Selective fluxing comes of age: a new fluxing technology offers advantages over conventional foam, spray and ultrasonic nozzle fluxers in wave soldering.

Electronics manufacturers are in an increasingly competitive environment that places continual pressure on margins. To ensure profitability, every part of a manufacturing process must be examined and improved to increase yields and reduce costs. Defects, downtime and added costs associated with out-of-control processes can no longer be ignored.

No exception is the wave soldering process, which has long been considered an inexact science. One of the most challenging and costly aspects of wave soldering is the application of liquid flux. Manufacturers have long sought a reliable technology to apply the minimum amount of liquid flux to printed circuit boards (PCBs), while providing solderability with acceptable yields.

For some years, traditional liquid flux application technologies such as foam, spray and conventional ultrasonic spray have been used to apply flux to the entire board. But these methods have significant drawbacks including high equipment maintenance, flux waste and imprecise flux application. The results have been sub-par solderability, downtime and excessive costs.

To accelerate production volume and efficiency, the electronics industry has moved to boards with a mix of surface-mount and through-hole components. Like pure through-hole assemblies, these mixed technology boards must be processed using wave-soldering machines. However, selective soldering pallets are needed to protect the surface-mount components from the soldering wave.

Process Considerations and Challenges

Selective soldering pallets enable standard wave soldering machines to process mixed technology boards efficiently because they protect the surface-mount components while exposing only the through-hole components to the solder wave. In many cases, only a small area of the overall board contains though-hole components. Thus, liquid flux only needs to be applied to these selected areas.

Environmental requirements are becoming more stringent. The environmental problems associated with liquid flux application include: volatile organic compound (VOC) emissions, flux disposal and waste from aqueous board cleaning systems. The VOC emissions are the by-product of any alcohol-based flux being applied by either foam fluxers, where the alcohol evaporates continuously, or conventional spray fluxers, where at least 50 percent of the flux is oversprayed and is exhausted "up the stack." All open bath fluxers must have the flux replaced periodically, necessitating the disposal of the old flux. Finally, the waste from the aqueous board cleaning process results from water-soluble fluxers that are over-applied to PCBs and pallets.

These complexities are placing more demands on the flux application system. The fluxer must be able to apply any liquid flux to the board surface and into the through holes uniformly and repeatably to ensure solderability with acceptable yields. With selective soldering pallets and tightening environmental rules, the ability to apply liquid flux where it is needed and in minimal amounts has become essential. Furthermore, assembly plants often operate 24 hours per day and seven days a week, so the system must be easy to set up and maintain for frequent process changes to provide the maximum time available for production.

Inline Selective Fluxing

To meet these challenges, a new flux application technology called inline selective fluxing (ISF) has been developed. This new technology surmounts the traditional challenges of precision fluxing and also delivers unprecedented process performance, control and flexibility while helping to meet environmental requirements.

Inline selective fluxing (ISF) uses ultrasonic spraying to produce a sheet-like spray of liquid flux, which is applied with enough force to obtain complete through-hole penetration (Figure 1). The spray provides a non-overlapping, uniform coating on the board regardless of board size or conveyor speed. With ISF, the motion of the spray head is synchronized with the conveyor speed; the spray is activated in one direction as the head moves from the fixed to the movable conveyor rail.

[FIGURE 1 OMITTED]

The ultrasonic spraying technology incorporated in ISF has three components: an ultrasonic transducer; a voltage-mode ultrasonic power generator; and an independent fluid applicator.

The ultrasonic transducer operates at a fixed frequency and produces vibrations with maximum amplitude at the tip of the spray forming head. The transducer consists of a convertor and spray forming head; each operates at the same frequency. The convertor uses disc-shaped piezo-electric elements, sandwiched between a cylindrical titanium front section and a steel back section, to convert high frequency electrical energy into high frequency mechanical energy. The spray forming head is a stepped block fabricated from titanium that concentrates the mechanical energy produced by the convertor at the atomizing tip.

The ultrasonic power generator provides the power required by the convertor; it supplies a constant sinusoidal voltage at the convertor's operating frequency. The power generator tracks the transducer's power demand by adjusting the operating frequency and power factor as required by temperature and load changes. Load changes occur when more or less liquid is being sprayed from the atomizing tip.

A microprocessor monitors these dynamic conditions and adjusts the generator to maintain optimum transducer operation. If an overload condition occurs, the power generator turns off and indicates a fault condition on its display.

The fluid applicator delivers liquid to the atomizing tip and directs airflow to shape and add momentum to the ultrasonically produced spray. The fluid applicator is comprised of a liquid applicator and two air directors.

The liquid applicator supplies liquid to the atomizing tip in an even flow to match the width of the spray forming head. The output surface of the liquid applicator is positioned adjacent to the spray forming head so that the liquid readily flows onto the atomizing tip. There, the liquid forms a film over the tip area and is disintegrated into small drops by the ultrasonic energy. The resulting spray pattern is a uniform sheet of spray equal to the width of the spray forming head. The ultrasonic energy propels the spray about one inch from the atomizing tip.

Air directors are then used to expand and propel the spray further so that the pattern can cover a larger width at distances greater than one inch from the spray head. The spray width expansion factor is 1.5 times per inch of distance from the head, up to eight inches. The air directors also add momentum to the ultrasonically produced spray to give the drops enough force to overcome ambient air currents and to penetrate through holes and vias in a PCB.

To maintain precise control over the speed and position of the spray head relative to the PCB, a servomotor-controlled linear actuator is used to traverse the spray head beneath the PCB. A motion controller controls the speed and position of the spray head. The controller also monitors the PCB's position and activates the flux flow to the spray head at programmed intervals based on the relative position of the spray head to the PCB.

Figure 2 depicts how inline selective fluxing works. The shaded areas of the PCB are where the flux is to be applied. For example, given a spray width of four inches, the spray head must take three passes beneath the PCB. During the first spray stroke, the flux flow is turned on when the spray head has traveled one inch and turned off at two inches (Figure 3).

[FIGURE 2-3 OMITTED]

Similarly, during the second stroke, the flux flow is turned on at one inch, turned off at two inches, turned on at three inches and turned off at six inches (Figure 4). During the third and final stroke, the flux flow is turned on at four inches and turned off at eight inches (Figure 5).

[FIGURE 4-5 OMITTED]

For the spray head to apply flux at the correct points, the user must program these points into the controller. This task is accomplished with a Windows-based operator interface (OI) that incorporates a computer-aided design (CAD) tool for selecting the targeted areas of the PCB. The OI is intuitive and can store many recipes for different selective fluxing patterns. Figure 6 shows the OI with a representative PCB with areas selected for flux application. The OI and controller then translate this drawing into programmed points for cycling the spray head on and off.

[FIGURE 6 OMITTED]

Conclusion

The advantages of inline selective fluxing include: uniform, efficient application of flux for maximum yields and predictable results; minimum flux use; minimal setup, cleaning and maintenance; and low total ownership cost. Inline selective fluxing overcomes the longstanding disadvantages of conventional spray fluxing systems and gives manufacturers unprecedented control and flexibility in applying liquid flux.

Stuart Erickson is vice president of sales and marketing and Christopher Cote is a systems engineer, both with Ultrasonic Systems Inc., Amesbury, MA; e-mail: [e-mail].