Soldering Oven reflow

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. 

Typically for oven reflow soldering, one profile board is prepared for the secondary-side reflow profile (in which only secondary-side components are mounted), then another for primary-side reflow profile (in which both secondary- and primary-side components are mounted). Another board is prepared for wave soldering (all SMDs loaded plus PTH parts). These boards also double as rework profile boards. 

Temperature Compatibility. If pin-in-paste soldering is to be used, check that components are temperature-compatible with the oven reflow process. The high temperatures and long exposures associated with oven reflow soldering may cause unsuited molded component bodies to melt or warp. Connector contact normal force may be affected if the molded connector body softens or distorts. Solder joints or wire bonds internal to some devices may become disbanded, and some, such as electrolytic capacitors, may leak or even explode as a result of an oven reflow cycle. Check the component manufacturer s specification for thermal hmits and compatibihty with oven reflow soldering. 

One attribute of this process is the speed at which solder joints are formed. Much quicker than oven reflow soldering, the wave-solder process allows little time for preheating, fluxing, and solder-joint formation, which also explains the variabiUty of this process. 

Y. S. Son, T. L. Bergman, and M. T. Hyun, Simulation of Heat Transfer in a Reflow Soldering Oven with Air and Nitrogen Injection, ASMEJ. Electronic Packaging, 117, pp. 317-322,1995. 

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. 

Use of N2for Increased Fluxing Effectiveness. The soldering environment is an important variable. Obviously the most common and least expensive soldering ambient is air, but many board assemblers introduce nitrogen to their reflow ovens, wave soldering... 

Early ovens utilized focused and unfocused IR lamps, mounted in the reflow oven tunnel. These bathe the solder paste-coated circuit boards and associated components with a broad spectrum of photonic energy heavily weighted to the IR end of the electromagnetic spectrum. The radiant energy absorbed by the printed wiring assembly (PWA) and related materials brings about the thermal increase needed to reflow solder. As the board travels beyond the last reflow heaters at the exit end of the oven, the absorbed heat is lost to the environment or the board is actively cooled by fans. The cooling results in resolidification of the molten solder and solder-joint formation. 

This is the preferred tool for controlled and reliable reflow soldering dependent on heated air recirculated at high velocity in the oven tunnel. The only direct irradiation of the PCBs is by the longest IR wavelengths emitted by the hot surfaces within the tunnel. 

In the case of a thermocouple sandwiched between a component lead and PWB pad (see Fig. 47.11) and an accidental junction formation nearby, the thermocouple will register the oven air temperature rather than the conducted temperature of the component lead, the solder, and the PW. Since the oven air temperature is generally much hotter than the PWB traveling through it, the resultant soldering time-temperature profile (based on the false thermocouple reading) may be cooler than desired and cold (under-reflowed) solder joints may result. 

An adjunct to the surface-mount process, this method, sometimes referred to as intrusive reflow, allows the soldering of some through-hole (solder-tail) parts into plated-through holes on the circuit board during SMT oven reflow. This process can eliminate or reduce the need for wave soldering—a step prone to defects. 

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. 

Oven reflow surface-mount technology (SMT) soldering... 

What reflow soldering oven is being used ... 

Reflow solder using a convection reflow oven ... 

It is standard practice to cure the printed circuits in a drying oven or reflow-soldering oven. There is a direct relationship between dwell time and curing temperature. Low temperatures necessitate long curing times to permit the ink to cure. This is the method of choice for substrates with low thermal durability. The dwell times for silver inks, for example, can be anything from 15 minutes up to 2 hours. The relationship between dwell time and temperature is also reflected in electrical conductivity, which tends to increase with sinter temperature. 

The processes generally employed to fix electronic components on three-dimensional circuit carriers are reflow soldering and electrically conductive-adhesive gluing. The first step in reflow soldering is to print solder paste on to the pads of the circuit trace (Fig. 4.1). The electronic components are subsequently placed in these solder depots. The module, complete with its electronic components, is then heated in a reflow oven, the solder particles of the paste melt, and the molten solder flows over... 

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