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Challenges and Laws of Lead-free Cleaning
December 31, 1969 |Estimated reading time: 7 minutes
Lead-free soldering implementation is closing in on the July 1, 2006 deadline. Characteristics of lead-free alloys are higher reflow temperatures and poor wetting compared to eutectic tin/lead soldering. To improve the process window, flux technologies must provide improved wetting, higher oxygen barriers, and higher thermal stability. Subsequently, flux residues pose a greater risk and are more difficult to clean. This article discusses the challenges of lead-free cleaning, and Identifies the laws affecting cleaning.
Lead-free cleaning? You might ask, “What happened to no-clean?” Water-soluble fluxes clean well with water, don’t they? These questions are resonating across the globe as lead-free becomes a reality to more operations. That reality is a bit different than the experiences we have had for the past 10 years, since water-soluble and then no-clean were the dominant solder technologies. This article identifies the many laws affecting cleaning - be they laws of government, customers, or nature.
Law #1: The Lead-free Revolution Will Affect Everyone in the Industry
Everyone will be affected. Consider your components and whether tin/lead and lead-free will be available throughout the industry. Consider soldering technology. How much product development time do you suspect has been devoted to tin/lead materials in the past few years? Given that lead-free will affect you, it may be safe to paraphrase some familiar advice given to your engineers: lead, follow, or get run over. There is no getting out of the way of lead-free.
Figure 1a. Typical no-clean residue.
You may ask how this ties into cleaning. After all, large segments of the industry stopped cleaning their product years ago, excluding stencils, misprints, and maintenance cleaning. To answer the question, start with soil: flux, or more generally, solder technology, and how these developments affect cleaning. The impact can be seen by comparing Figures 1a and b. Significantly more flux material can be seen on Figure 1a, highlighting the second law of lead-free.
Figure 1b. Lead-free ramp-to-spike reflow profile.
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Law #2: Residues are Always Undesirable
The presence of these substantial residues is causing some operations to consider their removal, and engineers to evaluate how the residue might be removed. This leads to the third law.
Law #3: The More Severe a Thermal Excursion, the Harder the Removal Challenge
Using Law #3, it is easy to understand why cleaning challenges tend to be more difficult when soak profiles are used compared to ramp-to-spike profiles. Of course, it is not the shape of the profile that is causing the problem, it is the peak temperature. The extra 20°-30°C exacerbates potential problems of the soak profile, making it an appropriate choice only after substantial process-validation testing. Extended high-temperature profiles may cause irreversible effects on the substrate itself. Figure 2 shows these potentially irreversible effects.
Figure 2. Clear flux outline after cleaning a lead-free soak-profile circuit board.
This takes us back to Law #1, cleaning corollary #1: If the soil and process conditions change, the cleaning process must change. We have described how key elements of soil (solder technology) have changed: alloys (and their oxides), flux packages (make-up and quantity), and temperature profiles. Essentially, every salient element of soldering technology that affects cleaning has changed with the transition to lead-free. It only makes sense that the cleaning process and technology must also change.
Lead-free Cleaning Data
As we explore the reality of corollary #1, it is helpful to have data available for analysis. Fortunately, more data sets exist that compare results of traditional or legacy cleaning technologies, which have been industry mainstays for more than a decade, and modern materials developed alongside modern lead-free materials (Figure 3a). Data must be carefully analyzed. Operating temperatures of cleaning systems pose some challenges. Lead-free products pose more of a challenge as they require higher soldering temperatures. Temperature is not only a sensitive indicator for clean-ability, but also a cost indicator. Results may vary, but cleaning at reduced operating temperatures can result in substantially lower requirements for cleaning materials. Figure 3b demonstrates this point. Legacy #2 cannot get the job done; however, Legacy #1 can succeed at the highest temperature and cost of ownership process conditions. Legacy #2 accomplishes the goal at relatively benign process conditions.
Figure 3a-c. Charts comparing the results of traditional (legacy) cleaning technologies and modern materials.
Figure 3c shows that Legacy #2 does not match up well with lead-free soldering materials. However, Legacy #1 shows suitable performance, while Modern #2 material performs better at the lowest temperature point. All pertinent data must be reviewed. Do not be satisfied with what you have heard or suspect your current process will achieve. Review the data, or have your supplier generate data. When updating benchmark data for new solder materials, it may seem as if there is an infinite selection of solder technology formulations. However, if a customer needs the data, any credible supplier will generate it. Now that the temperature factor is covered, let’s examine a soft critical factor - appearance.
Law #4: Solder Joints Should Always Have a Lustrous Glow
All customers want their product to look superb. This is one of the few technical aspects that the electronics business shares with other businesses. Since the days of CFC-113, solder joints have had the ability to look lustrous; and they still can (Figure 4a). Even lead-free alloys can demonstrate a better finish. Perhaps not the equal to that of tin/lead, but substantial improvements have been made. If your product or test substrates look like Figure 4b, caveat emptor - buyer beware.
Figure 4a. Solder joints with a lustrous glow.
The finish in Figure 4b is not specific to legacy technology. Until 10 or 12 years ago, all aqueous cleaners were hard on finishes, particularly if the cleaning cycle was extended for any reason. In the mid-1990s, aqueous technology made a substantial improvement in the area of finish appearance. Many legacy products on the market were developed before that technological leap, and may look similar to Figure 4b. Fortunately, the cleaning challenge of lead-free will help these technologies fade into the sunset over then next year or two as lead-free becomes the norm.
Figure 4b. Solder bumps without a lustrous glow.
However, it is not all about the chemicals. Hardware is an important part of the equation. If the cleaning agent simply will not clean, the hardware cannot make up for it. Likewise, a great cleaning agent in a poorly selected machine will also not perform. Assembly cleaning is dominated by aqueous cleaning equipment these days; and it is important to look at batch and inline systems each on their own merits.
On the batch side, the dominant design is cabinet washer-type systems. Batch systems of all types tend to have small footprints, modest facility requirements, and are less expensive than the smallest chemical-ready inline systems. They also have a much lower capacity. Typically, there are several models from which to choose, from explosion-proof, pneumatic-solvent systems through fully automatic, PLC-controlled, chemical-ready systems. As you might expect from cleaning data previously analyzed, lead-free cleaning is always challenging, and any system selected will need thermal and mechanical energy to accomplish these goals.
Inline, or conveyorized, cleaning systems are designed for high-volume, continuous operations. Systems offer an array of options: belt width, pump size, nozzle type, wash-zone length, and chemical isolation sections. Facility considerations are important when specifying these systems. Power, water, and ventilation, as well as adequate floor space for operations and routine maintenance, are essential. These increased requirements bring about vast throughput capacity, and the ability for parts to feed untouched off a wave or reflow oven, and automatically into a cleaner designed to match your required speed. Each manufacturer has a series of specially developed designs that have achieved success in the marketplace. The challenge for those designing the process is to choose from several high-performance options. For those implementing lead-free, these powerful systems generally provide a broad process window, allowing flexibility for future process requirements.
Law #5: Test
Thoroughly test the processes you are considering. Do not completely outsource this step to a supplier, but participate in testing. Developing a good Design of Experiment (DOE) program in concert with supplier candidates, both cleaning agent and hardware, is a learning experience - even for a seasoned engineer. Executing a well-thought-out DOE is always instructive. This results in a better-designed process and a better selection process. The DOE design-and-execution phase allows you to evaluate another aspect of your lead-free cleaning supplier, their technical prowess, and field-support acumen. Every complex process, such as a cleaning system, encounters a process upset. When that day occurs, you will want your supplier standing next to you to solve the problem. The DOE is a good time to evaluate their commitment to on-the-ground field support. Test the chemical. Test the machine. Test the machine and chemical combination. Test field-support capabilities of your candidates. Have the joy of an informed decision. It is hard work, but it will make your lead-free implementation that much smoother.
Tom Forsythe, vice president of marketing, Kyzen Corporation, may be contacted at (615) 831-0888; e-mail: tom_forsythe@kyzen.com.