Changes required and precautions to take for successful implementation of Pb-free manufacturing.

Key factors for successful Pb-free implementation are to understand process and equipment changes required and to ensure proper quality engineering practices to avoid cross-contamination. Particularly in the electronics manufacturing environment, these factors play a major role. As some products are currently exempt from legislative bans, companies will be manufacturing Pb-free and lead-based products for the near future.

Printing. Printing Pb-free paste is not very different from SnPb paste. Accurate alignment of the board to the stencil is very important for Pb-free manufacturing. Due to its high surface tension, Pb-free paste does not spread like SnPb paste; it remains where it was printed even after reflow.¹ Figure 1 elaborates the phenomenon. Current printers should work for Pb-free manufacturing, provided the alignment between stencil and board is accurate.

After July 2006, a product containing more than 0.1% by weight of lead in homogeneous material is not in compliance with legislation. Using separate tooling and fixtures for screen printing Pb-free paste is recommended. The use of common squeegee blades, spatulas and stencils provides an opportunity for cross-contamination. Pb-free paste does not wet like SnPb paste, however, and stencil design guidelines may need revising. (Author note: stencil design guidelines are beyond the scope of this article.) Coloring the tooling and fixtures with either a "green" color or label has been effective in avoiding cross-contamination. Storing Pb-free solder paste in a separate refrigerator could avoid possible cross-contamination.

Component placement. SnPb and Pb-free paste exhibit the same level of tackiness. Research shows that Pb-free paste exhibits less self-centering properties compared to SnPb paste.¹ Components placed off the pad will not move back to the center of the pad during reflow. Accurate component placement is required to avoid Pb-free assembly defects.

Component manufacturers are using various materials for component finishes such as matte tin, SnCu, NiPd or SnAgCu. Vision systems in pick-and-place machines may find these finishes different and may have trouble recognizing the components. The best practice could be to use a separate file for Pb-free components until the transition is complete. This would entail a different part numbering scheme and separate libraries for Pb-free parts.

Reflow. Pb-free alloys (SAC) melt at higher temperatures compared to eutectic SnPb. SnPb37 melts at 183^°C and a typical peak reflow oven temperature is 220^°C. Most SAC alloys melt at 217^°C with a peak temperature as high as 260^°C.

BoM analysis is a vital step in the Pb-free transition. The typical Pb-free process peak temperature is between 235° and 260^°C, depending upon the paste alloy and the board's thermal mass. This may be a maximum temperature to which some of the sensitive components can be exposed. Each component needs to be evaluated to verify that all components can sustain the higher peak temperature.

The Pb-free process window is more narrow than that of SnPb (Figure 2). Existing reflow ovens can be used for Pb-free manufacturing but the higher temperatures may increase preventive maintenance costs. Also, higher Pb-free temperatures will more quickly deteriorate reflow oven performance. The cooling zone of current ovens may not be able to cool the board. Larger boards with a high thermal mass content are most likely to present handling issues upon exit. Many equipment manufacturers are selling larger reflow ovens with larger cooling capacity specifically designed for Pb-free manufacturing.

The reflow profiling tool and process fixtures need to sustain the higher temperatures. A Pb-free test vehicle used to develop reflow profiles is shown in Figure 3 and the corresponding profile is in Appendix 1 (online).

Wave soldering. Wave soldering will face significant process and equipment changes, thus it requires careful monitoring. Over the years, substantial knowledge of SnPb wave soldering has been accumulated, while Pb-free wave soldering is a relatively new process. It seems the Pb-free alloys most successful for wave soldering are Sn96.5Ag3Cu0.5 and eutectic Sn99.3Cu0.7. The tin content in SnPb alloy is 63%, whereas it is more than 90% in Pb-free alloys. High tin content is a cause for concern for the solder pot and other hardware. The Pb-free alloy will corrode stainless steel solder pots and other hardware such as flow duct, pump, impellers, conveyer fingers, etc., in a short time (Figure 4²). It demonstrates pitting, and can eventually damage the pump and result in leaks. Therefore, a stainless steel solder pot cannot be used for Pb-free solder wave soldering. Major equipment manufacturers are offering special coatings for solder pots and other hardware that can withstand high tin Pb-free alloys. Some suppliers are offering equipment with double pots, which can be swapped for SnPb or Pb-free operation. This may slow production rates.

Pb-free wave soldering generates more dross than the SnPb process. The standard practice is to remove Cu6Sn5 intermetallics floating on the surface of pot. The density of Cu6Sn5 intermetallic is 8.28, while density of SnPb37 is 8.80, which causes SnCu intermetallics to float on the solder pot after cooling.³ However, the density of Pb-free solder is 7.39 which results in sinking of SnCu intermetallics in a Pb-free solder pot and makes them difficult to remove.³ If these intermetallics are not removed frequently, the solder becomes sluggish and does not reach the required wave height. The cost of running a Pb-free wave solder is higher because of the more frequent maintenance and the use of tin and silver in the alloy.

Pallets used in wave-soldering will be exposed to higher temperatures, slower conveyer speeds and longer contact times. Pallet material stability should be considered during material selection, with requirements communicated to the pallet supplier.

Pb-free solder bars should be stored in a different location than SnPb bars. A SnPb solder bar placed in a Pb-free solder pot will cause solder joint reliability issues and heavy maintenance costs. If the solder pot becomes contaminated, the entire pot must be drained, cleaned and refilled.

Many suppliers are manufacturing solder bars in different shapes (such as triangular) to distinguish SnPb bars from Pb-free. Pb-free surface-finished PCBs can be assembled using eutectic SnPb alloy; this practice has been performed for years. However, the use of SnPb HASL boards with SAC alloys may produce fillet lifting in through-hole joints.⁴

Hand soldering. Pb-free alloys melt at a higher temperature and have slower wetting characteristics, making them perform differently when hand soldering. Tip shape, tip condition, time on joint and optimum heat transfer are key factors in Pb-free hand soldering. The tip should have correct shape to maximize the contact area between the pad and lead. The contact time may rise between 2 to 5 sec.; however, the temperature does not increase much. Wetting characteristics of Pb-free alloys can be improved and process time can be shortened by increasing tip temperature, but it may affect the flux activation rate and the substrate. Most soldering iron suppliers are providing tips compatible for Pb-free soldering. Overall tip life will shrink during Pb-free soldering because of the higher tin content of the alloy and higher oxidation rates.

Hand soldering is an area highly susceptible to cross-contamination. It is highly recommended to use different workstations for Pb-free and SnPb products. Mistaken use of a SnPb soldering iron for Pb-free assembly may impact product reliability and compliance. Separate workstations with "Green" labeling on the soldering iron as well as the workbench are effective to avoid cross-contamination.

Material management. Successful Pb-free implementation depends upon joint efforts from engineering, production, material management and suppliers. Apart from process control, material logistics requires diligent monitoring during the transition. Some products are exempt at this point in legislation. As a result, most EMS firms will have to manage the processes side-by-side for the near term (at least). And managing material logistics will be a critical task. Some companies have not decided to go Pb-free, but may eventually be pushed to convert if SnPb-finish components cease to be available. EMS companies will experiment with Pb-free alloys and manufacture both chemistries. Some component vendors are not changing part numbers, which presents a potential problem for mixing of components. Companies will need to verify compliance with component vendors through established reporting systems such as Process Change Notices (PCN). Some consortia have recommended using new part numbers for Pb-free components to avoid potential mixups (and consequential reliability issues). Yet a number of component manufacturers have decided not to change their nomenclatures as the quantity of part numbers they will need to manage may be significant. Several component vendors are shipping components in Pb-free versions only, without prior notification. In this case, component compliance verification can be performed via production dates or lot numbers.

Incoming inspection. As use of SnPb components in Pb-free assembly will produce noncomplaint product, stringent incoming inspection is needed. Pb-free BGAs are not compatible with the SnPb process. If SnPb BGAs are placed on a Pb-free board, the resultant rework costs, lost hours and product reliability issues will be tremendous. Identifying and segregating leaded and Pb-free materials will be a challenge for incoming inspectors. Materials declaration standards (IPC-1751 and IPC-1752) pertaining to Pb-free components are being written. Component manufacturers and suppliers will be required to declare the material content of the component.

IPC has also defined an industry standard for Pb-free labeling: IPC-1066, "Marking, Symbols, and Labels for Identification of Pb-free and other Reportable Materials in Pb-free Assemblies, Components and Devices." These are guidelines for component vendors and assemblers for labeling assemblies, components and devices. This standard can be used to identify incoming Pb-free component specifications. However, the actual format of the label varies from supplier to supplier (Figure 5) and incoming inspectors need to be meticulous. Incoming inspectors will need to be trained to identify and understand Pb-free markings.

One method to avoid component mixups is a sampling plan. Sampling plans can be designed to test the conformance of various parts from a common vendor as well as common parts from different vendors. A low cost method to test lead content is the use of chemical swabs (Figure 6). If a product contains lead, then the swab changes color.

Compatibility. Component manufacturers are striving to make components available in Pb-free form. At this point not all components are Pb-free. During this transition, the following scenarios could occur:

• Pb-free paste and Pb-free components.

• Pb-free paste and SnPb components.

• SnPb paste and Pb-free components.

Backwards compatibility refers to the use of a Pb-free finish component in a SnPb solder process (Figure 7). Most Pb-free components are backwards compatible, except BGAs.

Imagine a Pb-free BGA placed on a board with SnPb paste. SnPb paste melts at 183^°C and the typical peak temperature of the process could be 220^°C. A Pb-free BGA ball melts at 217^°C and the process peak temperature may not be enough to form an intermetallic bond due to noncollapsing of the solder ball. If the peak temperature of the process is increased so that the Pb-free balls can be melted and form an intermetallic bond, other components on the board may sustain damage from the higher temperature. Use of Pb-free components with a SnBi finish in a SnPb assembly is not recommended, as the bismuth lead forms a low-temperature compound which melts around 96^°C.

Forward compatibility refers to the use of SnPb finish components in a Pb-free process (Figure 8). SnPb components used in a Pb-free process may not sustain the peak temperatures of the process and be damaged after reflow. Component specifications of SnPb components need to be verified before their inclusion in the process. As the two alloys have different melting points, voids may occur in the solder joint. Also, the product manufactured by this process may exceed the maximum Pb content permitted by the RoHS directive.

Assembly inspection. Pb-free solder joints look different than SnPb solder joints. They have a dull and grainy appearance, and higher wetting angles. Quality assurance personnel will need to be trained to inspect Pb-free solder joints. The revised IPC-A-610D depicts Pb-free solder joint acceptance criteria. Pb-free solder joints have a rough surface and cracks, which is normal and depends on the reflow profile (typically, the cooling rate). They could be misidentified as cold joints, and result in significant unnecessary rework.

Everyone involved in the manufacturing operation needs to be trained. This includes assembly operators, quality inspectors, final assembly operators, test and those in the return material area. Each shift should appoint a champion, an expert in identifying Pb-free joints, upon whom operators can turn for assistance. Differences in appearance of Pb-free and SnPb joints are shown in Apendix II (online).⁵

Pb-free assembly marking. IPC-1066 establishes guidelines for Pb-free assembly markings and symbols⁶:

Printed circuit assemblies shall be identified as being assembled with Pb-free solders and using components with Pb-free second-level interconnect leads/terminals. A Pb-free assembly shall be marked with the symbol [shown in Figure 9⁶].

In addition to the Pb-free symbol, the category of the solder used in the assembly shall be mentioned as shown in Figure 10⁶ and Table 1.⁶ The category can be marked with either a circle or an ellipse.

The label in Figure 11⁶ can be affixed on the container holding assemblies if boards are assembled with a Pb-free alloys and use components with Pb-free second-level interconnection leads/terminals. The category field in the label shall describe the solder used on the board assembly and the maximum temperature that the assembly can sustain.

The label in Figure 12⁶ can only be used if the board assembly is Pb-free per RoHS Directive 2002/95/EC; i.e., the Pb level in any of the raw materials and end-products is less than or equal to 0.1% by weight.

EMS companies will be manufacturing both Pb-free and SnPb assemblies under the same roof in the near future. Successful concurrent manufacturing is feasible, but requires careful understanding of the Pb-free process and taking precautions to avoid cross-contamination. Successful Pb-free implementation depends not only on the process but also on factors such as assessing equipment capability, material segregation, material logistics and retraining the workforce.

 

Acknowledgments: The authors wish to acknowledge Cookson Electronics for providing the test vehicle for this study.

References

1. A. Teredesai, D. Santos, J. Belmonte and P. Chouta, "A Study of Self-Centering Phenomenon in Lead-Free Surface Mount Assembly," SMTA Pan Pacific Symposium, January 2004.

2. K. Seelig and D. Suraski, A Practical Guide to Achieving Lead Free Electronics Assembly white paper, aimsolder.com, November 2003.

3. Aim Solder, A Study of Lead-free Wave Soldering white paper, aimsolder.com.

4. C. Handwerker, "Transitioning to Pb-Free Assemblies," Circuits Assembly, March 2005.

5. IPC-A-610D, "Acceptability of Electronic Assemblies," February 2005.

6. IPC-1066, "Marking, Symbols and Labels for Identification of Lead-Free and Other Reportable Materials in Lead-Free Assemblies, Components and Devices," December 2004.

Bibliography

1. P. Biocca, "How Do You Create a RoHS Compliant Pb-Free roadmap?" EMS Now, Dec. 13, 2004.

2. B. Gilbert, "Pb-Free Wave Soldering Experiences, A Six Year History," IPC/Jedec Lead-Free North America Conference, Dec. 3, 2004.

3. OK International, Hand Soldering with Lead-Free Alloys, okintl.com.

4. C. Shea, B. Barton, J. Belmonte and K. Kirby, "Practical Lead-free Implementation," IPC Apex Proceedings, February 2005.

5. F. Monette, "Material Control for Lead-Free Manufacturing," SMT, March 2005.

6. R. Robertson and J. Smetana, "Some Fundamental Concerns in Pb-free Implementation," SMTA Pb-free Symposium, June 2000.

7. M. Warwick, "Implementing Lead-Free Soldering Consortium Research," SMTA International, September 1999.

8. L. Whiteman, "Issues and Solutions to Implementing Lead Free Soldering," SMTA Lead Free Symposium, June 2000.

9. S. Yi, D. Geiger, M. Wang, H. Singh, A. Lee, W. Wong, "A Case Study of Pb-free Assembly Implementation in EMS Environment," SMTA Lead Free Symposium, June 2000.

10. JESD97, "Marking, Symbols, and Labels for Identification of Lead (Pb) Free Assemblies, Components, and Devices," May 2004.

Ed.: This article was first presented at SMTA International in September 2005 and is used with permission of the authors.

 

When this paper was written, Amey Teredesai worked with NuVisions Manufacturing LLC. Tony Batalha is vice president of engineering at Nu Visions Manufacturing LLC (nvems.com); tbatalha@nvems.com.

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