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CHAPTER 5

RANDOM VIBRATION TESTS ON DIP-PCB ASSEMBLY

5.1 INTRODUCTION

Random vibration tests are usually specified as acceptance,

screening and qualification tests by commercial, industrial, and military

manufacturers of electronic equipment. It has been shown that random

vibrations more closely represent the true vibration environment in which the

electronic equipment has to operate. Random vibration is also popular for

performing accelerated life tests, to estimate approximate fatigue life of

critical elements in electronic assemblies. Hence, random vibration continues

to gain wide acceptance for use in many different testing programs associated

with electronic equipment. The random vibration environments may be

observed in airplanes, missiles, automobiles, trucks, trains, petroleum drilling

machines, steel rolling mills, and numerically controlled machine tools.

Random vibration test has been proved to be a very powerful tool

for improving the manufacturing integrity of electronic equipment by

screening out defective components and defective assembly methods, which

result in a sharp improvement in the overall reliability of the system

(Steinberg 2001). Therefore, electronic packaging designers and engineers

must know the fundamental nature of random vibration and fatigue, in order

to design, develop, and manufacture cost-effective and lightweight electronic

structures that are capable of withstanding harsh random vibration

environments.

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Random vibrations are non-periodic and characterized in terms of

the bandwidth of the frequencies as it consists of many different sinusoidal

frequencies superimposed upon one another. All of the frequencies within the

bandwidth are present simultaneously at any instant of time for every

frequency. When the frequency bandwidth is from 20 to 1000 Hz, every

natural frequency of every structural member between 20 and 1000 Hz will be

excited at the same time. This includes every fundamental natural frequency,

and every higher harmonic of every structural member within that bandwidth.

Knowledge of the past history of random vibration is adequate to predict the

probability of occurrence of various acceleration and displacement

magnitudes, but it is not sufficient to predict the precise magnitude at a

specific instant.

Random vibration environments in the electronics industry

normally deal in terms of PSD (indicated as ‘P’) or mean squared acceleration

density. However, random vibration can also be expressed in terms of

velocity spectral density, and in terms of displacement spectral density.

Accelerations, velocities, and displacements for random vibration are

typically expressed in terms of root mean square (RMS) values.

Many standards such as MIL-810-F, JEDEC JESD22-B103B

(JEDEC, 2006) are used as qualification tests for electronic equipment. Many

electronic package manufacturers such as Motorola, Intel, and AMD follow

their own standards or adopt international standards to test their newly

designed packages.

This chapter deals with the random vibration experiments

conducted on DIP and PCB assembly, and investigations made on the

effectiveness of rubber spacers and pads as vibration isolating devices in a

random vibration environment.

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5.2 RANDOM VIBRATION TESTS WITH PCB ASSEMBLY

MOUNTED ON PLASTIC SPACERS

5.2.1 Experimental Procedure

The test vehicle used for random vibration tests consists of a PCB

and a DIP, having 16 lead wires (8 x 2 rows) as shown in Figure 4.2

(Chapter 4). The random vibration tests are conducted by exciting the PCB

assembly in Z direction (perpendicular to PCB plane) using an

electrodynamic shaker and random vibration software. The input acceleration

PSD used for tests is the condition “D” as specified by the JEDEC standard

and shown in Figure 5.1 and the frequency split up for this level is shown in

Table 5.1. The test was conducted for the duration of 30 minutes.

Figure 5.1 JEDEC test conditions

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Table 5.1 Frequency breakpoints of PSD of level ‘D’ of JEDEC

Frequency (Hz) PSD level, (G2/Hz)3 0.00017 0.003

40 0.00350 0.01370 0.013

200 0.001500 0.001

Input Grms = 1.11

As the random vibrations are non-periodic and follows the

Gaussian (normal) distribution, hence this was checked by collecting the

random vibration signals (acceleration) in time domain using LabVIEW

software and NI-4472 data acquisition (DAQ) card. From Figure 5.2 it is

evident that, the distribution follows normal or the Gaussian nature.

Figure 5.2 Time domain signals and corresponding normal distribution

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The complete test setup for conducting random vibration tests on

the DIP-PCB assembly is as shown in Figure 4.1 (Chapter 4). The PCB

assembly was mounted on an aluminum fixture using four plastic spacers and

steel fasteners. The natural frequencies of the PCB assembly and fixture were

first determined using logarithmic sine sweep test and they are tabulated in

Table 5.2. From the table it is observed that, the natural frequencies of PCB

and fixture are not close to one another.

Table 5.2 Natural frequencies of PCB assembly and Fixture

Frequency (Hz)

PCB Assembly Fixture 46.12 198.69 60.00 238.79 109.10 315.25 141.26 340.50 209.37 416.20 241.45 398.00 462.42

The reference PSD profile as per JEDEC standard, was

programmed using the random vibration test software and is as shown in

Figure 5.3. The input acceleration PSD profile was monitored and controlled

in a closed loop by using an accelerometer (B & K 4513, 8.6 g mass, 100

mV/G sensitivity) placed on the fixture. Another accelerometer (B & K 4517,

0.6 g mass, 10 mV/G sensitivity) placed near the component was used to

measure the response of the PCB assembly.

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Figure 5.3 Input acceleration PSD spectrum (control spectrum)

The response of the PCB assembly measured near the component

and in the Z direction is as shown in Figure 5.4. The area under the curve

(Figure 5.4) represents the root mean square acceleration (Grms) of the PCB

assembly, and from the figure, Grms value is found to be 7.99, and the

acceleration PSD at first resonant frequency is about 6 G2/Hz, whereas the

input PSD at the same frequency is 0.013 G2/Hz. Amplification of PSD level

at resonant frequency will lead to increased Grms and induce more stresses in

component lead wires. Figure 5.5 shows the transmissibility plot which is the

ratio of input acceleration power spectral density and output acceleration

power spectral density (PSDout/PSDin). From this figure it is seen that, the

acceleration PSD is amplified at resonant frequencies of 50 Hz, 210 Hz and

about 500 Hz.

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Figure 5.4 Response of the PCB mounted on plastic spacers

Figure 5.5 Transmissibility plot of the PCB mounted on plastic spacers

During 30 minutes of random vibration test, the failure of the lead

wires was monitored using a failure detecting circuit as shown in Figure 4.3

(Chapter 4). If at any instant of time, any of the lead wire fails; the LED

provided on the circuit will go off or glow intermittently due to opening of the

lead wire.

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2

8.9

nfrmsG

rmsZ

The repeatability of the test was checked by performing five trials

and the results obtained are tabulated in Table 5.3.

Table 5.3 Random vibration test results

Trial No. Grms

1 7.64 2 7.99 3 8.014 7.86 5 7.98

The single amplitude displacement Zrms, near the component at

resonant frequency fn is calculated using the Equation (5.1).

(5.1)

= mmx 79.025099.78.9

The RMS displacement (Zrms) experienced by the PCB assembly is

found to be 0.79 mm and the same is validated using spectrum analysis in

ANSYS.

5.2.2 ANSYS Simulation Results

To validate the experimental results, ANSYS, commercially

available finite element analysis software was used to simulate the random

vibration tests. Three-dimensional geometries of the PCB and the DIP along

with lead wires were modeled for the purpose of analysis. PCB, DIP body,

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and lead wires were meshed using 10 node solid 92 elements. The material

properties of the PCB assembly components used in the analysis are tabulated

in Table 4.9 (Chapter 4).

The spectrum analysis was performed by exciting the PCB

assembly using an input random vibration test profile as shown in Figure 5.6

(level D of JEDEC standard). The excitation PSD profile was applied at the

fixed nodes of the PCB holes in Z direction (perpendicular the PCB plane). A

uniform damping ratio of 0.026 was used during the spectrum analysis

(corresponding to the PCB assembly mounted on plastic spacers).

Figure 5.6 Input PSD test profile (level D of JEDEC)

Figure 5.7 shows 1 RMS acceleration (Grms) response of the PCB

assembly, and the Grms acceleration at the centre of the PCB is 8.73

(85.77 m/s2) which is close to the test data (7.99). Similarly, Figure 5.8 shows

the 1 displacement of the PCB assembly due to particular random vibration

input profile. The maximum 1 RMS displacement at the PCB centre is found

to be 0.79 mm which is equal to the displacement value obtained from the

experiment.

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Figure 5.7 1 RMS acceleration plot

Figure 5.8 1 RMS displacement plot

The response acceleration PSD measured at the centre of the PCB

is shown in Figure 5.9, and from this figure it is found that the PSD at first

resonant frequency is about 7 G2/Hz which is again close to the experimental

value.

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Figure 5.9 Response PSD captured at the PCB centre

The comparison of experimental and simulation results are

tabulated in Table 5.4. From the tabulated data it is observed that, the

simulation results are in close agreement with the experimental results.

Therefore, the finite element analysis results such as stress induced in lead

wires of the DIP due to random vibrations will be used to estimate the fatigue

life of lead wires.

Table 5.4 Comparison of Experimental and ANSYS results

Parameters Experimental ANSYS

1 RMS acceleration, Grms 7.89 8.73

1 RMS displacement, Zrms (mm) 0.79 0.79 PSD at first resonant frequency (G2/Hz) 6 7

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(Hz)

5.3 RANDOM VIBRATION TESTS WITH THE PCB

ASSEMBLY MOUNTED ON RUBBER SPACERS

5.3.1 Experimental Procedure

In an attempt to reduce the excessive PCB deflection and

corresponding damage due to random vibration loads, the PCB assembly was

mounted on four rubber spacers (Figure 4.6, Chapter 4) and subjected to,

D level random vibration test. Figure 5.10 shows the acceleration PSD

response when the PCB assembly is mounted on rubber spacers. From this

figure, the Grms acceleration and response PSD levels are found to be 2.72 and

0.7 G2/Hz.

From the test results it is seen that, the Grms acceleration is reduced

by 66%, the acceleration PSD is reduced by 87% compared to the

corresponding responses obtained when the PCB assembly was mounted on

plastic spacers.

Figure 5.10 Response of the PCB mounted on rubber spacers

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(Hz)

Figure 5.11 shows the transmissibility graph when PCB is mounted

on rubber spacers. Using the rubber spacers, the transmissibility ratio is

reduced by 16 % (at first frequency).

Figure 5.11 Transmissibility plot of the PCB mounted on rubber spacers

The RMS (Zrms) displacement at the centre of the PCB when it is

supported on rubber spacers is found to be 0.29 mm (Equation 5.1), on the

contrary, when PCB was mounted on plastic spacers the RMS displacement

was 0.79 mm. Therefore, from these results it can be concluded that, rubber

spacers can be effectively used to reduce the damage to the PCB and the

electronic packages mounted on it by reducing Grms and Zrms levels in a

random vibration environment. Also, the fatigue life of PCB assemblies can

be improved by mounting the PCB assembly on rubber spacers

5.3.2 ANSYS Simulation Results

Spectrum analysis, simulating the PCB assembly mounted on

rubber spacers was carried out in ANSYS following the procedure as

explained in section 5.2.2. A damping ratio of 0.038 (which is obtained from

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sine sweep test) was used in this spectrum analysis. The 1 RMS acceleration

(Grms) value obtained from the simulation is as shown in Figure 5.12, from

this figure the RMS acceleration at the PCB centre is found to be 3.1

(30.41 m/s2).

Figure 5.12 1 RMS acceleration plot

Similarly, 1 RMS displacement (Zrms) plot obtained from the

simulation is shown in Figure 5.13, from which the Zrms at the PCB centre is

found to be 0.29 mm which is close to the experimental value. Figure 5.14

shows the PSD response captured at the centre of the PCB. From this figure

the PSD value at fundamental frequency is found to be about 1 G2/Hz.

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Figure 5.13 1 RMS displacement plot

Figure 5.14 Response PSD captured at the PCB centre

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(Hz)

5.4 RANDOM VIBRATION TESTS WITH PCB ASSEMBLY

MOUNTED ON RUBBER PADS

In an another effort to reduce the PCB responses due to random

vibrations, the longer edges of the PCB were made to rest on the longer faces

of the two rubber pads and fastened to the fixture plate using fastening screws

(refer Figure 4.14 in chapter 4). Now the PCB assembly is subjected to

random vibrations as per JEDEC’s ‘D’ level (Figure 5.1) in a direction

perpendicular to plane of the PCB.

The response of the PCB assembly due to random vibration input is

as shown in Figure 5.15, from which it is observed that, the RMS acceleration

experienced by the PCB assembly is 1.95 and PSD level at first resonant

frequency is 0.4 G2/Hz. Therefore, by using the rubber pads the RMS

acceleration is reduced by 28% and PSD amplitude at first resonant frequency

43% when compared to the corresponding responses when the PCB assembly

was mounted on rubber spacers.

Figure 5.15 Response of PCB mounted on rubber pads

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(Hz)

The transmissibility plot as shown in Figure 5.16 is obtained when

the PCB assembly was mounted on rubber pads. From this figure it is

observed that, the transmissibility ratio at fundamental frequency is further

reduced by about 20 % when compared to the transmissibility of PCB

mounted on rubber spacers.

Figure 5.16 Transmissibility plot of PCB mounted on rubber pads

PCB responses obtained from random vibration tests for the three

mounting methods are tabulated in Table 5.5 for comparison. From the table it

is observed that, the rubber spacers and pads have effectively minimized the

RMS acceleration, RMS displacement and PSD amplitudes of the PCB

assembly in a random vibration environment.

Table 5.5 Comparison of random vibration tests with different mountings

PCB Mounting method

RMS acceleration (Grms)

PSD(G2/Hz)

RMS displacement (mm)

Four plastic spacers 7.99 6 0.79 Four rubber spacers 2.72 0.7 0.29

Rubber pads 1.95 0.4 0.05

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5.5 RESULTS AND DISCUSSIONS

Random vibration tests using level ‘D’ of JEDEC standard as input

profile in Z direction of the PCB assembly was conducted for 30 minutes. The

test results showed that, the 1 Grms acceleration, response PSD, and 1 RMS

displacement are 7.99, 6 G2/Hz and 0.79 mm respectively when the PCB

assembly was mounted on plastic spacers.

By mounting the DIP-PCB assembly on rubber spacers, the Grms

acceleration was reduced by 66%, PSD level by 88% and RMS displacement

by 63%. As the displacement values are reduced, the corresponding stress

amplitudes in lead wires will also be reduced and enhance the fatigue life of

electronic packages. Thus, the rubber spacers act as good vibration isolators

in random vibration environment also and are effective over entire frequency

band.

Other method of reducing the PCB response was that of clamping

the longer edges of the PCB assembly on rubber pads. By mounting PCB on

rubber pads, Grms level was reduced by 66%, PSD level by 93%, and

displacement by 99% (compared to response of PCB mounted on plastic

spacers). Thus, from Table 5.5 it is evident that, by using rubber spacers or

rubber pads, the PCB displacement is greatly reduced and the corresponding

life of electronic packages is improved.