Reflow oven

Optional and not required if adhesive cures in preheat zones of a reflow oven. 

The composition advantageously displays a thermal resistance effective to withstand the high temperatures encountered in a reflowing oven. Further, the high-temperature compositions more closely match the thermal shrinkage of PBT, which is presently used in solder connector applications. 

With flip-chip solder-bump attachment, solder bumps are deposited over the area of the silicon wafer. The wafer is then diced into individual die and flipped over the MCM substrate. The module is then placed in a reflow oven, where the solder makes a strong connection between the chip and the substrate. Flip-chip... 

After all parts are placed in the solder paste, the entire assembly enters a reflow oven to raise the temperature of the assembly high enough to reflow the solder paste and create acceptable solder joints at the... 

It should be noted at this point that there are three distinctly different solder balls referred to in this chapter and in publications discussing SMT. The solder sphere test refers to the ability of a volume of solder to form a ball shape due to its inherent surface tension when reflowed (melted). This ball formation is dependent on minimum oxides on the microscopic metal balls that make up the paste, the second type of solder ball. It is also dependent on the abUity of the flux to reduce the oxides that are present, as well as the ramp up of temperature during the preheat and drying phases of the reflow oven profile. Too steep a time/temperature slope can cause rapid escape of entrapped volatile solvents, resulting in expulsion of small amounts of metal, which will form undesirable solder balls of the third type, that is, small metal balls scattered around the solder joint(s) on the substrate itself rather than on the tinned metal of the joint. 

An important consideration about the pin transfer technique is that it requires an open bath of the adhesive or flux. Adhesives readily absorb water from the air. Fluxes lose vehicle (water or alcohol) and possibly other constituents through evaporation. By either mechanism, the material properties change, which affects the quantity of fluid retained on the pin and deposited at the site (including the fhp-chip process previously described). Adhesives must have sufficient wet strength, and the fluxes must have enough tack to hold the component in place for the duration of component placement activity as well as subsequent handling of the circuit board on its way to the curing oven or reflow oven. 

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... 

Even the simplest reflow ovens consist of several subsystems insulated tunnel, board conveyor, heater assemblies, cooling, and venting (see Fig. 47.1). 

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. 

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. 

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. 

Mesh Belt Conveyor. Mesh belts used in reflow ovens are generally fabricated of stainless steel links. Some are wide open with large spaces, many centimeters from link to link. Others are more like chain mail armor (Fig. 47.3). The widely spaced links allow more air flow to the bottom side of the board. 

There are several heating schemes used in reflow ovens, the result of years of technological evolution. Focused infrared (IR) lamps have given way to secondary emission panel heaters and, finally, to the forced hot-air convective ovens that are the de facto industry standard today. 

FIGURE 47.4 Three infrared heating schemes for reflow ovens (a) IR lamp (b) open resistance wire heater and (c) resistance rod (calrod-type cartridge heater). 

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. 

Heater Robustness. Check with the reflow oven manufactnrer to ensnre that the heater and fan assemblies (heater modules) will be compatible with the rigors of the higher oven temperatnres associated with Pb-free reflow. Although an oven may be able to reach the higher temperatnres, the heater modules may degrade or fail as a result of running close to their maximnm rated ontpnt. 

Firm ware. In some older reflow ovens, the compnter control heater firmware does not allow the heater modules to be set high enough for Pb-free reflow. If the rest of the oven is compatible for Pb-free soldering, check that heater modnles are settable for the higher peak temperatures required of Pb-free soldering. A firmware npgrade may be necessary. 

Seals and Others. Check with the reflow oven mannfacturer that soft seals and other oven components in older reflow equipment will be compatible with the higher temperatures of Pb-free soldering. 

There are four distinct steps in the reflow profile that must be accommodated by the reflow oven for successful soldering. Each step must be tailored for the solder paste composition and must be accomplished in proper, controllable, and repeatable reflow equipment. A misstep at any of the four stages of reflow can result in product loss. Refer to Fig. 47.8 for this discussion on thermal profile. 

As mentioned previously, the solder paste manufacturer will provide recommended profile guidelines, but those are only a starting point. The process engineer should optimize the reflow oven profile with product profile boards and analysis based on actual product runs. Board profiling will be covered in a subsequent section. 

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