Breaking down cleaning processes into basic building blocks reveals the optimal choice.

Post-reflow residues of SnPb and newer Pb-free soldering materials are more difficult to remove due to increases in component density, larger component packages, higher lead counts, finer lead spacing and lower stand off distances. While modern aqueous alkaline cleaning agents effectively remove these flux residues, achieving satisfactory cleanliness depends on the interplay of temperature, exposure time, chemical concentration and mechanical energy. At first glance, the path toward a perfect cleaning process may appear elusive and complex, if the process with all the associated variables is considered. As a chemist, I would equate the challenge of setting up a proper cleaning process to that of synthesizing a complex molecule. In fact, one approach to organic synthesis known as retrosynthetic analysis, or “the disconnection approach,” is also useful as a strategy for selecting the perfect cleaning process.

A disconnection approach is simply a way of breaking down a very complex target to make the best path to success more apparent. With this approach, a chemist will start with the structure of their target molecule and progressively go through the mental exercise of cutting bonds to create simpler molecules. The result is the complex molecule is reduced to individual building blocks where each can be judged as a viable or non-viable option by asking common sense questions: Is the building block readily available? Is it expensive? Is it safe to use? Once a viable list of building blocks is obtained, simply reversing this process gives a synthetic route to the desired target molecule from simpler and commercially available starting materials.

What does this have to do with selecting a cleaning process? More than you think. Like a chemist who faces the task of synthesizing a complex molecule, engineers developing a cost-effective, long-term cleaning process often feel overwhelmed. Naturally, they rely heavily on equipment and chemistry vendors to offer advice and support. Yet unless the customer fully understands the building blocks that make up the total cleaning process, as well as how each one can impact the process window, the result may be far from optimal, and expensive.

Before beginning any evaluation, it is important to have well-defined requirements or a “wish list” of what you would like to achieve. In other words, there are many factors to consider and rank, such as material compatibility, cost, consumption, throughput, etc. Some customers may need high belt speeds to handle high volumes of parts, thus avoiding bottlenecks in production. Others may place more value on low operating temperatures, which save energy and reduce evaporative losses, etc. The cleaning chemistry and cleaning equipment have to be considered in parallel, not selected sequentially. For example, selecting a cleaning chemistry that is not appropriate for use with high-pressure sprays can result in a shop full of bubbles.

Using a disconnection approach, one can break down cleaning processes into basic building blocks so that each can be tested against customer requirements. Basically, cleaning involves chemistry (or water), force, temperature and time, and each of these variables influences cleaning performance or material compatibility. Consequently, it is important to design your evaluation in such a way to optimize these factors to achieve your predefined goal. Communicate your objectives with each supplier openly and in advance so that they can share with you how they can contribute. For example, if your primary goal is to achieve the fastest belt speed, this will require achieving the desired cleaning results within short periods of exposure time. In the case of spray-in-air processes, contamination is partially dissolved or emulsified by the cleaning agent and partially washed away by the kinetic energy of the spray jet. Since chemical energy provided by the cleaning agent and the mechanical energy of the equipment will have significant impact on exposure time, it is important to design your experiments to test each factor independently.

The key factor for long-term success is to never recombine the building blocks during the evaluation until the impact of each important factor has been truly differentiated. For example, one should never allow vendors to change nozzles while doing a chemistry evaluation. Any change in nozzles would result in significant changes in mechanical energy, which will overshadow or negate otherwise significant differences in chemistry. One chemistry can be made to outperform another this way. Once the best chemistry is selected by keeping all parameters constant, explore improvements in mechanical energy until the best equipment is selected, etc. The same can be said of changing chemistry while testing different equipment configurations. This seems logical, but you would be surprised how often these mistakes occur.

In summary, you do not have to be an expert in solvency or in equipment design. However, you do need to understand the cleanliness and surface quality requirements of your own product line in the context of your process. You should also disconnect the process to take a good look at the impact of each building block. It is unwise to simply adopt a suggested strategy without question. Make sure the cleaning chemistry is truly optimal for your process. Likewise, independently confirm that the cleaning equipment is best suited to your product line.

Harald Wack, Ph.D., is president of Zestron (zestron.com); h.wack@zestronusa.com.

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