Optimizing Ultrasonic Cleaning for Disk Drive Components

C leanliness of parts and assemblies has always been an important factor for product yield in the disk drive industry. The challenge is that most disk drive parts are manufactured by conventional material processing methods, such as machining. stamping. and molding. Ultimately, the manufacturing process normally requiring the use of oils, coolants, and abrasive tools becomes the primary source of the part contamination, since manufacturing cannot be implemented in a cleanroom environment. The contamination from the parts may eventually infiltrate disk drives unless an appropriate cleaning process and component cleanliness inspection are implemented. Usually heavy cleaning, such as chemical etching, solvent cleaning, or degreasing, is done at parts suppliers. Final cleaning is normally done before parts are about to enter a disk drive assembly line. In efforts to reduce use of hazardous chemicals, aqueous cleaning has become the most desirable method for the final cleaning process. The main purpose of final parts cleaning is to remove loose particles as well as some ionic and organic contamination that could lx introduced during parts storage, shipping, and handling. Because cleanliness of parts assembled into a disk drive very much depends on the efficiency of the final wash process, it is critical that this process be well optimized. In the past few years, cleaning processes used in the disk drive industry have undergone significant changes, most due to elimination of CFCs and the shift to aqueous cleaning. Aqueous cleaners depend mainly on mechanical means of particle removal, such an ultrasonic agitation, and the use of different surfactants and water heating to enhance the cleaning action. However, it is not fully understood what variables in an aqueous cleaning process affect parts cleanliness the most. In general, the ultimate goal of cleaning is to achieve a better part cleanliness level. This, of course, depends on the method used to measure part cleanliness. Ultrasonic extraction is one of the methods currently used by IBM System Storage Division (SSD) to assess the cleanliness level of disk drive parts and assemblies. This method utilizes ultrasonic energy to extract particles from a part and puts them into aqueous suspension. Then, a liquid particle counter (LPC) is used to measure the particle concentration in the extraction solution. 1,2 There are two advantages of the ultrasonic extraction/LPC method: It can be used to measure cleanliness of irregular geometry parts. offering a way to assess the level to which the part can be cleaned with a given ultrasonic field. The second BY ROMAN GOUK