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.

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

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.

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

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.

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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