Circuits Assembly - November 2007 - (Page 35)

RoHS Transition for exposed copper on the board pads, particularly in pad corners and perimeters, thus increasing the potential for tombstoning or solder balling. As noted, the solder paste formulation plays heavily into the success of this process. So, too, does stencil aperture design. VirTex has found that rounded apertures behave significantly better, particularly when used with 0402 or 0201 size components. This produces a more uniform solder flow and facilitates alignment of the components. With continued component miniaturization, aperture openings and stencil alignment to board pads are critical for repeatable solder joint quality. Manufacturing flow. The overall SMT manufacturing flow is essentially the same sequence of operations used for leaded manufacturing. However, the applied reflow temperatures, to achieve SAC 305 solder reflow at 217°C, must be increased from 240° to 252°C. These increased temperatures and slower wetting cycles require the EMS fully understand the materials submitted for reflow. Significant damage to an assembly can occur if the laminate is not compatible with higher soldering temperatures. In the case of a Pb-free BGA device and a nonRoHS board, the EMS company should suggest to its customer that the product not be assembled. The potential for the board delaminating is very high and irreparable. Also, if reflowing a Pb-free BGA using a conventional leaded temperature of 215°C, ensure the balls on the BGA do not slump and a reliable solder joint is formed. It is imperative that the EMS verify all these packaging parameters prior to reflow. Rework. Pb-free rework processes have been developed during the past couple years. The applied temperatures from the focused hot air or gas can be as high as 280°C. Again, the materials used must be capable of withstanding these temperature levels. Pb-free board manufacturing has matured significantly during the past three years. Parts are now more common; board materials can handle the higher temperatures; component terminations and board finishes are compatible with the increased temperatures; solder paste formulations provide better reflow characteristics, and processes have been developed for all functions. Now we are quickly moving toward a time in which process refinements that enhance reliability will be the focus – just in time to support the new Chinese requirements. ■ Bibliography James Hall, “The Maturing of Lead-Free Assembly,” SMT, May 2007. Chrys Shea and Ranjit Pandher, “Optimizing Stencil Design for Lead Free SMT Processing,” SMTA International Proceedings, September 2004. Greg Caswell, Lead Free Manufacturing from a Contract Manufacturer’s Perspective, IPC Designers Council, February 2005. Greg Caswell is director, engineering services, at VirTex Assembly Services (virtexassembly.com); gregc@virtexassembly.com. Visit us at Productronica, November 13-16, Hall A5, Booth 312 circuitsassembly.com Circuits Assembly NOVEMBER 2007 35 http://virtexassembly.com http://www.valor.com http://www.valor.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - November 2007

Circuits Assembly - November 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Better Manufacturing Pb-Free Manufacturing from a Tier III EMS Perspective ‘Living Documents, Subject to Change’ Creating Ideal Solder Joints An Alternative Drying Process for MSDs ‘Customer Satisfaction is More than a Score’ Optoelectronic Substrates: Will They Happen? Tech Tips Wave Soldering Process Doctor Test and Inspection Getting Lean Materials World Equipment Advances Product Spotlight Assembly Insider Technical Abstracts Ad Index

Circuits Assembly - November 2007

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