In the previous discussion on lead-free solder alloys and PCB pad finishes, we focused on the solder side and the board-side metallurgy. This article turns to the other side of the joint: the component terminal finish materials, along with packaging concerns in lead-free assembly. Given the huge variety of components used on a PCBA, this discussion focuses on common passive components and semiconductor packages that are directly relevant to SMT assembly.
Key Challenges When Moving Components to Lead-Free
As with PCBs, components face two main classes of issues in the lead-free era. The first is the impact of higher temperatures on package materials. The second is compatibility between lead-free solders and component terminal finishes. While many terminal finishes such as AgPd, Ni/Au, and Sn were already lead-free in the SnPb era, the metallurgy and process windows on the solder side have changed substantially. Materials that worked well with SnPb may not behave identically with lead-free alloys, making compatibility a critical topic. This article discusses materials characteristics, market practices, plating thickness selection, and related considerations.
Component selection in a lead-free transition is not trivial. Components on a single PCBA come from many suppliers, and most will not accommodate arbitrary customer specifications for terminal plating materials, processes, and thicknesses. Achieving the best combination across suppliers to minimize manufacturing risk and assure quality requires significant effort. In other words, DFM work becomes notably more complex in lead-free manufacturing.
On the same PCBA, differences in component structure, thermal mass, and placement often lead to different optimal soldering conditions per component. In the SnPb era, many users never fully optimized these details, yet the wider process and quality window often masked issues. With lead-free's narrowed windows, the probability of problems increases. To avoid substantial quality losses, at minimum, critical parts and those with short life or high reliability requirements should be standardized and controlled in this regard.
Published research on component terminal finishes is scarcer than on solder alloys and PCB finishes. While a few alloys have been studied more deeply, available references still fall short for many users. Many studies originate from component suppliers, often with limited test matrices and sometimes differing conclusions. For example, Japanese reports frequently rate SnBi favorably, while many Western studies raise concerns. Another limitation is the heavy focus on BGA ball joints—widely considered the most demanding joint structure—so much of the solder, PCB finish, and component finish data centers on this package family, often with SAC balls. Yet joint reliability depends not only on alloy but also on terminal structure, package design, and process, making these results insufficient to fully cover the broad range of package types on a PCBA.
A further complication is the mixed-material reality during transition periods. Few supply chains convert from SnPb to lead-free in lockstep. Many users must assemble mixed SnPb and lead-free content in the same process window, which is often harder than pure lead-free. Users have limited practical experience; the process window may be smallest; and the coexistence of SnPb and lead-free creates significant complexity in documentation, process instructions, part numbering, inventory management, rework handling, and traceability.
Component Considerations at Elevated Reflow Temperatures
Lead-free processes commonly require higher temperatures and longer reflow times. Popular replacements for Sn37Pb have higher melting points. For SAC-class solders, the solidus is around 217 °C (varying with composition). NEMI recommended a peak reflow temperature not below 235 °C; many practitioners find 240–250 °C most suitable. This is typically 20–30 °C higher than traditional SnPb. Whether legacy component materials can withstand these higher thermal loads must be verified. Four areas need attention:
- Can the package materials tolerate higher peak and soak temperatures?
- Does increased thermal energy worsen moisture sensitivity and popcorning risk?
- Do terminal finish materials and thicknesses resist oxidation sufficiently to preserve solderability at higher temperatures?
- Does increased warpage at higher temperatures degrade coplanarity, causing opens or weak joints?
For thermal robustness, the IPC/JEDEC standard J-STD-020C provides guidance for package classification and reflow profiles that lead-free components should meet.
Different packages have different thermal masses. While some users prefer to unify on a 260 °C peak tolerance, this imposes higher requirements on suppliers. Where possible, align procurement with clear thermal criteria and, when necessary, perform internal qualification testing.
Moisture sensitivity is the second major consideration, particularly for plastic packages such as ICs. With higher reflow temperatures, moisture-induced damage becomes more severe. Practical experience indicates greater likelihood of package cracking and popcorning under lead-free profiles. Because vapor pressure increases disproportionately with temperature, a 10–20% temperature rise can yield a 63–86% increase in vapor pressure, greatly increasing destructive potential. Many practitioners consider that components may require one to two higher moisture sensitivity levels (MSL) under lead-free conditions, referring to IPC/JEDEC classification methods.
Moisture-related issues include popcorning, package cracks, and internal delamination. Depending on the location and severity, these defects are not always detectable by visual inspection. AOI struggles to detect them; manual AXI can identify some but is too slow and costly for production screening. Avoidance starts with supplier qualifications and pre-purchase component certification against relevant standards.
Although suppliers often recommend pre-bake procedures before assembly, better purchasing control and inventory conditions are preferred to avoid baking. Baking increases cost and handling and can exacerbate flux volatility and voiding in lead-free soldering.
The third consideration is the ability of terminal finishes to maintain solderability after exposure to elevated thermal energy. There is no universal standard for this; only supplier and user data exist. Many variables—material chemistry, plating process and parameters, thickness, storage conditions and duration, number of heat excursions, and specific reflow profiles—affect outcomes. Users should have internal capability to qualify finishes and collaborate closely with suppliers. Critically, when evaluating supplier reports, scrutinize the specific material combinations. Lead-free materials are diverse, and different combinations can behave very differently.
Coplanarity is a longstanding challenge and becomes more acute with increased warpage in lead-free profiles, especially for connectors and BGAs. Selecting appropriate terminal finishes and package materials, robust DFM rules (pad geometry and stencil design), and suitable heating profiles are all essential. In lead-free, the use of fixtures such as pallets, hold-downs, and support bars during reflow becomes more frequent than in SnPb assembly.
Lead-Free Terminal Finish Options for Component Leads
Within components, lead may exist not only on external leads but also inside (e.g., die attach). Here we focus on second-level interconnects—exposed leads and terminals relevant to SMT. Broadly, suppliers have pursued two paths for terminal finishes: Sn-based (e.g., pure Sn, SnCu, SnBi) and non-Sn finishes (e.g., Ni/Au, PdNi, Pd).
Finishes that have seen practical use include: 100% Sn, SnCu, Ni/Au, AgPd, SnAg, SnBi, Ni/Pd, Pd, Ag, SnZn, Ni/Pd/Au, Au, AgPt, SnIn, and SAC. Market preferences differ by region: many Western suppliers favor 100% Sn (often matte tin), while Japanese suppliers commonly use SnBi. Approximately 80% of Western suppliers offer pure Sn as the primary finish, whereas about 86% of Japanese suppliers primarily offer SnBi. Other popular finishes include, in rough order of prevalence: SnCu, Ni/Pd, Ni/Pd/Au, Ni/Au, SnAg, and SAC. A large Western user survey conducted in 2002 reflected these trends.
BGA, CSP, and other ball-attach packages are a special case. Globally, >70% of shipments use SAC spheres (with various compositions). Because these packages are generally more challenging to solder, aligning the joint metallurgy closely with the paste alloy helps reduce process variables. Be aware, however, that not all users will adopt SAC paste, and matching alloy systems may introduce other issues such as voiding.
Characteristics of Common Terminal Finishes
Compared with solder alloys and PCB finishes, published data on component terminal finishes are limited and concentrated on pure Sn, SnBi, and PdNi/Au-family finishes. Key characteristics are summarized below.
100% Sn (Matte Tin Recommended)
Advantages include generally good wettability to many lead-free alloys; similar processability to SnPb; joint reliability comparable to or better than SnPb in many reports (especially with SAC paste); no lead contamination risks; established electroplating processes with stable quality; abundant tin supply; and relatively low cost, often near SnPb levels.
Weaknesses are unresolved risks: tin whiskers, tin pest, and rapid IMC growth. While many reports show whisker mitigation by using matte tin, whiskers have also been observed on matte deposits in some studies. This remains an area of active research. Some suppliers promote semi-bright tin between bright and matte with reportedly better whisker resistance. Until the root mechanisms are fully understood and controlled, residual risk remains.
SnBi
Widely adopted in Japan, SnBi offers good soldering processability, extensive manufacturing experience, long joint life (comparable to or slightly better than SnPb), minimal whisker risk, and broad supplier support. Cost is modestly higher than SnPb (approximately 20–40%).
Challenges include higher susceptibility to lead contamination (quality variability), tight electroplating process control requirements (higher cost), difficult material reclamation (higher WEEE compliance costs), and essentially non-reworkable plating once defective. In wave soldering, Bi contamination of the pot can lead to pad or joint lift-off failures after reaching certain contamination levels. Due to low-melting IMC formation, high-temperature applications (e.g., automotive) generally do not accept SnBi finishes.
Many studies report excellent fatigue life and tensile strength for SnBi joints, and wettability better than pure Sn. However, SnBi's wettability with SnPb solder may not meet the fast wetting demands of wave soldering, while reflow soldering generally shows no issue and can outperform pure Sn finishes.
Other studies have observed faster IMC growth with SnBi plating versus pure Sn or SnCu. Reported activation energy (Q) driving IMC growth is about 2x that of pure Sn and 25% higher than SnCu. Thicker IMC layers degrade joint life, so this discrepancy remains an open question, possibly related to Bi altering Cu diffusion into Sn.
Comparative experiments of Sn, SnCu, and SnBi finishes also reported more voiding with SnBi joints, which is detrimental to reliability. Whether this factor was present in studies reporting superior SnBi reliability is unclear and warrants further analysis.
SnCu (typically Sn0.7Cu)
SnCu is attractive for its low cost (about 20% higher than SnPb) and compatibility with common wave-solder alloys. Because tin content is high, the concerns associated with pure Sn—whiskers, tin pest, and IMC growth—remain relevant. Cu composition strongly influences melting behavior, so plating and process control must maintain composition tightly to avoid variability. For copper-base leads, accurately measuring plating thickness can be difficult, impacting process control. Among the finish processes, SnCu plating is relatively harder to control, and discoloration during storage is a concern.
Reliability of SnCu finishes is generally acceptable—somewhat lower than pure Sn and SnBi, but comparable to SnPb—and aging tends to be less severe than other alloys.
Ni/Pd and Ni/Pd/Au
These finishes have long manufacturing and field experience (since around 1989), very long shelf life (reports up to eight years), good wettability (Ni/Pd/Au), high tensile strength (superior to SnPb), and no whisker reports. Constraints include high cost due to limited natural Pd supply, incompatibility with alloy 42 leads (corrosion), and risk of cracks at formed bends (mitigated by improved processes in some cases).
The Ni layer effectively blocks Cu diffusion into Sn, reducing voiding and improving joint life. Because Pd wets more slowly and dissolves more slowly than Au (more than 5x slower), many suppliers recommend Ni/Pd/Au rather than Ni/Pd alone. However, the added Au layer and its plating process increase cost. Since the late 1990s, Pd cost has risen sharply and fluctuates significantly, with occasional supply shortages.
Pd demonstrates good wettability, with faster wetting than SnPb but a larger wetting angle (about 20–30 degrees higher). This still meets common industry criteria (< 75 degrees). Plating thickness is critical: the process should ensure that the final joint forms onto the Ni surface, with Pd and Au fully dissolved into the solder. Users with stringent requirements should define thickness specifications and qualify the finish-process combination. When using Ni/Pd/Au terminations together with Au-bearing PCB finishes (e.g., ENIG), evaluate total Au content to avoid embrittlement.
Ni/Au
Ni/Au also has long application history, good shelf life, and good wettability. Reliability varies with plating thickness: too thin shortens shelf life; too thick can reduce joint life. Balancing thickness is essential, especially when the PCB pads also use Au-bearing finishes such as ENIG. In small-lot production with long storage cycles (e.g., beyond one year), users may choose Ni/Au for shelf life, yet these same applications often demand high field reliability. Sufficient thickness for shelf life may jeopardize service life. This challenge existed in the SnPb era and persists under lead-free.
On average, Ni/Au finish reliability is slightly lower than some alternatives due to Au-related brittle IMC. With careful Au control, however, these finishes can be highly reliable. The Ni layer slows Cu diffusion from the leadframe into the solder, and with SAC pastes, the resulting CuNiSn IMC can strengthen the joint, in some cases exceeding SnPb performance. Cost remains a barrier.
SnAg (typically Sn3.5Ag)
SnAg has long practical use and broad availability, with good reliability. Because Ag content strongly affects melting behavior, plating must control alloy composition strictly. The electroplating process is complex and waste treatment is more challenging environmentally. Ag is also highly toxic—second only to lead.
Wettability is poorer than SnPb and comparatively low among common lead-free finishes, so cosmetic acceptance criteria should be adjusted. Many reports indicate tensile strength and life comparable to or slightly better than SnPb. SnAg has not seen widespread use yet, with more studies conducted in Japan; broader use is more likely in DIP packages.
SAC (SnAgCu) for BGA and Other Ball-Grid Terminations
SAC is the predominant material for BGA solder balls and widely viewed as the mainstream reflow solder alloy moving forward. Using the same or closely related alloy for the ball and the paste reduces metallurgical variability in these high-difficulty joints.
One disadvantage is that the ball and paste have the same melting range, raising the bar for paste formulation and reflow profile control; otherwise voiding can increase. BGA reflow profiling is already challenging due to poor airflow under the package and large thermal mass differences, so users must master DFM and process integration.
During transition, a frequent problem arises when BGA suppliers switch to SAC balls while the assembler still uses SnPb paste. At typical SnPb peak temperatures (around 210–225 °C), SAC balls may not adequately melt and intermix, producing inhomogeneous microstructures and localized Pb-rich phases. Combined with the BGA's relatively "cold" bottom-side temperatures, achieving reliable BGA joints may require higher peaks that risk overheating other parts on the board. Where users have not fully qualified other components for lead-free temperatures, this can become unmanageable. In such cases, some resort to manual BGA rework, increasing cost, reducing throughput, and raising quality risk.
On the positive side, SAC joints exhibit high shear strength and much lower fatigue aging than SnPb under many conditions.
Plating Process Technologies
The most common processes for component lead and terminal finishes are:
- Hot-dip (immersion) tinning
- Electroplating
- Electroplating followed by reflow (melt) to heal defects
Hot-dip coatings are dense and relatively free of pinholes but have poor thickness uniformity and control, and cannot be used on fine-pitch leads. Electroplating offers good thickness control but is prone to pinholes. To address pinholes, the plate-and-reflow process was developed, at higher cost. Process choice affects quality, cost, and material compatibility and should be considered together with the finish selection.
For pure tin finishes, bright tin electroplating is generally not acceptable due to high residual internal stress and whisker risk. Post-plating reflow can help relieve internal stress and reduce whisker propensity. Electroplated tin finishes are also prone to micro-cracks at lead bends, which hurts solderability after storage and can trigger creep corrosion. That said, hot-dip processes cannot handle all components (e.g., fine-pitch leads). Users should therefore qualify specific combinations of materials and processes and document them in design and process standards.
As with PCB pad finishes, the plating process and control have a large impact on soldering quality. For example, variation in current density during Sn plating changes surface roughness, which influences whisker formation risk. Evaluate supplier process capability and quality controls carefully.
Defining Plating Thickness
Plating thickness intimately affects soldering behavior and reliability and must be controlled. Determining optimum thickness is nontrivial. Influencing factors include the finish material, plating process, solder alloy, product design complexity, quality and life requirements, component mix on the board, oven and process capabilities, and storage conditions. To make sound thickness decisions, users should:
- Understand the properties and failure modes of the selected finish and process, including shelf life; qualify as needed.
- Understand the manufacturing (plating) process characteristics and controllability.
- Understand soldering process requirements with the chosen paste chemistry and alloy (e.g., reflow window); where quality requirements are high, perform internal qualification rather than relying on generic data.
- Evaluate supplier process capability and willingness to provide different thickness options; audit and measure as needed.
- Define storage conditions and limits based on internal operations and manufacturing strategy.
- Compile finish thickness specifications per component type and material, based on the above.
- Define execution procedures and conduct reliability qualification on early builds, then adjust specifications as necessary before broader deployment. For high-reliability products, maintain ongoing oversight until confidence is high.
Finish Selection and Qualification
Given the complexity and variability of lead-free finishes, users should conduct targeted qualification trials when selecting materials and suppliers. All testing should reflect the actual solder alloy and specific paste product to be used. Qualification scope can include:
- Wettability (wetting time and wetting force)
- Joint strength (tensile and shear)
- Solderability and process window characterization (including IMC behavior and profile parameters)
- Plating uniformity
- Storage conditions and shelf life
- Thickness control capability
- Plating defects (pinholes, cracks)
- Appearance criteria
- Whisker risk
Technical and quality considerations may conflict with electrical performance or preferred suppliers, necessitating trade-offs and mitigations. Mitigations include better ovens, refined profiles, and DFM/process integration. When not every joint can be optimized, teams must decide which joints are mission-critical and ensure those have the necessary margin. This requires strong collaboration among design, core process engineering, and manufacturing engineering.
Supplier Engagement
During the lead-free transition, users may need to change suppliers due to differences in materials and capabilities. Even with incumbent suppliers, technology changes and requalification demand close communication. Before selecting or retaining suppliers, initiate detailed technical discussions.
Larger component manufacturers typically have documented lead-free strategies, timelines, and status. This information supports early screening of suppliers based on technical direction and schedule alignment. If internal preferences for terminal finishes are not yet fixed, supplier discussions can help users understand the rationale behind different finish choices.
A staged approach works well:
- Stage 1: Understand each supplier's lead-free direction and current status.
- Stage 2: Provide your technical requirements and standards, and discuss how closely the supplier can support them. Ask for data that matches your alloy and process combinations.
- Stage 3: For gaps, discuss additional support options such as joint qualifications, co-development, or site visits to reference users, and agree on a timeline that aligns with your internal plans.
There are many finishes and plating processes, and the workload to do this well is significant. Investing appropriate resources is necessary to manage risk and quality effectively.
Conclusion
From the component perspective, joint formation and reliability depend on terminal finish material, thickness, plating process, storage conditions and duration, solder alloy and flux chemistry, and reflow parameters (temperature and time). In certain combinations—such as fine-pitch Ni/Au terminations together with Au-bearing PCB pad finishes—pad finish thickness and composition must also be considered. The diversity of terminal finishes and supplier capabilities makes standardization, procurement, and quality control complex.
Many lead-free finishes have been used since the SnPb era, but changing the solder alloy alters joint metallurgy and process windows in ways that are not always predictable from first principles. Practical trials and usage experience are essential. Compared with solder alloys and PCB pad finishes, the industry's publicly available research on component finishes is limited and sometimes inconsistent across studies, complicating user decisions. Meanwhile, the narrowed process windows in lead-free increase the risk of poor choices and quality losses.
Robust supplier communication, process and quality qualification, and integrated engineering management are therefore critical. Organizations should determine the level of rigor appropriate to their product's reliability and life requirements and align teams accordingly.
Notes
1) SAC alloys are broadly expected to dominate lead-free reflow soldering, and both technical and commercial trends are moving in that direction.
2) Matte tin has 20–100x less organic/carbon content than bright tin, with smaller grains (~0.5–0.8 μm vs. 1–5 μm). Lower carbon and finer grains correlate with reduced whisker growth. Matte deposits are also typically thicker (3–30x), further mitigating whiskers.
3) In IMC growth models, the activation energy Q determines growth rate; higher Q yields thicker IMC for a given time and temperature.
4) Wetting angle depends not only on materials but also on terminal geometry, paste volume, flux chemistry, and process parameters.
5) BGA packages enable high miniaturization but are challenging for convection reflow due to poor airflow under the body and uneven joint temperatures, especially versus small low-mass joints. Lead-free's tighter process windows increase this challenge.
6) At lower peaks (approximately below 230 °C), SAC balls on BGAs may not fully intermix with SnPb paste, leading to inhomogeneous microstructures and localized Pb-rich regions that threaten joint life.
7) Qualification should target the specific paste product, not just alloy composition, because flux chemistry strongly affects wetting and voiding.
8) In lead-free assembly, carefully select high-performance reflow ovens. Price and performance vary by vendor; conduct thorough equipment evaluations.
