Many people believe that the conversion of tin-lead (SnPb) to lead-free can be achieved by directly adding lead-free solder to an existing wave soldering machine. Others believe that it is necessary to use a new type of wave soldering machine in the lead-free process.
In fact, due to the predominance of surface mount components, the current reflow soldering process has focused on lead-free manufacturing. Wave soldering must also be converted to lead-free technology to avoid mixing lead-containing alloys with lead-free alloys on the same component. This is also a problem that many producers need to consider.
For the lead-free wave soldering industry, the positive is that as lead-free production continues to grow, many manufacturers are looking for ways to save money by upgrading existing wave soldering machines to meet the needs of lead-free components. Methods. And many of them have been very successful, just like our company Shenzhen Weida Electronic Equipment Co., Ltd.
To make the lead-free peak soldering process a success, you must consider changing the entire process. Because most lead-free solders have good solderability, they exhibit a decrease in wettability compared to tin-lead solders. Since wettability is a key factor in soldering and is affected by multiple variables, the soldering process needs to be adjusted and these changes will affect most of the machine parameters. The company took this aspect into consideration when developing new products.
Since the wetting characteristics of lead-free alloys are not as good as those of tin-lead solders, the use of excellent flux compounds is critical. In addition, the higher temperatures required for lead-free soldering require solder compounds to withstand preheat temperatures up to 130 ° C and liquefaction solder temperatures of 280 ° C for up to 3 seconds. Water-based fluxes free of volatile organic compounds (VOC free) are generally recommended.
The tin content of lead-free alloys is significantly higher than that of tin-lead solders, and lead-free alloys require higher process temperatures. Many products are progressively turning to lead-free, and many manufacturers are producing wave soldering machines that are compatible with lead-free products.
According to the continuous testing of the company's R&D personnel, the preheating requirements for lead-free wave soldering require a heating zone with a transfer speed of 120 mm/min to reach a length of 1.8 m, while a heating zone with a transfer speed of more than 180 mm/min requires a length of 2.4 m. An effective upgrade method is to replace the existing flux sprayer with an external flux sprayer. This not only improves the quality of the flux application, but also frees up space inside the wave soldering machine.
At the same time, the developer also found that for a tin-silver-copper alloy (SnAgCu) with a melting point of 217 ° C, the temperature of the solder pot should be between 260 ° and 270 ° C. For high melting point alloys such as tin-copper (SnCu), the soldering temperature can be between 270° and 280°C.
Since the wettability of lead-free alloys is poor and requires long contact times, the characteristics of the laminar soldering tip are usually changed. The distance between the chip and the laminar flow is usually reduced to minimize the temperature drop between the contacts. Increasing the laminar wave length can improve the wettability while increasing the preheating, producing a similar effect. Reducing the pressure head height of the laminar flow wave can reduce the distance over which the solder overflows, thereby reducing the amount of dross.
Existing wave soldering machines require upgrades with newer flux sprinklers for processing volatile organic compound (VOC) water-based fluxes. Ultrasonic or nozzle type flux spray equipment works best because the droplet size of the flux is controllable and continuous uniform patterning can be applied across the printed circuit board (PCB). Using a flux free of volatile organic compounds, droplets of the smallest size can be obtained, and good through-hole penetration can be obtained.
In order to achieve higher temperatures and avoid overheating of the PCB when loading chips or laminar waves, a longer preheating zone is often required. Achieving an appropriate preheat temperature at the top of the PCB has the greatest impact on reducing soldering defects. Combining the method of far-infrared heating from the bottom of the board with the method of convective heating from the top of the board gives the PCB the best preheating effect.
Compared to tin-lead solders, most lead-free alloys oxidize quickly when the solder liquefies due to the increased tin content. Oxides and scums composed of tin oxide (SnO and SnO2) are formed very quickly due to the high process temperature. The inert nitrogen atmosphere in the solder pot allows the liquefied solder to be exposed to an oxygen atmosphere as little as possible, thereby reducing the amount of scum. Reducing the rate of oxidation and focusing on the scum can significantly improve solder performance.
The OSP coating on bare copper quickly replaced the traditional hot air leveling (HASL) board coating, making it necessary to weigh the potential impact of lead contamination and copper contamination. Since lead-free wave soldering is widely used, the high dissolution rate of copper in lead-free alloys requires an increase in the maintenance of the solder pot, causing a series of problems.
During long periods of operation, the flow of certain lead-free alloys in the solder pot gradually slowed down. This is due to the accumulation of copper-tin intermetallic compounds (CuSn) at the bottom of the solder pot. With standard tin-lead wave soldering, the copper-tin intermetallic compound is flowing, and such problems do not exist.
In a standard tin-lead wave soldering furnace, impurities such as copper form intermetallic compounds with tin as they accumulate. Lower the temperature of the solder pot, let the solder pot idle for several hours, remove the scum on its surface, and easily remove intermetallic compounds. This method works well because the copper-tin intermetallic compound (CuSn) has a density of 8.28, while the tin-lead (SnPb) density is 8.80, and the copper-tin intermetallic compound can flow. Periodic maintenance of the tin-lead soldering furnace maintains a copper content between 0.15 and 3%, which is acceptable. The density of tin-copper or tin-silver-copper lead-free solder alloys is less than the density of tin-lead solders.
The copper-tin intermetallic compound does not float on the surface, but penetrates into the lead-free alloy and is dispersed throughout the solder pot. In addition, some lead-free alloys dissolve copper faster than tin-lead. This is due to the fact that copper build-up and solder paste impurities are formed at a faster rate, so that the solder in the solder pot needs to be replaced frequently or thoroughly cleaned.
Based on the results of the study, we recommend that when the copper impurities in the solder pot containing the lead-free alloy reach 1.55%, the activity of the solder pot alloy must be lowered. Above this value, most lead-free alloys have low activity, and copper impurities above 1.9-2.0% can damage turbines, baffles, and solder pots.
Since tin is corrosive at high temperatures, many lead-free alloys can cause corrosion of the base metal used in turbines, baffles, and solder pots. Many base metals, such as stainless steel or cast iron, often have rust on the surface and begin to dissolve after prolonged contact with the lead-free alloy. Such a leaching process will release iron (Fe) particles, causing the solder alloy to be contaminated.
When using lead-free alloys, solder pot materials, such as stainless steel, are damaged after only a few months of operation. The use of high-grade stainless steel to some extent reduces this effect, and the anti-corrosive surface coating has been patented. However, because of the high level of maintenance required, the copper intermetallic compound immersed in the bottom of the solder pot can be removed and the surface coating may be scratched. After a period of time, an increase in scratches will damage the surface coating, resulting in corrosion of the base metal and contamination of the solder pot. In the absence of protection, the parts made of these base metals will be degraded to the extent that they need to be replaced after only one or two years of use.
Traditional base metal-made solder pumps and baffles, used in lead-free soldering, wear at a frequency that causes people to rethink the use of titanium. The retrofit box replaces the turbine, baffle and solder nozzles, and its components are made of titanium to meet the long-term requirements of lead-free applications.
The titanium welding furnace can be lined into the existing cast iron welding furnace, so that the existing wave soldering machine can be used in the lead-free process. This titanium furnace lining can be used for various types of wave soldering machines. Modification. This method does not have the problem of iron dissolution, which minimizes solder impurities and does not require periodic dumping of the solder pot.
Conclusion: The use of an external flux sprayer, a new preheating kit, a solder pot retrofit box, and a titanium soldering lining to upgrade existing wave soldering machines can reduce the cost of transitioning to lead-free wave soldering. Especially when applying an upgrade solution to an old wave soldering machine. This approach provides effective lead-free performance at a fraction of the cost of a new device.