder wave and PCB temperature require careful monitoring. The printed circuit board can be defective if the soldering environment is not right.
However, with reflow soldering, you don’t need to fret much about environment control. But with this being true, you must know that wave soldering is cheaper and faster than reflow soldering. And, in many applications, wave solder is the only useful way of soldering components onto the board.
You will notice that reflow is mostly used for small-scale applications. Such applications do not need reliable, cheap and fast mass production of PCBs. Surprisingly; you can also use a combination of reflow and wave solder. You can solder components on one side with wave soldering and can use reflow soldering on the other side.
So, these are some alternatives to wave soldering. However, there is still another type of soldering technique that the next chapter compares with wave soldering.
4、Selective Wave Soldering
4.1 Selective Soldering Machine
What if you have sensitive parts that can get damaged in the wave solder process or reflow oven? What do you suggest should be done to avoid the high temperatures? Would you want to try your luck with wave soldering or reflow soldering? Or, would you wish for any other way? Well, fortunately, that is where the selective wave soldering comes in.
You should go for selective soldering when you fear that your electronic components will not survive reflow or wave soldering. You will find a wide variety of selective wave solder machines in the market. There are nitrogen inserted standard machines, solder pot type machines and many others. Figure 5 demonstrates the selective wave soldering machine.
Image 5: Wave Soldering
4.2 Selective Wave Soldering Guidelines
When you buy a selective wave soldering machine, software and guidelines will come with them. Generally, you will need to do the following three steps to do work:
• You need to apply the liquid flux,
• You need to assemble the PCB or preheating,
• You need to solder using a “site-specific” solder nozzle.
4.3 Selective Soldering problems
In selective soldering, you can encounter the following problems:
1. Copper pad dissolution: High temperature can dissolve the copper pad into the molten solder.
2. Solder balling: Solder balls can be formed due to the sticking of solder mask at high temperature.
3. Solder bridging: Excess solder can make an additional connection between two pins.
4. Solder Stringing: Solder remains across the solder nozzle can cause it.
4.4 Selective Wave Soldering Machine Cost
If you want to compare selective soldering machine with wave soldering machine cost, you will be pleased to know that selective soldering will be five times cheaper. It will be because of less electricity requirement, less flux and solder consumption, no cleaning, less rework, and no protective taping.
Hopefully, you can now decide about the soldering technique that you want to use. But before you make your final decision, we have also mentioned the defects, cost, and problems of wave soldering in the next chapter.
5、Wave Soldering Defects and Problems
5.1 Wave Soldering Defects and Problems
If temperature and soldering environment are not properly controlled you will encounter the following defects after the wave soldering process:
• Poor Conductivity
• Inadequate solder thickness
And, these are the few problems with wave soldering:
• Higher consumption of electricity, flux, solder, and nitrogen
• Need for solder rework
• Extra sensitive points masking
• Extra cleaning of soldered assemblies wave solder aperture masks or pallets
Image 6: Wave Soldering
5.2 Wave Soldering Cost
So, now after reading the whole article, can you guess the operating costs of a wave soldering machine? You can guess concerning selective soldering. Well, because of the problems mentioned above and defects, wave soldering can be five times more expensive.
In this guide, we have explained everything related to the wave soldering process. We have even mentioned the alternates of soldering methods. We aimed to clear out any questions you may have regarding wave soldering. Now, you can easily decide which method of soldering is suitable for you and when.
Moreover, you can contact us if you want to manufacture PCB or want its assembly. You can even ask us about the techniques we use for PCB assembly. We will guide you in every possible way.
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