Printed Circuit Design & Fab - September 2008 - (Page 25)

ENvIRONMENTAL uPdatE Restricting the use of lead in PCB fabrication and assembly has required significant efforts both by material suppliers to reformulate basic products and by fabricators to tighten their processes to facilitate the use of these new materials. At this time, industry confidence and long-term reliability data on lead-free electronics does not match the substantial history that exists for traditional tin-lead products and processes. It will take more time, additional testing and substantial data collection to develop the real-time reliability information needed to assess the impact these changes have had on long-term product reliability. pCd&f MichaEl J. taylor works in research and development for Dynamic Details Inc. (DDi) and can be reached at mtaylor@ddiglobal. com. FiGurE 5. Lead-free assembly classifications for test purposes. Current lead-free solder formulations that meet the RoHS directives require higher temperatures to ensure reliable joints that are equivalent legacy tin-lead solder. The existing FR-4 material sets did not behave well in all designs, so the fabricators have had to conduct extensive testing to evaluate the impact of the change in the assembly process. In the case of DDi, testing protocol divided lead-free assembly applications into three groups, A, B and C, depending on reflow profile and board thickness, as shown in FiGurE 5. The tests included thermal stability, peel strength, solder float, electrical conductivity and the IST, HATS and CAF tests. In addition, they evaluated 10 different FR-4 materials and made several classifications of FR-4 boards by material properties, as shown in FiGurE 6. These properties include: ■ Resin system ■ Tg (glass transition temperature inflection point where the CTE changes due to increased molecular mobility) ■ Dk (dielectric constant), the relative ability of the insulator to hold a capacitive charge as compared to free space ■ df (dissipation factor), the decimal ratio of the irrecoverable to the recoverable electrical energy introduced into the insulating material when exposed to electromagnetic field Within the industry, the IPC has published Standard IPC-4101B with slash sheets to represent generic products which can meet lead-free assembly requirements. Specifically, /126 and /129 represent materials which meet the most rigorous requirements. The specifications address tests and methods, reproducible from one lab to the next, that provide a decision point on the product produced as a raw material for fabricating boards. Board fabricators see this as an SEPTEMBER 2008 FiGurE 6. Board classifications by material properties. entitlement (minimum of expected properties) but are more concerned with the integration of these materials, the design and value-added processes to provide a functional board. Summary The EU directives and other such global regulation are driving the evolution of green laminates and manufacturing processes. Solving environmental issues requires diligence in ensuring that downstream processes are not compromised. Ed note: This article has been adapted from the book “Green Electronics Design And Manufacturing: Implementing LeadFree and RoHS-Compliant Global Products,” by Sammy G. Shina (McGraw-Hill; 2008). printEd CirCuit dESign & fAB 25 http://www.chemcut.net http://www.chemcut.net

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - September 2008

Printed Circuit Design & Fab - August 2008 Contents Our Line Market Watch Around the World Happenings ROI Positive Plating Off the Shelf Marketplace Ad Index EMC for the Real World Final Finish Forum Design for Green: Laminates A Systematic Approach to Increasing Layer Count The NTI $100 Million Club Printable Nanocomposites BGA Bulletin

Printed Circuit Design & Fab - September 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.