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Green engineering challenges reliability, logistics

Posted: 17 Jul 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Gina Roos? EE Times? spotlight? green engineering? design for environment?

Chipmakers began transitioning to Pb-free finishes and "green" packaging years ago, prompted largely by the European Union's clampdown on six substancesincluding leadin electronics equipment. Eliminating lead has significantly affected the electronics industry. The biggest technical challenges by far are selecting a replacement for reliable tin/lead finishes and choosing a mold compound that could meet the higher reflow temperatures required by Pb-free processing.

But even as manufacturers addressed these reliability concerns, other major worries have surfaced, including logistics issues such as inventory management, part number changes for compliant parts and management of materials-content information.

Although it hasn't been easy, many IC suppliers have already implemented manufacturing strategies that mitigate the risks associated with Pb-free processes, especially those involving tin whiskers and solder-joint reliability.

One of the biggest challenges in the transition to Pb-free parts is tin whiskers, which can cause electrical shorts and system failures. Although matte tin finishes have been found to be the most suitable to replace tin/lead alloys in plastic packages, the whiskers, which are electrically conductive crystalline filaments, have been shown to grow in both pure and composition tin finishes.

The tin-whiskers problem was a key reason for the exemption of several high-reliability industriesmilitary/aerospace and some portions of industrial controls and telecommunicationsfrom the rohs directive for a fixed period. Component manufacturers continue to supply these industries with tin/lead plating or use gold plating for lead-frame terminals. But in some cases, chipmakers such as Texas Instruments have chosen to use a nickel/palladium/gold (NiPdAu) finish, which eliminates the tin whisker problem.

Matte tin was chosen for its electrical, mechanical and wetting properties, comparable to an industry-accepted finish, said Kathleen McLaughlin, a packaging and assembly engineer at Analog Devices Inc. "There are no backward- or forward-compatibility issues, provided that the board-level processing is optimized for the given components to ensure a reliable joint," she said.

Others agree. "Tin is a great metal, except for the whiskers," said Ray DiBugnara, director of technology at Microsemi Corp.

IC suppliers aren't turning a blind eye on NiPdAu plating, but a higher cost is associated with the transition. Besides tool modifications, process characterization and qualification work need to be done. Qualifying the different package types with the new plating is both costly and resource-intensive.

Mitigating tin whiskers
IC makers have implemented two key strategies to mitigate the formation of tin whiskers. One industry-accepted process is a post-anneal bake at 150C for one hour within 24hrs after plating.

The other strategy is to increase tin-plating thickness, typically from 7.6m to 10m. Most IC suppliers also closely monitor their plating processes to ensure that the impurity levels of the baths are minimized. While they have always monitored these processes, with the move to tin plating, suppliers said their efforts have become much more focused.

Another concern is raised with Pb-free BGAs that typically use solder balls of tin, silver and copper (Sn/Ag/Cu).

"The industry, in general, doesn't recommend using lead-free BGAs in a leaded reflow process because bad solder-ball joints are likely to occur," TI's lead-free program manager Ken Farrington said. "This is because lead-free solder balls use the same material as the lead-free solder paste (Sn/Ag/Cu) used on the boards. Thus, both need a high-temperature reflow process to melt/reflow properly. If a lead-free BGA was used on an old 'leaded' board and a low-temperature reflow process, the solder balls would not melt sufficiently to create a good solder joint."

Customers worried about "tin whiskering" will usually say the same thing about Pb-frame-based parts with matte tin finishes, Farrington said. "They often prefer to use the 'leaded' version in their 'leaded' systems, which doubles the work."

Another challenge is the need to support higher reflow temperatures of 260C. Lead-free solders typically require 30C higher soldering temperatures than tin/lead solder. If parts can't withstand the higher temperatures, problems like delamination, popcorning and material breakdown can occur. Moreover, higher temperatures have been found to degrade the moisture sensitivity level (MSL) ratings. Caveat: Just because a part is RoHS-compliant, it doesn't necessarily mean it can withstand the higher reflow temperature.

Analog Devices Inc. (ADI), like most chipmakers, considered the impact of higher reflow temperature on packaging. As a result, it changed its mold compound and die-attach materials to minimize delamination, said McLaughlin.

Suppliers must also ensure that new compliant packages maintain their MSL ratings as defined in Jedec J-STD-020. While most IC suppliers have managed to maintain the same MSL as equivalent parts that contain lead, some were reclassified because of the MSL degrading.

"If you change the MSL rating from 1 to 3, it requires the manufacturer to dry-pack and bake the products before selling them, which also adds cost," said Marji Baumann, Pb-free program director at ON Semiconductor.

Materials data
"OEMs usually have to provide a recycling report of all the materials used in their product, so they need material content information beyond the RoHS 6," ADI's Farrington said. "They need to track any potential hazardous materials in addition to any recyclable materials."

"We know how much die attach, gold bond wire and mold compound was used; the weight of the lead frame; weight of the NiPaAu finish; and we map this data back to the search tool," she said.

Customers must also be very clear about how each supplier defines RoHS-compliant, Pb-free and green. Much of the confusion surrounds the definition of green. Generally, green packages meet the RoHS restrictions, along with limiting bromine, chlorine and antimony.

Other headache-inducing issues for all supply chain partners include how companies are dealing with part numbers for RoHS-compliant/Pb-free versions, and keeping Pb-bearing and Pb-free inventories separate.

Without an industry-accepted way of designating Pb-free parts, suppliers are using their own judgment. In some instances, suppliers have kept the same part number, while others have added a suffix to existing part numbers. Some have also added special RoHS-compliant/Pb-free symbols to the packaging and Pb-free labels on shipping containers.

- Gina Roos
EE Times

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