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Lead-free Stencil and Misprint Cleaning
December 31, 1969 |Estimated reading time: 5 minutes
The emergence of lead-free products in the North American market and more innovative cleaning technologies have contributed to the growing attention toward stencil and misprinted circuit board cleaning. This article assesses the affect lead-free products might have on current cleaning processes.
By Umut Tosun
Increasing component density and the related increase in printing precision have given rise to better and more advanced stencil cleaning processes. Newer flux formulations and solder alloys, such as lead-free, are challenging these advancements. Earlier studies reveal that 60% of shortcomings in the SMT manufacturing process were the result of inappropriate solder paste printing. To reduce the number of defects significantly, electronic manufacturers invested in automated cleaning processes that could provide consistent and reliable cleaning results using common eutectic solder pastes. Depending on the user’s needs and requirements, solvent-, aqueous- or surfactant-based cleaning solutions were used. Despite some variation in the quality of cleaning results, the different cleaning technologies were capable of providing satisfactory results. With the move toward lead-free solder pastes, an extensive study focusing on more than 40 lead-free products from global manufacturers was completed.
Lead-free solder pastes contain more activators than eutectic products. These experiments focused on not only removing lead-free solder pastes from stencil and misprinted boards, but also ensuring that no activator residues remained on the substrates after cleaning. To accommodate this additional cleanliness requirement, new analytical methods were developed.
Testing Procedures and Requirements
Because most stencil and misprint cleaning is conducted in ultrasonic and spray-in-air cleaning processes, this lead-free cleaning evaluation was conducted in application technology centers throughout North America, Europe and Asia.
Prior to the trials, various lead-free solder pastes were printed on standard stencils. To better represent the printing applications available in the field, particular attention was placed on the selection of the aperture size, which ranged from 0.020" (0.508 mm) to 0.032" (0.813 mm).
Figure 1. Applied and cleaned raw solder paste on stencil and PCB.
Based on experience, worst-case scenarios were simulated. For example, the results obtained would provide the widest cleaning process window, thereby establishing useful conclusions. Each solder paste was applied manually and printed 24 hours prior to the cleaning process. During this timeframe, the solder paste was allowed to air-dry in an ambient, air-conditioned environment in a designated lab area. The amount of solder paste (350 mg/in2) that was applied on the surface of the metal foil was five times greater than the amount of solder paste that typically remains on production stencils (Figure 1). Based on these assumptions, each lead-free solder paste was printed on a standard test-board vehicle to simulate misprinted lead-free solder paste. Subsequent to the printing process, test boards were set aside for 24 hours prior to cleaning.
Testing Parameters
The ultrasonic cleaning equipment was outfitted with standard 40-KHz ultrasonic transducers and a power density of 25 W/L. The spray-in-air cleaning equipment used separate wash and rinse sections. Both sections were connected to rotating arms with about 22 psi of spray pressure. Depending on the cleaning agent, the substrates were rinsed with the properly filtered cleaning agent or DI-water (Tables 1 and 2).
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Cleaning Evaluation
After cleaning cycles were complete, stencils and misprinted boards were inspected visually under a microscope (×40) using differential-interference contrast. By using a polarized light source, contamination is easily visualized - especially on metal surfaces. The substrates were examined for any remaining raw solder paste residues using additional cleanliness methods:
- Surface inspection assisted by differential-interference contrast measurements;
- Manual wipe test;
- Ionic-contamination measurements;
- Contact angle measurements (CAM).
Stencil cleanliness was evaluated further using a manual wipe test. Specific fabrics were chosen to quantitatively assess the amount of solder paste residues remaining on a given surface area. Particular attention was paid to the actual fabric used, as well as the associated gravimetric-cleanliness assessment. Based on past observations, satisfactory cleaning results show a residue content of less than 0.1 mg/in2. Table 3 summarizes the achieved cleaning results.
Additional test procedures were followed for misprinted boards. Remaining residues were measured using ionic-contamination and contact-angle measurement. Values below 10.06 μg/in2 for ionic-contamination measurements are considered clean. For contact-angle measurements, experimental results between 38 and 42 mN/m correlate with high cleanliness levels compared to ionic-contamination measurements, flux test and other analytical values.
To provide meaningful results, this study used available SAC alloys from different solder paste manufacturers. This was necessary to determine the importance of different flux systems. For example, SAC 305 alloys were requested from different manufacturers because they vary greatly in flux technologies. It was concluded from the findings that no differences were found among various products in terms of removability. In other words, for solvent and water-based* cleaning agents, full removal of unsoldered pastes produced neither cleaning difficulties nor required additional process optimizations. Such findings were based on a combination of contact-angle measurement, ionic contamination and visual analysis. Only a combination of analytical tests would allow for an overall assessment on cleanliness. When examining misprinted boards, a novel analytical method was used to calculate surface-energy values. This procedure lets users assess cleanliness and determine the polar and non-polar constituents in the contamination. Due to a higher level of activators in lead-free solder paste, these values can offer important information that can be used to improve respective cleaning processes. Only overall surface-energy values were provided. Contrary to good cleaning results obtained with solvent and water-based* products, it was determined that for surfactant-based cleaning agents, some pastes were more difficult to remove. Differences in the amount of activators may contribute to the chemical mismatch between surfactants and residues to be cleaned.
Conclusion
This experiment confirms that despite the recent surge of customers switching to lead-free formulations, users can continue using existing (eutectic-based) stencil and misprint-cleaning processes without significant changes. This, in turn, means switching to lead-free will not incur additional process costs - a rarely observed occurrence. With the few unsatisfactory cleaning results observed, current users of surfactant-based processes should conduct additional testing before switching to lead-free; or consult a competent cleaning service provider for technical assistance.
* MPC-based cleaning agents, ZESTRON America, Ashburn, Va.
For a complete list of references, please contact the author.
Umut Tosun, application technology manager, ZESTRON America, may be contacted at (703) 589-1198, ext. 102; e-mail: U.Tosun@zestronusa.com.