A 'disconnection approach' to cleaning process selection: breaking down cleaning processes into basic building blocks reveals the optimal choice.

Author:Wack, Harald

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...

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