Popular flux systems are compared for PTH penetration, coverage accuracy, maintainability and chemical compatibility.

The ban on lead in electronics in Europe and pending laws elsewhere have set off a frenzy of development work on a reliable, defect-free Pb-free wave solder process. The flux delivery system’s ability to apply flux uniformly with through-hole penetration is critical to the rest of the process. All the fluxers tested were traversing systems that spray in only one direction. Data were collected across the width of the spray pattern to determine airflow velocity and uniformity at the board level. These characteristics affect flux penetration into through-holes. Uniformity and hole penetration were also measured using a fluxometer and pH paper on two test boards, one with 0.037" holes and the other with hole sizes of 0.013", 0.024" and 0.032".

Uniformity and through-hole penetration. Spray fluxer performance was based on three criteria: impact velocity at board level; uniformity at a 3" spray segment with a board with 0.037" holes; and visual hole penetration on pH paper through three different hole size patterns (0.013", 0.024" and 0.032"). Flux deposition was set to the same level on all systems for all tests and a no-clean VOC-free flux used during testing.

The 3" spray width is important because it sets how many spray strokes must be made to uniformly spray flux on a given circuit board. It also affects the speed that the system must traverse to return to the home position in time for the next spray stroke. This number was chosen because it gives systems the flexibility to process boards up to 24" in width at conveyor speeds of 5'/min. Smaller spray widths required to achieve uniformity and hole penetration will diminish this flexibility and reduce the board width or board processing conveyor speed.

Velocity test. A Plexiglas plate was manufactured (Figure 1) to analyze air velocity at board level. Ten holes were spaced equally over a 3.5" distance centered over the fluxer nozzle.


The flux head was manually positioned while reading air velocity at the pattern’s center hole. The head position was fixed when the maximum velocity reading was achieved. Measurements were then taken with an anemometer at each of the 10 holes; three readings at each hole were averaged. Figure 2 shows results of the air velocity measurements.


The 3" spray width is represented between the second and ninth hole in the test plate. Both velocity and uniformity are important in achieving maximum, reliable flux penetration in PTHs. The air atomized jet fluxer had the highest air velocity as well as good uniformity, and would be expected to be the best performer. Both ultrasonic fluxers had much lower air velocity, with the impact type showing the higher velocity of the two. The air atomized single-point fluxer had high velocity at the center but dropped at the edges.

Flux trajectory geometry. An important concern in Pb-free wave soldering flux application is hole penetration in complex, multilayer boards. The scale geometric sketches (Figures 3, 4 and 5) show that something as simple as the flux application angle and multi-point sourcing can go a long way toward ensuring flux finds its way to each barrel’s top. The sketches show theoretical flux application from single and multi-point sources at 3" below a 0.125" board. Flux dispense orientation is shown both perpendicular to the fluxer base and perpendicular to the board (6° angle). The sketches depict an overall flux application sketch and a magnified image of the flux dispersion at the board surface.


Single pass performance. Each fluxer was tested for performance in a single pass; results are shown in Figures 6 to 9. This test illustrates flux application width and the pass integrity at its extents. From field experience, we know that a minimum 3" pass is desirable, particularly for wide boards at relatively high conveyor speeds (to permit the servo to return the head to start position before the next pass must be triggered). We also know that sharp edges correspond to limited overlap in adjacent passes and reduced overspray at board edges.


Uniformity and hole penetration. PH paper was used to determine flux pattern uniformity with the fluxer set to traverse every 3" of forward board movement. Although air velocity and air uniformity are important fluxer performance factors, flux must be distributed uniformly within the 3" spray width to obtain uniform flux penetration at the board. Uniformity results are shown in Figures 10 to 13.


The air atomized jet had the best overall uniformity, followed by ultrasonic impact. The ultrasonic type A and air atomized single-point nozzle appeared to have more flux in the nozzle center and less on the sides of the 3" spray width.

Figures 14 to 17 show uniformity and hole penetration of a single spray pass through the three-hole patterns’ centers. The figures show the air atomized jet has the best uniformity and hole penetration, followed by the ultrasonic impact. Notice that the ultrasonic impact starts getting lighter on the outer edges on the 0.024" hole pattern and even more so on the 0.013" pattern. Both the ultrasonic type A and air atomized single-point nozzle showed weak hole penetration on the 0.013" pattern, although the air atomized single-point nozzle did show good uniformity across the 0.024" and 0.032" hole patterns.


Maintainability. The ultrasonic type A fluxer:

• Uses a stepper motor and a belt-driven actuator.
• Has wearable parts.
• Requires mechanical adjustments to the spray head.
• Uses an external cleaning method that results in a large amount of liquid waste.
• Has a self-tuning ultrasonic generator that requires manual tuning at times.

The ultrasonic impact:

• Uses a servo drive and a ball screw actuator.
• Has no wearable parts.
• Requires no mechanical adjustments to the spray head.
• Has a unique internal air orifice cleaning system that produces little or no waste.
• Has an closed loop ultrasonic generator that never needs tuning.

The air atomized jet:

• Uses a servo drive and a ball screw actuator.
• Has no wearable parts.
• Requires no mechanical adjustments to the spray head.
• Has a unique internal nozzle and air orifice cleaning that produces little or no waste.

The air atomized single-point nozzle:

• Uses a servo drive and a ball screw actuator.
• Has no wearable parts.
• Requires no mechanical adjustments to the spray head.
• Has a unique internal air orifice cleaning that produces little or no waste.

The listed items do not reflect each system’s complete maintenance requirements. Board production levels and operation frequency directly affect fluxer maintenance. Maintenance requirements and costs associated with operating the fluxer (flux consumption, waste disposal, etc.) play into cost of ownership and should be considered.

Chemical compatibility. All the systems tested were highly resistant to chemicals found in most types of flux. However, some fluxes contain chemicals that will damage any fluxer over a period of time. Fluxes that contain hydrochloric acid, hydrobromic acid and halogenated OA fluxes should be avoided, as they will damage both the fluxer and soldering machine over time.

Data and charts presented here show that, for equal flux volumes, each spray fluxer system tested had different performance levels. Individual fluxer performance can be improved by reducing 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. As such, a lower performance fluxer may be more than adequate in a specific production environment.

Further, this research shows that some flux systems inherently provide better uniformity and hole penetration because of their design and operating characteristics. In 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 be considered in the decision. The data collected show that for high-performance production environments, an air atomized jet flux system provides superior performance at a low cost of ownership.

Ed.: This was first presented at the Apex conference in February 2007 and is republished with permission.

Ken Kirby is applications engineer at Speedline Technologies (speedlinetech.com); kkirby@speedlinetech.com.