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47.2 MASS SOLDERING METHODS
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The mass flow or reflow methods are suited for high-volume manufacturing. The entire board is heated and large numbers of components on the board are soldered simultaneously. The two most common of these methods are oven reflow soldering and wave soldering. A third technique, vapor phase reflow soldering, has dwindled in popularity due to environmental concerns regarding the use of the chlorofluorocarbon-based solvents that were key to this process. Now, however, perfluorocarbons are substituted and the technique is still in use. The choice of soldering method is dictated by the types of components and boards being soldered, the required throughput rate, and requisite solder-joint properties. There are no clear-cut rules. Once the exclusive domain of wave soldering, some plated-through hole (PTH) components are being assembled along with surface-mount components in reflow ovens. Conversely, some surface-mount components are bonded by wave soldering.
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OVEN REFLOW SOLDERING
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Oven reflow is primarily used for surface-mount component soldering. To prepare a board for this process, solder paste is deposited and then forced through a metal stencil by either a metallic or polymeric squeegee. The paste contains both solder flux for preparing the metal surfaces for solder attachment and sufficient solder for joint formation. Components are placed in the solder paste on the board and the populated printed circuit board (PCB) is inserted into a conveyorized reflow oven. The oven is set to raise the circuit board and component temperatures gradually. Flux in the solder paste activates with the increase in temperature and strips metal surfaces on the components, board, and solder of oxides that inhibit solder joint formation. Finally, enough heat is imparted for the solder to flow (meaning to liquefy, also known as reflow). When built and implemented properly, the oven reflow process results in a controlled and predictable heating and cooling cycle and reproducible solder-joint formation. Rapid heating of the paste is widely known to be a source of solder ball formation. Solder balls (isolated spheres of solder not necessarily connected to the solder mass of the joint) can be problematic for printed circuit board assemblies. They can induce electrical shorts, especially with finer-pitch components where solder ball diameters may be on the order of component lead or printed wiring board (PWB) pad spacing. Solder paste heating rates must be controlled to preclude solder ball generation. Similarly, the soldering time-temperature profile must be carefully adjusted to prevent excessively high temperatures that can cause flux charring or caramelizing and solder-joint degradation. The circuit board itself may also fall victim to the reflow process and deteriorate if temperatures are not maintained properly or evenly. It is for this reason that much equipment development has ensued.
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Reflow Oven Subsystems Even the simplest reflow ovens consist of several subsystems: insulated tunnel, board conveyor, heater assemblies, cooling, and venting (see Fig. 47.1).
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FIGURE 47.1 Cross-sectional view of reflow oven with top and bottom heater assemblies, cooling module, vent stack, insulated tunnel, and printed circuit assemblies (PCAs) atop motorized conveyor.
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Reflow ovens have reached a high level of sophistication, and there are many other items that enhance oven suitability for the manufacturing floor. Those items, beyond the aforementioned reflow oven subsystems, are niceties, accessories, and gimmicks offered by oven manufacturers, but are not discussed in this section; however, those subsystems previously listed are reviewed to impart an understanding of the basics of oven construction and operation as well as the most advantageous configurations. 47.3.1.1 Tunnel. The tunnel is a thermally insulated passage through the length of the oven where the board is heated and cooled for a continuous reflow process. It serves to insulate the
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SOLDERING TECHNIQUES
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heaters and boards from the external (room) environment just as much as it is designed to preserve thermal conditions as prescribed by the process and demanded of the heaters. Boards are moved through the tunnel and past multiple heaters by an adjustable constantspeed conveyor permitting controlled and gradual preheating, reflow, and post-reflow cooling of the circuit board. Consideration of tunnel dimensions is critical for the reflow application. Short tunnel ovens may not permit a profile adequate for attaining prescribed reflow temperature-versustime profiles for larger, thermally massive PCB assemblies. Tunnel height dimensions must also be adequate to accommodate the tallest components or component heat sinks. Tunnel width will limit the size of the board that can be introduced to it. 47.3.1.2 Conveyors. There are two main conveyor systems used in reflow ovens: pin-chain and mesh belt. One is required and both are recommended for any reflow machine. 47.3.1.2.1 Pin-Chain Conveyor. The pin-chain conveyor, also known as an edge-hold conveyor, looks like a bicycle chain with a pin protruding inward from evenly spaced links, as shown in Fig. 47.2.
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