Managing lead-free compatibility: compatibility issues are critical to consider for a smooth transition to lead-free soldering.

The industry is now converging on a tin-silver-copper (Sn-Ag-Cu, or SAC) alloy for reflow and a Sn-Ag-Cu or Sn-Cu alloy for wave soldering. The lower cost of Sn-Cu as compared with Sn-Ag-Cu makes it an attractive alternative alloy for wave soldering, especially for cost-sensitive products. Further, because some products in volume production use SAC for reflow and Sn-Cu for wave soldering on the same board, methods for inspection, rework and accelerated testing must be compatible for both alloys (Figure 1).

[FIGURE 1 OMITTED]

Flux must be reformulated for a lead-free alloy to accommodate the characteristics of the lead-free alloy. The chemical reaction between the flux and the solder alloy affect the rheological characteristics of the solder paste. The density difference between the lead-free and the Sn-Pb alloys means that the metal loading of the solder paste needs to be different. The higher soldering temperature needed for the lead-free solder will require greater stability of the flux at higher temperatures. The performance of the flux residues after reflow, in terms of in-circuit test (ICT) probe ability and electromigration, is also an important consideration.

In terms of solderability for lead-free soldering, immersion tin (I-Sn) and electroless nickel/ immersion gold (ENIG) surface finishes provide the best wetting results on fresh boards, followed by immersion silver (I-Ag) and organic solderability preservative (OSP). However, after storage and heat exposures, the wetting of the I-Sn finish degrades the fastest, with less wetting degradation for the I-Ag and OSP finishes. The wetting of the ENIG finish remains excellent through various preconditioning treatments and heat exposures.

Fresh I-Ag boards can withstand up to four lead-free reflow cycles before the final reflow soldering process and at least two reflow cycles before the wave soldering process, whereas I-Sn finished boards cannot withstand multiple lead-free reflow cycles or a reflow cycle prior to wave soldering without significant degradation in wetting. For I-Ag, the exact chemistry, thickness, surface topography and the distribution of organic constituents within the silver layer must be carefully selected and specified. The Sn-Ag-Cu alloy is less sensitive to gold embrittlement than the Sn-Pb alloy.

As the soldering temperature increases, the z-axis coefficient of thermal expansion (CTE) mismatch among the laminate material, glass fiber and copper will exert greater stresses on the copper, potentially causing failures by cracking the copper of the plated vias--depending on the layer count and thickness of the printed circuit board (PCB), the laminate material, the soldering profile, the copper distribution and the via geometry. Other issues, such as delamination and blistering at higher soldering temperatures, are also areas to be studied. Much work is needed to determine under what conditions alternative laminate materials (such as high Tg, low CTE) may be needed for lead-free soldering. Lower cost materials may be able to be used with lead-free soldering; in fact, a variety of PCB laminate materials (such as CEM3, FR-2, FR-4, halogen-free and flexible circuits) have been used with lead-free solder in volume manufacturing, depending on the application.

Process Compatibility

Reflow, wave soldering and rework processes have been developed for lead-free solder. Detailed discussions on lead-free rework can be found in prior publications. For surface-mount and reflow, printability, tack, slump and solder balling depend on the solder paste formulation, not directly on the solder alloy. Very clear and consistent differences have been observed, however, in wettability between Sn-Pb and lead-free solder pastes.

In general, the wettability of lead-free solder paste is not as good as Sn-Pb solder paste. Lead-free solder paste exhibits very limited spreading on OSP during reflow, and exposed corners after reflow are quite common unless overprint or round corner pads are used.

For the SAC alloy, the minimum reflow peak temperature should be 235[degrees]C for volume manufacturing, considering process robustness, yield, variety of component finishes, oven thermal stability and tolerance. The reflow profile (in air or [N.sub.2]) may be straight ramp or with a preheat plateau for homogenizing the board temperature distribution.

For wave soldering with lead-free solder, a higher solder pot temperature, typically 255-270[degrees]C, will be required. Flux application and amount and preheat temperature and time must be optimized for each particular flux. A longer preheat may be needed to keep the thermal shock below 100[degrees]C to protect components. Inert atmosphere may be used to improve yield and reduce dross formation. The amounts of dross formed with SAC and Sn-Pb solders have been found to be very similar, while the Sn-Cu solder forms considerably more dross.

The overall influences of process parameters, such as conveyor speed, dwell time and contact length, component orientation and soldering direction, on the yield and quality for wave soldering are similar for lead-free and Sn-Pb solders. For lead-free wave soldering, the chemical composition of the molten solder in the pot needs to be closely monitored. For the Sn-Cu solder, the liquidus temperature will change by as much as 6[degrees]C when the copper composition changes by 0.2%. Such a change may cause significant change in the wave dynamics and the soldering quality.

Component Compatibility

A number of lead-free component termination finishes are available, including matte tin plating for passive components, matte tin (or tin alloys) or Ni/Pd/Au for leaded components, and SAC balls for area array packages (Figure 2). Tin and Ni/Pd/Au are forward compatible and backward compatible.

[FIGURE 2 OMITTED]

The issue of tin whisker growth has generated a great deal of concern, especially for fine-pitch quad flat packs (QFPs) and for long service life products. The root cause of tin whisker growth is the compressive stress that builds up in the tin plating. Several factors can help mitigate the risk of tin whisker growth, such as the use of matte tin, tin alloy, nickel barrier layer and reflowed tin.

The backward compatibility of SAC balls with the Sn-Pb solder, however, is very much questionable, primarily due to the fact that the SAC alloy will not always completely melt during reflow with the Sn-Pb solder, typically at reflow peak temperatures between 205 to 225[degrees]C. As such, little or no self-alignment will occur, which is critical, especially for finer pitch packages, with coplanarity issues further aggravating the situation due to the lack of collapse. Further, very little mixing takes place, leading to grossly segregated microstructures.

Poor interfacial bonding and increased voids are some of the issues, which, in combination with the process issues, render the SAC balled area array packages incompatible with the Sn-Pb solder. When the reflow temperature is high enough (>225[degrees]C), the reliability of the interconnect using Sn-Pb solder paste and SAC balls is not inferior to that of Sn-Pb solder paste and Sn-Pb balls.

Conscious efforts have been made to minimize the lead-free soldering temperature, but the soldering temperature must still be high enough to offer a robust process window that enables good yields for large volume production for a large variety of products. For reflow soldering, assuming the minimum peak temperature to be 235[degrees]C, the maximum temperature depends on the temperature delta across the board, which, in turn, depends on the board size, thickness, layer count, layout, copper distribution, component size and thermal mass, thermal capacity of the oven and certain unavoidable process variations. Large, thick boards with large complex components typically have temperature deltas as high as 20 to 25[degrees]C. Rework is another process that contributes to elevated temperatures. When all of the application requirements are taken into consideration, a 260[degrees]C peak temperature has been proposed as the temperature required for components for lead-free soldering. However, for small and/or medium form factor boards, a lower maximum temperature may be adequate.

Design Compatibility

No major changes in design rules are anticipated when switching to lead-free soldering. For wave soldering of through-hole components, some changes in the design may be necessary to accommodate the difference in the physical properties between lead-free and Sn-Pb solder alloys. The general guidelines, such as board orientation relative to the soldering direction, still apply to lead-free wave soldering. Optimizing board layout, component distribution and copper distribution in the PCB to minimize the temperature delta across the board is important.

Equipment Compatibility

For reflow ovens, the ability to minimize the temperature delta for large, complex boards is a key differentiator for lead-free soldering. The desire to minimize the temperature delta has generated renewed interests in certain, less popular, soldering methods such as vapor phase soldering. Localized heating may also be used for certain temperature-sensitive components, but the throughput will be fairly low.

Wave soldering machines must have adequate preheating capacity to keep the thermal shock below 100[degrees]C. For lead-free wave soldering, special materials for the solder pot and special coating for the pump impeller may be needed to prevent erosion. Most rework equipment can still be used for lead-free solder. Automated optical inspection (AOI), automated x-ray inspection (AXI) and ICT will need to be recalibrated and reprogrammed for lead-free solder boards.

Quality and Reliability Compatibility

The electrochemical reliability is determined by the resistance of the flux residue to electromigration and dendritic growth in no-clean applications; it needs to be assessed for lead-free solders because of the different flux systems and the higher soldering temperatures. The mechanical reliability of lead-free solder interconnects, under thermomechanical loading and dynamic mechanical loading, is still being studied. Under typical conditions, the SAC alloy is significantly more creep resistant than the Sn-Pb alloy due to differences in microstructure.

In many applications, the SAC solder interconnect has been found to offer greater reliability than the Sn-Pb solder. However, this result may not always be the case. Recent studies have noted that, below a certain stress threshold, the steady state creep strain rate is much lower for the SAC alloy than Sn-Pb; however, above the threshold, the trend is reversed. This result is believed to be the reason for the different reliability comparison between the SAC and Sn-Pb solders, under different thermal cycling conditions and for different components (Figure 3).

[FIGURE 3 OMITTED]

In typical conditions, the SAC solder is more reliable than the Sn-Pb solder. However, for very large components and components with a very large CTE mismatch with the substrate and/or under very severe thermal cycling conditions, the SAC alloy may not always be as reliable as the Sn-Pb solder. Moreover, the acceleration factor, which depends on materials properties, is different for SAC and Sn-Pb. Complicating the picture further is the use of different solder alloys on the same board (such as SAC for reflow and Sn-Cu for wave soldering).

Generally speaking, the IPC 610 standards are still valid for lead-free solder used in printed circuit board assembly (PCBA). Operator (and AOI) training is needed for lead-free solder because of the different appearance of the solder joints (Figure 4). Even though the types of defects for lead-free solders are the same as for Sn-Pb, lead-free solder has generally been found to generate more defects of voiding, tombstoning, solder beading, bridging and misalignment. Achieving the same yield for lead-free solder as for Sn-Pb solder, especially for wave soldering, takes considerable effort.

[FIGURE 4 OMITTED]

Business Compatibility

No simple formula assesses the precise cost impact for converting to lead-free solder because the field is still dynamic and fluid and volume dependency is an important factor. The cost differential between Sn-Pb and lead-free solder pastes is expected to decrease as the volume increases, because the metal cost is only a fraction of the solder paste cost. SAC bar solder and wire-core solder will cost more than the Sn-Pb solder because of the higher metal cost, and the Sn-Cu solder is much cheaper than the SAC alloy.

Components and possibly PCBs for higher soldering temperatures, as well as any special handling or baking for components, may also add to the cost. The difference in power consumption between Sn-Pb and lead-free reflow soldering is estimated to be 10% to 20% for boards of typical sizes and complexities. The actual cost increase for each board resulting from the increase in power consumption, however, may not be very significant. Yield is another factor that can significantly impact cost. Equipment upgrading, if needed, is also a contributor to initial cost increase. Other cost factors are due to the complexity in managing the transition, including engineering, qualification, training, materials handling and tracking.

Most companies are expected to go for a complete lead-free transition for the global market to leverage the economy of scale. Supply chain management becomes an important element for switching to lead free, and industry-wide cooperation is critical. Part number management, tracking and traceability and inventory management are important issues. Electronics manufacturing services (EMS) providers and original equipment manufacturers (OEMs) need to collaborate closely to manage the components on the same bill of materials (BOM) for lead free. Compliance and liability issues also need to be carefully worked out.

The transition to lead-free soldering is expected to have some impact on the equipment market. Reflow ovens with enhanced thermal capacity and stability and lead-free compatible wave soldering machines will become more attractive. Soldering equipment with localized heating will see increased demand to handle certain temperature-sensitive components.

Summary

Managing the compatibility issues is critical to the lead-free transition. The complete capability to handle lead-free conversion is expected to further differentiate the players in the EMS industry.

Dr. Dongkai Shangguan is the director for advanced process technology at Flextronics, San lose, CA; email: [e-mail].

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