What Is Clean Enough?

Suppliers of cleaning chemistry or cleaning equipment may claim their product “successfully cleans” heavy greases, particles, fluxes, and other soils. Private companies, military, and governmental agencies report successful cleaning in selecting a particular cleaning agent. What does “successful cleaning” actually mean for your manufacturing process? Success is a matter of perspective. “Clean enough” depends on the level of residue  that can be tolerated in the next step of the process. “Clean enough” depends on the hazards to product performance posed by residue left on the final product.  Here are four examples including cleaning electronics assemblies, LOX cleaning, cleaning to prepare surfaces for thermal spray, and cleaning medical devices during

Electronics assemblies
How clean do electronics assemblies need to be? Traditionally, the primary residue concern has been and continues to be with ionics.  If the assemblies “pass” SIR testing, all may be well. What does “pass” mean? We hear comments to the effect that everything passes ionics, but performance may not be adequate. While an SIR below 10 Megaohms may be adequate for some products, other more exacting applications may require a much higher resistivity.   In addition, high reliability products containing electronics assemblies may require controlling organic as well as inorganic contamination. 

There are many examples where passing the basic test is not enough. “Passing” the first grade may not be good enough, if you need the equivalent of a post-doctoral associate. Examples include miniature electronic assemblies, densely populated assemblies, assemblies with low standoff,  assemblies that will be sealed, products where poor performance can be catastrophic (medical, military applications),  and products that are expected to last for a long time.

In liquid oxygen (LOX) environments, particularly at later stages in the build process, the choice of cleaning agent is of necessity restricted to those that pass LOX testing; any residue on the surface of the product must not contribute to the potential for fire/explosion. If the surface of the component or product is already fairly clean, products used in LOX applications may not need to be particularly aggressive.  For LOX assemblies associated with breathing apparatus, the choice of cleaning agent and control of the cleaning process may take on additional dimensions – additional control of residue may be required.  Depending on the soils involved, remember that the fact that a cleaning agent has passed LOX testing  does not guarantee that it will adequately remove soils of concern. For this reason, cleaning and drying processes for high-value applications are often multi-step; and it is important to document those processes very carefully.

Engineered coatings
Consider what soils can be tolerated and what must be scrupulously removed.  In contrast with electronics assemblies, in thermal spray and other engineered coatings, organics wreak havoc with coating adherence. However, a totally “degreased” fingerprint containing ionic contaminants may be acceptable.  A cleaning agent that has passed  LOX testing might not be aggressive enough against oils and greases to  remove soils associated with parts to be coated.

Medical devices
Can you assume that a high level of soils can be tolerated? Maybe not.  In medical device manufacture and re-use, there used to be a pervasive, albeit erroneous, assumption that terminal sterilization would resolve any problems with residual soils.  Allow me to repeat an analogy I occasionally use.  Let’s suppose you place a batch of medical devices into an autoclave and run the validated, recommended sterilization procedure. You open the door to the autoclave; and  – lo and behold – resting in peace atop those devices is a mouse. Well, the devices may be sterilized; and the mouse may be sterilized. But you’d do well to pick that little mouse up by its tail, remove it from the devices, reclean the devices and then resterilize them! (You’d better also review housekeeping protocol, perhaps cleanroom protocol, and facilities management practices).

You have to clean to have successful sterilization.  Soil can interfere with sterilization; soil can interfere with device performance and compromise patient safety. People are beginning to realize the importance of critical cleaning both for device manufacture and for cleaning reusable devices. Implantable devices have become smaller and smaller and the expected lifetime of those devices (not to mention the device recipients) has increased.  Reusable medical devices are complex; and hospital staff may find complete disassembly, cleaning to specified instructions, and reassembly difficult, particularly in stressful situations..

How clean is clean enough? It’s a fine line. Too much cleaning has economic consequences; and over-cleaning can damage the product. Inadequate cleaning can result in catastrophic product failure. Assuring that the product is clean enough means taking a dispassionate look at the product and at the end-use requirements. Has the product changed? Have performance expectations changed? Are there new instructions, standards, specifications, and guidance documents?  Are current protocols adequate? Do we need to set up new cleaning protocols? Assuring that the product is clean enough is a matter of rational perspective; it also means taking responsibility for your product and manufacturing and re-cleaning process.

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