|VectraElite wavesolder system (ServoJet capable). |
The trend toward environmentally responsible electronics assembly is well-documented. Regulations banning the use of lead, such as Europe's RoHS directive, are driving similar initiatives throughout the world. While many manufacturers have implemented lead-free wave soldering, both equipment and process continue to evolve with the common goals of cost and defect reduction. From an equipment standpoint, wave soldering machines have undergone changes in all three major subsystems: the fluxer, preheat, and the solder module to optimize the lead-free process.
Since it occurs first in the soldering process, the flux delivery system's ability to apply flux uniformly with penetration into the through-holes is critical to the success of all subsequent operations. A variety of spray fluxer technologies are available for use in lead-free soldering, each with associated advantages and disadvantages. Ultrasonic (two types were evaluated in this particular study): air atomization nozzle and flux jet nozzle systems can be compared in terms of hole penetration and coverage accuracy. A series of tests was run to compare fluxer performance in these key areas.
One of the most important concerns in flux application for lead-free soldering is hole penetration in complex, multi-layer circuit boards. An important parameter of penetration is velocity of the flux during application. To evaluate this component, a Plexiglas plate was manufactured with 10 holes spaced equally over a 3.5-in. distance. The flux head was manually positioned while reading air velocity at the center hole of the pattern. The head position was fixed when the maximum velocity reading was achieved. Measurements were then taken at each of the 10 holes with an anemometer. Three readings were taken at each hole and averaged. Results of this test showed that flux jet technology generated the highest air velocity at the board level and performed very well in terms of velocity uniformity. While one of the two ultrasonic fluxers demonstrated good velocity uniformity, both yielded much lower air velocity readings. The air atomized fluxer produced high air velocity readings at the center, but they diminished at the edges.
In addition to the air velocity study, each spray fluxing system was evaluated on hole penetration and uniformity of coverage. Two test boards were used; one with 0.037-in. holes to evaluate uniformity and one with three different size holes, 0.013, 0.024 and 0.032-in., to visually evaluate penetration using pH paper. Flux deposition was set to the same level on all systems and a no-clean, VOC-free flux was used. A 3-in. spray width was used because it gives systems the flexibility to process boards up to 24-in. wide at conveyor speeds of up to 5 feet per minute. Smaller spray widths would reduce maximum board width and/or conveyor speeds.
For uniformity, test criteria include evenness of deposition across the width of the pattern and clear edge definition. Poor edge definition and/or center-focused nozzles generally do not perform well in uniformity testing. Because of its exceptional edge definition and air velocity consistency, the jet flux system had the best overall uniformity.
Flux Jet Technology
Flux jet technology again performed the best in the flux penetration evaluation on the test board with multiple hole sizes. Both ultrasonic systems and the air atomized single point nozzle showed weak hole penetration on the 0.013-in. pattern, although the air atomized single point nozzle did show good uniformity across the 0.024-in. and 0.032-in. hole patterns.
Based on test performance, ease of maintenance and competitive cost of ownership, flux jet technology would appear to be the best choice for spray fluxing systems used in the lead-free soldering process. The ServoJet system utilized in this study is a servo-controlled, reciprocating spray fluxer with eight flux jet-type nozzles controlled by two valves, with four nozzles per valve. The ServoJet combines high velocity jetted flux droplets with concentric air atomization to penetrate holes and deposit flux where it is required. Duty cycle-controlled, high frequency valves deliver pressurized flux through precision orifice nozzles. Flux droplets are jetted from the nozzles toward the circuit board. Air, concentrically surrounding the nozzles, atomizes the flux droplets to a fine mist.
|ServoJet spray fluxer. |
Flux spray is parallel to board holes and pallet openings to maximize hole penetration and minimize pallet edge shadowing. Flux application can be followed by an air knife blow-off to augment hole penetration and deposition uniformity. The nozzles are servo-driven by a ball screw actuator traveling at constant speed perpendicular to the conveyor. Conveyor speed synchronization and unidirectional flux application ensure there is no overlap between passes. Another key differentiator with the ServoJet system is the ability to vary flux deposition within a particular region on a board. This feature provides the ability to have greater flux deposition in an area historically prone to defects, such as around a heavy connector, if so desired. Variable selective fluxing can be software controlled via the wave soldering system.
Proper maintenance is key to success with any spray fluxer. Maintenance requirements and downtime also weigh heavily in the total cost of ownership. The air orifice of the flux jet system is self-cleaning and scheduled during non-production time. A high-speed valve opens to inject solvent into the air line for 1 to 2 seconds followed by a high-pressure air purge to blow out the solvent and clean the air orifice. A quick-clean feature is incorporated to run if the programmed cleaning delay time has been exceeded due to production constraints. No parts are required for replacement due to normal wear conditions and no mechanical adjustments are required for the spray head. The amount of maintenance a fluxer requires is directly affected by board production levels and operation frequency. Maintenance requirements and costs associated with operating the fluxer (flux consumption, waste disposal, etc.) are all part of the cost of ownership and should be considered when evaluating different fluxer alternatives.
|ServoJet nozzle detail. |
The tests performed in this evaluation show that, for equal flux volumes, each of the spray fluxer systems tested had different levels of performance. Individual fluxer performance can be improved by reducing the spray stroke timing to overlap the spray pattern. This will improve uniformity and hole penetration in situations where board width and conveyor speed are not a limiting factor. Other factors, such as increasing the flux volume, can also improve fluxing results.
When selecting a spray fluxing system, one must consider the complexity of the circuit boards being processed in a lead-free environment. Cost of operation (flux consumption, reliability and maintenance requirements) must also factor into the decision. The design and operating characteristics of some spray fluxer systems inherently provide better uniformity and hole penetration, making them especially suited to the lead-free wave soldering process. The data collected in these tests shows that for high performance production environments, flux jet technology provides superior performance and low cost of ownership.
See at IPC Midwest Booth #825.
See at IPC Midwest Booth #825. For more information, contact: Speedline Technologies, 16 Forge Park, Franklin, MA 02038 508-520-0083 fax: 508-520-2288 E-mail: email@example.com Web: http://www.speedlinetech.com