In order to eliminate the impact of lead on the environment during disposal or after disposal, the electronics manufacturing industry is striving to remove harmful lead from electronic packaging and reduce reliance on lead, the need for environmental protection, the introduction of government regulations and the market for lead-free electronic packaging Advantages drive the move to lead-free. There are many factors to consider when implementing lead-free assembly in a manufacturing process, and reflow soldering equipment is closely related to the ability to produce high-quality lead-free products.

The first step in implementing lead-free solder paste is to select solder paste. With so many options on the market today, the biggest hurdle is finding a direct replacement for the leaded materials currently in use. The biggest problem with solder paste and reflow ovens is the higher melting point, which makes finding direct replacements very difficult. So far, the more common lead-free solder paste contains alloys including tin, silver, bismuth and zinc and other elements.

The selection of lead-free solder pastes is the subject of many technical studies. Studies have suggested that the most effective solder paste melting point is between 217°C – 221°C. Compared with the 183°C melting point of traditional eutectic lead-containing solder paste, the temperature has obviously been greatly improved. The maximum peak temperature and maximum ramp-up/cool-down rates narrow the reflow process window due to the thermal performance of the components.

Temperature profile Due to the narrower temperature profile range specified by solder paste and the consideration of damage to components at higher process temperatures, in order to use lead-free materials, we must pay attention to different temperature profiles and machine settings. Generally, two types of reflow profiles are used in the reflow soldering process, they refer to the soak type and the tent type profile. Wet-type profiles are processes in which the component board is subjected to temperatures just below the melting point of the solder for a period of time to achieve a consistent board temperature. The tent profile is a continuous rise in temperature from the time the panel enters the furnace until it reaches the set top temperature. The ideal temperature profile varies depending on the type of solder paste used in the assembly. Depending on the chemical composition of the solder paste, the manufacturer will recommend an optimal temperature profile for optimal performance. For specific information, please contact the solder paste manufacturer.

Due to the high melting point of the lead-free solder paste alloy, the temperature profile requirements have changed, so the settings of the reflow equipment also need to be adjusted accordingly. A change that is often overlooked is a “flat” profile in solder paste reflow. As the process window narrows, peak temperature and TAL must be achieved without overheating the board or components. This requires longer reflow intervals and the ability to efficiently transfer heat to the product. This problem can be solved by using two heated zones as the reflow zone, or by using a peak-forward approach in the reflow zone. Using this method, set the temperature of the penultimate heating zone higher than the temperature of the last heating zone, which promotes faster heat transfer to the product. Then use the last heating zone to maintain a consistent temperature on the assembled board.

Equipment Considerations There are many issues to consider when transferring heat into lead-free materials. However, it cannot be considered that a high-temperature reflow furnace is required just based on the reflow furnace and higher process temperature requirements. More important is how efficiently the machine transfers heat to the component boards.

Speedline Technologies’ newly released OmniExcel recirculation system series includes a new heat transfer concept, that is, a uniformly mixed gas mixture surrounds the product to enhance heat transfer capabilities, and the heating design at the air inlet allows three separate areas to draw air for mixing.

The central air intake uses a surface area of over 900 square inches, fin-shaped heater unit to transfer heat from the heater to the air. This method can reduce the direct heat transfer power to reduce energy consumption. This heating design has reduced energy consumption by more than 50% compared to traditional reflow solutions.

The gas heated by the heating unit is mixed by a fan, creating a moderate back pressure behind the pressure plate. Moderate pressure behind the diffuser creates concentric circles of air that overlap evenly on the process surface, providing excellent heat transfer to the product. Another design advantage is the exceptionally good isolation between heating zones, allowing better control over the product process temperature profile.

In reflow soldering, product cooling is as important as heating up. Prolonged time above liquidus temperature and extreme peak temperatures can result in product and component damage. The system must be designed according to the scheme of controlling cooling parameters. In systems using nitrogen, a cooling medium such as water is ideal for removing heat from the product. The cooling water design system needs to be easy to implement in the design. For example, the heat exchanger through which the cooling water flows is positioned vertically, allowing residual flux to drain naturally by gravity into a flux collection bottle. Water connections can be made with quick couplings without tools. The reflow system should be designed so that the cooling system can be operated independently without opening the heated oven chamber. This allows for faster scheduled maintenance downtimes and reduces the energy required to return to operating temperature.

Nitrogen The benefits of nitrogen in reflow environments have been widely debated for many years. With the introduction of lead-free solder paste, this is again a factor.

Nitrogen is used for different purposes in the reflow environment. It can protect the surface of the board through multiple reflows, prevent the oxidation of pads and pins, make the pins more wettable, and solder joints brighter. These are more pronounced in lead-free processes. Higher lead-free temperatures accelerate oxidation. Nitrogen will act as protection against oxidation. Although it is not required in lead-free processes, the use of nitrogen increases the process window and many lead-free reflow producers are using it. Nitrogen reduces oxidation on the surface, allowing for better wetting of the solder joints.

When considering reflux equipment, it is a very good choice to consider air or inert gas configuration design. Balanced and steady air flow in the heating zone reduces turbulence in the furnace and lowers nitrogen consumption. Time should be spent on heat transfer design and airflow balance stabilization concepts considered by the manufacturer. Closed loop fan control and variable speed fan technology are also used in the return system design to both enhance performance and reduce nitrogen consumption.

However, there is also a downside to using nitrogen. It includes the initial purchase cost of the machine, the cost of nitrogen and the additional maintenance cost of the reflow equipment to collect the volatilized flux in the machine. The reflux system and the use of nitrogen need to be considered for efficient system design.

Another factor that needs to be considered in flux evaporation management is flux evaporation management. The furnace should be able to purge the flux-laden gas outside the furnace chamber and return the clean gas back to the furnace chamber.

One example is the recently developed patent-pending secondary filtration/separation system that incorporates self-cleaning features to reduce maintenance. The first stage utilizes a mesh filter in the box. The flux gas enters the box and begins to expand, changing from a gaseous state to a small liquid droplet, which will break out of the air flow once it has grown to a certain extent. The residue of the gas passes through the filter, which separates out the large and heavy particles. These particles mainly consist of collected metals, rosin and resins, which adhere to the outside of the filter. This partially precludes the formation of highly viscous and difficult-to-clean run-down residues in the machine.

The cleaning function of the filter is realized through the regular rotation of the motor attached to the machine. Centrifugal force overcomes the adhesion of the particles to the filter and throws them outward against the walls of the enclosure. Since this system has no cooling function, the entire system remains hot due to the passage of hot chamber gases. This allows the heavy liquid attached to the tank wall to “run” into the collection bottle at the bottom of the tank.

The second stage is filled with stainless steel balls in the box. Those small and light particles, mainly alcohol and solvent, contained in the gas passing through the first stage, again began to expand into droplets. The gas passes through the bottom and collides with the steel balls in large numbers. Because the gas contains liquid, it will spread to the surface of the steel balls, and these balls will be wetted. Therefore, on the initial collision of the particle with the sphere, heterogeneous crystal nucleation occurs, covering the sphere with a liquid film. Once the ball is completely covered by the liquid film, the particles in the gas continue to collide with these liquid films. Because these are like substances, forming homogeneous crystal nuclei, the liquid beads up into droplets, which flow into the flux collection bottle and are very easy to clean up.

Energy Efficiency Reflow ovens are one of the most concerned devices when it comes to energy consumption in electronics manufacturing. This is easy to understand, since it is characteristic of the process to load heat onto the product, and then use energy to cool it down. Efficient heat transfer techniques must be considered in the design of reflow equipment to provide proper reflow profiles for leaded and lead-free processes. But based on energy consumption considerations, it becomes even more important.

Compared to other reflow products that have been handed down and tested, there have been huge improvements in thermal efficiency and reduced energy consumption through improved heat transfer capabilities and improved airflow in newer equipment designs. In the original test of the reflow system, the average power consumption per hour of running the lead curve in standby mode was 21KW. Compared with the recently launched backflow system, the energy consumption has been reduced from 21KW per hour to 12.2KW per hour due to the new system design. 41% energy saving under similar curve performance.

Transmission system considerations

the

Due to the improvement of product technology and production requirements, the reflow oven transmission system also needs to be improved and enhanced, and different transmission methods and options are available to meet these requirements. The most basic is the mesh belt transmission system.

The rail transfer system is the most common method of transporting electronic component boards through the reflow system. A concern is the ability to maintain track strength due to higher operating temperatures in lead-free processes. Exposed track support shafts and small track structures can be more stressed at higher operating temperatures and larger format component boards. The proven track design allows for thermal expansion and keeps the track system parallel, reducing the chance of PCBs dropping or jamming. The design incorporates a multi-angle rail extrusion construction that reduces rail bending during thermal expansion.

Due to the higher process temperature in lead-free reflow, electronic assemblies may be more prone to board warping and board drop issues. A higher reflow temperature closer to the transition temperature of the laminate, coupled with more components will cause more problems down the process line.

A central board support (CBS) transfer system can do the trick. CBS can provide better product support in the high temperature of lead-free reflow and overcome the problems of board deformation and board drop. This option will also become more common as manufacturers move to lead-free processes.

Before jumping to conclusions about lead-free reflow soldering, it is very important to carefully review the full characteristics of the furnace. When considering conversion to lead-free materials, determine material and temperature profile requirements. Once a selection has been made, enlist the help of the reflow oven manufacturer to set the desired profile for your product. Combined with some survey preparation, this can initially be done with a tent-shaped temperature profile or a peak-forward profile. Then refer to the number of components and layout, you can get the real effective temperature curve. Return system options also require careful consideration. Nitrogen process environment, cooling and conveying system, and overall system operating costs need to be discussed with the reflow oven supplier.

Send us an inquiry , we will get back to you soon ,thank you !