Tester Loadboards
A loadboard or performance-board is a mechanical and electrical interface that
connects the test system’s test head to the device under test(DUT). The
loadboard often mates to a wafer prober, device handler or other test
hardware. The loadboard routes electrical signals from the pin electronics
cards to the device under test.

During wafer test, the loadboard interfaces with the probe card. When testing
devices by hand, the test socket is mounted on the loadboard. For high volume
production testing, the loadboard interfaces with a device handler. Because
there are many different types of equipment that the tester must mechanically
and electrically mate with, there are a wide variety of loadboards.

When testing high speed or high current devices, a custom loadboard is
required. These high performance custom circuit boards must be impedance
matched to insure signal integrity; they often have matched Line lengths
(signal paths) to insure timing integrity. Custom Ioadboards can be very
expensive and often take months to design and manufacture.

Wafer Probing
During Wafer Test, the test head is mated directly to the wafer prober. The
test head is flipped upside-down and connects with a prober interface assembly
which contains the probe card (this type of testing is often called flip head
sort). A single wafer is placed onto the chuck and the wafer is aligned with
the probe card to insure a perfect fit. The probe card is then used to make
contact with each die on a wafer. The probe card is not visible in Figure
2-12, but it is located in the center of the prober (near the chuck).

In the early years, wafers were loaded one at a time onto the chuck, and the
alignment of the wafer was performed manually by a skilled probe operator. The
probing area was open, as seen in Figure 2-12, allowing both light and
contamination to reach the wafers. We now know that some test parameters can
be affected by light, which may cause a device to fail. in recent years,
contamination has become a larger concern due to the smaller geometry of the
components on a die. Contamination can cause parameters to drift, devices to
fail and in general, shorten the life of a device.

Wafer probers are now designed so that light and contamination can no longer
reach the wafers. The wafers are fully enclosed and protected the during the
testing process.

When the wafers leave the fabrication area they are placed in a protective
container called a boat. A boat typically holds about 25 wafers. During the
testing process the boat of wafers is placed inside the prober and all wafer
handling and alignment functions are performed automatically by the prober.

Modern probers can ink defective dice, but many companies prefer to create
wafer maps that store the locations of good and bad dice electronically. These
maps are then used during the assembly process to separate the good dice from
the defective dice. Electronic wafer mapping eliminates problems associated
with prober inking attachments.

Modern probers are highly reliable and require very little attention. They
have greatly eliminated the need for skilled probe operators.

Probe Cards
Probe cards connect the test head electronics to the individual pads of a die.
In Figure 2-14, the probe card is physically part of the load board. In some
cases the probe card attaches to the loadboard via a socket.
The probe card mechanically attaches to the test head.
Tester resources are routed through springy pogo pins that make electrical
connection to the bottom of the probe card. Signals then travel through traces
within the circuit board to the individual probes.

Device Handlers
Just as there are a wide variety of semiconductor package types, there are as
many different styles of handlers to accommodate them. Figure 2-16 illustrates
a handler which tests DIP packages (dual in-line package).

After device assembly is complete, packaged devices are placed in tubes or
trays so that they can be taken to the test area. During production testing,
untested devices are loaded into the handler storage bin. The handler then
selects an untested device, moves it to the contact area, which interfaces to
the test head, and inserts the device in the contactors. On a signal from the
handler that all is ready, the test system performs the required tests and
makes a pass/fail decision.

Information is sent from the test system to the handler regarding the proper
bin assignment and the device is moved and stored in the appropriate bin
location. Bins are used to group devices together based on their test results.
A device handler must have a minimum of three bins: an untested bin, a good
device bin and a reject bin. Other bins are also useful—for instance, it is
convenient to separate rejects into categories such as: DC rejects, AC rejects
and Functional rejects. There may also be various grades of good devices based
on a parameter such as operating speed, e.g. 100MHZ, 166MHZ and 200MHZ
devices. The maximum number of bins a handler supports is limited by its
physical size.

Many handlers also offer environmental chambers for testing devices at
temperatures different from room temperature.

Temperature Forcing Units
Some devices must be tested at various temperatures. For example, a device
specifications may state that the circuit must perform properly between 55°C
and +125° C.

In the past, small hand held thermal probes were used to heat devices to 70 °C
or 125° C. This method was time consuming and troublesome because the handle
of the thermal probe would become excessively hot and it was often difficult
to make good thermal contact with the device under test. Hand held probes also
do not accurately control device temperature.

For low temperature testing, dry ice or liquid nitrogen was often used to cool
a device. Using dry ice is not an accurate method of cooling a device because
the temperature of dry ice is approximately -60° C and device specifications
typically call for -55° C or 0° C. Another problem with dry ice was simply
having it available when it was needed.

Temperature forcing systems are generally the best method of heating or
cooling devices when testing in a hand socket. Temperature forcing systems are
programmable, accurate and fully self contained, requiring only external
electrical power.

这里的内容转载于：

The Fundamentals Of Digital Semiconductor Testing（Soft Test）