Vapor-Phase Reflow Soldering

The second process used in the assembly of printed wiring boards with Pb-Sn eutectic or neareutectic solder is referred to as vapor phase or condensation soldering. This method uses the heat of vaporization released when a vapor condenses into its liquid phase, to heat the assembly and reflow the Pb n solder. Vapor phase reflow is a batch process [5]. The Pb-Sn solder is printed as a paste on the circuit board the components are placed on the solder paste deposits then, the assembly is placed into the equipment. A preheat step is performed on the assembly, typically in a separate chamber. Entry of the assembly into the vapor results in relatively fast heating rates for the solder joints, approximately 7° to 10°C/sec. Similarly, the joints experience fast cooling rates upon withdrawal from the fluid. The working fluid vapor is not a very efficient barrier against O2. 

Before the widespread use of solder mask, a primary method for protecting copper circuitry was to electroplate a coating of tin or tin-lead on the copper. After component insertion, the tin plating could be liquefied with oven or vapor-phase reflow to form solderjoints.The use of solder mask over bare copper to take advantage of surface mount, wave soldering, and mixed assembly restricted the use of tin plate finishing. For simple technology product, tin or tin-lead plate and reflow remains a viable fabrication and assembly method. 

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. 

Because of safety and environmental concerns and comphance with the Montreal Protocol for the reduction of ozone-depleting chemicals, this soldering technique had fallen out of favor. With the advent of Pb-free soldering, there is increased interest in vapor-phase soldering for SMT applications, but it is likely that it will remain a niche application. Due to its continued diminished status, vapor-phase reflow is covered only briefly. 

Although vapor phase reflow has the appearance of being a fast process, solder paste reflow recommendations must still be followed. If the paste is heated too rapidly, paste volatiles may boil, resulting in explosive solder ball formation. As in other soldering methods. 

Besides legislated environmental issues, there are health risks associated with vapor-phase reflow soldering. With the continued recycled use of HFCs, toxic materials such as hydrofluoric acid and perfluorisobutylene are formed.These must be neutralized and their by-products must be disposed of properly. 

Reflow solder using a vapor phase reflow oven ... 

A vapor phase reflow oven was used for tombstoning evaluation of the pastes. A vapor phase reflow fluid with a boiling point of 260°C was used for the SnAgCu pastes and a fluid with a boiling point of 215°C was used for the SnPb paste. For each target paste, eight boards were reflowed. There were 1,352 chips total soldered for each paste. 

Figure 1. Tombstoning rate of solder pastes with vapor phase reflow process. SnAgCu and SnPb pastes were reflowed at 215° and 260°C, respectively.

Generalized time and temperature parameters have been identified for a number of Pb-Sn solder assembly processes. Those parameters are listed below for reflow furnace soldering, vapor phase soldering, wave soldering, and hand soldering. 

Because flux is such a concern, one contractor is exploring ultrasonic bonding (10) using prepunched aluminum interconnects that are attached to electroplated copper cell metallization with a seam welder. Others are examining fluxless bonding concepts, such as vapor-phase solder reflow. 

Once SMDs have been placed in solder paste, the assembly will be reflow soldered. Thk can be done in either batch-type ovens, or conveyorized continuous-process ovens. The choice depends primarily on the board throughput/hour required. Whereas many early ovens were of the vapor phase type, most ovens today use IR heating, convection heating, or a combination of the two. The ovens are zoned to provide a thermal profile necessary for successful SMD soldering. An example of an oven thermal profile k shown in Fig. 11.26. 

The vapor phase lead-free solder process operates at a maximum temperature of 230°C, whereas many convection or IR reflow processes range from 250°C to over 300°C in some zones of the reflow process, depending on the type of PCBAs being processed. Vapor phase is compatible with typical PCB finishes, including HASL, bare copper with OSP, copper/nickel/gold, immersion tin, and immersion silver. 

The heat transfer mechanisms utilized for the mass reflow soldering of electronic assemblies include convection, radiation (infrared sources), and condensation (vapor phase). The most popular solder reflow method is based on forced convection. 

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