1. A Novel Method to Identify Faulty Socket for Semiconductor
Packaged Device Testing Using Contact Impedance Tester
Abdul Azeem bin Mohd Mohideen
Intern (PE-IP)
Altera Corporation (M) Sdn. Bhd.
Penang, Malaysia.
+6012-4252786
azeem040@gmail.com
Ker Harn Lim
Section Head (PE-IP)
Altera Corporation (M) Sdn. Bhd.
Penang, Malaysia.
+6017-5500338
khlim@altera.com
Abstract - A contactor socket is an essential tool for
semiconductor packaged device testing. Over time after
long hours of usage and repetitive insertions, the socket
will be defective and socket pins will need to be
replaced. Currently, there are several methods
available in the market yet some better and effective
method to identify faulty socket pins required to
determine the end-life of a contactor socket pins earlier
and save numerous hour spent for the hardware
debugging process in performing a test. The most
recent method used in semiconductor industry is by
measuring the contact resistance unfortunately the
presence of parasitic components such as inductance and
capacitance on the contactor pin could not be measured
effectively. Hence, semiconductor industries still
searching for the best method to solve the faulty
contactor pin problem diligently. In this paper, a new
method is proposed where a quantitative indicator,
contact impedance is introduced.
Keywords - Contactor, Impedance, Pin, Resistance,
Socket, Tester
I. INTRODUCTION
In semiconductor testing, contactor is one of the test
accessories that make the actual physical contact with the
device under test (DUT) to create the necessary electrical
connection between the ATE and the DUT. The contactor
assembly is generally a part of the test handler. Thus,
before production testing can begin, the test handler must
first be fitted with the test contactor assembly or contactor
block suitable for the device to be tested. [1]
A contactor has a set of contact elements that are
usually in the form of metal fingers (also known as 'contact
fingers') or spring-loaded pins as shown in Figure 1. These
contact elements are the ones that come into contact with
the leads or solder balls of the DUT during electrical
testing. The crown on top of the contactor is the part where
it come into contact with Ball Grid Array (BGA), where
else the bottom part of the spring probe known as pin and it
create contact to the Printed Circuit Board (PCB). Contact
elements are commonly composed of a beryllium-
copper base metal with gold-plating on the surface. [2]
The proper selection of contactors for electrical testing
has a great impact on test yields, device grading,
repeatability and reproducibility of testing, and
productivity. However, over time after long hours of usage,
exposure to changing temperatures and repetitive
insertions, the socket will be defective and socket pins will
need to be replaced. A defective contactor socket with
faulty pins can eventually lead to contact problems that
cause invalid failures or test miscorrelations, which in turn
can result in product verification downtimes, unexplained
yield problems, and even customer’s dissatisfaction. [5]
To overcome this problem, the most common method
used in the industry is by inspecting the pin condition using
naked eye or microscope and it is up to the person’s
discretion to determine the pin is faulty or still fit to be
used. The next common method used is to leverage ATE
and perform an open/short test. Through this method,
faulty pins can be localized at the exact location easily, but
it is still qualitatively represented, either good or bad pin.
Almost end-of-life pin will not be detected. In this paper, a
new method is proposed where a quantitative indicator,
contact impedance measurement is introduced.
Figure 1: Contactor Pin (Spring Probe) Architecture
This paper is organized as follows. The disadvantage of
existing faulty socket pin identification methods are
described in Section 2. The proposed contact impedance
testing methodology discussed in Section 3. The
conceptual tester design is presented in Section 4 and the
conclusion is given in Section 5.

2. II. FAULTY SOCKET PIN IDENTIFICATION
METHODS
A. Visual Inspection
Visual inspection is a common method used for quality
control, data acquisition, and data analysis and majorly
used in maintenance of facilities, mean inspection of
equipment and structures using either or all of human
senses such as vision, hearing, touches and smell. Typically
visual inspection means inspection using raw human senses
and/or any non-specialized inspection equipment such
microscope, magnifying glass and etc. [3]
Normally the contactor pins visually inspected to identify
the physical damage on contactor socket pins by using bare
naked eyes or with the aid of microscope. Figure 2
describes how visual inspection conducted using
microscope; where contactor pin with label 1 represent a
good contactor pin where else contactor pin with label 2
represents a deformed or defective contactor pin
Unfortunately this inspection method is limited to identify
the mechanical damage of socket pins but unable to
indicate the actual number of faulty pins and the faulty
pin’s location on the contactor.
Figure 2: Visual inspection using microscope
B. Open/Short Circuit Test
Usually open/short circuit test conducted to detect open
or shorted device pins and to verify proper connections
between the test system and the DUT. This method is also
known as continuity or contact test and it helps to
determine whether a device has shorted pins, missing bond
wires, a pin damaged from static electricity, a
manufacturing defects. [3] This method only evaluates the
device but it does not evaluate the contactor pins
separately. This method produce DC measurement and
measured value indicates whether the failure was caused by
a shorted condition or open condition inside the device
using Red and Green colour indicators on ATE’s graphical
user interface (GUI). Red indicates a failing contact and
green indicate a good contact.
Sometime the failure might be caused by a defective
contactor pins instead of the device itself. The only way to
verify whether the failure is related with contactor or the
device is by repeat the test process with same contactor but
with another working device. [5] If the failure reproduced
identically at the same pin even with different working
device, it verifies the failure is related to contactor pins.
The Figure 3 shows a custom made DUT card used for
performing open/short circuit test on each pin of FPGAs
contact by routing it to the edge of the DUT card and
connects it to an ATE. This method is time consuming and
the result also qualitative and not accurate.
Figure 3: DUT card for open/short circuit testing
C. Contact Resistance Tester
Currently, contact resistance tester has been used in
semiconductor industry to expedite root cause analysis in
test setup, to enable efficient trouble-shooting by
identifying the locations of open or high resistance contacts
within the contactor socket pin array. Besides that, this
method also helps to verify the contactor pin’s electrical
integrity for preventive maintenance. Unfortunately, the
contact resistance testing method does not consider the
presence of parasitic components such as inductance and
capacitance on the contactor pin. These parasitic
components will affect the integrity of the AC signal in
characterization process of a device.
To improvise the existing contact resistance method, this
paper is introducing the contact impedance method which
identifies the faulty contactor socket pin by evaluating the
impedance of the contactor pin qualitatively and verify the
electrical integrity of pins in term of AC and DC, with
considering the presence of parasitic according to the

3. Equation (1) explains the relationship between voltage,
current and impedance and the components of impedance
verified in Equation (2), where else Equation (3) explains
the concept of contact resistance by assuming the
reactance, X is absence or neglected on the contactor pin.
V = I Z (1)
V is voltage,
I is current,
Z is impedance.
Z = R + jX (2)
R is resistance,
X is reactance.
When assume jX = 0
Z = R (3)
Basically the Figure 4 explains the methodology used for
developing the contact resistance tester by integrating a
Digital Multi Meter (DMM) to the contactor pin probe
card. The resistance value is measured by using the Ohm’s
Law according to Equation (4). [5][6] Where sense voltage
(Vsense) divided by force current will provide the contact
resistance.
If substitute Equation (3) into Equation (1)
V = I R
R = V sense / I force (4)
Figure 4: Contact resistance tester methodology
III. PROPOSED IMPEDANCE TESTING
METHOD
A. Impedance
Impedance is defined as the frequency domain ratio of
the voltage to the current. In general, impedance will be a
complex number, with the same units as resistance, for
which the SI unit is the ohm (Ω). For a sinusoidal current
or voltage input, the polar form of the complex impedance
relates the amplitude and phase of the voltage and current.
The two impeding mechanisms to be taken into account
in AC circuits: the induction of voltages in conductors self-
induced by the magnetic fields of currents (inductance),
and the electrostatic storage of charge induced by voltages
between conductors (capacitance). The impedance caused
by these two effects is collectively referred to
as reactance and forms the imaginary part of complex
impedance whereas resistance forms the real part.[2][6]
The symbol for impedance is usually and it may be
represented by writing its magnitude and phase as shown in
the form .
B. Impedance Tester Experiment Setup
Figure 5 shows the contactor pin test setup to measure
the contact impedance of each pin using LCR meter and
PC. The measured data by LCR meter will be saved into
PC via USB and the test result displayed using specific
software for identifying the faulty contactor pin.
Figure 5: Contactor pin impedance test setup
C. Proposed Test Procedure
 Setup test equipments according to Figure 5.
 LCR meter and laptop connected using USB
cable.
 Kelvin clip probe connects each contactor pin
designated for testing to the LCR meter.
 Test equipments turned “ON” and the LCR meter
interfacing software launched in PC.
 Each contactor pin tested using Kelvin clip probe
to measure its impedance.

4.  Measured impedance values recorded into PC and
analyzed by specific software.
 The test result for each contactor pins displayed
qualitatively and quantitatively through specific
software on PC.
D. Impedance Measurement Methodology
Kelvin sensing approach proposed for this impedance
testing method as shown in Figure 6. Kelvin sensing is
also known as Four-terminal sensing, an approach
discovered by Lord Kelvin, who invented the Kelvin
Bridge in 1861 to measure very low resistances. Each
two-wire connection can be called a Kelvin connection. A
pair of contacts that is designed to connect a force-and-
sense pair to a single terminal or lead simultaneously is
called a Kelvin contact.
Four-terminal sensing (4T sensing), 4-wire sensing,
or 4-point probes method is an electrical measuring
technique that uses separate pairs of current-carrying
and voltage-sensing electrodes to make more accurate
measurements than traditional two-terminal (2T) sensing.
The key advantage of four-terminal sensing is that the
separation of current and voltage electrodes eliminates the
impedance contribution of the wiring and contact
resistances but increases the conductance. [7]
Figure 6: Kelvin probe architecture
IV. CONCEPTUAL TESTER DESIGN
A. Hardware Design
The automated impedance tester designed by integrating
impedance measurement features to the contactor pin test
fixture or electronic architecture as shown above in Figure
7. The automated impedance tester could able to perform
test on all the contactor pin in few minutes automatically
and display the test results on the PC.
The tester main board require a multi-layer PCB to test
each pin of contactor using 4 trace, where 2 trace will
provide constant 25mA current supply using (Force +)
and (Force -) where else another 2 trace used for
(Sense+) and (Sense-) to measure the voltage drop across
each pair of contactor pin and let the microprocessor to
compute the impedance by dividing the sense voltage with
force current according to the Equation 5.
Z = V sense / I force (5)
The tester is proposed to design using Motorola
MC68020 32-bit Embedded Microprocessor, which
enables the tester to measure the impedance of every each
contactor pins simultaneously using preprogrammed
algorithm. The microprocessor also automates the test
process and sends the impedance measurement to the host
PC via USB cable as shown in Figure 7.
Figure 7: Impedance measurement processor circuit

5. The Daisy-Chain device is also recognized as shorting
device which made out of gold used to short all the
contactor pins to enable the microprocessor to test each
pair of pins in sequence as shown in Figure 8. Besides that
the tester will be using 4-Wire Reference Point for each
pin of the socket to implement the Kelvin sensing
approach for better impedance measurement.
Figure 8: Daisy-Chain configuration circuit for impedance measurement
The tester’s electronics design in Figure 7 and Figure 8
able to successfully measure the impedance and send the
impedance measurement result to PC and the software
plays an important role to organize and display the test
result qualitatively and quantitatively using specific
software as shown in Figure 9. The software design will
be discussed in details on Section 4.2.
Figure 9: Complete Impedance Tester setup
B. Software Design
The test result displayed on PC using software designed
specifically to represent the test results qualitatively and
quantitatively with simple user-friendly GUI as shown in
Figure 8. The software proposed to be developed using
Visual C++ and Lab VIEW. The software enables the user
to locate the faulty contactor pins visibly and by using
colour indication and provide the user with the averaged
impedance value of specific contactor pin. Red colour
indicates that the contactor pin might be open circuit or
short circuit where else the green colour indicates good
pins.

6. The software also allows the user to define the
impedance level manually to enables it to filter which pins
fails and which pin have acceptable impedance range. The
software GUI layout will look similar to Figure 8. The
software also enables user to save previous test result and
compare the new result to identify which pins impedance
have increased and might cause failure in future.
Figure 10: Expected Output Display
V. CONCLUSION
This paper proposed an effective method of identifying
faulty contactor socket pins using impedance
measurement technique to improvise the commonly
practiced test methods available in the semiconductor
industry. This impedance measurement method also
improvise the existing contact resistance tester that
available in the market to a better overall solution for
identifying the faulty contactor pin in semiconductor
packaged device testing to reduce inefficient debugging
time.
Besides saving debugging time, this proposed method
also able to diagnose the end-life of the contactor pin
earlier and avoid debugging down time, increase the
effectiveness and efficiency of DUT testing and assure the
reliability of characterization result.
Accurate identification of defective contactor pins, help
reduces 50% of contactor replacement cost by changing
only identified faulty pins. This testing method also saves
75% of debugging time if an engineer require
approximately 2 hours to debug a contactor issue but with
this tester they could able to identify which pin is faulty
and identify whether the faulty pin will affect their test
within 30 minutes only.
This contact impedance tester can be further improvised
by adding more measurement features besides impedance
and reduce the tester hardware size.
VI. ACKNOWLEDGEMENT
We would like to express our greatest gratitude to all the
people who have helped & supported us throughout our
research and in completing this paper. A special thank
goes to Hong Hai Teh and Ah Kah who helped us in our
experiments. At last but not least we want to thank our
friends who appreciated our work and motivated us and
finally to God who made all the things possible.
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