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| A publication of the National Electronics Manufacturing Center of Excellence | March 2006 |
When converting to lead-free solders questions arise regarding the selection and use of lead-free surface finishes. Whether the board assembler contributes to the surface finish selection process or the surface finish is pre-selected, most producers and manufacturers have been affected by the conversion to environmentally friendly materials and processes. The conversion to lead-free materials has resulted in changes in surface finishes for both the component manufacturers and the bare board suppliers. While the majority of the lead-free surface finishes themselves are not new, the problems of materials compatibility and supply chain management are re-introduced to manufacturers who have neglected these areas in the past. The reality is that large manufacturers with extensive R&D budgets have been using alternative surface finishes for decades. Lucent and Texas Instruments, for example, had performed reliability testing on gold and palladium finishes well before the WEEE and ROHs directives were introduced. Pure tin finishes have been in use by the low, mid, and upper level manufacturers without consequence for years. However, today, surface finishes are getting as much if not more attention in the lead-free conversion process as the actual solder alloy. Manufacturing and product engineers have recognized the effect that surface finishes have on the producability and performance of the hardware. The surface finish selection process can have significant impact on four main areas:
These areas influence the performance of both the component and the bare circuit board. The components and circuit boards share many of the same performance concerns, however, some problem areas have a more significant affect on components while other problem areas have considerable impact on the performance of bare boards. Components
For now, market demands will determine the alternative finish of choice. However, the change in surface finish materials from SnPb to alternative finishes has brought the component supply chain under increased scrutiny. Despite having clear guidelines for labeling components (IPC-1066 / JEDEC-97 Labeling Standard), suppliers have not demonstrated consistency in identifying component finishes. In many cases the component manufacturer or supplier cannot provide accurate information about the component surface finish. The unknown surface finish issue is of particular concern for discrete components like chip resistors and chip capacitors where part tracking can be more difficult. In one case, the technical support service line was contacted for information regarding the surface finish on a single lot of chip resistors. Not only did the technical service representative fail to provide accurate information regarding the surface finish, he could not provide a list of available surface finishes for that particular product. This example is unfortunatly not as atypical as one might think. The negative effects of surface finish uncertainty can be catastrophic. Materials interactions can significantly reduce reliability. For example, a low temperature alloys is created when bismuth interacts with SnPb, which can negatively affect product reliability. Additionally, some surface finishes are sensitive to atmospheric conditions and require controlled environments for both short and long term storage. These are some of the main reasons that part tracking and inventory control have become secondary challenges associated with lead-free conversions. However, whisker related risk is greater in high reliability electronics that are used in harsh environments. A recent EMPF study showed that most manufacturers are producing lead-free components with a matte tin finish despite industry concerns of tin whisker formation. Component manufacturers cite the low cost of converting existing equipment, compatibility with the current group of popular lead-free solders, and low risk of whisker related failure in consumer products as the main reasons for producing tin finished components. The problem of tin whiskers on component leads has been well documented. While there is limited information on the exact growth mechanism, there are a number of mitigation strategies available. These strategies range from simple processes such as annealing the tin finish, to more complex processes such as plating underneath the tin finish with a stress reducing under-plating. One common option for high-reliability applications that are exempt from the European legislation is the refinishing of pure tin-plated components with a SnPb finish (3 weight % Pb or greater). A well performed lead tinning process can provide a consistent, even coating of SnPb solder to all leads on a component. Solder tinning is a long standing process outlined in MIL-PRF-38535 that provides a finish in compliance with the IPC J-STD-001/J-STD-002. Leads are re-tinned in Sn63 solder in most cases. Components are de-taped, treated, and re-taped to ensure a minimal loss in productivity. A good tinning shop will utilize ESD precautions and have no trouble refinishing any leaded SMT component, regardless of size or configuration. Lead-free tinning services have not yet emerged in the mainstream market, limiting the mitigation options for those forced to comply with the EU directives. |
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Bare boards
Each finish has benefits and liabilities that would impact the selection process. Immersion tin finishes have proven to be one of the most logical and economical bare board finishes. The immersion tin finishes have also shown to provide excellent reliability. However, solderability concerns from oxidized tin and the fear of tin whisker formation at unsoldered areas prevent immersion tin finishes from gaining a higher degree of acceptance among electronic manufacturing engineers. Good co-planarity characteristics and favorable intermetallics are some of the benefits of immersion silver surface finishes. ImAg finishes are costly and require controlled storage conditions to prevent tarnish that can severely reduce solderability. Organic solder preservative finishes have gained popularity as a robust surface finish. Changes have been made to improve solderability, particularly after multiple reflows or long term storage. Hot Air Solder Level (HASL) finishes using lead-free solder alloys are being developed and are currently available from select suppliers. These surface finishes present the same challenges of SnPb HASL such as co-planarity. They also present new challenges unique to lead-free such as compatibility of the surface and solder compositions. Table 3-1 shows a comparison of the surface finish properties, including the quoted cost for preparing an EMPF-009 board.
ENIG finishes can be susceptible to high phosphorous levels in the nickel layers that create a weak connection between the solder joint and pad (Black pad). However, the excellent wetting characteristics and good reliability of ENIG finishes have made ENIG boards one of the most popular lead-free replacements. Additionally, ENIG finishes have an excellent track record with SnPb solder joints.
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| The American Competitiveness Institute - - www.aciusa.org - - (610)362-1200 |  |
When surveyed, the majority of organizations that attend the EMPF's lead-free soldering course select ENIG as the finish of choice when converting to lead-free finishes. Those new to alternative finishes have based their decisions on the wetting performance and reliability results published in papers and presented at conferences and courses. However, many of the attendees underestimate the impact that improperly processed ENIG boards can have. The problems associated with oxidized nickel and black pad can be undetectable until the board is in service, requiring a recall of product in the field. Figure 3-1 shows the solder joint from a recalled black pad board. The manufacturer must be aware of the latent failure risk of ENIG boards. The recommendation is to require quality data for each lot of incoming boards. Despite these concerns, the reliability risks do not seem to be an impedance to ENIG as one of the most popular Sn-Pb replacement finishes.