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Lead-Free Reflow Process Control
December 31, 1969 |Estimated reading time: 3 minutes
The changeover to lead-free solders due to environmental concerns and legislation presents new challenges for the solder reflow process.
By Fred Dimock and Karl Fischbeck
Today's climate demands that a company look at the bigger picture instead of just reducing product defects, with the goal of stopping defects before they happen. Superior process control is within every manufacturer's reach if they apply good statistical process control (SPC) techniques.
Shrinking Lead-free Reflow Process Windows
Standard tin-lead (SnPb) eutectic solder has a melting point of 183°C with a generous reflow window, meaning many manufacturers have been able to use just one or two thermal profiles to process a wide range of board assemblies. However, when dealing with lead-free solders, the process window for reflow has shrunk considerably. The longer soak times and increased melting point of lead-free solders (220°C) approaches component maximum temperature of 240°C. This means that tighter thermal control and more profiles are required to ensure good quality solder joints without damaging components.
Process Control
The days of taking process data by gathering, compilation, sorting, separating and matching, and then feeding the results into various charting formats for analysis are gone. Reflow processes now can be run using advanced SPC software, data acquisition equipment and precision oven capabilities.
Today's precision reflow ovens have advanced closed-loop systems and control algorithms that limit variations. These self-controlling systems provide feedback so the reflow oven can adjust itself to remain within the process window. Sophisticated mathematical algorithms determine when adjustments are needed and how much change is needed to keep parameters on target. Closed-loop control of factors such as convection, belt speed and zone temperatures provides for stable process platforms, the foundation of an effective SPC program.
Data acquisition equipment includes data logging and RF product profilers, machine quality management (MQM) tools, and 24-hour reflow monitoring. Each has built-in, sophisticated data collection, charting and SPC analysis, and gather the data and create process control metrics. They then compare the results of the process profile with predetermined control parameters, automatically reporting any out-of-spec trends before product quality is negatively affected.
All typically integrate with standard production or plant management SPC software packages. To be truly useful, the data logging and SPC analysis software must be intuitive and have a short learning curve while providing a full complement of useful features, such as SPC analysis, data storage, retrieval, process modeling and reporting.
The goal of process qualification tools is to "know" good parts will be made before the actual product is run. Building a database of control charts that contain most common causes of variability accomplishes this objective. Data are entered into the oven computer from either an MQM tool or a continuous oven monitoring system. Comparing actual process data to these charts allows an "out of control" situation to be caught and corrected at its outset.
The Whole SPC Picture and Reflow Action Plan
The main reason to link the reflow oven SPC into the overall production line quality control system is to ensure good product, critical during the transition to lead-free solder formulas. Reflow data can join with other production data to create full accountability. If issues surface regarding a particular product lot, even after a year or more of use, SPC oven/product data, along with good record keeping will identify the paste used, the profile the product saw, and the parameters of the oven during the time the parts were manufactured.
Pinpointing and correcting a problem before it exists cannot be done without a good control action plan. Action plans can be as simple as a written instruction to turn off a process and call an engineer or as detailed as a step-by-step guide, depending on the amount of responsibility a company wants an individual operator to have and the workforce skill level.
Everyone involved must know exactly what corrective steps are to be taken when an out-of-control situation is identified. If all this is in place and running smoothly, it translates into lower overall manufacturing cost, increased product quality, higher yield, and even increased life for the reflow oven.
Fred Dimock, senior process engineer, BTU International, and Karl Fischbeck, director of North American sales, Electronic Controls Design Inc., may be contacted at (978) 667-4111, ext. 207, E-mail: fdimock@btu.com and (607) 786-5788, E-mail: ecdeast@ pronetisp.net, respectively.