Particle Problems – Process or Machine?

What happens when parts exit the cleaning process dirty?  First of all, PANIC, especially in the case of critical cleaning for optics, semiconductor, disc drive and other industries where particles that can’t even be seen with a microscope render product worthless and unrecoverable!  Then there is the inevitable quest for, “Why this is happening?”  Although this scenario is surprisingly common and often even expected, especially in new installations, there are cases where proven, validated equipment and processes all of a sudden “go South” for no obvious reason.

It is natural that the cleaning machine is usually at the focus of suspicion.  However, the first logical step in diagnosis should be to first eliminate other potential problem sources that may be easy find and remedy.  Things to look for include a process change prior to cleaning, introduction of a new product or contaminant, new equipment or equipment modifications, repairs or upgrades, a change in material handling or flow, etc.

Once the problem is isolated to the cleaning machine, the first and arguably illogical step is to determine if the problem is that the cleaning machine just isn’t cleaning the parts or if it is the cleaning machine that is making them dirty.  A simple way to make this determination is to introduce some known clean parts into the cleaning machine and see if they exit the machine as clean as when they went in or if they become “dirty” in the cleaning process.  If the parts are as clean after cleaning as they were before, there is one set of things to look at.  Another path is indicated if the parts are dirtier after cleaning.

If the “clean” parts exit the cleaning process “clean,” it means that there is a process problem (Time, Temperature, Chemistry, or Agitation).

  • Temperature too high or too low
  • Process times too short
  • Improper chemical or concentration
  • Ultrasonics, agitation, or flow or spray insufficiency

Although any of the above could be a result of equipment malfunction, they are still process issues.

However, if the “clean” parts exit cleaning “dirty,” it means that there is another kind of equipment problem.  At this point, if the means are available to identify the contaminant, this may provide helpful clues.  Although with very small particles it may not be possible to quickly get an exhaustive analysis, even generalities like “glass,” “hydrocarbon,” “metallic,” etc. may provide useful direction.  Knowing what the contaminant is may lead directly to the source.  For example, metallic particles might indicate that there is a worn valve or pump shaft that is creating metallic particles.  Rubber might indicate a disintegrating “O” ring.  But even if the contaminant can not be identified, it is possible to pursue a probable source by isolating the various parts of the overall cleaning process.

The best methodology is to work backward through the system if possible by first introducing “clean” parts into the final stage of the process which might be rinsing or drying and eliminating each stage in succession.  If, for example parts introduced at the second rinse come out clean but parts introduced in the first rinse come out dirty, the target stage is the first rinse or earlier.  The problem could be an improperly seated filter, worn seals on a valve or pump, or perhaps a case of something unexpected being introduced in to the tank.  There might also be a buildup of particles or other contaminant in the first rinse due to an inoperative pump, clogged filter, unintentionally closed valve or other machine issue.  In any event, you know where to focus the attention.

The biggest mistake in diagnosing cleaning problems is to change a number of “suspect” variables at once!  If you “shotgun” by changing chemical concentration, temperature and ultrasonic power all at once, even if the exercise is successful, you don’t know which action gave the desired result.  If the effort is not successful, there is no way to know that one change didn’t negate the other.  Even in panic mode, it is worth the effort to work methodically approach the problem to help prevent its re-occurrence in the future.

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