WaferSawing.doc

Wafer Sawing
There are several forms which the customer may prefer to receive their end product, for example, waffle pack, gel-pak, wafer sawn on ring-frame etc. For each case in order to convert the wafer into a useable form it must be sawn. Due to the delicate nature of the raw material, consideration must be given to several factors which effect the overall yield and quality of the separated die. Factors such as blade speed, cutting patterns, or even the electrical charge carried by the filtering water used can have a significant impact on the output of the sawn dice. There is also a great deal of both skill and experience associated with wafer sawing as products can vary significantly in both their size and composition material. E.g. A silicon saw process would be very different from a Galium Arsenide process.

Wafer Sawing - The Concept

Wafer Saw - The Process
Before sawing can take place the wafers must be mounted onto a ringframe. This is achieved by use of a mounting chuck which stretches a specialized foil across the backside of the wafer. The foil is roll-pressed to ensure even coverage across the wafer's backside surface and the excess foil is then removed. An important factor to consider at this stage would be the die size in relation to the resistivity factor of the foil. Once the wafer is suitably attached to the ring frame it is ready for sawing.

The wafer on ringframe is then secured on the moving saw chuck where it's position is then sensed and calibrated by the saw's internal configuration program. The configuration program automatically presets the saw's X and Y co-ordinates to follow the wafer streets (empty tracks between each die). This automated handling of the wafer enhances accuracy and reduces the likelihood of human error. After the wafer position is calibrated the actual cutting occurs by moving the chuck into contact with a rapidly spinning diamond stud blade which is mounted on a spindle above the wafer. The saw moves and rotates the chuck in accordance with its program until all streets have been cut. The wafer on foil is then removed, washed in a specialised solution and then dried via infra-red in order to remove remnants of raw material dust from the saw process.

Foil and Tape - The Process
Most die that are shipped on tape only have a short shelf life (average three weeks) before the tape deteriorates. The tape will start to lose adhesion allowing die to fall off in transit or at the pick and place operation, in addition with longer life the adhesive strength will increase on the tape making the die difficult to remove and will contribute to back surface rejects. At Ditech we use a tape that if stored correctly at room temperature will release the die at a consistent force for a period of 10 years. This allows long term storage, eliminates damage on removal and speeds the pick and place rate.

Wafer Saw - Equipment/Capabilities
< Our precision saw allows us to saw completely through wafer on film
< Wafers are cleaned by nitrogen/de-ionised water rinse wafer washer
< We can saw wafers up to 8"/200mm in size
< Automatic handling
＜Single or part wafer flexibility
＜Sawing of customers own product

Pick and Place
Pick and Place - The Concept
Careful die handling is paramount to maintain overall quality and good die yields from the wafer once it has been sawn. Due to the delicate nature of the product (particularly on the active side of the die), using a Pick and Place machine allows us to apply the absolute minimum contact and relative force required to remove die from the foil ring frame after saw. Although there are circumstances where a degree of manual removal is necessary it is always preferable to opt for an automated process by using a Pick and Place machine as the possibility mechanical damage to the die is greatly reduced. Pick &; Place automation is also necessary to deal with high volumes and enables us to offer fast output turn-arounds which are simply not possible otherwise. A good example to use would be a small transistor, even our most dextrous and experienced clean room operative could not hope to manually remove 1,750 die in an hour! Lastly, the use of an automatic Pick and place machine also enables us to guarantee 100% die orientation, this ensures complete compatibility with end customer equipment where once again high-speed production is required.

Pick and Place - The Process
The sawn wafer remains on the ring frame and is fixed to a plate in the centre of the Pick and Place machine. The foil supporting the sawn wafer is now stretched to increase the space between each die. The operator manually adjusts the X & Y co-ordinates to centre on a die by viewing it on a magnified screen. The actual die are removed from the foil through the use of single or multiple ejector pins which push up from under the foil and release the die by applying pressure to the backside. To collect the raised die, a vacuum powered mechanical arm sweeps across, picks up the die and releases it into the appropriate carrier. Although the Pick and Place machine requires expert calibration to set-up, once this is done an operator can systematically remove the die one by one from the foil and deposit them into their chosen output carrier very quickly.

Pick and Place - Capabilities
2x DE35i Manual Pick and Place
1x System A45 Automatic die handling pick and place system
Capable of traying 5000 bare die per hour
Offers improved die handling - enhance die yield - ensure die reliability
Delicate die handling
Up to 8" (200mm) wafer diameter capability
Picks die from 0.008" to over 0.6" sq.
Safe handling of 'bumped' die for 'flip-chip' applications
Die can be sorted into waffle or vacuum-release (Gelpack) trays
Automatic die orientation
Fast turn-around for volume orders