iNEMI Lead-Free Assembly & Rework Project Reports Results at IPC Printed Circuit Expo/APEX/Designers Summit; Project team reviews conclusions of three-year effort.

ANAHEIM, Calif. -- The International Electronics Manufacturing Initiative's (iNEMI's) Lead-Free Assembly & Rework Project is featured as one of the free forums at this week's IPC Printed Circuit Expo/APEX/Designers Summit conference. The project team will report on their three-year collaboration to develop lead-free assembly and rework processes for large, double-sided, thick IPC Class 2 printed wiring boards (PWBs). The forum (#F06) is scheduled from 3:00 to 5:00 p.m. PST on Wednesday, February 23.

"This project closely followed the initial work and results developed by the first iNEMI lead-free development team," says Jerry Gleason, R&D engineer for HP and co-chair of the iNEMI project team. "We focused on furthering lead-free process development, including second-level assembly and rework processes. We also evaluated manufacturability, toolsets and components. All assembly and component rework processes were conducted on production equipment to ensure that test results reflected 'real-world' processes."

Thick boards, such as the ones used by the Lead-Free Assembly & Rework Project, pose significant challenges for lead-free processing. They require more heat, have greater Delta-T across assemblies(1), and may require a slower reflow oven line speed. As overall board size increases, process windows tend to shrink.

Rework poses additional challenges. The higher temperatures used for lead-free processing place greater stress on components and boards, which is exacerbated by cumulative heat exposures. The combined reflow and rework heat cycles can result in a single assembly being exposed to as many as five thermal heat excursions (two reflow passes, a wave solder pass and two local hot gas rework passes). Internal package structures within components and PWBs must survive all processes and still provide long-term reliability. Component effects include increased moisture sensitivity levels (MSLs) and resulting shorter exposed floor life; while PWB laminates must minimize internal layer delamination, via cracking, and board warpage.

"The key to reliable surface mount reflow as well as hot gas rework processing is to understand the effect temperature has on components and PWBs, while controlling the manufacturing line to newly specified operating windows," explains Charlie Reynolds, an engineer for IBM Microelectronics Division and co-chair of the Lead-Free Assembly & Rework Project. "The iNEMI project was organized to investigate these effects and we successfully developed processes for both reflow and rework that provide for the assembly of large, thick IPC Class 2 boards with a wide range of components using the lead-free iNEMI-recommended SnAgCu solder alloy."

The project team designed a special reliability test board utilizing high-temperature laminate intended for Pb-free soldering. These large (7" X 17"), thick (0.135"), double-sided boards had 14 layers, and were assembled with SMT and wave assembly processes, using the iNEMI-recommended Sn3.9Ag0.6Cu solder.

Half of the test boards were passed through a series of representative component rework protocols. Each build group was then subjected to a series of mechanical bend tests and 5700 cycles of accelerated thermal fatigue reliability stress tests (0 degrees to 100 degrees C), followed by failure analysis.

Highlights of project findings include:

1. IPC Class 2 lead-free SMT reflow process windows will shrink when compared to current tin-lead processes. The lead-free SnAgCu-based alloy systems have a near-eutectic melting point approximately 34 degrees C higher than the tin-lead eutectic point.

2. Multiple heat cycles caused laminate and via damage. The higher lead-free processing temperatures placed greater stress on components and boards and exacerbated the cumulative heat exposures. The board thermal performance observations have been shared with the IPC's Printed Board Base Laminate Materials Committee to highlight the concerns and challenges lead-free processing entails.

3. Based on iNEMI test results, lead-free SMT first-pass reflow processing typically had the following characteristics:

--Four- to six-minute cycle times.

--Temperatures ranging from 230 degrees C to 250 degrees C for joints and body temperatures.

--Delta-T ranges from 5 degrees C to 20 degrees C on a single assembly.

--Time above liquidous ranging from 60 to 90 seconds.

4. The use of nitrogen was shown to promote wetting, and created shinier looking solder joints (improved aesthetics). If there are aesthetic concerns, the use of nitrogen will ensure lead-free soldered joints "look better." The study between air vs. nitrogen showed no significant difference in microstructure formation and reliability performance (based on pull/shear tests) for SMT assembly.

5. SMT reflow using an air atmosphere was shown to produce properly formed and reliable solder joints.

6. Trials and testing recommended that the coldest solder joint on any lead-free assembly should be no less than 230 degrees C.

The Lead-Free Assembly & Rework Project team will discuss these findings and more at the Wednesday forum. The team is also scheduled to give a final report at IEEE's Electronic Components and Technology Conference (ECTC), May 31-June 3 (http://www.ectc.net/). The project final report will be available on CD in June ($150 for non-members). Watch www.inemi.org for details.

About iNEMI

The International Electronics Manufacturing Initiative's mission is to assure leadership of the global electronics manufacturing supply chain. Based in Herndon, Va., the industry-led consortium is made up of approximately 70 manufacturers, suppliers, industry associations and consortia, government agencies and universities. iNEMI roadmaps the needs of the electronics industry, identifies gaps in the technology infrastructure, establishes implementation projects to eliminate these gaps (both business and technical), and stimulates standards activities to speed the introduction of new technologies. The consortium also works with government, universities and other funding agencies to set priorities for future industry needs and R&D initiatives. For additional information about iNEMI, visit www.iNEMI.org.

(1) "Delta-T across assemblies" refers to the difference between the "coolest" solder joint and the "hottest." iNEMI measurements indicate that T can range from 5 degrees C to 25 degrees C, depending on the size and thermal mass of the assembly.