Reflow convection soldering

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. 

Just as in the reflow soldering oven, there are multiple solder-wave preheater styles, but only two are in prevalent use radiant preheaters (direct and indirect IR) and forced-air convective preheaters. Both are effective and both have their advantages. In fact, the best configuration is a combination of the two. Some wave-soidering machines can be equipped with both top and bottom preheaters. This can be advantageous for thermally massive boards. 

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. 

Reflow solder using a convection reflow oven ... 

In reflow soldering, heat is transferred by heat transport and can also be transferred by conduction, convection, condensation, or radiation. It is characterized by the coefficients of thermal transfer listed for the various heat-transfer mechanisms in Table 5.6. 

In April 1999, Marconi Communications initiated production of a telephone manufactured with lead-free solder, hailed as the first lead-free line in Europe. The telephone utilized a Sn-Ag-Cu alloy that was introduced to Marconi s existing surface mount assembly line. The only modihcation required was a slight alteration to the convection reflow profile [21]. 

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. 

FIG. 30 Conceptual drawing of the RSF-4, four-zone convection reflow oven. The nozzle manifold and scirroco fans improve heat transfer efficiency and reduce the temperature gradients across the PCBs during soldering. (Courtesy of Panasonic.)... 

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