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Keeping Lead Away from Lead-free During Cleaning
December 31, 1969 |Estimated reading time: 6 minutes
While the deadline for conversion is fast approaching, it does not mean that everyone will become lead-free all at once. Many manufacturers are processing lead-free and lead-bearing assemblies. The concern for contamination is serious, even in the cleaning process. This article looks at ways assemblers can reduce contamination risks in the cleaning process.
By Jon Phelps
The deadline for lead-free conversion is approaching, but that does not mean everyone will convert to lead-free all at once - or that lead-free soldering will cease to exist. During the transition, many electronics manufacturers will process lead-free and lead-bearing assemblies. In cases where military electronics are manufactured, lead-bearing solders will continue to be used. The concern for cross contamination is a serious issue that will affect product reliability and cost. It also is a matter of real concern for wave soldering, where the contamination of small amounts of lead into a lead-free solder pot can mandate discarding all contents of the pot, at a considerable cost.
A large part of any cleaning process is dedicated to the rinsing of wash effluent and the drying of rinse effluent. With the introduction of lead-free processing to a eutectic environment, the cleaner operator must keep the two unique processes separate. The risk is that eutectic solder can contaminate a lead-free process through the cleaner. Managing this risk in the cleaning process, and keeping the processes separate, requires an understanding of potential problems and an appropriate action plan.
When managing risk, it is a good idea to be aware of what limits apply to the process. For the purposes of RoHS regulations, a maximum concentration of up to 0.1% by weight in homogeneous materials for lead… will be permitted in the manufacture of new Electrical and Electronic Equipment (EEE). Homogeneous material means a material that cannot be mechanically disjointed into different materials. A coating can be removed by scraping or abrading, making it a non-homogeneous material. Lead, a homogeneous material, cannot be separated physically from the tin in the solder alloy. The ability to detect lead is key in preventing lead contamination. One recommended method is the use of swabs with dyes. When swabs are rubbed onto soldered assemblies and exposed to certain reactive chemical dyes, the swab will turn from pink to red when lead is present. This test method can be applied across an entire assembly, or to a specific solder joint. These swabs and other simple methods can detect lead below the 0.1% RoHS limit.
The criterion for maximum concentration of lead is 0.1% by weight. Consider, for example, if the solder inspection occurred at a lead-free via site. What happens if a eutectic solder sphere is deposited into this lead-free via, and the assembly is reflowed? A measured sample of the homogenous material in this via would likely exceed the 0.1% maximum concentration.
Many engineers are beginning to consider this possible risk, as well as where cross contamination can occur. In terms of the cleaner, for example, it is common to use the same machine for PCB and stencil cleaning with eutectic and lead-free processes. One must ask the following questions given current manufacturing setup and practices:
- Is cross contamination a possibility?
- How can a process that eliminates risk associated with exceeding the 0.1% maximum concentration at a specific site on the assembly be verified?
Cross contamination can occur when solder particles/balls from the eutectic cleaning process are deposited onto lead-free assemblies. Imagine an immersion bath cleaning process. PCBs and stencils with eutectic solder paste and spheres are introduced to the bath. The product is cleaned under immersion, and perhaps all solder paste and solder balls are removed. PCBs and stencils with lead-free solder paste and spheres are then introduced into this bath, which now contains both eutectic and lead-free contaminants. With the agitation and stirring of the wash bath, the potential for eutectic solder particles lodging into fine-pitch leads and microvias is significant. With an associated weak-rinsing process, these solder particles likely will remain. Subsequent printing processes and reflows will form these previously eutectic and lead-free non-homogenous materials into one homogenous material that is subject to the 0.1% maximum lead concentration restrictions.
We then must ask, relative to the cleaning process, “How can a process be verified that eliminates risk associated with exceeding the 0.1% maximum lead concentration at a specific site on the assembly?” In a cleaning system that accepts both eutectic and lead-free products, the wash and rinse solutions are considered a potential medium for the transfer of residual lead to lead-free products. To eliminate this potential, filtering is key to cleaning leaded and lead-free paste in the same cleaner.
To filter the wash solution successfully, a filtration loop must be established. The filtration loop must ensure complete filtration of all wash and rinse solution that enters the process chamber, and comes in contact with the part being cleaned. This is achieved by creating a closed-loop process. Wash solution is pumped from a wash reservoir, through a micron-size filter, and into the process chamber. From the process chamber, the wash solution is routed back to the wash reservoir - closing the loop.
Rinse solution can be close-looped using a similar process. In an open-loop process, fresh rinse solution is delivered to the process chamber from the facility and routed to the municipal drain directly through particulate filtration.
In the process chamber, solder paste and solder balls are washed away from the product with high impingement energy. Solder paste and particles are routed out of the process chamber to the wash reservoir. Upon recirculation, the solder paste and balls are filtered. No solder particles and solder balls are re-deposited onto current or subsequent products being cleaned.
In immersion processes, including ultrasonics, solder paste and solder balls never leave the process chamber, and have a real potential for being redeposited on a product. Ultrasonic energy can also dislodge solder paste and solder balls, but does not significantly carry the solder balls or paste away from the stencil or electronic assembly being cleaned. Loosened solder balls have a propensity to fall onto a stencil frame or into the leads of components.
With an associated high-energy, spray-in-air rinse process, wash effluent is rinsed away completely. Cross contamination is prevented using this process. Testing has shown that the combination of spray-in-air cleaning and a closed-loop-filtered system will minimize the possibility of cross-contamination with lead-bearing products. Other issues and considerations affect this combination.
Conclusion
No process or system is completely foolproof; there is some element of risk associated with processing lead-bearing eutectic and lead-free assemblies in the same cleaner. Combined spray-in-air and closed-loop filtration systems reduce the risk of cross contamination. However, there are limits to the filter’s capability. While there is a limit for the percentage of smallest particles, there is no specification for the smallest particle size. Solder-balling problems attributed to sub-micron solder-particle content in paste have resulted in paste manufacturing processes that have been improved to ensure that sub-micron particles are not generated. A low-micron filtration system will prevent almost all lead particles from transferring through. An appropriately sized, improved filtration system will also capture nominal particle sizes. With continued use, filter ability improves.
References
For a complete list of references, please contact the author.
Jon Phelps, regional manager, Austin American Technology, may be contacted at (512) 335-6400, ext. 22; e-mail: jrphelps@aat-corp.com.