Circuits Assembly - February 2008 - (Page 33)

Profiling for Large BGA Attachment A series of studies reveal the critical factors. he profiling basics for attachment of large defined amount of time within this range. The flux ball grid array packages using industrymaterials used for Pb-free rework conditions are specific precision systems consist of bottom typically referred to as gel flux or tacky flux. These heaters (commonly referred to as preheaters), and materials are stable to higher temperatures than a top heater (also referred to as a nozzle heater). those of traditional solvent-based flux materials. The convection settings of temperature and airSAC 305 has a liquidus temperature of 217°C flow rate are controlled by profiles within systemand is reflowed at peak temperatures of 235°C. specific software. When processing on a rework Zones three and four are where the nozzle temsystem, profiles are also created. The process is perature is raised in conjunction with an increased similar to that of a reflow oven; however, the airflow rate. For relatively large BGAs – 35-50 mm board is not conveyed through the temperature on a side – airflows will generally be high for zones zones but remains in place, and the system heaters three and four. are ramped instead. To summarize the sequence of events: From a system viewpoint, the process attaches • The substrate is preheated to 150°C using pria large BGA to a PCB composed of epoxy, fibermarily bottom heat. glass, and many layers of copper metal. The sub• The substrate is heated to within 20°C of the strate will also have various components already liquidus. attached. Each of these materials has specific • The component is directly heated using a highthermal properties. Some components are large, er nozzle temperature and airflow rates. some small. • The attachment material reaches liquid temAs heated air is applied to the board botperature at the reflow zone. tom by the preheaters, conduction takes place • The solder densifies to a standoff height. within the materials. There are two valCopper is an extraordiues to consider for temnary conductor of heat perature differences. with a thermal conducThe first is across the tivity of 401 W·m -1·K -1 component, and is measured from the center (at 25°C). With large of the component to a boards and high thercorner. Faster profiles mal mass, the input heat must be significant to Figure 1. Target end-of-zone temperatures for thermocouple will generally create a larger temperature difheat the localized area reading at component. ference across the comof interest. For a Pb-free ponent. This is important because when the grid solder attach profile, a typical preheat will target begins to reflow, the center may liquefy first, 150°-180°C at the end-of-zone temperature. This followed by the edges. For Pb-free components, temperature is read at the component/substrate a differential as low as 5-10°C is required. The interface, S. A consistent preheat is important second temperature gradient is measured from the because temperature variations in environment component top to the substrate bottom. A value will be present, and elimination of this unconfor this is directly measured from the placement trolled factor is best kept in check. of monitoring thermocouples on the top of the lid For large substrates, the preheater will apply and at a secure attachment on the board bottom, most of the heat in order to minimize the temcentered below the component. A value of 10°C is perature difference across a component. This is generally required and is important for fine-pitch accomplished by having the preheater temperacomponents. ture higher than the nozzle temperature. This will The justification for using process parameters be the case for the first two or three zones. This with a higher bottom heat relative to the nozzle practice also prevents the drying or early activatemperature is to prevent component warping. tion of flux materials. Figure 1 shows an example Component packaging is considered safe below of a four-zone ramp. Zone two should have a 250°C. However, with a higher preheat, the board target end-of-zone temperature of 190°C. This is is another limiting factor. Excessive heat will cause where flux is activated and specifications require a Tech Tips T The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly. circuitsassembly.com Circuits Assembly FEBRUARY 2008 33 http://www.aciusa.org http://circuitsassembly.com

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Circuits Assembly - February 2008 Contents Caveat Lector Letters Industry News Market Watch Global Sourcing Better Manufacturing Maximizing Lean Copper As a Viable Solution for IC Packaging Embedded Active Components for High-Rel Products Cover Story: XRF Equipment As a RoHS Screening Tool Tech Tips Selective Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - February 2008

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