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along the length of the furnace and the conveyor speed determine the time-temperature profile. More zones (five to seven are typical) provide for better control of the soldering profile. Inert atmospheres, typically nitrogen (N2) to minimize cost, can be maintained to better than 20 ppm oxygen (O2). However, control of both the time-temperature profile as well as atmosphere cannot match that of a batch furnace. Also, vacuum conditions are not possible with most inline equipment. Nevertheless, inline furnaces are well suited for high production volumes and are the most widely used furnace type for electronics assembly. Whether batch or inline, furnace selection is based not only on throughput rates, but is also determined by the type of product being assembled. A greater complexity to the circuit board requires more control of the time-temperature profile to ensure that all of the solder joints are completed at minimum defect levels. In some applications, the extra furnace zones are used to control the cooling rate of the soldered circuit board to prevent thermal shock damage to sensitive components or substrates. The introduction of Pb-free solders has impacted reflow soldering, less so in terms of actual equipment temperature capabilities than in the development of a suitable time-temperature profile. The heat source technology (infrared, convection, or mixed) can provide the higher reflow temperatures of the Sn-Ag-XCu alloys (Tmelt = 217 C versus 183 C for the traditional Sn-Pb solder). Of course, higher energy usage and maintenance costs are likely. Two generalized reflow profiles are used for Pb-free soldering. They are illustrated graphically in Fig. 40.23. The soak-reflow profile in Fig. 40.23a derives from the traditional Sn-Pb eutectic solder profile, but with the reflow spike increased for the higher melting Sn-Ag-Cu alloys. The soak step provides for activation of the flux as well as heatup of the circuit board and components. The continuous ramp or hat profile in Fig. 40.23b allows for a more rapid heatup rate, which reduces the time that heat-sensitive components and materials spend at elevated temperatures. On the other hand, the relatively faster heatup rate can increase the chances of thermal shock damage to some components for example, larger plastic and ceramic devices or to circuit board structures such as vias.
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FIGURE 40.23 Generalized time-temperature profiles for Pb-free reflow soldering: (a) the soak-reflow profile; (b) the continuous ramp or hat profile. Peak temperatures will vary, depending on the particular circuit board product.
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In terms of optimizing the time-temperature profile, a balance must be established between achieving sufficiently high temperatures that will reflow the solder of every component (size and shape) and preventing thermal damage to other components or the substrate. Therefore, the higher melting temperatures of Pb-free solders make it more challenging to develop a time-temperature profile that will successfully melt the solder paste of larger components without causing thermal damage to smaller devices or to the circuit board material.
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Therefore, in the case of some very-high-mix products, it may be necessary to solder larger packages in a separate operation (e.g., hand soldering or selective soldering). Under this circumstance, the latter packages would be considered as odd-form components. 40.3.3.5 Wave Soldering. Wave soldering is used when surface-mount technology is mixed with through-hole components on the circuit board. (See Figs. 40.7 40.9 showing wave soldering as it is used for through-hole circuit boards.) Surface-mount devices that are present on the same side as the wave are soldered as well, being held in place by an adhesive. The surface-mount components must be resistant to thermal shock because of the temperature spike experienced upon entering and leaving the molten solder wave. The temperature on the topside surface of the circuit board typically remains well below the solder solidus temperature, thereby preventing the reflow of any solder joints present there. Wave-soldering equipment can be used either in an inline or in a batch process since the equipment has generally the same construction. The inline approach is used to support high production volumes. Small systems that are used in a batch-like mode include those used for selective soldering. This equipment includes miniwaves or solder fountains and are used to solder only localized areas of the circuit board, such as when attaching through-hole connectors, transformers, or switches. The advantage of selective (wave) soldering is that the entire circuit board need not be exposed to an elevated temperature. An important consideration in wave soldering is the supporting fixtures that hold the circuit board on of the conveyor. In reflow soldering, the conveyor can support the entire bottom side of the circuit board. However, in wave soldering, the conveyor does not offer such support, as it must allow the wave to contact the circuit board bottom side. Therefore, it may be necessary to provide an additional fixture to prevent warpage and sagging of larger circuit boards. The impact of Pb-free technology on wave soldering has largely occurred in the equipment performance. It has been determined that the same solder bath temperatures that are used for Sn-Pb processes (250 to 270 C) are suitable for the Sn-Ag-XCu Pb-free alloys. Therefore, excessive dross formation and flux residue removal have not become a significant problem during equipment operation. The lack of shiny fillets with the Sn-Ag-XCu alloys has been addressed by modified alloys having Ni and Ge additions that alter the solidification process, which leads to shinier fillet surfaces. The Pb-free alloy compositions are more prone to erode the wave machine s structures, such as the impeller, baffles, and pot walls. New wave-soldering machines address this problem through the use of alternative steel alloys and ceramic coatings. 40.3.3.6 Condensation (Vapor Phase) Soldering. Condensation soldering, also referred to a vapor phase soldering, uses a working fluid s heat of condensation to reflow the solder paste or solder preform to make the joints. Because the condensation reflow process had its origins in the early days of surface-mount technology when production runs were more limited in volume, these machines were primarily batch-type units. Subsequently, inline equipment was developed to attach the vapor phase reflow machine to the back end of the placement equipment for higher production volumes. Two particular attributes of condensation soldering are (a) the temperature of the product cannot exceed the vaporization (or condensation) temperature of the working fluid, thereby preventing overheating of temperature-sensitive materials; and (b) the temperature is very uniform over all of the components and the substrate, thereby minimizing temperature gradients that could warp or crack component or laminate materials. This process fell out of favor in the early 1990s, for two reasons. First, the working fluid for Sn-Pb solder, Dupont s Freon TMF, was categorized as an ozone-depleting substance (ODS). Its use was initially restricted and then prohibited by the Montreal Protocols. Second, the increasingly more complex circuit boards that were being designed with surface-mount technology required more precise control of the time-temperature profile, which lead to the development of multizone reflow furnaces having convective and infrared heating capabilities. There has been a resurgence in the use of condensation soldering. Alternative working fluids have been developed for both Sn-Pb and Pb-free processing that are compatible with
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