Wafer Probing
Wafer 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.
- 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 Mintech products are 100%
electrically tested and visually inspected to the requirements of MIL-STD-750
and MIL-STD-883 as appropriate.
- 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.
- 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.
- 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.
- 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
- We have 2 wafer probers
- 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
- Multimap wafer mapping probing and traying system