Introduction:

Electronic designers have been moving towards higher [part]{.mark}
~~circuit~~ density in multi-chip modules (MCMs) for aerospace
applications. This ~~circuit~~ density can be achieved by combining
multiple functions on a single die, including more dice in one package
(single substrate) and by extending the substrate (die-to-die
interconnect) into the z-direction. As [density]{.mark} ~~chip
functionality~~ increases, the number of inputs and outputs ~~tends to~~
increase[s,]{.mark} creating a wire bond configuration that is driven by
minimum distances between the bond pads on the die and those on the
package. This ~~tends to~~ create[s]{.mark} long wire bonds in the x-y
plane. Stacked dice ~~inside of a single cavity package~~ will have long
bond wires in the z-direction.

The fracture strength of aluminum wire is well understood because the
material is [a]{.mark} standard [construction material]{.mark}
throughout the industry ~~and is not changing chemically~~. Gold alloy
wire is manufacturer unique[,]{.mark} and the exact composition and
quantity of the additives is considered proprietary by most (if not all)
of the manufacturers. These additives are in the 10 ppm range and are
used to enhance elongation, heat affected zone (HAZ) characteristics,
and tensile, fatigue and fracture strength. The presence of the
additives makes it very difficult to predict the fatigue strength of the
gold wire used to make long bond wires[,]{.mark} and [this]{.mark}
~~so~~ limits one\'s ability to ~~generically~~ predict the electronic
part\'s reliability. Knowing the susceptibility of long bond wires to
fatigue fracture and their resonant frequency will enable reliability
predictions of electronic system that may be exposed to shock and
vibrations.

Background

Component failure rate apportionment studies show that wire bond failure
rates have been the cause of up to 30% of electronic packaging failures
\[3\]. A significant amount of research has been done on wire bond
processes and reliability. Absent from this research has been a thorough
investigation of the strength not of the bond, but of the wire itself.
This was identified during the resolution of a failure of NASA hardware
that was detected during vibration testing.

Recent failures of wire bonds in NASA solid state recorders (SSR)
highlighted the fact that bond wires can experience bending stress
induced by self-resonance if the wires are long enough and the vibration
frequency spectrum includes corresponding frequencies\[4,6\]. The bond
wires used in the failed memory modules (which used stacks of memory
dice) were between approximately 161 mils and 240 mils. Bond wires
applied in packages with a single die are generally around 40 to 60 mils
long. During final box-level random vibration testing with frequencies
up to 850Hz and total spectral power of 6 Grms, four of the memory
modules used in the SSR failed due to failed wire[s]{.mark} [and]{.mark}
bonds.

The memory used for the SSR is an array of 72 memory modules. Each
memory module consists of two stacks of DRAM dice stacked 12 high. This
yields 384 Mbits of memory per module. Due to the height of the die
stack and placement of the bond pads on the die, this application
requires unusually long bond wires, 161 to 240 mils in length (Figure 1,
2). The usual rule for bond wire length state[s]{.mark} that the ratio
of length to diameter of the wire should not exceed 100\[6\]. In this
application[,]{.mark} the ratio is approximately 200.

During the failure investigation the wire bonds were inspected. Seven of
the 100 wire bonds inspected were torn, lifted from the bond pad, or
both for up to 33% of the width of the bond. As a result of the bond
damage and long bond wire, the resonant frequency of the wire was
dropped into the bandwidth of the box-level random vibration testing.
During testing the wire was excited into resonance and the resulting
stresses seen at the bond were sufficient to rupture the bond.

The cracks in the wire bond were found to be a result of a number of
conditions including alignment of the bond, wire loop radius, wire
length and the bond pad surface condition. Due to the number of
variables contributing to this defect and the difficulty associated with
analyzing them, Finite Element Analysis (FEA) was used to predict the
resonant frequencies of the wires.

![](media/image1.png){width="4.350694444444445in"
height="4.423611111111111in"}

**Figure 1. Variation in the shape of bond wires observed in stacked
memory modules.**

Using the predicted values as a starting point, testing was performed
which showed that the measured resonant frequency values were within 15%
of those calculated using FEA. Thus it was verified that FEA could play
a significant role in bond wire failure/reliability predictions. With
known fatigue and fracture strength data for a given wire, FEA can be
used to identify the resonant frequency for a given wire length and
shape allowing users to design for reliability and avoid overstressing
the wires during acceptance testing.

![](media/image2.wmf)

# Figure 2. SEM photo of long bond wires in the stacked memory module

Test methods used to evaluate bond quality include destructive and
non-destructive testing. The tests used by the military to qualify high
reliability hardware are destructive and non-destructive bond pull and
are documented in MIL-STD-883, Method 2011.7, and 2023 respectively.
These methods have been adopted by industry as repeatable and reliable
methods for evaluating bond quality. Destructive bond pull drives the
cost of using the part through the destruction of samples and the cost
of the labor involved. Non-destructive bond pull has been used to avoid
wasting good parts but it is also labor intensive.

Research was performed to find other non-destructive methods for
evaluating bond strength. Unisys has developed a test fixture for
non-destructively evaluating the quality of bond wires based on
excitation of the bond wire in a varying electromagnetic field (Figure
3). The sample is placed in the field of a permanent magnet which has a
recessed area machined into it. The ~~position of the~~ bond wires on
the sample are perpendicular to the magnetic (B) field while in the
fixture. A electrical alternating current is passed through the wires of
interest which causes a motional displacement of the wire due to the
interaction between the permanent and induced B fields. The advantage of
this system is that the ~~motional~~ displacement can varied by varying
the current. In addition, this fixture has the ability to record the
displacement (this value is calculated) by measuring the voltage drop on
the wire caused by cutting the flux lines of the permanent magnet.

This test fixture is intended to be a non-destructive, "in-line",
production level test to evaluate bond quality. It has potential for use
as a fatigue strength tester since it uses self -resonance to exercise
the wires. The draw back of this fixture is the cost and complexity if
used for evaluation purposes only. The Unisys design would be more
suited for a manufacturing environment where the cost could be justified
by production volume.

![](media/image3.png){width="5.948611111111111in"
height="3.6020833333333333in"}

##  Figure 3. Non-Destructive Bond Wire Tester Based on wire excitation

The critical region determining the mechanical reliability of a wedge
bond is the heel joint. This critical region is about 25 microns in
length and is therefore very difficult and impractical to exercise
\[3\]. Researchers at Cornell University designed a fixture to test wire
fatigue strength in long sections of wire by inducing a
tension/compression cycle \[1\]. This was achieved by passing a section
of wire through a pair of mandrels from one pick-up reel to another
(Figure 4, 5). The fixture was capable of sensing the take-up reel
position so that after the length of wire had passed completely through
the mandrel assembly in one direction the DC motor was switched to
rotate in the opposite direction. Two gold and one aluminum wire sample
was used. Three different strain amplitudes were applied; 0.7%, 5%, and
10%.

The strain on the wire was varied by varying the gear reducer and
mandrel diameter.

The strain on the wire is given by:

> σ = E \* r / R

Where: σ = Strain

E = Young's Modulus

r = Wire Diameter

R= Mandrel Diameter

In addition to varying the strain on the wire, testing was performed
over temperature. Testing was done at 20°C, 75°C, and 125°C. Results of
the testing showed that cycle-to-failure time increased with decreasing
strain, and decreased with increasing temperature.

This approach to the analysis of wire fatigue failure is good but in
itself is not enough. The disadvantage of this technique is that the
central area of the wire is neutral. Examining the equation given for
strain indicates that the strain decreases with decreasing wire
diameter, that is, as the center of the wire is approached, the strain
approaches zero. This testing needs to be complemented with one that
exerts force over the entire cross-section. The results of this analysis
are useful to our evaluation as a starting point, but should be expanded
to include cold temperature (-55°C to 25°C), and samples from numerous
vendors. This would make the data more useful to aerospace and high
reliability users.

![](media/image4.png){width="4.999305555555556in"
height="3.9993055555555554in"}

Figure 4. Wire path Schematic

![](media/image5.png)

# Figure 5. Fatigue Machine

Project Objectives

Our evaluation will use a ~~much~~ simplified version of test fixture
[as in]{.mark} ~~used by (~~Figure 6~~)~~ and a second ~~fixutre~~
[fixture]{.mark} which exerts force across the cross-section of the wire
(figure 7).

To address the need for information about the strength of gold wires
being used in electronic devices the following ~~is~~ [are]{.mark} the
objective[s]{.mark} of research that has been started through NASA\'s
Advanced Interconnect Program.

[\[Make this the second bullet\]]{.mark}

-   Determine the first order statistics (average, mean, median) for the
    number of bending and tensile stress cycles required to produce
    fatigue-fracture in gold alloy wire. Determine the second order
    statistics for this value (number of cycles to fatigue-fracture)
    where meaningful. Determine the difference in the cycles to
    fatigue-fracture statistics for samples with different heat
    treatments and cold working processes. Determine the cycles to
    fatigue-fracture for samples with different alloy compositions and
    diameters.

[\[Make this the first bullet\]]{.mark}

-   Build test fixtures and develop test procedures for producing
    ~~this~~ data [on the number of bending and tensile stress cycles
    required to produce fatigue-fracture in gold alloy wire]{.mark} and
    providing repeatable results. The test procedure should be designed
    such that it can be performed by individuals with varying technical
    abilities with a minimum of special training. The test procedure
    should reference ASTM or other industry recognized standards for
    identifying the compositions and dimensions of the samples tested.
    Wherever possible, other test parameters, such as mandrel diameters,
    should be identified by reference to industry standard definitions
    and terms. Electrical and mechanical conditions, such as mechanical
    pre-loads must be calibrated.

# FY98 Accomplishments

Research has been done regarding the best strategy for accomplishing the
objectives and the best test fixture that can be implemented reasonably
in terms of cost and schedule. Two fixtures were designed and are in the
process of construction (Figures 6,7). Both of these fixtures are based
on the test apparatus used by Deyhim, Yost, Lii and Li. These fixtures
were developed to facilitate the cycles to fatigue fracture analysis in
ambient conditions and over temperature.

The two fixtures designed share the same base and equipment. Figure 6
consists of a DC vibrator motor, strain gauge (in the gram range), and
adjustable plate for setting the pre-load. The wire is epoxied to a
flange mounted in the chuck of the motor and threaded through the
mandrels. The free end of the wire is then epoxied to a piece of RTV.
The RTV will allow the wire to follow the motion of the vibrator motor
without allowing slack that will result in backlash. The wire will be
preloaded with approximately 1 gram for testing. The pre-load will be
measured with the strain gauge. The pre-load measurement will be
accomplished by tacking the wire to the strain gauge with epoxy.

Figure 7 is essentially the same as 6 except that the wire is not passed
through a pair of mandrels. This will allow the strain or force exerted
by the motor to be exerted across the entire cross-sectional area of the
wire.

Both of these fixtures will be modified to allow automated cycle count
and wire failure monitoring.

![](media/image6.png){width="5.3493055555555555in"
height="3.970833333333333in"}

**Figure 6. Fatigue Fixture Incorporating the use of Mandrels.**

![](media/image7.png){width="4.947916666666667in"
height="3.5194444444444444in"}

**Figure 7. Fatigue Fixture, applying force across the cross section of
the wire.**

# FY99 Objectives

The following are the tasks planned for FY99:

-   Obtain the gold wire samples. To reduce cost, gold wire already on
    hand will be used. It is 1 mil and 2 mil in diameter and 99.99% pure
    gold. A material analysis will be done to try to identify the trace
    materials.

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-   Build the test fixtures.

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-   Produce a test plan and procedures.

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-   Baseline the repeatability of the procedure and test fixtures using
    0.001\"gold alloy (99.99% pure) wire.

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-   Complete the testing and publish the results.

FY00 Plans

The following are the tasks being proposed for FY00:

-   Adapt the fixture designs to operate in simulated space environments
    ( i.e. thermal vacuum and thermal cycling chamber with mechanical
    excitation applied).

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-   Obtain additional wire samples from a variety of manufacturers and
    repeat testing and reporting.

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-   Duplicate experiments documented in the scientific literature to
    validate existing conclusions.

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-   Complete testing and publish the results

# References

\[1\] A. Deyhim, B. Yost, Mirng-Ji Lii, and Chc-Yu Li "Characterization
of the Fatigue Properties of Bonding Wires" 1996 IEEE Electronic
Components and Technology Conference.

\[2\] J.I. Tustaniwskyj, R.J. Usell, Jr., and S. A. Smiley of Unisys
Corporation. " Progress Towards a Cost- Effective 100% Wire Bond Quality
Screen", IEEE

\[3\] Jun Ming Hu, M. Pecht, A. Dasguta. " A Probabilistic Approach for
Predicting Thermal Fatigue Life of Wire Bonding in Microelectronics'. J.
Electronic Packaging v. 113 (September 1991) p. 275.

\[4\] H. Leidecker, S. Hull, J. Plante, "Analysis of Wire Bond Failure
in Stacked Memory Modules"

\[5\] Tustaniwskyj et al., U.S. Patent number 4,667,370, Method of
testing wire bonds for defects using a magnetic field.

\[6\] H. Leidecker, S. Hull, Vibration Induced Fatigue Failure in
Bonding Wires Used in Stacked Chip Modules, 1998.