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CHAPTER 5

FREE FALL DROP IMPACT ANALYSIS OF BOARD

LEVEL ELECTRONIC PACKAGE

5.1 INTRODUCTION

The handheld electronic products such as mobiles, cameras, PDAs etc. are more prone to being dropped during their useful service life because of their size and weight. The dropping event can, not only cause mechanical failures in the housing of the device but also create electronic failures in the PCB assemblies mounted inside the housing due to transfer of energy through the PCB supports. The electronic failures may result from various failure modes such as cracking of PCB or the cracking of solder joint etc.

JEDEC has suggested two different test methods for evaluating the performance of hand held electronic components when subjected to drop impacts. JEDEC test standard JESD22-B104C, 2004 is a standard intended to evaluate electronic components to determine the compatibility of the components to withstand moderately severe shocks as a result of suddenly applied forces or abrupt change in motion produced by handling, transportation or field operation. This test is a destructive test intended for component qualification.

JEDEC test standard JESD22-B111, 2003 is a standard for performing board level drop test and is intended to evaluate and compare performance of surface mount electronic components for hand held electronic product applications in an accelerated test environment. This test is meant to identify the failure modes of the surface mount electronic packages. This test

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is not a component qualification test and is also not meant to replace any system level drop test that may be needed to qualify a specific handheld electronic product. The method is also not meant to cover the drop test required to simulate shipping and handling related shock of electronic components or PCB assemblies.

Because of the shear varieties in size and complexities of various handheld electronic products available in the market, it is practically impossible to pin point the vulnerable elements in a surface mount electronic package using JESD22-B104C test standard. Further, as mentioned earlier, JESD22-B104C is a product qualification test standard. JESD22-B111, 2003, board level drop test is convenient to characterize the performance of surface mount electronic packages, because it is more controllable than product level drop test. Hence, JESD22-B111, 2003, test standard is used in this research.

JEDEC standard (JESD22-B111, 2003) has proposed eight different service conditions for which board level drop test has to be carried out for hand held electronic products. The service conditions proposed by JEDEC are shown in Table 5.1

Table 5.1 Service conditions as per JEDEC for board level drop test

Service condition Acceleration peak (G) Pulse duration (ms)

H 2900 0.3

G 2000 0.4

B 1500 0.5

F 900 0.7

A 500 1.0

E 340 1.2

D 200 1.5

C 100 2.0

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In this research, the dynamic response characteristics of an

electronic package when subjected to board level drop impacts are

investigated. The PCB used for this research is similar to the ones reported in

chapter 3. Initially, board level drop tests are conducted on a custom made

PCB as per JEDEC standard using a drop test apparatus. A FE model of the

drop apparatus is developed and drop test is carried out using Drop Test

Module of ANSYS v 10.0 software. The dynamic response characteristics of

the PCB and the package are compared to validate the FE method. Critical

elements in the package vulnerable to failure due to dropping events are

identified and validated by experiments. Two simplified FE methods for drop

impact analysis reported in the literature are compared with experimental

results to identify the most suitable FE method in terms of accuracy of the

results produced when compared with experimental results. Using the

identified FE method, the dynamic responses of PCBs with three different

support configurations are analyzed to identify the best possible support

configuration that can with stand drop impact loads better.

5.2 EXPERIMENTAL FREE FALL DROP TEST

5.2.1 Experimental setup

Free fall drop impact tests were conducted using the drop impact

test equipment (M/Rad Corporation, Model 0505-20). Drop impact test

equipment consists of the following items:

a) Drop table

b) Base plate

c) Guide rods

d) Rigid base

e) Strike surface

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f) Accelerometer

g) Strain gauge

h) Signal conditioner

i) Data Acquisition System

Figure 5.1 shows a typical drop test apparatus. The drop table is

lifted to the necessary height using separate lifting mechanisms. The drop

table when released from a certain height travels down on guide rods and

strikes the strike surface which is mounted over the rigid base. The strike

surface is covered with nylon material to achieve the desirable pulse and

acceleration levels during drop tests.

A base plate with standoffs is rigidly mounted on the drop table.

The PCB assembly is mounted to the base plate standoffs using 4 screws, one

at each corner of the PCB. The PCB is mounted on the base plate in the

horizontal orientation with the electronic packages mounted on the PCB

facing downwards. An accelerometer (B&K 4517) is mounted on the drop

table near the supporting screws in order to measure the acceleration pulse

generated when the drop table hits the strike surface. A strain gauge is

mounted on the centre of the PCB to measure the strain produced along the

longitudinal direction of the PCB during the dropping event. The signals from

the accelerometer and the strain gauge are captured using separate Data

Acquisition Systems (DAQ cards). The DAQ cards are mounted on a NI PXI-

1042Q Chassis. The signals captured from the sensors are processed using

LabVIEW 8.6 software supported on the NI PXI-1042Q Chassis.

Typical impact pulse suggested for drop test by JEDEC (JESD22-

B111, 2003) is as shown in Figure 5.2.The pulse shown in Figure 5.2 is half

sine pulse and its equation is shown in equation 5.1. The shock pulse

measured using the accelerometer should resemble a half sine pulse.

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Figure 5.1 Free fall drop impact test equipment

Figure 5.2 Half Sine Impact pulse

(5.1)

Accelerometer DAQ Strain gage

DAQ

NI PXI Chassis

PCB

Strain

Accelerometer

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5.2.2 Experimental procedure

Figure 5.3 shows the actual drop test apparatus. Before performing

the drop test, it is important to check the drop test equipment for repeatability

of the impact pulse generated during drop impact tests. Hence, before the

PCB is mounted on to the base plate, an accelerometer (B&K 4517) was

mounted on the base plate to measure the impact pulse generated during drop

impact and the drop block was dropped from a fixed height several times and

the impact pulse generated was monitored during each drop.

Figure 5.3 Actual experimental set up for free fall drop impact

Guide rod

Limit switch for setting

drop height

Drop table

Machine base

Strike surface

PCB mounted with packages facing down

Strain gauge Base plate Accelerometer Drop table

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The impact pulses measured when the drop block was dropped from several fixed heights are shown in Figure 5.4 to 5.7. From these figures it is clear that the drop impact equipment is able to generate impact pulses consistently. The maximum variation in the pulses measured from various drop heights was found to be ±6%.

Figure 5.4 Pulse measured when drop table dropped from a height of

400 mm

Figure 5.5 Pulse measured when drop table dropped from a height of

700 mm

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Figure 5.6 Pulse measured when drop table dropped from a height of

900 mm

Figure 5.7 Pulse measured when drop table dropped from a height of

1000mm

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Once the repeatability of the drop impact equipment was

established, the PCB was mounted on to the base plate and drop impact test

was conducted. As shown in Figures 5.4 to 5.7, different drop heights produce

different shock pulses with different pulse duration. Figure 5.7 shows an

average peak pulse of 1500G and pulse duration of 0.5 ms. This pulse

produced matches with the service condition B of JEDEC standard (JESD22-

B111, 2003). It was the severest shock pulse that can be produced by the test

equipment. Hence, it was decided to perform the drop test from the height of

1000 mm as shown in Figure 5.7. The impact pulse generated during one such

drop and the strain measured at the centre of the PCB are shown in Figure 5.8

and Figure 5.9 respectively.

Figure 5.8 Impact pulse measured at the centre of the PCB during

drop test

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Figure 5.9 Strain measured at the centre of the PCB during drop test

5.3 FREE FALL DROP FINITE ELEMENT MODEL OF PCB

It usually takes more than four months (including board design, fabrication, assembly and testing) and involves significant amount of cost to conduct an actual drop test for package qualification or design analysis. Due to pressure of short time-to-market, testing has become a bottleneck for semiconductor and telecommunication industry. Therefore, there is a need for a faster and cheaper solution, i.e., validated drop impact model, which is accurate, reliable, and enables understanding of physics-of-failure for design improvement. In general, a validated drop impact model should have good correlation of dynamic responses of PCB (system’s structural behavior) with the experimental results.

A FE model of the PCB for free fall drop impact was developed based on actual drop test equipment. The FE model consists of six different parts and is shown in Figure 5.10. The six different parts and its material properties are listed in the Table 5.2. The PCB is mounted on the drop table with four mounting screws. A stand-off distance of 10 mm is maintained between the PCB and the drop table as in the case of experiment. The BGA

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package is mounted on the PCB using 15 solder balls as shown in Figure 5.10. The PCB is mounted on the drop block with the package facing downwards. A layer of nylon in the form of absorbing surface is placed above the strike surface in order to generate the necessary sinusoidal impact pulse (JEDEC, JESD22-B111, 2003). Free fall drop impact analysis was carried out using Drop Test Module of ANSYS V 10.0 FEA software.

In this FE analysis, initially it is proposed to achieve a shock pulse given in service condition B. Even though much severe shock pulses (service conditions G and H) were achievable in the analysis, because of limitation in the achievable pulse in actual experimental set-up it was decided to restrict the shock pulse to service condition B. Once the FE results were correlated with experimental results, FE analysis can be carried out for higher shock pulse conditions to study the performance of the packages.

Figure 5.10 Detailed FE model of the PCB for free fall drop impact analysis

Package PCStand off

Absorbing material

Strike surface

Drop table

Solders Package

PCB

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Table.5.1 Material properties of parts used in FE model

S.No Material Description Density,

(kg/m3)

Young’s modulus, E

(N/m2)

Poisson’s ratio

()

1 Steel Strike surface 7800 2.2 x 1011 0.3

2 Nylon Absorbing surface 1140 1.6 x 109 0.3

3 Aluminium Drop table 2700 7 x 1010 0.3

4 Plastic Package 2100 2.2 x 1010 0.29

5 Lead Solder balls 8440 3.2 x 1010 0.38

6 FR-4 PCB 2200 1.7 x 1010 0.35

Before starting the analysis, the drop table is brought close to the

absorbing surface and is dropped from this height as shown in Figure 5.11.

The actual height from which the drop table is to be dropped is accounted for

by giving an initial velocity to the drop table at the beginning of the analysis.

The velocity of the drop table at the beginning of the analysis is computed

using equation 5.2 from the energy conservation principle.

Velocity of the drop table = v = gh2 (5.2)

where, g = acceleration due to gravity (9.81 m/s2)

h = height from which the block is to be dropped in metres

This will reduce the solution time for the drop impact analysis. The

drop table and the other components are modeled using SOLID 164 element

in this explicit dynamic analysis.

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Figure 5.11Drop table brought closer to strike surface before analysis

The results obtained from the FE analysis are shown in Figure 5.12.

From this figure it is clear that as the drop table strikes the strike surface, it is

abruptly comes to rest from the free fall condition which generates an

acceleration pulse in a direction opposite to that of drop direction. This

acceleration pulse is transmitted to the PCB through the four screws used for

fastening the PCB to the drop table. Even though the drop table is stopped

abruptly, the PCB, because of inertia continues to travel down except at the

locations where it is rigidly fastened to the drop table as shown in Figure 5.12

(a). The PCB then springs back up and then down again which results in the

PCB vibrating up and down till the vibration dies down because of its

inherent damping. Because of this bending action of the PCB, the solder joints

Actual drop height (1000 mm)

Height from which drop

table is dropped

Drop table with PCB

Absorbing surface

Strike surface

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at the corner of the package mounted at the centre of the board are subjected

to maximum stress as shown in Figure 5.12 (b) and (c). Hence it can be

inferred from this analysis that in a free fall drop impact process, the failure of

the solder joints is due to the combination of the sudden acceleration pulse

transmitted to the PCB and the bending action of the PCB.

Figure 5.12 Results from FEA showing maximum stress at corner solder

balls due to axial force

In order to validate the FE model, the impact pulse transmitted to

the PCB through the fastening screws is obtained from a location on the drop

table close to the screws and is shown in Figure 5.13. The strain developed at

the centre of the PCB on its back side was also obtained from the FE model

and is shown in Figure 5.14.

PCB bending down due to inertia

Corner solder balls subjected to axial pull

because of bending action of the PCB

Maximum stress at the corner solder

ball

(c)

(b) (a)

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Figure 5.13 Impact pulse generated on the FE model of the drop table

Figure 5.14 Strain obtained from the centre of the FE model of the PCB

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5.4 COMPARISON OF FE AND EXPERIMENTAL METHODS

The results obtained from the FE and experiment methods are

tabulated in Table 5.3. From Table 5.3, it is clear that the results from the FE

method correlate well with the experimental results. The variation between

the pulse generated from the experiment and that from the FE method was

found to be 6.8%. This variation can be attributed to the fact that in the

experiment the PCB was mounted on the base plate which in turn is mounted

on the drop table. In FE model, the base plate is ignored and the size of the

drop table is increased to accommodate for the absence of base plate.

Including the base plate will increase the complexity of the FE model and will

further increase the computation time to solve the model. The variation in the

pulse generated is well within the acceptable limits of ± 10% (JESD22-

B104C, 2004).

Table 5.3 Impact pulse generated and the strain produced from FE

and Experiment methods

Input

acceleration (G)

Pulse duration

(ms)

Strain at the centre of the board (microstrain)

Full Drop FEA model

1410 0.6 81.5

Experiment 1514 0.5 98.2

JEDEC standard (JEDEC, JESD22-B111, 2003) has recommended

that during a drop test the PCB has to be dropped 30 times continuously or till

80% of all packages have failed. In this research, the actual drop test was

performed by dropping the PCB from the same height continuously. After

each drop, each package was tested using the dedicated circuit designed and

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built for each package on the PCB as described in chapter 4. After 26 drops, it

was found that there was no signal from the package at the centre of the board

whereas other packages were giving clear signals. From the results of FE

method, it is clear that maximum stress occurs at the corner solder joint of the

package mounted at the centre of the PCB. SEM photograph of the corner

solder joint in the BGA package mounted at the centre of the PCB were taken

and is shown in Figure 5.15. From this figure it is clear that the corner most

solder joint is the vulnerable one as the solder joint is found to have cracked

at the interface between the solder joint and the PCB. This further validates

the FE model as the FE results show (Figure 5.12 (b)) that maximum stress

occurs at the corner solder joint at the interface between the solder joint and

the PCB.

Figure 5.15 SEM photograph of failed solder joint

5.5 DEVELOPMENT OF VALIDATED DROP IMPACT MODEL

From the discussions in the previous sections, it is clear that the

validated full drop test model can be used to evaluate the performance of

packages under various drop test conditions specified by JEDEC. One of the

major hindrances in using the full drop test model is the amount of time taken

to arrive at a solution by the model. The analysis in the previous sections had

PCB Crack

Solder ball

Package

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taken nearly 48 hrs to arrive at a solution. In order to reduce the time taken

during the analysis, researchers have suggested two simplified FE drop test

methods to evaluate the performance of packages during drop impacts. The

first method is called the Input – G (Tee T.Y. et al, 2004(a)) and the other

method is called Support Excitation Scheme (Yeh C.L. et al, 2006). In the

subsequent sections, the performance of the PCBs subjected to drop impacts

was evaluated using the above two drop methods. The results from these

methods were then compared with experimental results. Once the suitability

of these models to analyze the performance of PCBs subjected to drop

impacts was established, the models were used to investigate the performance

of PCBs and packages when subjected to various drop test conditions

specified by JEDEC. The results were compared and conclusions were arrived

at based on the variation on the results obtained and the time taken for the

analysis.

5.5.1 Input – G method for drop impact analysis

During actual drop tests, the impact pulse generated on the drop

table was continuously monitored. This pulse is transferred to the PCB in the

form a shock load through the four screws used for fastening the PCB to the

drop block. Hence, this impact pulse measured in the experiment can be given

as input to the FE model of the PCB directly through the four support

locations. By this way, the drop block, absorbing surface and strike surface

can be eliminated completely from the FE model. Only the model of the PCB

is enough for performing the analysis. Also, by using the impact pulse

measured in the actual drop test for analysis, the complex variations in

friction in the guide rods of the drop test apparatus, contact conditions

between the drop block and the strike surface and other unknown testing

parameters were considered in the analysis indirectly. In case of full drop

model, the friction in the guide rods of the drop test apparatus is not

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considered for analysis. Input-G method simplifies the drop test model and

will produce consistent and realistic results (Tee T.Y. et al, 2004(a)).

Figure 5.16 shows the Input – G model of the PCB. In order to reduce

the time taken for the analysis, only quarter model of the PCB was developed

and symmetric boundary conditions were given to simulate the full PCB. The

impact pulse given as input is the one measured during the actual drop test

and is shown in Figure 5.8 and the values of the acceleration pulse developed

at various time intervals are shown in Table 5.4. This pulse is given as input

to the PCB at the four corner support locations used for fastening the PCB to

the drop block in the upward ‘z’ direction.

Figure 5.16 Input – G method for a PCB with four support locations

with package facing down

The deflection and the strain obtained from the centre of the PCB

are shown in Figure 5.17 and Figure 5.18 respectively. From these figures it is

clear that for the given shock pulse, the PCB bends down and springs up. The

maximum deflection and maximum strain observed at the centre of the PCB

were found to be 0.97 mm and 98.7 microstrains respectively.

G

G

G

G

Quarter model of the PCB used in the analysis

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Table 5.4 Impact pulse measured during experiment

Time (sec) Acceleration (G)

0 0 5E-05 7.320677 1E-04 16.7878

0.00015 40.05489 0.0002 72.6562

0.00025 130.3723 0.0003 225.2403

0.00035 386.3693 0.0004 659.6877

0.00045 1115.451 0.0005 1514.745

0.00055 1397.309 0.0006 989.3601

0.00065 442.0388 0.00070 74.92156 0.00075 0

Figure 5.17 Strain produced at the centre of the PCB in Input – G

method

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Figure 5.18 Deflection produced at the centre of the PCB in Input – G

method

The corner solder joints of the package mounted at the centre of the

PCB was found to experience the maximum stress. The maximum stress was

found to be 70 MPa. Only four solder joints are shown in Figure 5.19 as only

quarter model of the PCB was used in the analysis.

Figure 5.19 Maximum stress at the corner solder joint in Input – G

method

Length Direction

Width Direction

Corner solder ball

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5.5.2 Support Excitation Scheme

The drop test apparatus as shown in Figure 5.1 can be simplified as

a structural system of multiple degrees of system (MDOF), for which

components regarded as lumped masses are connected by springs and dash

pots of different coefficients. The simplified MDOF system of the drop test

apparatus is shown in Figure 5.20. According to this scheme, the MDOF

structural system can be separated into two independent structural systems

and an effective support excitation load can be applied to the PCB as shown

in Figure 5.21 (Yeh C.L. et al, 2006).

Figure 5.20 MDOF structural systems of drop test apparatus

Figure 5.21 Separated independent structural system

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The new structural system after separation consists of only the

standoff, the test board (PCB) and the mounted packages. From the known

mass of these structural systems, the load acting on these systems can be

computed using the measured acceleration pulse. These loads can then be

given as input load to the PCB, the packages and the solder balls for

performing the drop test analysis. Hence in this analysis also, only the PCB,

the packages and the solder joints are to be modeled. Other items like the drop

table, the strike surface etc., are eliminated by this scheme. The computed

loads acting on the PCB, the packages and the solder balls for the measured

acceleration pulse (Figure 5.8) are shown in Table 5.5.

Table 5.5 Support excitation load for the measured pulse

Time (sec) Acceleration

(G) Force_PCB

(N) Force_BGA

(N) Force_Pack

(N) 0 0 0 0 0

5E-05 7.320677 3.597E-05 1.91941E-08 1.15258E-07 1E-04 16.7878 8.24864E-05 4.4016E-08 2.6431E-07

0.00015 40.05489 0.000196809 1.0502E-07 6.30632E-07 0.0002 72.6562 0.000356994 1.90498E-07 1.14391E-06 0.00025 130.3723 0.000640581 3.41824E-07 2.05261E-06 0.0003 225.2403 0.001106713 5.90559E-07 3.54623E-06 0.00035 386.3693 0.001898416 1.01302E-06 6.08307E-06 0.0004 659.6877 0.003241359 1.72964E-06 1.03862E-05 0.00045 1115.451 0.00548074 2.92461E-06 1.75619E-05 0.0005 1514.745 0.006290037 3.97152E-06 2.38484E-05 0.00055 1397.309 0.005802379 3.66361E-06 2.19995E-05 0.0006 989.3601 0.004108356 2.59401E-06 1.55767E-05 0.00065 442.0388 0.001835583 1.15898E-06 6.95954E-06 0.00070 74.92156 0.000311115 1.96437E-07 1.17958E-06 0.00075 0 0 0 0

128

This load is distributed to all the nodes in the respective structural

systems. Support excitation scheme can be carried out with damping and

without damping. Both the conditions were investigated in this research. A

damping ratio of 0.012 (1.2%) measured at the first natural frequency

(Chapter 3) was given as input when analyzed with damping.

Figure 5.22 shows the FE model of the PCB with support excitation

load. The deflection and strain obtained from the centre of the board for both

with and without damping conditions are shown in Figure 5.23 and Figure

5.24 respectively. This model also validates that the solder balls at the corner

of the package mounted at the centre of the board is subjected to maximum

stress as shown in Figure 5.25.

Figure 5.22 FE model of the PCB for support excitation scheme

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Figure 5.23 Strain produced at the centre of the PCB for support

excitation scheme

Figure 5.24 Deflection produced at the centre of the PCB for support

excitation scheme

5.5.3 Comparison of the drop test analysis methods

The two drop test analysis methods discussed in sections 5.5.1 and

5.5.2 were compared to determine the validated drop test analysis method.

Since the dynamic response measured during the actual drop test is the strain

produced at the centre of the PCB on its backside, the same has been used for

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comparison with the drop test methods. The results from the three methods

are tabulated in Table 5.6.

Figure 5.25 Maximum stress at the corner solder ball

Table 5.6 Comparison of the two FE methods with experiment

Input

acceleration (G)

Pulse duration

(ms)

Strain at the centre of the

board

(microstrain)

Pulse duration of the strain

produced (ms)

Experiment 1514 0.5 98.2 1.2

Input-G 1514 0.5 98.7 1.5

Support Excitation

1514 0.5 79.02 1.0

The maximum strain produced in the Input-G method was found to

be 98.7 microstrains and that from the experiment was found to be 98.2

microstrains. In case of support excitation scheme, the maximum strain with

damping was found to be quite low (79.02 microstrains) when compared with

Corner solder ball

Length Direction

Width Direction

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the measured strain from experiment. The variation in the strain measured

from the experiments and that from the analysis is in the time duration of the

strain pulse. In case of experiments, the pulse duration of the strain produced

was found to be 1.2 milliseconds and that from Input-G was found to be 1.5

milliseconds. In case of support excitation scheme, the pulse duration was

found to be 1 millisecond.

The failure of the solder joints during a dropping event is due to the

bending action of the PCB. This bending action produces the maximum strain

at the centre of the PCB. Since the maximum strain produced at the centre of

the PCB in the experiment and in the Input-G method are found to correlate

very well, it can be concluded that the Input-G method can be considered as a

validated drop test method.

5.6 COMPARISON OF DYNAMIC RESPONSES FOR

DIFFERENT MOUNTING CONFIGURATIONS OF THE

PCB

Once a validated drop test method was established using Input-G

method, the dynamic responses of the PCB for three different mounting

configurations were analyzed using the Input-G method. The three mounting

configurations considered for the analysis are the following:

Four screw corner mounting (Figure 5.26 (a))

Four screw diamond mounting (Figure 5.26 (b))

Six screw corner mounting (Figure 5.26 (c))

The analysis was carried out for two different pulses. One is the

measured pulse from the experiment (Figure 5.8) and the other is the Service

condition H specified by JEDEC (JESD22-B111, 2003). Service condition H

shown in Figure 5.27 is chosen because of is the severest drop impact

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condition that a board level package is expected to withstand during its

service life.

Figure 5.26 Various mounting configurations of the PCB used for drop

test analysis

Figure 5.27 Acceleration pulse as per service condition H of JEDEC

standard

(a) (b)

(c)

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5.6.1 PCB subjected to measured impact pulse from experiment

The strain and deflection produced at the centre of the PCBs for the

three different support configurations are shown in Figure 5.28 and Figure

5.29 respectively. From these figures, it is clear that the PCB with four corner

supports is found to be subjected to maximum strain and deflection at its

centre. For the other two configurations, the strain produced at the centre was

found to be reduced by 85% for the PCB with diamond configuration and

95% for the PCB with six screw corner configuration.

Figure 5.28 Strain produced at the centre of the PCB for the three

configurations

The deflection produced at the centre of the PCB with four corner

screws was found to be 0.97 mm. For diamond configuration, the deflection

was found to be 0.34 mm (65% reduction) and for six screw corner

configuration it was found to be 0.36 mm (63% reduction). From the above

discussions it can be observed that the PCB with six screw corner

configuration was found to have the minimum strain. Even though the

diamond configuration of the PCB has only four support locations like the

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four screw corner configuration, there is a considerable reduction in the

deflection and the strain produced at the centre of the PCB with diamond

configuration when compared with the PCB with four screw corner

configuration. This can be attributed to the fact that the PCB is constrained

from bending along both the length and width directions because of the

presence of two support locations each at the centre line of the PCB along the

length and width directions.

Figure 5.29 Deflection produced at the centre of the PCB for the three

configurations

The equivalent stress produced at the critical solder joint for the

three configurations is shown in Figure 5.30. The stresses are also found to be

considerably reduced for the diamond and six screw corner configurations

when compared with the four screw corner configuration. The maximum

stress at the critical solder joint was found to be 64.9 MPa for the PCB with

four screw corner configuration. The maximum stress was found to be 13.8

MPa (78.7% reduction) for PCB with diamond configuration and 9.3 MPa

(85.7% reduction) for PCB with six screw corner configuration.

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Figure 5.30 Equivalent stress produced at critical solder joint

5.6.2 PCB subjected to service condition H

The strain and the deflection produced at the centre of the PCB for

the three configurations are shown in Figure 5.31and Figure 5.32 respectively.

The maximum strain for the four screw corner configuration was found to be

836.87 microstrains. For diamond and six screw corner configurations, it was

found to be 148 microstrains (82% reduction) and 14.5 microstrains (98%

reduction) respectively. The maximum deflection at the centre of the PCB for

the six screw corner configuration was found to be smallest (0.47 mm)

compared to the other two configurations (1.13 mm for four screw corner and

0.52 mm for diamond).

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Figure 5.31 Strain produced at the centre of the PCB for the three

configurations

Figure 5.32 Deflection produced at the centre of the PCB for the three

configurations

137

Similar trend was observed in the stresses produced at the critical

solder joints as shown in Figure 5.33. The maximum equivalent stress at the

critical solder joint was found to be 23.05 MPa for the diamond configuration,

23.03 MPa for six screw corner configuration and 77.2 MPa for four screw

corner configuration.

Figure 5.33 Equivalent stress produced at the critical solder joint

5.7 CONCLUDING REMARKS

In this chapter, a comprehensive investigation was carried out to

evaluate the performance of a PCB with mounted IC packages subjected to

free fall drop impact loads. The investigation was carried out by using both

experiments and FE methods.

Initially, a complete free fall drop FE model of the PCB was

developed. The FE model consists of the drop block, mounting screws, PCB,

and packages. Analysis of the FE model was carried out using the Drop Test

Module of Ansys Ver. 10 software. The impact pulse produced at the drop

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block near the mounting screws and the strain produced at the centre of the

PCB were captured from the FE model.

Experiments were carried out using the free fall drop impact test

apparatus on an actual PCB having the same configuration as used in the FE

model. The impact pulse at the drop block near the mounting screws and the

strain produced at the centre of the PCB were captured using an accelerometer

and strain gauge respectively. Comparison of the two results showed that the

FE model correlates well with experiments. The critical solder joint in the

PCB was identified from the FE model and it was found to be the one located

at the corner of the package mounted at the centre of the PCB. This was

further verified by experiments. SEM photograph of the solder joints revealed

that the corner solder joint of the package mounted at the centre of the PCB

has cracked because of the bending action of the PCB during drop impact.

The need for a simplified but validated FE method for drop test was

identified because of the large amount of time taken by the full drop FE

model to arrive at a solution. Two FE methods proposed in the literature for

analyzing the performance of packages subjected to drop impact loads were

compared to determine a validated but simplified FE method. The result from

the Input-G method was found to be in agreement with experimental results

and hence was considered to be a validated FE method.

Using the validated FE method, further investigations were carried

out to compare the dynamic response of the PCB for three different mounting

configurations of the PCB when subjected to two different drop impact

pulses. The results revealed that the six screw corner configuration was able

to with stand impact loads better than other two configurations for all pulse

levels.