Circuits Assembly - June 2008 - (Page 32)

Wave Control 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 solFigure 2. Distribution of samples at set point 0.000”. der 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 Figure 3. Sensor coil mounted in conis within +/-0.006" of set point with veyor rail. 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 Figure 4. The blue and red lines show 0.240" below the “V” of the conveyor dwell time and defect rates.1 fingers to 0.220" above. A minimum board spacing of 8.0" is required for accurate wave control. With lead The control system compensates for clearance and conveyor speed fixed, changes in solder level, dross accuthis system directly controls the mulation and mechanical variation dwell time and immersion depth of due to heat or changes made durthe board through the solder wave. ing maintenance, so that a set point Design details. Mounting the senentered in a particular recipe will sor into the conveyor rail close to always produce the same results. The the PCB being soldered minimizes sensor is positioned as closely to the measurement error (Figure 3). Wave PCB as possible to facilitate measureheight control software is integrated ment of wave height at the board. with board tracking software to comThe system uses eddy current pensate for distortion in wave height technology that has a high degree readings as a result of the board passof immunity to the hostile opering through the wave. ating environment near the solder Board tracking enables running wave. Measurement of the solder 32 Circuits Assembly JUNE 2008 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. n 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. circuitsassembly.com http://speedlinetech.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - June 2008

Circuits Assembly - June 2008 Contents Letters Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing China Goes ‘Upmarket’ Out of the Garage Improving Production Line Performance Manufacturing Steps Onto the Enterprise IT Stage Measuring and Controlling Wave Height and Dwell Time Wave Soldering Tech Tips Test and Inspection Process Doctor Pb-Free Lessons Learned Getting Lean Eastern Advances Semicon West Product Preview Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - June 2008

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