Structural Analysis on Stretchable Electronics Joints of ...thevibrationlab.eng.usm.my/images/ICVSSD/FullPaper/... · solder, a lead-free solder and a conductive adhesive. The lead-based

Proceedings of the International Conference on Vibration, Sound and System Dynamics Penang, 2 August 2017

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Structural Analysis on Stretchable Electronics Joints of Automotive LED

Lighting under Vibration Loading

M.F.M. Sharif

A.A. Saad

N. Mustafa School of Mechanical Engineering, Universiti Sains Malaysia, Penang, MALAYSIA

F.C. Ani

M.Y.T. Ali Jabil Circuit Sdn Bhd, Bayan Lepas Industrial Park, 11900 Penang, MALAYSIA

The purpose of this paper is to investigate the effect of vibration loading on structural integrity of LED joints strength in

manufacturing of automotive lighting. Ten LEDs were separately assembled on printed electronic circuit by dispensing a

conductive adhesive. There were five different build matrices constructed for the conductive adhesive and each matrix had two

samples. A platform was fabricated and mounted to the shaker to run the samples altogether in vertical and horizontal

positions. The samples were run under random vibration in frequency range of 3 Hz to 500 Hz for 4 hours. The assembly of

LED joints was further analyzed using finite element simulation to understand their stress-strain behavior under vibration

loading. The quality and integrity of LED joints were investigated using microscopy observation and x-ray analysis at the LED

joints. The results show there is no failure generated from the vibration test of applied period. The experimental result was

supported by finite element analysis on detail stress-strain behavior during the test.

Keywords: LED joints; Stretchable electronics; Vibration fatigue.

1 INTRODUCTION

Random vibration is the most related vibration

condition subjected in the environment conditions

due to destabilization which created by rotational and

tumbling masses. Automobiles and portable

electronic components are mostly exposed to random

vibration. A system can vibrate in a standard manner

that is known as vibration modes representing the

degree of freedom of the vibration in the system with

certain natural frequency. Range of frequency varied

according to its application, it had been tested that 2.5

Ton truck provide acceleration level 15 G and 19 G

with frequency 15 Hz – 40 Hz at speed of 10 mph -

15 mph (Steinberg & Wiley, 2001). In the

automobiles vibration frequency range is varied with

the system, vibration while driving, suspension

system, and steering system, which the vibration

range from 3 Hz – 1 KHz (Izumi, n.d.). A range of

frequency is excited at the same time, which means

that frequency loading correspond to all masses are

represented at the same time which produce large

displacement amplitude to produce impact of failure

to analysis system. To quantify random frequency,

Power Spectral Density (PSD) input is needed. PSD

is the input curve of acceleration units per Hz² versus

range of frequencies. PSD curve input is translated to

the Root Mean Square (RMS) value which the value

of each curve is formulated to a single quantity. As

the random vibration is the nature response statistic it

is represented in zero mean distribution where the

result is presented in three standard deviations of

RMS value in the presented value of stress, strain,

acceleration and velocity (Kumar & Engineer, 2008).

Several types of interconnection materials have been

used in electronic packaging such as a lead-based

solder, a lead-free solder and a conductive adhesive.

The lead-based solder was the main interconnection

material in electronics industry as it is requires low

temperature and less issues regarding to its quality.

However due to restriction in law and new

requirements for non-toxic materials, lead free solder

and conductive adhesive are used as alternative (Li &

Wong, 2006). Lead free solder is a tin based alloy,

common composition of lead free solder used are Sn-

Ag, Sn-Cu, Sn-Zn and Sn-Ag-Cu. Conductive

adhesives are a composite of adhesive epoxy and a

conductive metal such as silver, silver, tin oxide or

indium. Concept of the composition is that soft

material epoxy filler promote good contact within

and other particles as it deform and shrink during

curing process. Most common material for the epoxy

filler is silver as it is a good conductor, commercially

available and has moderate cost. Conductive adhesive

is applied as the electronics interconnections by using


22

similar method as the solder paste, where it is

dispensed to the circuit before placement of

electronic component. Conductive adhesive does not

need a specified pressure for curing and curing

temperature is between 130ºC - 180ºC which suitable

for the thermal sensitive electronic components

(Siow & Lin, 2016).

The finite element analysis is used to predict stress

strain behaviour of the solder joint and fatigue

lifetime. There are two models that have been used in

other research which are detailed FE models and

smeared FE models. Detailed FE models however are

not recommended since for a commercial case as the

time required to build and solve such a model is

excessive when simplified models produce data of

appropriate accuracy much more quickly and with

less effort (Amy & Aglietti, 2009). The simulation of

FEA used to check the stresses on solder balls when

conducting a fatigue life assessment of the

component, the boundary condition setting that

identical to the vibration test is used in the analysis.

The variation in mesh densities is applied in the

model in order to examine the convergence of the

analysed frequency results. For verification of FEA

model, natural frequencies test vehicle are examined

experimentally with modal testing method (Chen,

Wang, & Yang, 2008). Besides, FEA is used to

obtain stresses for the fatigue estimation. The

analysis then was validated with experimental modal

analysis to determine natural frequencies and mode

shapes (Yu, Al-Yafawi, Nguyen, Park, & Chung,

2011). Researcher also, employed FEA to get

numerical deflection of solder joints similar to the

out-of-plane displacement measured with the

scanning vibrometer. The package is accurately

modelled. The modal analysis is first performed on

the global model and compared with experimental

results. Equivalent strain in the critical area of solder

joints is then determined with global-local approach

(Libot et al., 2016).

Aim of this paper is to investigate the effect of

vibration loading on structural integrity of LED joints

strength in manufacturing of automotive lighting.

The experimental work is supported by simulation

study in order to determine the stress generated due

to the vibration loading.

2 METHODOLOGY

2.1 Experimental procedure

The conductive adhesive (Loctite Ablestik ABP

2032S) that contained in syringe was taken out from

freezer and left in room temperature at least around

20-30 minutes for thawing process which depended

on volume of the adhesive in the syringe (reference).

The syringe with needle size 0.5 mm in diameter was

mounted to Liquid Dispenser (JB1113N) machine

and the machine was set at pressure 50 psi with

duration timing 1 s.

The adhesive was dispensed using five build matrices

and each build matrix had two samples in order to

investigate strength of the interconnections between

LEDs joints and the circuit as depicted in Fig. 1. The

circuit printed on polycarbonate sheet was cut into

small pieces and each piece consisted of one LED.

All of the samples were attached on a customized

platform using Araldite epoxy adhesive where five

samples were in vertical position and another five

samples were in horizontal position as illustrated in

Fig. 2. The platform was mounted on shaker to run

the vibration loading on the samples altogether. The

shaker was set at frequency range of 3 Hz – 500 Hz

using random vibration with running time duration 4

hours. Before the samples were tested, the shaker’s

amplitude was calibrated by mounting an

accelerometer on it and gets the desired amplitude.

The bonding between the joints and the circuits was

observed at every time interval 30 minutes and the

final inspection was done by using Hi-Scope

Advance (Hirox) and x-ray scanning. The assembly

of LED joints was further analyzed using finite

element simulation to understand their stress-strain

behavior under vibration loading. The quality and

integrity of LED joints were investigated using

microscopy observation and x-ray analysis at the

LED joints.

2.2 Simulation Procedure

Two 3-dimensional (3D) CAD models were

developed based on the assembly prototype model

that had been used in experiment as shown in Fig. 1.

CAD model of LED was developed by referring to

the actual component specifications from

manufacturer. The actual dimension of the adhesive

cannot be determined as it was not a geometry

structure, therefore, the adhesive 3D CAD model was

developed approximately as the actual prototype as

illustrated in Fig. 3. Whereas, models of the circuit

and polycarbonate base was modified to cater the

needs of simulation as shown in Table 1. The

complete assembly model as shown in Fig. 4.


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(a) (b)

(c) (d)

(e)

Figure 1: The build matrices of a) 3 dots at bottom + 1 dot

at top of the joints b) 2 dots at bottom + 1 dot at top of the

joints c) 1 dot at bottom + 1 dot at top of the joints d) 3

dots surround the joints e) 2 dots surround the joints

Figure 2: The vibration test setup for LEDs joint

(a) (b)

Figure 3: Assemble model for LED with adhesive for (a)

build matrix no. 1 and (b) build matrix no. 4.

Table 1: Dimension for each parts of geometry model

Parts Dimensions (mm)

Base of the lead 5.5×2.5×0.5

Adhesive geometry

model 1

6.5×3.5×0.5

Adhesive geometry

model 2

6.5×3.5×1.2

Silver circuit pad 20×15×0.08

Polycarbonate base 150×150×1

Figure 4: Complete assembly model

Materials properties were defined based on the

material used in the experimental prototype model.

Materials’ mechanical properties were provided from

the ANSYS materials library and manufacturers’

datasheets. All materials were assumed to be elastic-

plastic materials as summarized in Table 2.

2.3 Random Vibration Parameter Input

The solution data history from the modal analysis

was used in random vibration analysis. The number

of 12 mode shapes used and 0.02 of constant

damping ratio need to be set up in the analysis

settings. Random vibration is a non-deterministic

motion. The vibration pattern would be varied, to

quantify the frequency excitation PSD input need to

be assigned in the simulation. In this analysis band

limited white noise has been used which the spectral

density has a constant value over a quantified

frequency range as shown in Figure 5. Frequency

range from 3 Hz - 500Hz would be excited at the

same time. The PSD input was referred from the

JEDEC standard for vibration and variable frequency

to test the reliability of package devices under

various levels of application vibration to which

component can be exposed (Lai, 2009). The PSD

input applied in this analysis was based on level

where the component can be exposed to the most


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severe condition. PSD excitation was applied on the

model’s fixed support in Y-axis direction which was

perpendicular to the model plane.

Figure 5: Power Spectral Density input pattern and values

Table 2: Mechanical material properties

Parts Density

(kg/m3)

Young’s

modulus

(MPa)

Poisson

ratio

Yield

strength

(MPa)

Tangent

modulus

(MPa)

Ultimate

tensile

strength

(MPa)

Circuit pad 8900 130000 0.34 120 125 210

Polycarbonate

base 1200 2506 0.38 63 0.05 65

Adhesive 4500 4140 0.32 24.1 38.62 34.5

Lead 8300 110000 0.34 280 1150 491

3 RESULT AND DISCUSSION

3.1 Visual Inspection of LED Joints

Based on visual inspection, there was no

disconnection occurred at adhesive/circuit interface

after the vibration loading for both LEDs in vertical

and horizontal positions as shown in Fig. 6 and Fig.

7. Neither LED joints nor adhesive peeled off from

the circuit. It shows that strength of the adhesive was

enough to withstand the vibration loading even the

LEDs were positioned horizontally where more

internal force loading exerted on the adhesive

bonding at the LEDs joints since mass of the LED’s

head included as gravitational force.

Figures 8 and 9 show x-ray images of bottom view of

the adhesive bonding at the LEDs joints. There was

no defect such as crack propagated during the

vibration loading at the adhesive bonding. However,

some small voids were visualized in the adhesive for

a certain bonding. These voids formed during the

dispensing process of the adhesive at the joint and

they stayed in the adhesive after curing process. The

formation of the void did not contribute a significant

effect to the bonding as the bonding had not

experienced any failure after vibration loading.

Figure 9: X-ray images of bottom view for (a) build

matrix 1 (b) build matrix 2 (c) build matrix 3 (d)

build matrix 4 (e) build matrix 5 which placed in

horizontal position.

6.2 Random Vibration Analysis

Random vibration is presented in the manner of

Gaussian probability distribution with zero-mean and

the RMS response is described in the form of sigma

values, 1σ, 2σ and 3σ, where 1σ, 2σ and 3σ are lying

between the G acceleration loading with probability

68.3%, 95.4% and 99.7% of occurrence over the time

respectively.

From the overall results for both models, maximum

equivalent stress occurred at the circuit pad. As


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shown in Figure 10 (a) and (b) the stress on the

circuit pad for model 2 distributed more uniform than

model 1 which the stress was only generated in

certain spot. This was occurred due to solder joint of

model 2 had more surface contact area with the

circuit pad than model 1. The stress contour plot

obtained as in Figure 11 (a) and (b) for the solder

joint shows that stress was generated at the surface

below the solder joint which contact with the circuit

pad. Figure 12 (a) and (b) shows that stress was also

generated at the LED lead where the maximum

equivalent stress for model 1 was concentrated at the

corner of the LED lead and for model 2 the stress was

distributed from the corner of the LED lead to the

LED lead base.

Figure 6: Visual test results of vibration loading for

(a) build matrix no. 1 (b) build matrix no. 2 (c) build

matrix no. 3 (d) build matrix no. 4 (e) build matrix

no. 5 which placed in vertical position.

Figure 7: Visual test results of vibration loading for

(a) build matrix no. 1 (b) build matrix no. 2 (c) build

matrix no. 3 (d) build matrix no. 4 and (e) build

matrix no. 5 which placed in horizontal position.

Figure 8: X-ray images of bottom view for (a) build

matrix 1 (b) build matrix 2 (c) build matrix 3 (d)

build matrix 4 (e) build matrix 5 which placed in

vertical position. (a) (b) (c) (d) (e)

(a) (b) (c) (d) (e)


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(a) (b)

(c) (d)

(e)

Figure 9: X-ray images of bottom view for (a) build matrix

1 (b) build matrix 2 (c) build matrix 3 (d) build matrix 4

(e) build matrix 5 which placed in horizontal position.

3.2 Random Vibration Analysis

Random vibration is presented in the manner of

Gaussian probability distribution with zero-mean and

the RMS response is described in the form of sigma

values, 1σ, 2σ and 3σ, where 1σ, 2σ and 3σ are lying

between the G acceleration loading with probability

68.3%, 95.4% and 99.7% of occurrence over the time

respectively.

From the overall results for both models, maximum

equivalent stress occurred at the circuit pad. As

shown in Figure 10 (a) and (b) the stress on the

circuit pad for model 2 distributed more uniform than

model 1 which the stress was only generated in

certain spot. This was occurred due to solder joint of

model 2 had more surface contact area with the

circuit pad than model 1. The stress contour plot

obtained as in Figure 11 (a) and (b) for the solder

joint shows that stress was generated at the surface

below the solder joint which contact with the circuit

pad. Figure 12 (a) and (b) shows that stress was also

generated at the LED lead where the maximum

equivalent stress for model 1 was concentrated at the

corner of the LED lead and for model 2 the stress was

distributed from the corner of the LED lead to the

LED lead base.

The analysis showed that the most critical area for

both models when subjected to the vibration loading

are at the connection of solder joint and circuit pad,

which means that there are high probability that the

solder joints are cracked or peeled off, either from the

silver circuit pad or together with silver circuit pad.

(a)

(b)

Figure 10: Maximum equivalent stress contour plot at

circuit pad for (a) model 1 and (b) model 2.

(a)


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(b)

Figure 11: Maximum equivalent stress contour plot at

solder joint for (a) model 1 and (b) model 2.

(a)

(b)

Figure 12: Maximum equivalent stress contour plot on

LED lead for (a) model 1 and (b) model 2.

Table 3: Maximum equivalent stress at circuit pad

Model 1 Model 2

Sigma

value

Maximum

equivalent

stress (MPa)

Maximum

equivalent

stress (MPa)

1σ 224.19 80.018

2σ 448.39 160.04

3σ 672.58 240.06

Table 4: Maximum equivalent stress value at solder joint

Model 1 Model 2

Sigma

value

Maximum

equivalent

stress (MPa)

Maximum

equivalent

stress (MPa)

1σ 60.857 41.316

2σ 121.71 82.631

3σ 182.57 123.95

Table 5: Maximum equivalent stress value at LED lead

Model 1 Model 2

Sigma

value

Maximum

equivalent

stress (MPa)

Maximum

equivalent

stress (MPa)

1σ 159.09 15.463

2σ 318.19 30.926

3σ 477.28 56.388

4 CONCLUSION

The automotive LED joints could withstand when

tested under random vibration in frequency range of

3 Hz to 500 Hz for 4 hours. There was no failure

observed at the bonding between the conductive

adhesive and the circuit pad or between the

conductive adhesive and the LED lead. The stress

generated at the LED joints also could be determined

for each model to support the experimental results.

These results show that the interconnection method

could be utilized in automotive lighting since there is

no effect on the structural integrity of the LED joints

after going through severe test.

ACKNOWLEGEMENT

The authors would like to thank Collaborative

Research in Engineering, Science & Technology

(CREST) for research grant P24C1-2015. Also, the

authors would like to acknowledge the support from

ETS Department of Jabil Penang and

TheVibrationLab of USM.


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Structural Analysis on Stretchable Electronics Joints of ...thevibrationlab.eng.usm.my/images/ICVSSD/FullPaper/... · solder, a lead-free solder and a conductive adhesive. The lead-based