Engineer in a Class 100 Clean Lab
Explained simply, a clean room is a highly purified and controlled room to avoid contamination during work process. By technical definition, a Class 100 clean room has no more than 100 particles larger than 0.5 microns in a cubic foot of air. Basically, a Class 100 clean room is required to ensure that the surrounding air is of acceptable quality so the disk could be opened up to perform the necessary repair work. To be able to at least open up your crashed disk for further servicing, it is sufficient and necessary to ensure that the vendor has a Class 100 clean room. Otherwise the contaminants may stray and stick on the exposed platters creating further random crashes with the read write head during spinning.
Thus, in order to ensure that the vendor could open up your hard disk for servicing, you have to ensure that he has the necessary Class 100 clean room environment. Interestingly, even if the answer from the vendor is "yes", you will most likely be told that the Class 100 clean room is located some where and not accessible to the public. You may at least ask for some video or photographic shots of the Class 100 clean room with the engineers at work. No pictures? No problem, why not pass your hand-phone or digital cameral for him to take one simple shot? Such proof is so easy to come by and often overlooked. If they could not even produce such a simple proof, you simply need to think twice.
On the other hand, one should not over-emphasize on just "air-cleanliness" alone. Class 100 clean room is just the basic and appropriate environment to open up and service a damage disk. It should not be mistaken by many as the only "solution" to data recovery as the success of recovery still depends on a great number of other factors, such as the experience of engineers, the availability of parts, the technical know-how etc.
You may like to know that a human on average disperses 1 million particles equal to or greater than 0.5 microns every minute! As human is the main source of contaminant, it is impossible or simply not feasible to get better air quality such as (Class 10 or Class 1) if a human is to be allowed to work within. For hard disk work, Class 100 has been the acceptable standard in manufacturing or repair work. Typically Class 10 or cleaner air quality is used for semiconductor manufacturing where the slightest amount of contamination will cause product failure. Here, the workers have to wear complete suits with sealed face masks. Even breathing is a main source of contaminants and must be done through a respirator. In even more stringent conditions, probably only robots could stay and work within the environment.