Circuits Assembly - June 2008 - (Page 30)

Wave Control Measuring and Controlling Wave Height and Dwell Time Ken Kirby A direct measurement, closed-loop system is effective for regulating critical parameters. n 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 30 Circuits Assembly JUNE 2008 A Figure 1. Impact of immersion depth. 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. circuitsassembly.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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