At first glance, the wavesolder machine and the process seem to be very simple since there are only four basic functions. Convey the board through the process of applying flux, preheating and soldering. However, when you combine chemistry, thermal, physics, board materials, board design and layout, you now have a very complex process with many variables that interact with one another. Let's look at some of the basic functions of the wavesolder machine and how each one can affect the process.   CONVEYOR: The conveyor system function is to convey the board into the machine, through the process and out of the machine. The conveyor must hold the board firmly while transporting the board smoothly at a constant speed. The process parameters that are controlled by conveyor speed are preheat ramp rate, time over preheat and dwell time in the solder wave. Typical conveyor speeds are 4-6 ft/min.   FLUXER: The fluxer is a system to apply flux to the board. There are three types of fluxer systems in use, the foam fluxer, wave fluxer and the spray fluxer. The spray fluxer is the preferred system for no-clean because it provides the best control of flux deposition and elimates evaporation or contamination as the flux is in sealed containers. Flux is responsible for removing the oxides from the parts to be soldered and to promote wetting. In a no-clean process the amount of flux put on the board is critical to get good soldering and minimize the residues left on the board. There are currently two categories of no-clean flux in use, alcohol based and VOC free or water based. Because of its low surface tension the alcohol based flux wets well and is easy to dry but may not hold up to long preheat times and dual waves. The VOC free or water based flux is generally more robust and requires forced convection preheat for good drying. Although, because of the high surface tension of water this flux does not wet as well unless a surfactant has been added to its chemistry.   PREHEAT: The purpose of preheating is to dry the flux carrier, promote activation, reduce thermal shock to the board and components and to began the process of transferring thermal energy to the board in preparation for the soldering process. The typical topside board temperature ranges between 180o F and 240o F.   IR PREHEAT: The system uses IR tubes as the primary source of energy. The tubes are covered with pyroceram glass, which becomes a secondary emitter. This method provides a medium length wavelength that is emitted from the IR tubes and because the glass is a lower temperature than the tubes, it emits at a longer wavelength. The combined range is around 5-7 micron. This will provide a uniform temperature across a circuit board better than preheats that have only a primary emitter. There is, however, two disadvantages with the IR preheat, they are color selective and do not dry water very well. Dark colors absorb more IR energy than lighter colors so, if the color of the solder mask changes, the amount of IR absorbed will change. Water does not absorb much of the IR and is mostly heated by the circuit board. This makes it difficult to dry the water based no-clean flux.   FORCED CONVECTION PREHEAT: In this system, we use a heated diffuser plate that heats the air as it passes through the holes in the plate. Blowers re-circulate the heated air back into a chamber and through the diffuser plate. The board, components and flux absorb the energy from the heated air equally so the board is heated more uniformly. Since convection is a much more effective way to transfer energy, its ability to dry water based flux is increased many times over that of the IR preheat. Because forced convection is not color sensitive, changes in solder mask color will have little or no effect on board temperature.   SOLDER WAVES: The solder waves are undoubtably the heart of the machine and the soldering process. The solder waves consist of the main wave and the chip or turbulent wave. The main wave in an air process is a laminar wave with a controlled exit flow. In a N2 process the main wave is an "A" wave called the coN2tour. Inerting the process will open the process window, increase wetting, reduce the amount of flux required and yield nice shiny joints. The chip or turbulent wave is typically used when there is bottomside surface mount on the board. The turbulence of the chip wave helps to break up the flux gas bubbles and push the solder around the surface mount leads. The wave height is a critical parameter as it affects dwell time and exit peel off. As a general rule the wave should come half way up the thickness of the board as the board enters the wave. The temperature of the wave is typically held between 470o F and 500o F.   HOT AIRKNIFE For high-density circuit and tight geometry assemblies, or when processing at high throughput rates, the patented debridging hot airknife can be fitted, either as standard equipment or as a field retrofit option. The airknife, located at the exit side of the solder pot,provides several distinct advantages. Airknife Advantages  The airknife acts upon the still molten solder joints as the PWA exits the solder wave. The jet of heated air, directed at the underside of the PWA, disrupts cohesively-bonded joints, sculpts the fillets, fills voids in plated through-holes, debridges joints, removes excess solder, exposes wetting problems, and repairs fractured joints. By debridging solder joints on high density and tight geometry circuit assemblies, you can increase productivity by decreasing the amount of rework. Defect rates as low as 2-20 PPM can be achieved in heavy production environments on systems equipped with the debridging hot airknife.  Equally important is inspection. Solder joints which appear sound and meet visual inspection criteria can mask voids in the barrel of a plated through-hole, leading to premature assembly circuit failure when the assembly is stressed in the field. The hot airknife “stress tests” every soldered joint, with the heated air jet directed at the joints,at a rate of 100 joints per second or faster. Applying a jet of heated air at about 1,000 ergs/cm2 (the approximate force required to blow off excess solder) will not disturb a sound, adhesively-bonded joint, but will expose the weak cohesively-bonded joint which perhaps masks a void. The airknife will cause refilling of plated through-hole joints that might have an outgassing problem. This refilling is caused by the void being exposed by the hot airknife and the still molten solder wicking back into the barrel of the hole. Joints refilled in this manner will appear as an unsoldered pad around the leg of the component. Upon closer inspection, however, solder will be observed filling the barrel of the through-hole to some degree. Depending upon the inspection criteria used, if good wetting of the lead and barrel is observed, the joint is sound. Another airknife advantage is apparent when soldering SMDs. Expansion coefficients for the glass/ epoxy substrate, the component leads, pads and the solder are all different. Large amounts of solder on the leads and pads will cause excessive stress of the fillet that will lead to premature joint failure in the field. The hot airknife will sculpt the fillets, removing excess solder and making the joint more ductile, and thus less susceptible to thermal and mechanical stress fatigue. Airknife Dynamics The hot airknife directs a narrow (0.018") sheet of heated air to the underside of an assembly. The airknife temperature is typically adjusted to 750° F to 850° F, and pressure between 4 and 20 psi, depending upon the population density, component type and orientation on your board. A number of airknife parameters can be adjusted for optimum process efficacy: Impingement angle (40 to 90° from horizontal) and distance (as close as possible) to the underside of the circuit assembly. If solder is observed splattering from the top of the board, the air pressure applied to the airknife should be reduced in single psi increments until debridging is achieved without blow through. Air pressure of five to ten psi, temperature of 800° F (426° C) and a angle of 65° as the initial airknife setup parameters gives excellent results on all types of circuit assemblies.