Lead free soldering will influence commercial-off-the-shelf (COTS) implementation and long-term program sustainment. To appreciate the dilemma aerospace manufacturers are under, when considering lead free soldering for program sustainment, one has to understand the environment in which aerospace manufacturers operate. In the aerospace environment, military systems can be in the field for over 30 years. These systems are constantly being upgraded and repaired during their product life cycle. All aerospace hardware is manufactured with SnPb solders. According to the IPC, aerospace electronics represents less than 1% of the total electronics manufacturing market.

The Federal Acquisition Streamlining Act (FASA) of 1994 requires that, to the maximum extent practicable, contract requirements and market research should facilitate use of commercial items. Aerospace electronic manufacturers are qualifying COTS hardware to meet FASA goals. COTS components are currently being manufactured with SnPb.

Component manufacturers, such as Motorola, Tyco, Texas Instruments and Amkor, are converting their production lines to lead free finishes to meet commercial market demands. It is unlikely that component manufacturers will support SnPb component production to meet aerospace rework and repair operations due to aerospace electronics' small market share. It is conceivable that solder manufacturers will stop supporting SnPb solders since the bulk of their market, the commercial market, will be transitioning to lead free solders.

In a commercial electronics environment, commercial manufacturers do not perform rework and repair operations to hardware in the field. Due to the goals stated by the European Union's Waste Electronic and Electrical Equipment (WEEE) Directive and the Japanese Ministry of Industry and Trade Institute (MITI), commercial manufacturers will aggressively convert their production lines to lead free solders. Epson, Hewlett-Packard, Northern Telecom and Panasonic are a few of the commercial manufacturers that are beginning to convert their production line to lead free solders.

Legislation Status
The European Union's WEEE Directive and the Restriction of Hazardous Substances (RoHS) legislation stipulate that electronic equipment sold to European consumers be lead free as of July 1, 2006. In Asia, MITI called for lead usage to be reduced by 67% by 2005. In the United States, the Environmental Protection Agency's (EPA) Toxic Release Inventory Status requires electronic manufacturers to report lead usage above 100 pounds annually. Therefore, one can conclude that electronics will be lead free in the near future.

What Do We Know?
For first piece production, it is feasible to manufacture hardware with lead free solders. ACI has successfully implemented several lead free production runs which require hand soldering, wave soldering, and SMT reflow soldering. In general, lead free solders require higher soldering temperatures and thermal profiles, depending upon the alloy (Table 1). Various consortiums have proven that it is feasible to meet high reliability requirements. The Lead Free Components Focus Group and the National Electronics Manufacturing Initiative (NEMI) proved that one could meet IPC Class 2 and Class 3 inspection requirements.

There are specific challenges to introducing lead free solders in a production environment. Lead free solders do not wet as well as tin lead (SnPb). Aggressive solder fluxes and nitrogen are recommended to improve solderability. Components and board materials are more sensitive to moisture due to lead free solder's higher processing temperatures. From an inspection perspective, because lead free solder joints have a grainy dull appearance, the IPC is revising their visual inspection requirements to compensate for lead free solder joint differences.

Program Sustainment: What Do We Need To Know?
The process variables for performing rework and repair operations with lead free solders need identification and quantification. It is acknowledged that most lead free alloys have higher soldering temperatures than SnPb. The impact of higher temperatures on the board, components, and the assemblies requires investigation. There are concerns about the quality and reliability of lead free solders that undergo rework and repair processes.

Lead (Pb) contamination presents new process variables. Pb contamination restricts the use of specific families of lead free solders. There is evidence that Pb and Bismuth (Bi) can form an intermetallic with a melting point of 96oC. Therefore, lead free alloys that contain Bi may not be considered for specific applications where intermixing SnPb and Bi lead free solders is possible.

AIM Solders documented the possibility that Pb contamination will adversely affect solder joint reliability, based on their metallurgical studies. It is AIM Solders' contention that lead contamination will migrate to the last area of the joint to cool. For a leaded device, this is at the base of the solder joint (see Figure 2). AIM Solders performed tests with Pb-contaminated Tin Silver Copper (SnAgCu) solder paste, based on test method ASTM E606. Pb-contaminated lead free solder pastes failed this test, using 10,000 cycles as a pass / fail criteria (see Table 2).

This contradicts data from the Lead Free Solder Component Focus Group. The Lead Free Solder Components Focus Group manufactured hardware mixing SnPb with lead free components, board finishes, and solder paste. The hardware underwent thermal cycling tests, for up to 2,000 thermal cycles, from -55oC to 125oC. It was determined that the lead free soldered hardware, the SnPb soldered hardware, and mixed hardware had equivalent reliability performance (see Figure 3).

ACI’S 2003 Lead Free Action Plans
ACI plans to continue research and development activities in lead free solders. ACI is developing a core competency program to document the process variables for performing rework and repair operations with lead free solders. ACI will perform rework and repair operations on hardware mixing lead free and SnPb solders, component finishes, and board finishes. Upon completion, hardware will undergo cross-section analysis to determine the metallurgy of rework solder joints. ACI will perform thermal cycling testing, based on MIL-STD 810F, from -55oC to 125oC to discover the lead free reworked solder joints’ reliability.

Another project under consideration is to determine if Pb contamination presents a reliability concern. As with the AIM Solders’ experiment, ACI plans to build lead free hardware with advanced electronic packages, using "doped" lead free solder contaminated with a known quantity of Pb. Cross-section analysis will verify the hardware's metallurgy. Thermal cycle testing will determine the solder joint reliability.

ACI continues to support the JG-PP Lead Free Solder Team. The JG-PP Lead Free Solder Team will be building lead free solder, using Surface Mount Technology (SMT) and Through Hole Technology. The program will be testing first piece production hardware and rework hardware to simulate sustainment activities. A variety of Environmental Stress Screening (ESS) tests will be performed to assure that lead free solders meet aerospace reliability requirements.

ACI is currently developing a Lead Free Manufacturing course. The course's objective is to introduce the technical variables associated with implementing lead free solders in a production environment. The course will be a hands-on course, utilizing ACI's Demonstration Factory. Participants will be encouraged to bring samples of their hardware to build with lead free solders.

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.