Circuits Assembly - November 2007 - (Page 34)

RoHS Transition datasheets provide the maximum temperature to which a component can be subjected. Finally, as BGA or other area array packages go, so goes the assembly. If the balls on any BGA are Pb-free, then the entire assembly must be produced with Pb-free materials (e.g., solder paste, board materials, etc.) to ensure solder joint reliability. In this circumstance, it is vital for the EMS firm to verify the BGA balls are Pb-free. This can be accomplished through a simple swabbing operation or XRF (x-ray fluorescence). In many instances, customers did not realize that a Pb-free BGA required significant changes in other materials, particularly the printed wiring board, as an FR-4 board may not survive the higher reflow temperatures. The EMS company must guide the customer in order to avoid product failures. PWB fabrication. As noted, standard FR-4 materials are not designed to handle the 250°C exposure required of Pb-free reflow. To remain competitive, PWB suppliers have switched to RoHS-compliant materials. This involved the removal of polybromide-biphenyls (PBB) or polybromide-biphenyloxides (PBBO) from the fabrication process. In 2004, this was not the case. It was vital that the EMS interact with the supplier to identify, or specify, the laminate and B-stage materials required. Most suppliers now use materials such as IS410 or Nelco-4000 to meet Pb-free requirements. As a result of the switch, users must pay attention to a new parameter, CAF (conductive anodic filament) resistance. This involves how close traces can be designed before the propensity for a dendrite to grow between the traces increases. EMS companies must obtain this information from their board suppliers to manage bare board procurement. Surface finishes. Circa 2004, a HASL finish was typical on most FR-4 boards. RoHS mandated the removal of lead. As such, surface finishes like electroless-nickel/immersion gold (ENIG), immersion silver (ImAg), or organic surface protectant (OSP) are now among the common technologies. More recently, some fabricators are using a high temperature HASL finish, which is also viable. It is necessary to provide this information to your supplier. VirTex prefers the finishes in the order noted, as the ENIG approach provides the most uniform surface for reflow; silver does well, and OSP offers some challenges in our experience. ImAg boards must also be properly stored so that the finish does not oxidize. We place a sheet of Daubert Cromwell anti-tarnish paper between boards and seal them in nitrogen. All other boards are either sealed in nitrogen or with desiccant to eliminate oxidation. OSP is removed from the surface of the board during reflow, and the combination of flux in the paste and the reflow temperatures activate its removal. Because of the slow wetting of Pb-free solder pastes, VirTex has experienced issues with achieving complete OSP removal. This can result in pad opens where the material was not completely removed. Stencil design. Spreading characteristics of leaded vs. Pb-free solder pastes are considerably different, with the newer Pb-free materials having a much slower wetting cycle. This variance in the material characteristics provides a reliability concern because of the potential Individual Component Analysis ©2007 Thermo Fisher Scientific Inc. All rights reserved. All trademarks are the property of Thermo Fisher Scientific Inc. and its subsidiaries. Incoming Material Screening Pb-free Veriﬁcation The World’s #1 RoHS Compliance Screening Tools* Thermo Scientific NITON XL3 Series RoHS Analyzers • • • • • Fast, reliable, nondestructive elemental analysis Revolutionary small-spot focus for isolation and screening of small components Quantifies total Pb, Cd, Hg, Cr, Br and more in seconds Integrated color CCD camera and sample imaging system VIP™ tilting color touch-screen display for easy viewing of result Asia Central, Hong Kong Tel: +852 2869 6669 niton.asia@thermofisher.com USA Billerica, MA USA Tel: +1 978 670-7460 niton@thermofisher.com *Market position statements are based on publicly available information as well as private sources. Thermo Fisher does not endorse these sources as being accurate, comprehensive or complete with regard to product, market size or share. Europe Munich, Germany Tel: +49 89 3681 380 niton.eur@thermofisher.com Sales & Service Worldwide www.thermo.com/niton Part of Thermo Fisher Scientific 34 Circuits Assembly NOVEMBER 2007 circuitsassembly.com http://www.thermo.com/niton http://www.thermo.com/niton 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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