Die Preparation

   

Die preparation is the process by which the wafer is singulated into individual dice in preparation for assembly.   Die preparation consists of two major steps, namely, wafer mounting and wafer saw.

   

Wafer mounting is the process of providing support to the wafer to facilitate the processing of the wafer from Wafer Saw through Die Attach. During wafer mounting, the wafer and a wafer frame are simultaneously attached on a wafer or dicing tape. The wafer frame may be made of plastic or metal, but it should be resistant to warping, bending, corrosion, and heat.  The dicing tape (also referred to as a wafer film) is just a PVC sheet with synthetic adhesive on one side to hold both the wafer frame and the wafer. Typically measuring 3 mils thick, it should be flexible yet tough and strong, and with low impurity levels as well.



Using a special wafer mounting machine, wafer mounting consists of the following steps:  1) frame loading; 2) wafer loading; 3) application of tape to the wafer and wafer frame; 4) cutting of the excess tape; and 5) unloading of the mounted wafer.

                           

Fig. 1.  Photos of wafers mounted on wafer frames

Fig. 2.  Wafer films for mounting wafers on wafer frames

        

During wafer mounting, the following concerns must be prevented:  wafer cracking or breakage, bubble trapping on the adhesive side of the tape, scratches on the active side of the wafer, and non-uniform tape tension which can result in tape wrinkles.

   

Wafer saw follows wafer mounting and is the step that actually cuts the wafer into individual dice for assembly in IC packages.  The wafer saw process consists of the following steps:  1) the frame-mounted wafer is automatically aligned into position for cutting; 2) the wafer is then cut thru its thickness according to the programmed die dimensions using a resin-bonded diamond wheel (see Fig. 3) rotating at a very high rpm; and 3) the wafer goes through a cleaning process using high pressure DI water sprayed on the rotating workpiece and then dried by air-blowing.

 

Fig. 3.  Photos of Wafer Saw Blades



Kerf width is defined as the average width of the cutline plus the error attributed to microchipping. The singulated dice may be left attached to the wafer tape for pick-and-place during die attach or may be individually kept in waffle packs for future assembly.

          

Important parameters for consideration during wafer saw include the following:  cut mode (direction and manner of cutting), feed speed (speed at which the wafer is being introduced to the blade), spindle rev (speed of revolution of the cutting wheel), blade height, and cutting water flow.  Important parameters for the washing step include the following: wash time, wash rpm, DI water pressure, dry time, dry rpm, temperature and air flow rate.

   

   

Front-End Assembly Links:  Wafer Backgrind;  Die Preparation;  Die Attach;  Wirebonding;  Die Overcoat

Back-End Assembly Links:  Molding;  Sealing;  Marking;  DTFS;  Leadfinish          

See Also:  Die Preparation Failure Mechanisms;  IC Manufacturing;  Assembly Equipment

      

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