Printed Circuit Design & Fab - March 2008 - (Page 36)

INNERLAYER PROCESSING Improved INNERLAYER BONDING for Sequential Lamination Multiple lamination cycles add cumulative thermal stress to the innerlayer bonds. by DR. JEAN RASMUSSEN, DR. ABAYOMI I. OWEI, DANIS ISIK, DR. AXEL DOMBERT, and DAVID ORMEROD Industry requirements to meet the demands of lead-free applications have challenged many areas of PCB fabrication and design. These demands have initiated a rapid influx of new dielectric materials, which, despite increased thermal stress, must remain effectively bonded to copper innerlayers. Delamination failures in advanced multilayer and sequential-build products, caused by the conversion to lead-free soldering, have also driven the need for improved oxide replacement technology. Such products must resist higher peak temperatures, some 30˚C greater than eutectic applications. Based on statistically designed experiments, a new process has been developed that delivers a more resilient copper conversion coating. The process has shown improved bonding performance and greater stability at higher temperatures, thus improving its capability for more thermal excursions (assembly-reflow cycles at >260˚C) without failure. This article focuses on the product development criteria and performance evaluation required to develop a process that is suitable for this challenging environment. The testing and qualification will include data on peel strength, solder float and IR-reflow testing. Over the past decade, traditional alkaline black oxide (or brown oxide) bonding processes needed replacement due to a range of technical and environmental issues as well as heightened cost pressures. The acidic peroxide-sulfuric (oxide-replacement) processes quickly found favor due to lower cost and more environmentally friendly chemistry. These peroxide-sulfuric technologies, which use a primary organic additive to enhance the surface texturing, were shown to deliver excellent bond integrity in the pre-RoHS assembly setting. However, it comes as no surprise that continuing advances in PCB fabrication are now pushing the performance requirements of these processes as well. These include: ■ Advanced board constructions / sequential lamination / microvia capable build-up processes ■ New halogen free, high Tg and filled phenolic cured dielectric materials ■ Demanding, high temperature lead-free assembly conditions FIGURE 1. Schematic of the alternative oxide reaction. 36 FIGURE 2. XPS spectra of the alternative oxide and surface composition. MARCH 2008 PRINTED CIRCUIT DESIGN & FAB

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

Printed Circuit Design & Fab - March 2008 Contents Our Line Market Watch Around the World Happenings ROI EMC for the Real World Positive Plating FPGA/PCB Co-design Increases Fabrication Yields Optoelectronics Comes of Age, Part 2 Implementation of Buried Capacitance in High-Speed Designs Ad Index Improved Innerlayer Bonding for Sequential Lamination Off the Shelf Marketplace BGA Bulletin

Printed Circuit Design & Fab - March 2008

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