Jack Shandle and Lee Goldberg
TechOnline

By the end of this year, chip packaging, solder, solder paste, and other aspects of the electronics industry will start getting more environmentally friendly.

	Pb-Free: The Solder Situation Board-Level Manufacturing Can Pose Problems Although packaging engineers will redesign and requalify literally thousands of products to meet Pb-free goals, it is the manufacturing sector where the rubber will really meet the road. The higher temperatures required for Pb-free solder alternatives are the primary reason for concern because of the additional thermal stresses placed on the PC board and other components. Because Pb-based solder has been used in SMT (surface-mount technology) PC board manufacturing for more than three decades, its mechanical, thermal, and electrical characteristics have—to a great extent—driven the characteristics required of other parts of the PC-board subsystem. "A PC-board is a symbiotic design," says Bill Trumble, an independent SMT consultant based in Kanata, ON. "Every component depends on every other component. They have been designed for mutual compatibility." As a result, when a major part of the system such as solder is replaced, the change ripples through the entire system. Over the past few years, there has been a spirited debate about the effects of Pb-free components on overall PC-board quality. Jennie Hwang, an independent consultant based in Cleveland, OH, says manufacturing problems can be avoided by employing the best practices that have evolved over the past decade of experimenting with Pb-free solders and components. Nevertheless, in order to observe best practices, it is important to anticipate the problems that could occur. There are seven key issues that could cause problems, according to Trumble's review of the Pb-SMT literature. Wetting times Pb-free solders require a higher temperature and more time to wet the solder surface. Together, these conditions promote oxidation of the solder surfaces and inhibit wetting of surfaces on both board and components. Board warping Higher oven temperatures will heat the PC board beyond its normal design range, induce plastic flow in the epoxies typically used in FR-4 board material, and perhaps allow the board to sag out of its fixture. These conditions can impose stress on the components and distort the dimensions of the PC board. Distorted through holes Higher temperatures will make the board expand significantly in the Z direction. This lengthens the plated holes and causes significant strain on the plating in the hole. This can increase the time required for the solder to flow through the hole. It also applies compressive stress on the components during cooling. Conductive anode filaments Higher process temperatures can cause the bond between the glass fiber and the epoxy in the board to break down. The resulting gap can create a conductive path for corrosion from an anodic element to a cathodic element and eventually cause a short. Solderability after multiple reflows Higher process temperatures can significantly deteriorate the solderability of the solder pad on the board with each pass. Effect on components When the PC board assembly cools the dimensional difference will cause a strain on all elements of the solder joint that may cause joint failure. Moisture considerations PC boards and components will have to be dried to a lower moisture content to avoid "pop-corning" components due to moisture trapped under the component.

It's the result of legislation passed by the European Parliament in 2003 to reduce the amounts of heavy metals such as lead (Pb), mercury (Hg), cadmium (Cd), and chromium (Cr-IV) in the environment. The new rules go into effect July 1, 2006, which leaves precious little time for slow learners in the electronics industry to get the lead out—or get out of the multi-billion dollar European Community marketplace.

The harmful effects of lead are well documented and have, for example, resulted in the removal of lead additives in gasoline and the elimination of lead-based paint from the market in many countries.

Two classes of brominated compounds—poly brominated biphenyls (PBBs) and poly brominated diphenyl ethers (PBDEs) are also subject to the law. But for the time being at least, Pb is the primary concern for semiconductor and board assembly companies because of its use in solder (the primary source of lead in electronics), lead-frame packages, ball-grid arrays (BGAs), and other subsystem-level electronic products.

All industries—not just the electronics industry—are subject to the regulations, which apply to products sold within the European Community (EC). Since the EC is a large market, however, any electronics company that sells globally almost certainly already has a Pb-free program in the works.

For the record, the electronics industry adds approximately 18,500 tons of lead to the environment each year, according to consultant Ronald C. Lasky of Indium but this pales in comparison to lead batteries, which add about 4 million tons a year - more than 200 times a much.

The relatively small amount of lead in electronics gear prompted suggestions from business leaders—as well as some environmentalists—that the billions of dollars spent in the conversion to Pb-free electronics could be used more effectively for recycling and other efforts. This view was not adopted by the EC, however, perhaps because there was no politically viable way to access the billions of dollars that would not be spent if there was no Pb-free directive.

Europe's three major semiconductor vendors—Infineon Technologies, Philips Semiconductors, and ST Microelectronics—immediately saw the value of developing Pb-free standards together and speaking about their Pb-free programs with a single voice. As a result, they formed the E3 Pb-Free Initiative.

One result was to define Pb-free for electronic components as products having less than 1000 parts per million of lead by weight. In 2004, U.S.-based Freescale Semiconductor joined the Initiative and the E3 became the E4. Research results and costs are shared by E4 members. They have also been an effective lobby that helped move the original hard deadline for compliance back from 2004 to 2006.

Another goal is to be sure the package and manufacturing solutions have little to no impact on design engineers. According to quality managers at Philips Semiconductors, ST Microelectronics, and Freescale Semiconductor, this goal will be met for the vast majority of designs. "The challenge lies much more with the packaging engineers," says Jan van de Water, Program Manager for Pb-free at Philips. "The issues are mostly mechanical and physical. We have to design Pb-free packages for both existing and new package types."

The cost of new packaging and systems manufactured with it will be affected—but the cost of the new alloys and solders is not the primary driving factor. As the programs ramp up, both IC companies and contract manufacturers will have to maintain both Pb-free and conventional lines costs, says Mike Thomas, Director of Quality at Freescale.

Some companies might be able to keep the price of the finished product stable by absorbing the additional cost, cutting costs elsewhere, by managing the transition successfully, or, some combination of these and other tactics. The other alternative, of course, is to pass the cost of Pb-free on to customers.

Pb-free and conventional lines will co-exist for longer than just a transition period, however, because some industries—telecommunications, medical, and automotive, are the most prominent examples—have been given five-year waivers for complying with the regulations.

The waivers were granted because these industries successfully made the case that reliability of Pb-free components must be proven by years of field testing for system-level products that are mission critical and can have a direct impact on human life and health. Consumer, commercial, and other products will have to be content in relying on laboratory testing that indicates that Pb-free electronic products have reliability ratings equal to those of conventional products, according to quality engineers.

The replacement metals or alloys that ST Microelectronics—as well as the other three E4 Initiative companies—have chosen are shown in Table 1. References in the table to "whiskers" refer to the growth of very thin metallic filaments on the leads after the plating process and during operation.

Whiskers are extrusions of tin that are formed due to internal stress between copper and tin on the coating layer. Once considered a reliability issue, the E4 companies consider the whisker problem solved. A more detailed discussion of whiskers will be presented later this article.

Ball grid arrays and flipchip packages require different alloys and these are shown in Table 2. Glass seals in ceramic EEPROMs and high-lead (Pb > 85%) alloys used for power die bonding are exempted from the EC's Pb-free requirements because no suitable replacements have been found to date. Although the information in Table 2 was provided by ST Microelectronics, it represents the view of all four companies in the E4 Initiative and is widely accepted by other companies as well.

So finding Pb-free replacements involves a combination of materials science and clever process control. Process control is, in fact, the key. "It is getting tougher to make a reliable joint," says Philips' van de Water. "The window for success (in the soldering process) is getting smaller."

Semiconductor companies have four primary problems to solve when designing Pb-free packages: solderability, reliability, whiskers, and moisture sensitivity. Not surprisingly, the attributes are interrelated.

Solderability relates largely to the ability to melt the Pb-free solder at temperatures close to those of lead-based solder and lead coatings. Compatibility with the equipment used in most of today's wave-soldering assembly lines is an imperative if it to be kept in service with a minimum of retrofitting. Reliability addresses the strength of the joint; and, moisture sensitivity determines how long the component may be kept in storage before it is attached to a board.

Solderability
Conventional Pb-based soldering takes place in a range of 215° and 240°C for lead frame devices. Due to higher melting temperatures, matte tin requires a range of 235° to 260°C and this potentially has an impact on reliability. Tests have shown that tightly managing the temperature profile during the soldering process provides an acceptable solution, says van der Water. Other package types have similar results.

Reliability
The reliability of a solder joint can be compromised by changes in temperature while the chip is in service and is known as thermo-mechanical solder fatigue. Joint failure follows a well known process that begins with diffusion and re-crystallization. Crack initiation and growth follow until the fracture can actually be observed.

Testing the reliability of solder joints has been conducted by cycling the product through a range of temperatures from -40° to 125°C for 10,000 cycles. Solder fatigue failure is visualized and analyzed using a technique called Weibull statistics. Many package types have been tested. Reliability has been comparable to conventional connectors and solder pastes.

Whiskers
As previously mentioned, when pure tin is used for plating a lead frame, the growth of tin filaments has been identified as a potential reliability problem. If these "whiskers" grow long enough they can conceivably short circuit two pins. Irregular intermetallic growth at the copper-tin interface causes stresses that extrude the tin whisker from the surface. Whisker growth is not immediately apparent. It occurs during storage at ambient temperatures—not during the soldering process—so countermeasures must be taken.

One approach is to make the tin layer thicker. This dramatically reduces the length of whiskers because a thicker tin layer can absorb more stress. In test results reported by ST Microelectronics, maximum whisker length decreased 160 microns to less than 10 by increasing the thickness of the tin layer from 1.82 microns to 10 microns.

Another approach is to post-bake the component at 150°C for an hour. It was found that the higher temperature created a bulk diffusion in the material and regular intermetallics. No whisker growth has been observed under these circumstances.

Still another approach involves chemically pre-treating the tin surface of the lead frame to create a matte—as opposed to shiny—finish. A matte finish has proven to be less susceptible to whisker formation that a shiny tin finish. There is no reason for choosing just one of these solutions because they are, in fact, compatible.

Moisture Sensitivity
If any component is stored outside a dry pack for a significant amount of time moisture can accumulate which will change its solderability and reliability characteristics. Pb-free components are more inclined to be susceptible to moisture but using a different soldering profile (245°C for packages greater than 350 mm² and 250°C for packages smaller than 350 mm²) helps alleviate the problem, says Freescale's Mike Thomas.

Every Pb-free SMD package will have to be re-qualified according to JEDEC standards, however, and the Moisture Sensitivity (MSL) classification of some will drop, which means more care will have to be taken when they are stored and used in board manufacturing facilities.

• For lead-frame packages, matte tin is the preferred in-house post-plating process for four reasons.
  • Drop-in alternative to the conventional SnPb process
  • Reasonable cost and the highest productivity among the options
  • Easiest process control
  • Solution to whiskers problem includes making the tin layer thicker than 7 microns and baking for 1 hour at 150°C.
• For components where pre-plating is the most cost effective option such as when a lead frame is purchased from a supplier, AuPdAg is the most desirable Pb-free alloy. The lead frames are already available from suppliers and were actually marketed in 1998 and 1999 but discontinued due to cost issues.
• SnAgCu is the choice for BGAs, largely because of its bond strength
• SnAgCu, AgCu and gold bumps are the best choices for flipchip packaging and the choice depends on applications and bumping techniques.

During the next few months, Pb-free will face its biggest challenge: industry-wide implementation. Unanticipated problems almost always crop up in real-world experiences and only time will tell just how critical these unforeseen problems will be.