Lead has been linked to various medical disorders and physical development problems, especially in children. Historically, contamination has been a consequence of improper disposal of leaded waste, residues left over from internal combustion engines or the direct leaching of lead into potable water sources. The regulatory agencies within the United States have begun to act in accord with the Environmental Protection Agency's (EPA) 1973 proclamation to gradually reduce lead in gasoline. In addition, the Consumer Product Safety Commission banned the sale of paint with greater than 0.06% lead in 1978. Within the electronics industry, the EPA's Toxic Release Inventory implemented a reduction in reporting thresholds for lead amounts from 10,000 lbs to 100 lbs per year starting in 1999. Outside the US, the European Council and European Parliament have set July 1, 2006 as a target date for a ban on hazardous materials such as lead as per the Waste Electrical and Electronic Equipment (WEEE) Directive and the Restriction of Hazardous Substances (RoHS) Directive. The Japanese Ministry of Industry and Trade proclaimed that lead usage will be reduced 67% by 2005. As a result, the shift in PWA manufacturing overseas will impact domestic customers [1].

The effects of Lead regulatory changes on the Electronics Manufacturing Industry
Solder consisting of tin/lead (63/37 wt %, also known as SN63) is considered a eutectic. In metallurgical terms eutectic refers to the lowest melting mixture of the two metals (183°C). Historically, this ratio of tin to lead solder has dictated the types of materials which could be used in PWB base material and component packages.

* This alloy has been identified as a primary combination with two specific variations coming from Japan (96.5% tin/3.0% silver/0.5% copper) and North American Electronics Manufacturing Initiative (NEMI) (95.5% tin/3.9% silver/0.6 % copper). [3]

In combinations other than that of eutectic, solder does not melt or become liquidis at a specific temperature but has a melting range. As a result, the use of non-eutectic combinations would influence the soldering process since partial melting would occur. The alloys shown in Table 2-1 are being evaluated by many manufacturers for future production applications and in some selected cases, are being used in the production process.

However, it is critical to the electronics manufacturing industry that there be a consensus for the metallurgical makeup of solder as well as a recognized Design for Manufacturing (DFM). The DFM must include solderability testing, as acceptable wetting will be affected by the solder alloy. The current specifications for solderability testing within the US are IPC J-STD-002B "Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires" and IPC J-STD-003A "Solderability Tests for Printed Boards".

There are a number of tests for evaluation of solderability. Dip & Look and Wetting Balance analysis are the most common and the focus of this article. According to Wassink [7] there are two groups of tests: Those that evaluate the parameters which create good wetting (i.e. rate of wetting and wetting angle) and those that evaluate the results of these parameters. Acceptable visual solderability is indicated by a "continuous solder coating free from defects for a minimum of 95% of the critical area. Anomalies other than de-wetting, non-wetting and pin holes are not cause for rejection." [7] Another trait of acceptable solderability is a shiny appearance, as newly flowed eutectic tin/lead solder gleams in light. In contrast, lead free solder is often dull in appearance due to the fact that tin is often the major component and the oxides of tin are more stable than those of lead (i.e. lower heat of formation- Hf [4]) (SnO -69 cal/mol, SnO2 -143cal/mol, PbO -53cal/mol, PbO2-67cal/mol).

**Note: This graph is from an actual wetting balance analysis and is portrayed opposite of the direction of the actual forces involved. (Buoyancy is an upward force while solder wetting is a downward force).

Quantitative solderability testing (Wetting Balance testing) measures the wetting force (Figure 3-1) which is dependent upon the density and surface tension of the solder. This test does not have established accept/reject criteria but does provide suggested evaluation criteria (Table 2-2).

The time to reach the maximum wetting force along with the absolute value of the force are of great importance as both will be impacted by the ability of the flux to remove oxide layers common in lead free solder.

Considerations for Lead Free Testing
In addition to evaluation criteria, test requirements could be affected by lead regulation. Currently the two standards mentioned earlier utilize an RMA type flux. Since surface oxides are more likely with the lead free systems, higher activity fluxes, which leave residues that require more aggressive cleaning, may be required. Additionally, performing the testing in a blanket of nitrogen may be necessary since dross removal is more frequently needed with lead free solders.

The current pot temperature for eutectic tin/lead solder is 245 ± 5°C (62 degrees above the melting point). Referencing the melting temperatures of the various solders listed in Table 2-1, the necessary pot temperatures would average 272 °C. As a result, contamination from some of the substrate materials shown below in Table 2-3 could occur as these materials may be susceptible to thermal degradation despite the short contact times.

The requirement to steam age parts has been designated for
those components and connectors that experience extended time periods between testing and soldering (>6 months) and have either limited thermal exposure or multiple thermal exposures before soldering. This current requirement assumes that "properly applied tin and tin/lead coatings can withstand the steam conditioning environment well beyond the eight hours specified and may survive natural aging well beyond 12 months". [5], [6] Steam aging samples prior to testing may become more critical as the shelf-life of components and boards is reduced.

During the transition from lead-based to lead free solders, vendors will also find themselves maintaining multiple solder baths to avoid cross-contamination.

What has been done so far to answer these questions?
In 2000, the Lead Free Component Focus Group (an industry consortium) compared the component solderability of tin, tin/copper, nickel/palladium, and nickel/palladium/gold finishes and the PWB solderability of Hot Air Solder Leveling ( HASAL ), Organic Solder Preserve (OSP) and Electro less Nickel Immersion Gold (ENIG) finishes. Standard RMA flux and three solder systems (Sn63 / Pb37, Sn95.5 / Ag4.0 / Cu0.5, Sn99.3 / Cu0.7) were used in the study. The results are as follows:

• The four component finishes are suitable for lead free systems.
• The "Dip and Look" test lacks the robustness to accurately predict board level solderability.
• Wetting times and wetting forces were degraded for various leaded packages using Sn/Ag/Cu solder vs. Sn/Pb at 235 °C.
• ENIG and OSP are acceptable as lead-free finishes.
• Temperature has a direct influence over solderability as wetting times improved with all the alloys in comparison to SN63 (Figures 3-2, 3-3 & 3-4).

The affect of temperature on board solderability was also confirmed by NSF Center for Advanced Vehicle Electronics, Auburn Univ. [9]. The push for VOC free fluxes and their influence in lead-free systems was investigated [10]. This study showed that the lead-free systems did not perform as well as the incumbent eutectic tin/lead control. It was observed that better wetting was observed in systems which contained less Cu, confirming the consensus that the addition of copper degrades wet-ability.

As mentioned earlier, the consensus of the NEMI and JAPAN is a solder of composition Sn/Ag/Cu. Their recent study examined how subtle variations of silver content (3 to 4%) may influence solderability testing. The findings of the IPC study showed no significant difference in wetting balance parameters for the variations.

What does this mean for the future of electronic assemblies?
The technical staff at the EMPF has researched the manufacturing challenges associated with lead-free soldering for several years and has developed a considerable level of expertise in lead-free solder alternatives, with an emphasis on military applications. A technical core competency focus at the EMPF will continue to be working with and finding solutions for the difficulties associated with the inevitable transition to lead free solders. The need to establish a consensus on the composition of a lead free solder is critical, as this will determine solderability test requirements (i.e. flux & temperature). Criteria for accepting/ rejecting components must also be determined.

There is a likelihood that Class I applications will switch readily over to lead free solders in the near-term, as the reliability requirements are not as stringent as that of Class II and Class III applications. The EMPF will remain actively involved in representing the specific needs of military applications at the board level (Class III) during the development of solderability standards. Overseas, the commercial industry appears to be ahead of its domestic counter parts as approved lead free solders have already appeared on the market.

References
1) SMTA Class, 3/8/2003 "Lead Free Soldering in a Production Environment".
2) Surface Mount Technology, Principles and Practice, 2nd edition, Ray P. Prasad, 1997.
3) "IPC-SPVC-WP-006 Round Robin Testing and Analysis Lead-Free Alloys Tin, Silver and Copper", July 2003.
4) CRC Handbook of Chemistry and Physics, 65th Edition 1984-1985
5) IPC J-STD-002B "Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires".
6) IPC J-STD-003A "Solderability Tests for Printed Boards".
7) Soldering in Electronics, 2nd edition, R.J. Klein Wasssink, 1989.
8) "Test Results from the Lead-Free Component Focus Group (ACI, Intersil™, AmKor Technology; Inc., Photocircuits, Rockwell Collins, Shipley Ronal and Technic; Inc.).
9) "Wetting Performance vs. Board Finish and Flux for Several Pb-Free Solder Alloys", S. V. Sattiraju; R.W. Johnson; D.Z. Genc and M. J. Bozack, NSF Center for Advanced Vehicle Electronics, Auburn Univ., 26th IEMT Symposium.
10) "Solderability Assessment of Pb-free Alloys Using VOC-free Flux", Global SMT and Packaging Journal; Krystyna Bukat, Janusz Sitek, Leszek Hozer and Ronald Bulwith, December 2002.

For more information concering Lead Free processes and surrounding issues, please stop by ACI's new Lead Free Manufacturing Page to download articles contributed to ACI by some of the industry's most knowledgable individuals and organizations, as well as material generated by ACI, and documents on the legislation surrounding the Lead Free issue.