Wave soldering process:
Insert components into corresponding component holes → precoat flux → prebake (temperature 90- 1000C, length 1- 1.2m) → wave soldering (220-240 c)→ cut off redundant pins → check.
Reflow soldering technology is to re-melt the paste solder pre-distributed on the printed board pad, and realize the mechanical and electrical connection between the soldering terminals or pins of surface assembly components and the printed board pad.
Wave soldering is a new welding technology with the enhancement of people's awareness of environmental protection. Tin-lead alloy was used before, but lead is a heavy metal, which is very harmful to human body. So now there is a lead-free process. It uses * Sn-Ag-Cu alloy * and special flux, and it needs higher welding temperature and higher preheating temperature. After the PCB board passes through the welding zone, a cooling zone workstation is also needed. This aspect is to prevent thermal shock, if there is ICT, it will have an impact on the detection.
In most products that do not need miniaturization, TH or hybrid technology circuit boards are still used, such as televisions, home audio-visual equipment and digital set-top boxes to be launched soon. Perforated components are still in use, so wave soldering is needed. From the technological point of view, wave soldering machine can only provide a little adjustment of the most basic equipment operation parameters.
First, the production process
After the circuit board enters the wave soldering machine through the conveyor belt, it will pass through some form of flux coating device, where the flux is coated on the circuit board in the form of wave crest, foaming or spraying. Because most fluxes must reach and maintain the activation temperature in the welding process to ensure the complete penetration of solder joints, the circuit board must pass through the preheating zone before entering the trough. Preheating after flux coating can gradually increase the temperature of PCB and activate flux, and this process can also reduce the thermal shock when assembly enters the peak. It can also be used to evaporate all the water that may be absorbed or dilute the carrier solvent of flux. If these things are not removed, it will boil over the peak, causing solder sputtering, or leaving steam in the solder, forming empty solder joints or sand holes. The length of preheating section of wave soldering machine is determined by the output and the speed of conveyor belt. The higher the output, the longer the preheating period is, so that the board can reach the required wetting temperature. In addition, due to the large heat capacity of double panels and multi-layers, they need higher preheating temperature than single panels.
At present, wave soldering machines basically adopt thermal radiation preheating, and the most commonly used wave soldering preheating methods include forced hot air convection, electric heating plate convection, electric heating rod heating, infrared heating and so on. Among these methods, forced hot air convection is generally considered as the most effective heat transfer method of wave soldering machine in most processes. After preheating, the circuit board is welded by single wave (λ wave) or double wave (spoiler wave and λ wave). For perforated assemblies, a single wave is sufficient. When the circuit board enters the wave crest, the flow direction of solder is opposite to the moving direction of the circuit board, which may cause eddy current around the component pins. This is like a kind of cleaning, which removes all the residual flux and oxide film on it and forms wetting when the solder joint reaches the wetting temperature.
For the assembly of hybrid technology, generally, spoiler waves are also used in λ wavefront. This kind of wave is narrow, and the vertical pressure is high when disturbed, which can make the solder well penetrate between the closely placed lead and the die pad, and then use λ wave to complete the formation of the solder joint. Before making any evaluation of future equipment and suppliers, it is necessary to determine all the technical specifications of wave soldering plate, because these can determine the performance of the required machine.
Second, avoid defects.
With smaller components and denser PCBs, the possibility of bridging and short-circuiting between solder joints increases. But there are some effective ways to solve this problem, one of which is to use air knife technology. This is to blow a bunch of hot air or nitrogen to the melted solder joint with an air knife when the PCB leaves the peak. This kind of air knife with the same width as PCB can comprehensively check the quality of the whole PCB width, eliminate bridging or short circuit, and reduce the operating cost. Other possible defects include virtual soldering or missing soldering, also known as open circuit, which will happen if the PCB is not coated with flux. If the flux is insufficient or the operation in the preheating stage is incorrect, the wettability of the upper surface will be poor. Although the welding bridge connection or short circuit can be found in the post-welding test, it is necessary to know that the virtual welding will also pass the test in the post-welding quality inspection, but there will be problems in the future use. Problems in use will seriously affect the set minimum profit index, not only because of the expenses incurred in field replacement, but also because customers find quality problems, which will have an impact on future sales.
In the wave soldering stage, PCB must be immersed in the wave crest to coat solder on the solder joint, so the height control of the wave crest is a very important parameter. A closed-loop control can be attached to the peak to keep the height of the peak unchanged. Sensors can be installed on the guide rail of the transmission chain above the peak to measure the height of the peak relative to the PCB, and then the correct tin immersion height can be maintained by increasing or decreasing the speed of the tin pump. The accumulation of tin slag is harmful to wave soldering. If tin slag accumulates in the tin bath, the possibility of tin slag entering the peak will increase. This problem can be avoided by designing a tin pump system to extract tin from the bottom of tin bath instead of the top of tin slag accumulation. Using inert gas can also reduce tin slag and save money.
Third, inert welding.
Nitrogen welding can reduce tin slag and save cost, but users must bear the cost of nitrogen and the upfront investment of transportation system. It is usually necessary to compromise the above two factors, so it is necessary to determine the cost savings of reducing maintenance and reducing the reject rate because of better solder joint infiltration. In addition, the low residue process can also be used, at this time, some flux will remain on the circuit board, which is acceptable according to the requirements of products or customers. Users, such as contract manufacturers, will not have overall control over the design of the products they weld, so they need to seek a wider range of processes, which can be achieved by using corrosive flux and then cleaning. Although there will be initial equipment investment, in most cases, this is the lowest cost method, because the products from the production line are of high quality and do not need to be reworked.
Fourth, the issue of productivity.
Many users use automated online equipment for manufacturing and assembly seven days a week. Therefore, the issue of productivity is more important than before, and all equipment must have the highest possible normal operation time. When choosing wave soldering equipment, the MTBF (mean time between failure) and MTTR (Mean Time to Repair) of each system must be considered. If the system adopts liftable panel, foldable back door and fully operational desktop access door, and has high maintainability, it can achieve lower MTTR. Similarly, in view of reducing the maintenance of the solder module and the flux coating device, a shorter maintenance time can be obtained.
5. What wave soldering method is used?
The adoption of wave soldering method or process depends on the complexity and output of the product. If you want to make complex products with high output, you can consider the nitriding process such as CoN▼2▼Tour peak to reduce tin slag and improve the wettability of solder joints. If medium-sized machines are used, their technologies can be divided into nitrogen technology and air technology. Users can still handle complex circuit boards in an air environment. In this case, corrosive flux can be used according to customer requirements, and then cleaned after welding, or low solid flux can be used.
Six, air knife bridge technology
There are many advanced auxiliary options in various machine types. For example, Speedline ELECTROVERT provides a patented hot air knife bridging technology to eliminate bridging and conduct nondestructive stress testing on solder joints. The air knife is located at the exit of the welding groove, and it sprays 0.4572mm narrow hot air to the welding spot at an angle of 40 ~ 90 with the horizontal plane. It can make all the punched solder joints that are not welded well for the first time because of air residue, and will not affect the normal solder joints. However, it must be noted that in order to significantly improve the quality of solder joints, it is not necessary to set more options on wave soldering equipment. And for all production equipment, it is also very important to check the authenticity and accuracy of each engineering data. The best way is to run the board with the machine before buying it.
Seven, the choice of machine
According to the price and output, wave soldering machines can be roughly divided into three categories.
40,000 to 55,000 dollars can buy an entry-level vertical machine with medium and low output. Although there are cheaper desktop models, these are only suitable for R&D or prototyping, because they are not enough to meet the growing demand of manufacturers. A typical machine of this type has a conveyor belt output speed of about 0.8 m/min to 1 m/min, and uses foaming or spraying flux coating equipment. There may be no convection preheating device, but most suppliers will provide machines with single wave and double wave performance.
You can buy a medium-sized machine for $48,000 to $80,000. The preheating zone is about 1.22m to 1.83m, and the production speed is about 1.2m/min to 1.5m/min. In addition to the standard configuration of double peaks, it also provides more advanced configuration, such as inert gas environment.
In the high-end market, you can buy high-yield machines for $95,000 to190,000, which can run 24 hours a day with almost no human intervention. Generally, the preheating length of 1.83 m to 2.44 m can be used, and the output of more than 2 m/min can be obtained. At the same time, it also includes many advanced functions, such as statistical process control and remote monitoring devices, as well as spraying, foaming and peak flux coating systems in the same machine, and may also have three-peak performance.