Probing - The concept

Probing is the most direct approach for mass testing of un-encapsulated semiconductor die in wafer form.  Wafers are traditionally received from the manufacturers fab pre-probed to a basic level, the majority of the time this comprises of a DC test at room temperature to guarantee that the parts meet the basic parameters according to the manufacturer’s spec.  However, there are other circumstances where there are important if not vital factors that drive the need for a second level of probing tests.

1.  Some manufacturers will only sample pre-probe the wafer.  Often the customer requires wafers 100% probed to indicate that every die has been tested and passes the manufacturers spec.  This can also depend on the visual inspection standard the product is to be released to.  All Die Technology products are 100% electrically tested and visually inspected to the requirements of MIL-STD-750 and MIL-STD-883 as appropriate.

2. The customer may wish to guarantee that each individual die will perform to the released specification or higher.  This type of probing may be suitable for those with a need for high reliability & yields or special requirements such as probing at high temperatures.

3.  Individual die never perform 100% alike, sometimes the customer may wish to specify a higher electronic performance for the die to be tested at in order to 'cherry pick' the higher performance die.  This type of selection is particularly appropriate for those customers looking for differing grades of end product.

4.  The customer may find that the required product is now obsolete, however an option may still be available to obtain that part through 'Upgrading' of a part deemed to be of a lower performance factor but is derived from the same core family.  Once again the wafer probe cherry picks the higher performing devices by testing them against the required parameters to find the compatible parts.

5.  If a customer wishes to receive product in wafer form they may wish for die which don't pass to their set specification to be marked with an ink dot so that their own equipment can clearly recognise the rejects when the wafer is processed in house.  It is also possible that wafers are supplied from the manufacturer without the rejects being inked.  In these cases rejects are identified by the use of an electronic wafer map.  For customers that do not have capability to use a wafer map, a second level of standard probing is necessary to add the ink dots in.

Probing - The Process

The probing process is very delicate and requires a great degree of expertise and experience.  The wafer is carefully mounted onto a movable plate.  The wafer can then be moved either manually or automatically by the machine in both vertical and horizontal directions.  The actual electronic connection is made via a 'probe card' which is a custom built PCB circuitboard designed to match the bonding pad geometry of each die and connect it to the test equipment. 
The probe card has thin metal probes attached to it which make the connection between the card's circuit and the die bonding pads.  By lowering these probes onto the pads of each die the connection is made and the test program runs to determine the pass / fail status of the die.  After the test completes the probes then lift, if the die is reject it is marked with an ink dot in the centre and the wafer is then moved into position for the next die, by this method each die on the wafer is probed.  The process leaves a mark on each pad and it is this mark which is used as evidence to demonstrate the die have been tested.

Equipment/Capabilities

• Electroglas Model EG2001CX Industry Standard Prober

• 3" to 6" (75mm - 150mm) diameter wafer capability

• Hot Probe facility (25 to 130 deg C) (up to 200°C for limited tests only)

• Off-line inking facility

• Customer probing & upscreening of customers own product