A direct measurement, closed-loop system is effective for regulating critical parameters.

An unfortunate, somewhat rhetorical, utterance often heard at the wave soldering work center is “I don’t understand this. One day this board runs perfectly and the next it’s loaded with defects!” This generally indicates something has changed in the process, and the solder wave is often the culprit.

The solder wave is at the heart of the process, and its parameters must be optimized and controlled to minimize defects and ensure repeatable results. Operating parameters include:

• Dwell time, or the amount of time a component lead is in the solder wave. This should be controlled in 0.1-sec. increments.
  
  
• Immersion depth, or how deep a board sinks into the wave. Even the best designs have some variation in wave height, so this parameter should be gauged relative to a process window.
  
  
• Contact length, or the distance a lead travels through the wave.

Immersion depth affects contact length, which in turn determines dwell time (Figure 1). This implies conveyor speed is not the sole operating parameter that regulates dwell time. There must, in fact, be a means for accurately measuring and controlling immersion depth.


Wave height. Immersion depth directly affects contact length and dwell time. If wave height changes, immersion depth changes. Therefore, direct, accurate wave height measurement is critical. Solder pump speed controls wave height, which also can change as solder is removed or added to the pot. Even the amount of dross present can make a difference.

Conveyor speed, lead clearance and wave height are controllable parameters that also impact dwell time and immersion depth. Wave height is by far the most influential variable of the three, so it becomes imperative to control this parameter with Six Sigma methodology to maintain a consistent process.

The novel wave height control system is designed to control the distance between the conveyor rail and solder wave. The PCB being soldered is held at a fixed distance below the rail to maintain accurate contact between the board and solder wave. The control system compensates for changes in solder level, dross accumulation and mechanical variation due to heat or changes made during maintenance, so that a set point entered in a particular recipe will always produce the same results. The sensor is positioned as closely to the PCB as possible to facilitate measurement of wave height at the board.

The system uses eddy current technology that has a high degree of immunity to the hostile operating environment near the solder wave. Measurement of the solder wave’s intrusion into the sensor’s magnetic field is converted to a voltage proportional to the distance between the sensor and solder wave. This voltage is interpreted by software and converted to a reading that represents the solder wave surface in relation to a PCB supported by the conveyor fingers. When wave height control is enabled, the computer adjusts solder pump speed so that the sensor reading matches the wave height set point entered by the operator. Figure 2 illustrates a distribution of 450 samples with a wave height set point of 0.000". The overall control range is within +/-0.006" of set point with the clear majority falling between +/-0.002".


A number of methods can be used to determine the proper wave height control set point (glass plate, wave analyzer, trial and error). When the desired setting is found, it is entered as a set point in the process recipe. Every time the recipe is run, the solder wave-to-board contact will be identical. The control range is from 0.240" below the “V” of the conveyor fingers to 0.220" above. A minimum board spacing of 8.0" is required for accurate wave control. With lead clearance and conveyor speed fixed, this system directly controls the dwell time and immersion depth of the board through the solder wave.

Design details. Mounting the sensor into the conveyor rail close to the PCB being soldered minimizes measurement error (Figure 3). Wave height control software is integrated with board tracking software to compensate for distortion in wave height readings as a result of the board passing through the wave.


Board tracking enables running the machine in standby mode, in which the solder wave is shut off when no boards are in the system. To bring the wave quickly back to the correct height, the last pump speed before the wave was shut off is stored. When the board reaches a set distance (as entered by the operator) from the wave, the pump is brought back to the stored speed. The wave is permitted to stabilize for approximately 4 sec.; then the software initiates height measurement and control.

Dwell time baseline. A design of experiments (DoE) was conducted to determine optimum dwell time and to illustrate how wave height variability, which affects dwell time, will also affect defect rates. Two different boards were used to show the optimum dwell time, for one board may not be the same as another. In the DoE, boards were run at different dwell times in half-second increments, from 0.5 sec. to 5.0 sec., correlating each dwell time with the defect rate it produced.

Figure 4 shows defect rates at various dwell times for two different boards. It was determined the lowest defect rate for one board was between 2.5 and 3.0 sec. A second DoE was conducted to refine the dwell time, and it was determined defects are consistently lowest at 2.8 sec. The optimal dwell time for the second board was 3.6 sec., reinforcing the theory that one size does not fit all.


Reference

1. Technology Information Corp., 1999.

Bibliography

1. Gary Jimenez, Martin Ingall, Nissim Sasson, “Critical Parameters in Wave Solder Optimization,” Circuits Assembly, April 1999.

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

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