CHAPTER 4 SINE SWEEP TESTS ON DIP-PCB...

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91 CHAPTER 4 SINE SWEEP TESTS ON DIP-PCB ASSEMBLY 4.1 INTRODUCTION Sinusoidal vibrations can induce very high acceleration levels in lightly damped structures, when their natural frequencies are excited. Transmissibility values can be greatly magnified, resulting in very high displacements, forces, accelerations, and stresses, which often result in electrical malfunctions and failures. High displacements often result in impacting between adjacent structural members such as circuit boards, resulting in cracked components, cracked solder joints, broken electrical lead wires and broken connector pins. High forces can produce high stresses in load carrying elements such as screws, rivets, and ribs, which may become loose or may fracture. High accelerations can cause relays to chatter, crystal oscillators to malfunction, and potentiometers to lose their calibration accuracy. High stresses typically result in very rapid fatigue failures in various electronic elements from aluminum housings to cables and harnesses (Steinberg 2001). The sine sweep test involves a logarithmic frequency sweep usually through a range of 5-1000 Hz, holding a specified acceleration constant at the base of the test article or its mounting bosses on the fixture. A control feedback accelerometer is mounted in the desired position on the fixture and the level is maintained as the frequency of vibration is swept. This method ensures excitation at all frequencies between the sweep end frequencies. This

Transcript of CHAPTER 4 SINE SWEEP TESTS ON DIP-PCB...

91

CHAPTER 4

SINE SWEEP TESTS ON DIP-PCB ASSEMBLY

4.1 INTRODUCTION

Sinusoidal vibrations can induce very high acceleration levels in

lightly damped structures, when their natural frequencies are excited.

Transmissibility values can be greatly magnified, resulting in very high

displacements, forces, accelerations, and stresses, which often result in

electrical malfunctions and failures. High displacements often result in

impacting between adjacent structural members such as circuit boards,

resulting in cracked components, cracked solder joints, broken electrical lead

wires and broken connector pins. High forces can produce high stresses in

load carrying elements such as screws, rivets, and ribs, which may become

loose or may fracture. High accelerations can cause relays to chatter, crystal

oscillators to malfunction, and potentiometers to lose their calibration

accuracy. High stresses typically result in very rapid fatigue failures in

various electronic elements from aluminum housings to cables and harnesses

(Steinberg 2001).

The sine sweep test involves a logarithmic frequency sweep usually

through a range of 5-1000 Hz, holding a specified acceleration constant at the

base of the test article or its mounting bosses on the fixture. A control

feedback accelerometer is mounted in the desired position on the fixture and

the level is maintained as the frequency of vibration is swept. This method

ensures excitation at all frequencies between the sweep end frequencies. This

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type of testing usually will cycle up and down repetitively between frequency

limits for a specified time or number of sweep cycles to ensure that adequate

reliability levels are attained.

By comparing the acceleration, usually measured in G’s, to

frequency one can determine the natural frequencies of the test item. When

the acceleration is plotted against the frequency one might observe large

peaks at certain frequencies. The frequencies where these peaks occur are the

natural frequencies. One would need to either re-design the structure to

change the natural frequencies of the test item or design the tested item to

withstand the increased accelerations due to experiencing resonance. By

limiting the displacement, component stresses will be reduced

correspondingly, increasing the vibration lifetime of the equipment (Sloan

1985).

This chapter deals with the sine sweep tests conducted on DIP and

PCB assembly subjected to sinusoidal vibrations at a constant input

acceleration of 1G, 2G, and 3G. The dynamic responses of the PCB assembly

mounted on plastic spacers at different input acceleration levels are studied

and the influence of rubber spacers and rubber pads as vibration isolators (or

damping mechanism) on the dynamic responses is investigated by

experimental and numerical methods. Before explaining about the

experimental procedure, a brief introduction to the relationship between

acceleration level, natural frequency & displacement, and the transmissibility

ratio is given in the following sections.

4.1.1 Relation of Displacement to Frequency and Acceleration

Dynamic displacements that are developed during vibration

conditions are often very important since they can be used to determine the

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dynamic stresses, which are needed for the calculation of the expected fatigue

life of the critical electronic structural members. Dynamic displacements are

very difficult to observe and to measure. Acceleration levels are easy to

obtain with the use of small accelerometers, and frequencies are easy to

obtain directly from the electrodynamic shakers performing the vibration.

The dynamic displacement (single amplitude in meters) of the

vibrating structure may be determined using Equation (4.1). The detailed

procedure used to derive Equation (4.1) is given in a book by Steinberg

(2001).

2*2485.0

0nf

GZ (4.1)

Also Equation (4.1) may be written in the following form;

2

**2485.00

nf

QinGZ

(4.2)

where Q = (Gout/Gin) is the transmissibility ratio.

When the input acceleration level is expressed in gravity units, or

G’s, the maximum single amplitude displacement Z0 will also represent the

input displacement. When the output or response is expressed in G gravity

units, the displacement Z0 will represent the output or response displacement.

Above equation is often considered to be the most important equation in the

field of dynamics. It shows that the displacement, acceleration level, and the

frequency are locked together. Any two values automatically determine the

third value. This equation is valid for sine vibration, random vibration, shock,

and acoustic noise.

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4.1.2 Transmissibility

General rules for estimating the transmissibility values for different

electronic structures in vibration environments are difficult to establish. This

is because there are so many different types, arrangements, and combinations

of electronic hardware being used in so many different commercial, industrial,

and military applications. Extensive data collected over a period of several

years on transmissibility values for a wide variety of electronic systems shows

a lot of scatter. An average transmissibility equation was developed from the

data that can be very useful in obtaining a good approximation of the

expected transmissibility for some common structures used for packaging

electronic equipment (Steinberg 2001). This transmissibility equation is given

by:

0.76

0.6)( in

n

GfJQ (4.3)

where, J = 0.50 for a plate type of structure

fn = Natural frequency of the structure (Hz)

Gin = Input acceleration G level in dimensionless gravity units

4.2 EXPERIMENTAL WORK

The experimental setup for conducting the sine-sweep test is as

shown in Figure 4.1. The setup mainly consists of an electrodynamic shaker

(DEV-001, 50 kg-f, 12 mm peak-to-peak displacement) for exciting the PCB

assembly at constant input acceleration using sinusoidal vibration controller

software. An aluminum fixture (300 mm x 300 mm x 5 mm) is bolted onto the

shaker head for mounting the PCB made of glass-epoxy material and

measuring 240 mm x 210 mm x 1.6 mm in size. The PCB assembly is

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mounted on the fixture using four fastening screws and plastic stand-offs

(spacers) placed at the corners of the PCB.

Two accelerometers (B&K 4513-001 and B&K 4517), used for

controlling and monitoring the input acceleration level (Gin) in a closed loop

and monitoring the output acceleration (Gout) are interfaced with a four

channel signal conditioner. The first accelerometer is placed near the

fastening screw on the base plate (fixture) and the second accelerometer is

placed near the component on PCB as shown in Figure 4.1.

Figure 4.1 Experimental setup for conducting sine-sweep tests

The electronic package used for the test is a through-hole mounted

16 pin dual in-line package (DIP-53C539H, 8 pins x 2 rows) mounted at the

centre of PCB with its longer sides parallel to the shorter side of the PCB as

shown in Figure 4.2.

AccelerometersPCB

Power Amplifier

4-Channel Signal Conditioner

PC and Digital Vibration Control Software

Fixture Shaker

Plastic spacer

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A failure detecting circuit for detecting the failure of component

lead wires or solder joints during the test is shown in Figure 4.3. In case, any

of the lead wire or solder joint fails, the LED provided on the circuit will go

off.

Figure 4.2 Electronic package mounted on PCB

Figure 4.3 Failure detecting circuit

4.2.1 Sine Sweep Test with PCB Assembly Mounted on Plastic

Spacers

The logarithmic sine-sweep at the rate of one octave per minute

was programmed using sinusoidal vibration control software. Sine sweep tests

were conducted on the PCB assembly by mounting it on four plastic spacers

(encircled in Figure 4.1) placed at the corners of the PCB in the frequency

range 20-500 Hz (JEDEC sine sweep test standard, service condition 4) at a

constant input acceleration of 1G, 2G and 3G. The displacement of PCB

assembly at the resonant frequencies was noted from the display window of

16 pin DIP

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the vibration controller software and is shown in Figure A1.20 in Appendix 1.

Also, the displacement values at predominant natural frequencies may be

measured using a pen type digital vibrometer (Figure 4.4) while holding the

sweep (software facilitates this feature).

Figure 4.4 Pen type digital vibrometer

The frequency response of the PCB assembly when mounted on

plastic spacers is as shown in Figure 4.5. From Figure 4.5 it is seen that, the

first resonant frequency of PCB assembly is 46 Hz and peak acceleration at

this frequency is 25G. The single-amplitude displacement at 46 Hz frequency

is 2.93 mm (Table 4.3). Second prominent resonant frequency is at 210 Hz

with peak acceleration of 70G. From the data it is clear that, the first resonant

frequency of 46 Hz will lead to maximum PCB displacement inducing

maximum stresses in component lead wires. This will lead to reduced fatigue

life and early failure of electronic packages. Hence, it is necessary to reduce

the dynamic displacement of PCB assembly by using alternative spacer

material which is having high energy dissipating capacity such as neoprene.

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Figure 4.5 Response of the PCB mounted on plastic spacers due to 1G

input

The natural frequencies of the PCB assembly obtained from the

logarithmic sine sweep tests at 1G, 2G, and 3G input acceleration levels are as

tabulated in Table 4.1. From Table 4.1 it is observed that, the resonant

frequencies of the PCB assembly are same when it is excited at different input

acceleration levels. Similarly, the output acceleration levels and displacement

values at resonant frequencies are as tabulated in Table 4.2 and Table 4.3

respectively. From Table 4.2 it is evident that the output acceleration levels at

the resonant frequencies increase with increase in input acceleration levels.

From the data tabulated in Table 4.3 it is observed that, the displacement

levels are high at lower natural frequencies and decrease with increase in

natural frequencies.

The responses of the PCB assembly mounted on plastic spacers due

to 2G, and 3G input sine-sweep tests are shown in Appendix 1.

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Table 4.1 Resonant frequencies of PCB assembly mounted on plastic

spacers

Mode No.

Frequency (Hz) due to: 1G input 2G input 3G input

1 46 46 46

2 60 60 60

3 109 109 834 140 140 140

5 210 210 210

6 240 240 2407 340 340 340

8 398 400 430

9 460 460 460

Table 4.2 Acceleration values (Peak –G) of the PCB assembly mounted on

plastic spacers

ModeNo.

Output acceleration (G) due to:

1G input 2G input 3G input 1 25 35 602 6 14 303 4 4 7.5 4 4.5 9 385 70 100 150 6 7 17 167 2.5 5 258 7 15 209 18 40 52

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Table 4.3 Displacement values of the PCB assembly mounted on plastic

spacers (measured at PCB centre)

ModeNo.

Single amplitude displacement (mm) due to: 1G input 2G input 3G input

1 2.93 4.11 7.04 2 0.41 0.97 2.07 3 0.09 0.10 0.26 4 0.06 0.11 0.48 5 0.39 0.56 0.79 6 0.03 0.07 0.07 7 0.01 0.01 0.05 8 0.01 0.02 0.03 9 0.02 0.05 0.06

4.2.2 Sine Sweep Test with PCB Assembly Mounted on Rubber

Spacers

In an attempt to reduce the transmissibility ratio, output

acceleration levels, and displacement amplitudes, and enhance the fatigue life

of electronic packages, the Neoprene rubber having the desired properties

(please refer Appendix 3) of an good vibration isolator was chosen as a spacer

material for mounting the PCB assembly (Figure 4.6). The neoprene rubber

also possesses some damping properties. All the fastening screws were

tightened uniformly using a torque wrench so that all the rubber spacers are

equally compressed (Salvatore Ligoure 1995).

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Figure 4.6 PCB assembly mounted on rubber spacers

The sine sweep test at an input acceleration level of 1G, 2G, and 3G

were conducted when the PCB assembly was mounted on rubber spacers

(dimensions are given in Appendix 1). The response of PCB assembly to 1G

input acceleration when mounted on rubber spacers is as shown in Figure 4.7.

From the figure it is observed that, the peak acceleration levels at first and

fifth mode are reduced by 28% and 29% respectively. Similarly, the peak

acceleration levels at higher frequencies are also reduced by considerable

amount. The natural frequencies, output acceleration levels, and displacement

values due to different input acceleration levels are tabulated in Tables 4.4,

4.5, and 4.6 respectively. The response curves of the PCB assembly due to

2G, and 3G input accelerations are shown in Appendix 1.

Rubber spacer

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Figure 4.7 Response of the PCB mounted on rubber spacers due to 1G

input

Table 4.4 Resonant frequencies of the PCB assembly mounted on rubber

spacers

ModeNo.

Frequency (Hz) due to:1G input 2G input 3G input

1 46 45 422 60 60 60

3 109 109 108

4 140 140 140 5 210 210 210

6 240 240 240

7 350 340 340 8 410 410 410 9 460 460 460

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Table 4.5 Acceleration values (Peak –G) of the PCB assembly mounted

on rubber spacers

ModeNo.

Output acceleration (G) due to:1G input 2G input 3G input

1 18 (28)* 26 (26)* 42 (30)* 2 5.5 (8.3) 12 (14) 15 (50)3 3 (25) 3 (25) 4.5 (40)4 1.5 (67) 2.5 (72) 4 (89.5)5 50 (29) 80 (20) 100 (29) 6 4 (43) 7 (61) 10 (88)7 3 (-20) 5 (--) 8 (68)8 2.2 (68) 5 (67) 9 (55)9 12 (33) 20 (95) 29 (44)

* Figures within bracket indicate percentage of variation in peak acceleration compared to plastic spacer

Table 4.6 Displacement values of the PCB assembly mounted on rubber

spacers (measured at PCB centre)

ModeNo.

Single amplitude displacement (mm) due to: 1G input 2G input 3G input

1 2.01 (31)* 3.14 (24)* 4.72 (33)* 2 0.31 (24) 0.79 (19) 1.04 (50) 3 0.07 (25) 0.07 (30) 0.11 (58) 4 0.02 (67) 0.03 (73) 0.05 (90) 5 0.23 (41) 0.42 (25) 0.51 (35) 6 0.02 (43) 0.03 (63) 0.04 (89) 7 0.01 (00) 0.01 (00) 0.02 (60) 8 0.00 (100) 0.01 (50) 0.01 (67) 9 0.01 (50) 0.00 (100) 0.04 (33)

* Figures within bracket indicate percentage of reduction in displacement compared to plastic spacer

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From Table 4.1 and 4.4 it is observed that the fundamental

frequency and higher mode resonant frequencies are almost same for both the

mounting types. From Figure 4.7 it is observed that the acceleration levels at

all the resonant frequencies are reduced and the percentage of reduction is

tabulated in Table 4.5. A reduction of 28% in peak response acceleration

(Table 4.5) and single amplitude displacement (Table 4.6) at the first

frequency are noticed when the PCB assembly was mounted on rubber

spacers.

Figures 4.8 and 4.9 show the comparison of the peak acceleration

(output) levels for the PCB mounted on plastic spacers and rubber spacers for

all the input acceleration levels. From Figure 4.8 it is evident that, the output

acceleration levels increases with increase in input acceleration and are

amplified at resonant frequencies. So, by using rubber spacers, the resonance

amplification is reduced and this fact is evident from Figure 4.9. The

acceleration amplitudes at all the resonant frequencies are reduced by a

considerable amount.

Figure 4.8 Peak acceleration levels

with the PCB mounted on plastic

spacers

Figure 4.9 Peak acceleration levels

with the PCB mounted on rubber

spacers

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Similarly, Figure 4.10 and 4.11 shows the comparison of

transmissibility ratios (Q) of the PCB assembly mounted on plastic and rubber

spacers respectively. From these figures, it is seen that there is a reduction of

about 28% in the transmissibility ratio at first frequency (46 Hz) and fifth

mode frequency (210 Hz). The transmissibility ratio decreases with increase

in input acceleration, which is evident from the figures 4.10 and 4.11 (also

refer Equation 4.3) and also agrees with the findings of John H.L (2004).

From transmissibility plot (Figure 4.11) it is also observed that, the

transmissibility ratios are reduced over the entire frequency range.

Figure 4.10 Transmissibility ratios

with the PCB mounted on plastic

spacers

Figure 4.11 Transmissibility ratios

with the PCB mounted on rubber

spacers

The displacement (measured at the centre of the PCB) of the PCB

assembly when mounted on plastic spacers at three input levels of

acceleration is shown in Figure 4.12. Similarly, Figure 4.13 shows the

displacement of PCB assembly when mounted on rubber spacers. The

displacement level compared to the PCB mounted on plastic spacers was

reduced by about 31% at the first resonant frequency and the same trend is

observed at higher frequencies also.

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Figure 4.12 Displacement levels

with the PCB mounted on plastic

spacers

Figure 4.13 Displacement levels

with the PCB mounted on rubber

spacers

From the above results it is seen that, the rubber spacers help in

reducing the transmissibility ratio, output acceleration levels, and

displacements of the PCB assembly, compared to the response of the PCB

assembly mounted on plastic spacers.

4.2.3 Sine Sweep Tests with the PCB Assembly Mounted on Rubber

Pads

In order to reduce the dynamic displacement of the PCB assembly

to a still lower level, it was decided to mount the PCB on two rubber pads

each measuring 300 mm x 20 mm x 12 mm in size (Figure 4.14). The longer

edges of the PCB were made to rest on the rubber pads and the PCB was

rigidly clamped using six fastening screws (passing through fixture plate,

rubber pad, and PCB). All the nuts were uniformly tightened using a torque

wrench. Again, to find out the dynamic responses of the PCB assembly, a

sine-sweep test (as described earlier) was conducted. The response obtained

due to 1G input is as shown in Figure 4.15.

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Figure 4.14 PCB assembly mounted on rubber pads

Figure 4.15 Response of the PCB mounted on rubber pads due to 1G

input

From Figure 4.15, it is observed that, the first prominent resonant

frequency is about 90 Hz, and peak acceleration at this frequency is 18G. The

PCB displacement measured at 90 Hz was found to be 0.55 mm. Thus, by

mounting the PCB assembly on rubber pads the fundamental frequency was

increased by 195% but, the peak acceleration was limited to 18G which is

same as that of the PCB mounted rubber spacers and displacement was

Rubber pad Nuts

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reduced by 81%. The responses of the PCB assembly using the three methods

(due to1G input acceleration level) are tabulated in Table 4.7 for comparison.

Table 4.7 Comparison of responses of all mounting methods (1G input)

PCB Mounting method

Fundamental frequency (Hz)

Peak acceleration (G)

Displacement

(mm)

Four plastic spacers 46 25 2.93

Four rubber spacers 46 18 (28)* 2.01(31)*

Rubber pads 90 (195)* 18 (28)* 0.55 (81)*

* indicates percentage of variation with respect to plastic spacer data

4.3 CALCULATION OF DAMPING RATIO

Damping in a vibrating structure plays an important role in

establishing the magnitude of the transmissibility Q that will be developed

when the structure is excited at its natural frequency. When the damping is

increased, more kinetic energy is converted into heat so there is less energy

available to do work on the structure, which results in a decreased

transmissibility. When the damping is decreased, less kinetic energy is

converted into heat so there is more energy available to do work on the

structure, which results in an increased transmissibility. This can also be

related to dynamic displacements and stresses. The estimation of the damping

ratio of the system (PCB assembly) for different types of PCB mounting

methods is explained in the following paragraphs.

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The damping ratios of the PCB assembly when mounted on plastic

spacers, rubber spacers and rubber pads (Figure 4.16) were estimated using

half power bandwidth method (Andy Perkins et al 2008). The calculation of

damping ratio for the PCB mounted on plastic spacers is illustrated here.

Figure 4.16 Plastic spacer, Rubber spacer and Rubber pad

The response of PCB mounted on plastic spacers (Figure 4.5) is

reproduced (Figure 4.17) for calculating damping ratio. Referring to

Figure 4.17, Gfn is the peak acceleration level at first natural frequency fn. The

half power points Gfn are defined as 0.707 times the peak acceleration Gfn.

Bandwidth f of the half power points Gfn is defined as f = fh - fl . From

Figure 4.17, the peak acceleration Gfn at first resonant frequency (fn =46 Hz)

is 25G and Gfn will be 0.707 x 25G =17.67G. Bandwidth f around first

frequency is 2.4 Hz. The damping ratio is calculated using the following

relation.

026.046*24.2

*2 nff

(4.4)

110

fl fh

Figure 4.17 Damping ratio calculation with PCB mounted on plastic

spacers

Similarly, the damping ratios for the PCB assembly mounted on

rubber spacers and rubber pads are calculated by considering the acceleration

responses (Figure 4.7 and Figure 4.15) and the values are tabulated in Table

4.8. The procedure of calculating the damping ratio for other mounting

methods is illustrated in Appendix 1.

Table 4.8 Damping ratios for different PCB mountings

PCB Mounting method Damping ratio ( )

Four plastic spacers 0.026

Four rubber spacers 0.038

Rubber pads 0.044

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From Table 4.8 it is seen that, the damping ratio for the PCB

mounted on rubber spacers is 0.038 i.e., an increase of about 32% in the

damping ratio when compared to damping ratio of PCB mounted on plastic

spacers. Similarly, the damping ratio for the PCB mounted on rubber pads is

found to be 0.044 which is about 41% more compared to plastic spacers and

14% more compared to rubber spacers. Thus, increase in damping ratio of the

system has resulted in reduced transmissibility ratio and displacement.

4.4 ANSYS SIMULATION RESULTS

The experimental test results were validated by doing harmonic

analysis in the frequency range of 20-500 Hz at input accelerations of 1G, 2G,

and 3G using finite element analysis. For finite element simulation, three

dimensional model of the PCB assembly was created using ANSYS. The

material properties of the PCB and DIP package used for simulation are

tabulated in Table 4.9. Solid-92, 10 node elements were used to mesh all the

volumes. All the nodal degrees of freedom at the holes were arrested to

simulate the actual mounting method of the PCB. The harmonic analysis of

the PCB mounted on plastic spacers was carried out at input acceleration

loads of 1G, 2G, and 3G. During numerical analysis, a damping ratio of 0.026

(corresponding to PCB mounted on plastic spacers) was used.

Table 4.9 Properties of the components of PCB assembly

Component Young’s modulus (GPa)

Poisson’s ratio

Density (kg/m3)

PCB (FR-4) 24 0.284 2269

DIP body 17 0.3 2200

Lead wires 121 0.34 8954

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The displacement contour plot at the first frequency of 46 Hz is

shown in Figure 4.18, and from the plot, the maximum displacement at the

centre is found to be 2.76 mm, which is close to the displacement magnitude

obtained from experimental test (2.93 mm).

The finite element simulation results of PCB mounted on plastic

spacers and subjected to input acceleration loads of 2G and 3G are shown in

Appendix 1 and the summary of analysis results is tabulated in Table 4.10.

Figure 4.18 Displacement plot (1G input-plastic spacers)

The simulation was carried out to simulate the PCB assembly

mounted on rubber spacers and subjected to harmonic excitation. In this

harmonic simulation, an input acceleration of 1G and a damping ratio of

0.038 were used. Simulation results revealed a displacement of 2.18 mm at

the centre of the PCB (Figure 4.19) which is close to the experimental value

of 2.01 mm.

Finite element simulation of the PCB mounted on rubber pads was

also done at an input acceleration of 1G using a damping ratio of 0.044. The

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displacement plot obtained at fundamental frequency of 90 Hz is shown in

Figure 4.20. The maximum displacement at the PCB centre is found to be

0.53 mm which is close to the experimental value of 0.55 mm.

Figure 4.19 Displacement plot (1G input-rubber spacers)

Figure 4.20 Displacement plot (1G input-rubber pads)

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The summary of finite element simulation results when PCB was

mounted on plastic spacers, rubber spacers and rubber pads is given in

Table 4.10. From Table 4.10 it is evident that the simulation results are in

close agreement with the experimental results.

Table 4.10 Comparison of experimental and ANSYS results

Procedure

Displacement in mm(at fundamental frequency due to 1G input)

Plastic Spacer Rubber Spacer Rubber Pads

Experimental 2.93 2.01 0.55

ANSYS 2.76 2.18 0.53

4.5 RESULTS AND DISCUSSIONS

The dynamic responses of the DIP-PCB assembly, subjected to

sinusoidal excitation at an input acceleration loads of 1G, 2G, and 3G were

obtained experimentally. The dynamic responses (at 1G input) obtained from

the PCB assembly when mounted on plastic spacers, revealed a fundamental

frequency of 46 Hz, peak acceleration of 25G, single amplitude displacement

of 2.93 mm, and a damping ratio of 0.026.

In order to reduce the transmissibility ratio and the dynamic

displacements further, rubber spacers were chosen to support the PCB

assembly. The test results revealed a fundamental frequency of 46 Hz same as

before but, the peak acceleration and displacement levels were reduced to

18G and 2.01mm respectively. The damping ratio of the system obtained by

mounting the PCB assembly on rubber spacers was 0.038.

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The displacement value of 2.01 mm is again a too large value and

not safe for the PCB assembly. Hence, the second proposed method

(clamping longer edges of PCB on rubber pads) was used to reduce the PCB

deflections.

The test results showed an improvement in the natural frequency

(90 Hz) and the PCB deflection was reduced to 0.55 mm when rubber pads

were used. The damping ratio obtained by mounting PCB on rubber pads was

0.044. Thus, mounting PCB assembly on rubber spacers reduced the dynamic

displacement by 31%, peak acceleration level by 28% and transmissibility

ratio by 28%. Similarly, PCB mounted on rubber pads increased the

fundamental frequency by 195% and reduced the dynamic displacement by

81%.

Thus, from the experimental results it can be concluded that the

rubber spacers and pads used to mount the PCB assembly act as good

vibration isolators and reduce the dynamic displacements, peak accelerations,

and transmissibility ratios at resonant frequencies of the system. As a result of

reduced acceleration and displacement levels, the life of the electronic

assemblies will be improved.

During the sine sweep tests, it was observed that the plastic spacers

started failing at higher input accelerations (3G) and it is shown in

Figure 4.21. Therefore, at higher input acceleration levels plastic spacers will

not last longer and rubber spacers are the alternative solution.

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Figure 4.21 Good and failed plastic spacers

Finite element simulation was carried out in ANSYS to validate the

experimental results. Results obtained from simulation are in close agreement

with the experimental values.

Deformed face of plastic spacer