Table 1 summarizes the various material properties, based on vendor technical data information sheets. It should be noted these values are based on averages of the data collected and variations exist.


Coating equipment. The second process requirement that must be understood is the coating process/equipment. Two coating equipment sets were evaluated for functional and complete coverage processes. The main difference between the two coverage types is the requirement for side coverage on nonconductive components. In that, for both coverage types, all conductive, metallic or lead surfaces are required to provide full conformal coating coverage. However, functional coverage will permit nonconductive, hermetically sealed areas to have material dewetting or creep from these locations. This initial requirement will then dictate the application process type required of either select film coating or an atomized spray pattern, with the latter required for complete coverage. Sample images for both process types are included within Appendix Sections 1 and 2.


Requirements to evaluate the coating process/equipment were:

• Initial equipment costs.
• Yearly equipment expenses.
• Processing cycle time.
• Service and maintenance.
• System capability.

Initial equipment costs. For this ROIC evaluation, initial equipment costs were not based on the system costs only, but included all tooling and accessories required to perform the required coating processing requirements. An interesting finding was that Vendor B’s base machine costs were lower than Vendor A’s, yet after all tooling and accessories were added, Vendor A’s initial equipment costs were lower. This is a critical aspect that must be understood when initial equipment analyses are completed. If not included upon initial capital assessments, either more tooling would be required or inaccurate tooling may be in place at higher costs than planned.

Yearly equipment expenses. We can focus on a different angle for higher resolution. This factor would be the floor space cost as based on equipment size and factory floor space cost (which varies per geography). For this evaluation, floor costs were considered comparable, with the key difference that Vendor B’s system footprint was slightly larger, providing a higher yearly equipment cost versus Vendor A’s.

Processing cycle time. With two distinctly different application types – functional and complete coverage – we need to select a single process for comparison. This noted, the following labor cost values will be assessed on a functional select coating operation: if both systems would require full assembly masking for a spray process with comparable material and labor costs incurred. An interesting finding in this section relates to the functional select coating process. This factor is the pattern definition of the film pattern, as compared to the spray pattern. The difference in the film pattern definition resulted in a higher level of pattern control for the application, leading to various cost increases (Table 2).


Service and maintenance. Both vendors provide excellent service upon request, along with required regular maintenance. Furthermore, continuous service depends on the process being completed, with higher service required for complete coverage, as compared to functional coverage. Tooling upgrades for maintenance can be acquired within reasonable timeframes to request. The sole support issue noted for Vendor B could be related to communication between site and vendor.

System capability. Various capability requirements were identified and evaluated independently of each other to provide an unbiased rating on the process. Functional and complete coverage, along with dispensing requirements, were evaluated (Figure 9). Overall, the two systems provided distinctly different strengths and weaknesses. Vendor A had four specific items considered strengths, including systems controls in place, pattern control and transfer effectiveness. This would suggest this system was more capable for select coating. However, Vendor B had one aspect that was slightly stronger: The system nozzle design could apply high viscosity material flow with tilt and rotation techniques. That, however, negatively affected placement and control accuracy.


Procedure/Results

A clear description should be provided as to which materials and processes will be evaluated (Table 3). The green squares are the two sections that were evaluated in this report.


A three-variable, two-level matrix is run to establish a process window based on customer requirements for coverage and thickness.

Five CC-Tango assemblies are run with their process requirements compared from process-to-process / material-to-material, as shown in the results for an acrylic process using both film and spray processes.

Two evaluations were completed, one for select coat process and one for atomized spray process.

As noted in Figure 10 and Table 4, there were distinct line definitions within the accuracy, along with the overall glossy appearance, that are common properties for film coat applications.


Figure 11 and Table 5 show the inconsistency within the accuracy section, along with the overall matte appearance, common properties for atomized spray coat applications. A key point is that the inconsistent/fuzzy edges in the accuracy section are created by the atomized portion of the coating that is also an advantage in being able to provide more uniform coverage to odd-shaped surfaces, materials and edges.


With the comparison between the two processes film and spray (Figure 12), there are direct advantages and obstacles for each process:


1) Cycle time. The overall cycle time for the running of the CC-Tango TV was 7.36 times longer for the spray process, as compared to the film process. Provided volumes are low, a spray process could match a film process in capital requirements. However, if volumes were of larger formats for either an inline or batch process, additional capital could be required, based on the assembly sizes or volume requirements, and would lead to a per cost increase due to capital and additional space requirements. Furthermore, as noted, a spray process requires additional masking that also increases the overall cycle time for the process.

2) Missing/dewets. There was a higher quantity of missing/dewetted locations: approximately 11 times greater for the film compared to the spray process. Most defects could be modified within an inline touchup process prior to final assembly inspection and shipment. The cost of this increase could result in hours of additional rework, depending on coverage requirements. A sample of these defects is shown in Appendix 3.


3) Bubble/voids. No bubbles or voids were found within the spray process application; however, four were found within the film. With finer process optimization for the film process, this value of zero defects could also be obtained. Sample bubble defects are shown in Appendix 5.


4) Wicking defects. The overall defects for this section were 297 to 3 for the spray process versus the film process, respectively. The main concept obtained from this data collection was that manual application of masking is required for a spray process to prevent coating of keep-out locations, and can be optimized and not included for a film process saving both in material and labor costs, along with cycle time. The labor for this masking process increase as a result of component times could lead to ranges between 5-30 min. per assembly more, compared to a film process. Defects are shown in Appendix 4.


5) Drainage results. There was no significant difference between the film and spray process penetration standings. However, the film process was applied slightly thicker than the spray and would require a via/hole of diameter 0.041", as compared to just over 0.035", to permit such flow and potential defect creation.

6) Thickness. The thickness for both processes was within operating limits with no signs of deviation from this requirement.

Overall, the three main process variables as described are the coverage requirements (missing/dewets), thickness and adhesion of coating to required areas. With these three process variables met, a stable process can be defined. These six process defects and requirements are summarized in Table 6.


Conclusion

Many of the most complex electronics assemblies have conformal coating processes in place that may not be optimized. Conformal coating results from this TV permit the effect of coating variations on reliability to be assessed, and should be useful in developing site specifications similar to IPC-CC-830 for use in standardizing reliability testing for conformal coating within a company.

The CC-Tango TV procedure and process has demonstrated that, by using this procedure, a comparison between various cleaning agents, masking materials, coating materials and application equipment can be completed. Comparisons between film and spray processes were completed with pros and cons provided for each.

In summary, if a functional conformal coating process was required, a film process could save 7.36 times in application cycle time, and 5-30 min. in masking labor time and materials, compared to spray.

If a complete conformal coating process was required for a high-complexity topology assembly, a spray process could save 11 times the rework/inspection loops compared to film.

Future Work

A version of the CC-Tango TV will be created with Pb-free solder to work with new fluxes being implemented to accommodate density increases and transition to Pb-free solders, along with different component packages to deal with the higher required interface properties for the increased density components. These changes have produced various interactions with the conformal coating materials and processes used.

A version of the CC-Tango TV with the inclusion of taller transformer packages for testing. Components of this nature would provide an extra obstacle for the coverage requirement to overcome, such as the shadow effect of smaller components, and evaluate the specific process.

A version of the CC-Tango TV with the inclusion of 0403 and 0201 components for cleanliness testing. Components of this nature would provide an extra obstacle for the cleanliness requirement to overcome and evaluate the specific process.

Acknowledgments
The author gratefully acknowledges the contribution of Hector Barrera of Celestica, Mexico and Ti Loon Ang of Celestica, Malaysia, in the data collection for the report, along with the assistance of material and equipment suppliers in the qualification installation and testing of the CC-Tango test vehicles. Finally, the author wishes to thank Jeffrey Kennedy of Celestica, Arden Hills, for assistance in the overall review and support in the designing and data collection for the CC-Tango process.

References

1. Jason Keeping, “Process Development and Optimization Using a Newly Designed Conformal Coating Test Vehicle” SMTAI, October 2007.
   
   
2. IPC, IPC-TM-650, “Test Methods Manual,” method 2.6.1.1, Fungus Resistance – Conformal Coating, July 2000.
   
   
3. IPC, IPC-TM-650, “Test Methods Manual,” method 2.6.7.1, Thermal Shock – Conformal Coating, July 2000.
   
   
4. IPC, IPC-TM-650, “Test Methods Manual,” method 2.6.11.1, Hydrolytic Stability – Conformal Coating, July 2000.
   
   
5. IPC, IPC-TM-650, “Test Methods Manual,” method 2.5.7.1, Dielectric Withstanding Voltage – Polymeric Conformal Coating, July 2000.
   
   
6. UL, “Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances,” October 1996.
   
   
7. IPC, IPC-TM-650, “Test Methods Manual,” method 2.6.3.4, Moisture and Insulation Resistance – Conformal Coating, July 2003.
   
   
8. IPC, IPC-TM-650, “Test Methods Manual,” method 2.4.1.6, Adhesion, Polymer Coating, July 1995.
   
   
9. IPC, IPC-SC-60, “Post Solder Solvent Cleaning Handbook,” August 1999.
   
   
10. IPC, IPC-SA-61, “Post Solder Semi-Aqueous Cleaning Handbook,” June 2002.
    
    
11. IPC, IPC-AC-62, “Post Solder Aqueous Cleaning Handbook,” January 1996.
    
    
12. IPC, IPC-CC-830B, “Qualification and Performance of Electrical Insulating Compound for Printed Wiring Assemblies,” August 2002.

Ed.: This paper was first published at the SMTA Pan Pac Symposium in January 2008 and is used here with permission.

Jason Keeping is project manager, conformal coating/Potting Sector, at Celestica Inc. (celestica.com); jkeeping@celestica.com.

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