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Reflow Oven Flexibility
Every aspect of a reflow oven should be considered when determining true oven flexibility.
A flexible reflow oven is required to accommodate multiple SMT manufacturing processes, functions and applications. Manufacturers using high-performance reflow ovens achieve increased productivity, throughput and profits via decreased production downtime.
Flexible reflow systems require a unified set of parts and product options working together to optimize the conveyor system, heating and cooling systems, machine accessibility, and the control system. Options such as flux management and inert systems must also be flexible and work efficiently in conjunction with the rest of the machine.
The flexibility of a conveyor system is determined by the various applications accommodated within the system. A robust conveyor system ensures printed circuit boards (PCB) will not be dropped inside the machine and is evident in the parallelism of the conveyor rails and repeatability of the width adjustment.
Offering various types of conveyor belts is one option that allows for flexibility of product sizes and the ability to accommodate different manufacturing processes. Most machines should allow standard, larger pitch belts, as well as smaller pitch belts. These smaller, fine-pitch belts allow for the processing of smaller PCBs by providing more support per unit area of the board.
Belt conveyors that allow for full width utilization for board processing increase machine throughput. This configuration benefits manufacturers that place boards across the entire width of the belt, allowing side-by-side processing.
Combination pin-chain/belt conveyor systems provide flexibility and the ability to use either the rail or the belt conveyor. The rail conveyor may be used for processes involving double-sided surface mount, while the belt may be used for thinner products that need full bottomside support to prevent "sagging." Additionally, this dual-purpose conveyor system provides the means for immediate changeover to other processes, eliminating unnecessary downtime.
A dual-rail/dual-conveyor option offers two parallel conveyors positioned side-by-side to process product simultaneously through the oven (Figure 1). The greatest advantage of this type of system is that it doubles product throughput. With independent drive systems for each rail/belt combination, this option provides additional flexibility to run different products side-by-side at different speeds, ensuring that the profiles of each product remain optimized.
It is equally important that rail conveyor systems, just as belt conveyors, accommodate various board types and sizes. This allows optimization of pin length for sufficient support vs. clearance needed for components close to the board edge.
One final consideration for conveyor system flexibility is center-board support. This option helps maintain board integrity during the processing of thinner or heavier boards that may have a tendency to sag when heated. Retractable supports offer more flexibility by allowing the user to raise or lower the mechanism for boards needing additional or less support. Productivity is improved through multiple product processing capabilities and negligible time loss associated with PCB design changes.
Heating systems are at the "heart" of the reflow process. It is important to understand that each heating application employs a different technology; the key to maximizing any system is based on the implementation of a solution that addresses the particular needs for each unique process element.
Current heating systems are either higher or lower mass convection systems. Higher mass systems provide an extremely stable temperature environment within the oven using convection heat. These elements are beneficial for the processing of high-mass boards or increasing the throughput of low-mass boards at the expense of energy. On the other hand, lower mass systems provide quick temperature stabilization upon startup or when changing recipes to run different products. They are best suited for low-mass boards because they are less likely to maintain thermal uniformity under extreme loading conditions. Whether high or low mass, any competitive reflow system must accommodate flexible configurations to suit various applications. Regardless of the heating technology required for a specific application, the option of various heated lengths (number of heated zones) provides flexibility to fit the desired throughput. Machines with multiple heated zones may require more floor space, but they also offer a higher yield because they increase conveyor speeds to achieve profiles similar to machines with fewer heated zones (Figure 2). The ability to provide each manufacturer with the optimum heated length to attain required throughput becomes the key factor.
Heating systems must also process different board and component sizes, which is achieved by providing variable convection blower speeds to help optimize each process, product and application. These higher speeds allow for high convection-heating rates of large, densely populated boards. Lower speeds provide less turbulence within the oven, which can potentially cause lightweight components to shift out of position.
Cooling systems within reflow ovens also play a vital role within the process by determining the liquidus-to-solidus transition of the solder joint, as well as the exit temperature of the PCB. This point in the solder phase transition is the determining factor in solder time above liquidus (TAL), which, in conjunction with the cooling rate, determines the microstructure and intermetalic layer of the solder joint. Offering various cooling options allows for flexibility in achieving the necessary TAL and exit-board temperatures.
Blowing air across a heat exchanger is one viable cooling system option. This provides adequate cooling for most applications; however, in a nitrogen environment, the system maintenance required may be slightly more than desired. Implementing gas amplifier knives as an alternative provides lower maintenance cooling, shortened TALs and increased cooling rates for different solder joint microstructures.
Machine accessibility impacts maintenance times and mean time to repair (MTTR). Providing easy access to the internal workings of the machine drastically lowers the MTTR and aids in the installation of future retrofits for different processes or applications (Figure 3). This reduces machine downtime and increases overall productivity.
Ease of access for internal maintenance is determined by the ability to quickly open and close the heating and cooling chambers. Given their large size, heating chambers are opened and closed easiest by the use of a mechanical lift/lower device that requires no human assistance, other than an electrical switch to raise or lower. Cooling chambers are opened and closed by a mechanical assist device, such as a gas shock, and are locked into the closed position using quick clamps. Easy access to heaters, heat exchangers and cooling devices reduces downtime and related costs.
External maintenance, as well as the addition of future retrofits, require easy removal of external panels. Front panels that lift off to allow access underneath the heating/cooling chambers, or electrical box doors with quick, one-fourth turn-tool-operated latches, provide the necessary easy access.
Perhaps the most important factor determining reflow machine flexibility is the control system. It should be designed to provide user-friendly operation and troubleshooting to minimize downtime, and is accomplished through features such as real-time graphical animation, product flow tracking, on-screen help and system debug screens.
Process flexibility is attained through an operating system that provides multitasking and networking capabilities, as well as storage for multiple recipes and programmable machine timers.
Atmosphere Control Options
Atmosphere control within a reflow oven can impact soldering performance during specific processes. Controlling these conditions extends beyond the temperature of the heating and cooling sections - it involves providing an inert environment for necessary processes and managing flux contamination within the machine chamber.
There are various options available, such as nitrogen inertion, to eliminate lead/pad oxide formation within the heated chamber. Switching between air and nitrogen operation, and controlling the level of atmospheric purity via a flow-control valve and an on-board oxygen analyzer, allow the most efficient use of nitrogen. If the plan is to initially run with air, a nitrogen-ready package should offer seamless transition to future inert processes. Atmosphere controls within the oven, via a flux management system, can reduce contamination and maintenance from flux buildup inside the chamber and should offer the ability to isolate the system from the rest of the machine. This allows for maintenance and cleaning to take place without affecting production, eliminating maintenance downtime.
Every aspect of a reflow oven (conveyor, heating technology, cooling system accessibility and atmosphere control) should be considered when determining true oven flexibility. Machine flexibility has a direct impact on maintaining and increasing production levels, as well as achieving overall business goals. SMT
JON DAUTENHAHN is a mechanical engineer and senior project manager for new pro- duct development of reflow equipment for Speedline/ELECTROVERT, 16 Forge Park, Franklin, MA 02038; (508) 520-0083; Fax: (508) 520-2288; E-mail: firstname.lastname@example.org.
Figure 3. Machine accessibility impacts MTTR.
Figure 2. Conveyor speed vs. number of heated zones.