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provide information as to the specific gravity-per-flux target and also recommend an appropriate thinning formulation. The flux reservoir should be emptied, cleaned thoroughly, and refilled with a fresh charge of flux periodically. Sometimes the flux develops a residue that changes the flux s surface tension characteristics, clogs nozzles, or dispenses pores in foaming systems. In addition, flux may become contaminated with debris conveyed by the PCB to be soldered, which may impact assembly quality. Since the PTH relies on a minute capillary to be filled first by flux and ultimately by solder from the solder-wave process, any small particulates entrained in the flux or the solder may impede PTH barrel fill. Also, while the flux reservoir is empty, it is a good idea to inspect it to ensure that the materials of construction are holding up to the rigors of system operation and prolonged contact with fluxing agents that may prove corrosive in the long term. An inspection of the materials of construction for the entire system should be made prior to committing to the purchase of a wave-soldering machine. Unfamiliar or untested materials should be avoided unless there is sufficient literature, test results, or customer experience to ensure compatibility with fluxes that will be used in that system.
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Preheating Preheating the PWB and components serves three purposes in the wave-soldering process:
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Helps the board and components reach a temperature sufficient for the flux to activate and clean surfaces to be joined in preparation for wave soldering Lessens thermal differentials and decreases the likelihood of components cracking due to thermal shock when impacted by the intense heat of the molten solder wave Allows the board to ramp quickly to solder-melting and -wetting temperatures when it comes in contact with the solder wave
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A component s resident time in the solder wave is 10 to 30 times shorter than for a comparable solder joint formed during reflow oven soldering, so everything in wave soldering has to happen very rapidly. On the other hand, heating time-temperature ramps must be observed in accordance with the component manufacturer s specifications. Components and PWB bonding pads must be fluxed sufficiently to allow adequate solder-joint formation, yet there must be enough flux left on the circuit board after preheating to protect the newly fluxed surfaces until the assembly reaches the solder wave. 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-soldering machines can be equipped with both top and bottom preheaters. This can be advantageous for thermally massive boards. 47.4.4 The Wave Numerous solder-wave configurations are available. Rather than provide a detailed discussion of each, this section offers a basic overview of the elements of the process in relation to the wave.As previously mentioned, the molten solder is pumped to form a standing wave.This is accomplished by a spinning impeller on the bottom or side of the solder reservoir. Once the solder is molten, the impeller motor is activated and the solder wells up between baffles and nozzle that reside within the solder reservoir.A combination of impeller speed and baffles-and-nozzle configuration dictates overall standing-wave characteristics. The nozzles and baffles are generally adjustable, as are impeller speed, molten solder temperature, board introduction angle, and board conveyor velocity. These, along with preheater settings, define the profile parameters or process variables that must be tamed to accomplish high-yield wave soldering.
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Since the solder is a molten, turbulent liquid, theoretically it can make intimate contact with the underside of the circuit board. Therefore, thermal uniformity is generally not difficult to control as long as the wave contact area along the width of the PWB s underside is uniform. Since the solder is in contact with the underside of the PWB and since the underside of the PWB is coated with flux, the molten solder has the opportunity to wet to PWB lands and component leads. As the process progresses, the solder wicks between the pin and barrel. After leaving the wave, the board cools rapidly, liquid solder solidifies, and solder joints result. 47.4.5 Wave-Soldering Accessories Solder-wave manufacturers have devised a number of options that may be helpful to the process. Some of these have extended wave-soldering capability to allow finer-pitch soldering or have permitted wave soldering of thicker PWBs. 47.4.5.1 Gas Knife. The gas knife more commonly called the air knife directs a highvelocity stream of heated air or nitrogen at a glancing angle to the bottom side of the board directly after the board emerges from the wave. It can be effective in relieving solder bridges from tight interstitial pin fields or closely spaced passive device fields. When the gas knife is used with air instead of nitrogen, increased rates of dross formation may result. Also, if the angle is wrong and the velocity is too high, the exit gases could disrupt the solder wave, resulting in solder opens or shorts. If it is set too cool, the knife will have little effect or may exacerbate solder bridging. Since the advent of the air knife, there has been less dependence on the teardrop-shaped trailing pads used to minimize trailing edge solder bridges. 47.4.5.2 Sonic Assist. Although not in common use, another accessory imparts highamplitude sonic pulses into the solder wave.This can help drive solder into PTHs and increase wetting, especially on thick printed wiring boards. Care must be taken during setup, as very high amplitudes may pump so much solder up the barrel that it may result in topside solder bridging or solder splashing. This feature can help with barrel fill on thick PWBs. Its value is limited, since it does not affect board heating, which is the primary factor in barrel fill. 47.4.6 Wave-Solder Diagnostics As in all cases of mass soldering, the use of a product-specific, fully populated time-temperature profile board is necessary to ensure that critical areas of the board to be soldered are maintained at proper temperatures for each stage of the process. Topside SMT component solder joints must be kept well below the melting point of the solder used to join them to the PWB. At the same time, PTH barrels and PTH components must surpass liquidus temperature to guarantee sufficient wetting and capillary rise of the molten solder. With the complexity of today s mixed-mount boards, this is more of a challenge than ever. A profile board should be constructed and instrumented just as for oven reflow soldering, but thermocouples should be attached to both sides of the board and monitored simultaneously during the wave-solder profile run. Topside SMT solder joints should be monitored to ensure that they stay beneath the reflow temperature of the solder alloy being used. Connector bodies should be monitored to ensure that they are not overheated. Generally, for 63Sn/Pb solder, a temperature of approximately 120 C is chosen for maximum preheat to preclude reaching reflow temperature (183 C) on the top side of the board when the board touches the molten wave. Of course, this thermal limit varies greatly with board size, thickness, component type, number of board layers, number of connections to inner planes, thickness of inner planes, component layout density, etc. In the case of Pb-free solder, the preheat temperature should be about 50 to 70 C less than the liquidus temperature of the soldering alloy used for SMT. The use of wave pallets (shields) in the context of selective wave soldering will be discussed later.
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