1. INTRODUCTION
1.1 Background Information
The purpose of this project is to take a look at some of the major issues with electronics
packaging in the aerospace industry today. Namely, the application of Ball Grid Arrays
(BGAs) on rack style printed wiring boards (PWBs). There are many factors that affect
the BGA components on the circuit board: placement on the PWB, vibration level,
temperature levels, and solder type. In addition to all of these variables, every design is
slightly different with different resonances, and requirements. Rack card assemblies do
not have the ability to control the PWB resonances around the BGAs like other PWB
designs without significant design improvements; the goal is to keep the design simple.
The BGA that will be analyzed is the ACTEL 484 BGA. BGAs are used for programmable logic devices as well as processors for high amounts of inputs and outputs. These
are essentially the brains of the electronics, and it is critical to make sure they operate
under all conditions. Lastly, BGA’s are unreliable in higher vibration environments so
proper understanding of how each of the above variables affects the component is
critical to having a good design the first time, and to not run development testing under
common environments that are shown in this paper.
1.2 Lead vs. Lead Free Solder
The main reason for looking at lead free solder is to transform the aerospace electronics
to green lead free solder. Each of the packages in the ACTEL 484 BGA are built as lead
free. This is because these components are not only for aerospace applications, but also
for cell phones or computers too. The aerospace field is looking to go to lead free
components for many reasons including green compliance, ROHS compliance, and to
avoid obsolescence of key parts, mostly due to the fact that aerospace products life is up
to 20 years of service. The difficulty is that high volume products whose life is around 12 years, which means high risk or a part obsolescence, drive the electronics industry. In
addition to different mechanical and thermal properties, lead free solder has concerns
with tin whisker growth, which will not be explained in this paper. The last reason that
aerospace is behind in making the change to lead free solder is due to the different
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techniques and procedures to assemble these parts to the PWBs. Lead based solder will
not be around forever, hence the importance of studying the differences of lead vs lead
free solders and their mechanical responses in BGAs.
1.3 Random Vibration for Fixed Wing Aircraft
Electronic controllers are not structural members of the aircraft and thus cannot be
analyzed by simple static analysis. The mechanical stress analysis is governed by the
requirements from RTCA-DO-160, which is an aerospace standard for environmental
tests. Obviously one cannot test the electronics under the actual loads of the aircraft
because it would take years to find out a result. RTCA DO-160 accelerates the vibration
environments to test each axis in 1-5 hours on average. The random analysis is used
primarily to solve frequency responses and find resonances/mode shapes of the PWBs.
Typically this analysis is matched with test data to calculate the number of cycles to
failure. The analysis is intended to be detailed and show a correlation of failure cycles to
vibration level, solder type and placement. More detail will be discussed in the theory
and methodology and chapter.
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2. THEORY AND METHODOLOGY
2.1 Analytical Model: PCB Normal Modes and Displacement
The analysis will done based on a 6”x9” Printed Circuit Board (PCB). Of course there
are many sizes and configurations that can be used to optimize the board design, but
boards of this size are common in the lower fuselage electronics bays of aircraft. Placements will include the center of the board, upper right corner, lower left corner and left
center. This is shown by the figure below:
Figure 2.1-1: PWB BGA Placement
Each placement on the board will have different results based on the curvature of the
board and the board displacement at that point. This means that every single solder ball
will have a unique max stress based on the curvature and placement. This is one key
item to find in the analysis.
The first thing about doing a Finite Element Model, analytical calculations are invaluable to determining if the solution is correct. There are some simple equations from
Steinberg’s Vibration Analysis for Electronic Equipment. The first equation is used to
solve for the first mode natural frequency of a PWB that is fixed on 3 edges and has 1
free edge (fixed means that the edge is controlled in all 6 degrees of freedom). The FEM
boundary conditions are modeled to be exactly like a circuit card in a chassis. The fixed
positions are from the connectors at one end of the PWB that connect to the interconnect, and also the two card guides on the long edge of the PWB.
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Interconnect
Fixed Edge (6DOF) (Card Guide)
b
Connectors
Fixed Edge (6DOF)
Free Edge
a
a
Fixed Edge (6DOF) (Card Guide)
Figure 2.1-2: PWB and Inteconnect Diagram
fn 
 D .75
2
12 
   4  2 2  4  [3]
a
3 
a b b 
D
E * h3
[3]
12 * (1   2 )

The first mode natural frequency is based upon the Young’s Modulous, thickness,
poisson’s ration, which is the plate stiffness D, the density and the length and width; a

and b respectively. The second equation is to solve for the displacement of the PWB
based off the first mode natural frequency at the center of the board. :
Z RMS 
G

2
9.81* G
[3]
fn2
* P * Q* f n [3]

The displacement is really a dynamic single amplitude response based on the first mode
natural frequency and G, or GRMS. GRMS is the response of the PWB based off the

transmissibility, Q, first mode natural frequency, and the power spectral density value
where the first mode is. The transmissibility is then calculated through the random
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analysis. These results are then compared for accuracy based on the FEM. Although the
displacement calculated is accurate, it does not give you the worst case. Steinberg uses a
3 band method approach to determine max displacements of the PWB which is used for
the high cycle fatigue life. This is the point where maximum damage will occur in the
electrical components. Although based on this statistical approach will only occur 4.33%
of the time, it must be considered in the overall damage calculations.
Figure 2-1-3: Gaussian Distribution for Stienberg 3 Band Method
The basic idea is that when the number of cycles reaches the number of allowed, or
calculated cycles, the part will ideally fail. Traditionally, this calculation is used to
ensure that this never happens for any electrical component, BGA’s do not always
follow the same “simple calculations” and have relatively short cycle lives. There is a lot
that can go wrong with a single BGA: improper installation, solder joint failures due to
thermal or structural inputs, and for each of these failures, every solder ball can be
slightly different. Controlling this environment is crucial as well as the processes involved in assembling the BGA’s to PWB’s.
2.2 Finite Element Model: Random Vibration
The FEM will be created in PATRAN with MD enabled version 2010 and solved with
MD NASTRAN 2008. The FEM is composed of a few different types of elements to try
and decrease the model size. The PWB consists of 2-D Hex-8 shell elements, as well as
the BGA body. The Solder balls in the BGA will be modeled with 3-d Hex-20 elements
for detail and accuracy. All of the elements are tied together using a glue constraint for
deformable bodies through PATRAN. The Glue constraint makes it much easier to
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connect multiple bodies and greatly reduce the number of user input REB2 or REB3
Multi point constraints.
The method that is used to solve the problem of ball grid arrays and common aerospace
random vibration environments is composed of multiple independent steps. The first is
to simply run a modal analysis of the PWB with no BGA attached. Having the BGA
attached to the PWB will not affect the stiffness to a degree of concern and the model
can now be much smaller. The modal analysis should line up with the calculated results;
this is the success criterion for this step. The next step involves developing a model for
random vibration cases. Again the simple PWB model used for the modal analysis is
used to keep the model size down during dynamic analysis. The result that is desired
here is to find the max displacement of the PWB and then compare this to the hand
calculations, again which should line up for each vibration case. The difference here is
that for each random vibration case, although the natural frequencies are going to be the
same as long as the boundary conditions are the same, the transmissibility will not be.
This is the term that is calculated from the room mean square (RMS) of the vibration
response of the board. From the modal analysis, it is easy to narrow down the area where
the Q will be the highest and thus the highest displacement. The hand calculations are
modified and again compared with the FEM results to validate this step. In the idea of
keeping the model as light as possible and the iterations as few as possible, it was not
possible to run the full model with the BGA on the PWB. Instead, taking the results from
the random acceleration analysis, determining the max displacement, it is just as easy to
run a static inertial analysis on the full FEM. This dramatically reduces the computation
time for the study at hand. Although this deviates slightly from the original plan of
running dynamic simulation for each case, the results are very close to one another. This
is a standard industry practice that is creative and very time forgiving.
Now that the thought process is given for how the FEM will be solved, the boundary
conditions must be analyzed. The most important boundary condition or load case is of
course the random vibration environments that are analyzed. The figure below from
RTCA DO-160F shows the vibration curves for fixed wing aircraft:
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Figure 2.2-1: RTCA DO-160 Vibration Levels for Fixed Wing Aircraft [1]
The PWB boundary conditions are simple as described in Steinberg for the 3 Fixed edge
case. This approach is chosen after a popular robust design in aerospace rack card
assemblies. This design includes the use of wedge lock card guides. Below is a picture
representation of what they are and how they work:
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Figure 2.2-2: Wedge Lock Card Guides [3]
The wedge lock card guide essential keeps the board edge fixed (all 6 degrees of freedom). In turn this gives the designer a much more stiffness that will have less
displacement at the board center due to higher first modes. The last edge that is fixed
comes from connectors on the daughterboard connecting to the motherboard as seen in
Figure 2.1-2. Although this connection may not always be completely fixed due to
tolerances in the rack assembly and improper mating with an interconnect, it is assumed
that the mating conditions are fixed.
The last boundary condition for this model is the mating condition of the solder balls to
the PWB and the solder balls to the BGA package. There are several ways to do this
which include user defined Multi Point Constraints (MPCs), either REB2 or REB3,
Mesh matching, or lastly the Glue constraint. The REB2 MPC is used as a rigid or bolted
connection which would be far to stiff for this application. The REB3 element would be
idea but very cumbersome, about 968 connections to match the solder balls to the PWB
and the BGa body. Mesh matching is typically a very good option, although for this
case, the mesh is very fine, .008 inches, which results in a model too large to solve for
either dynamic or static modeling with the computer hardware available. Lastly the Glue
boundary can be applied to the solid geometry to which the mesh is associated. This
boundary sets up the model with REB3 connections that are ideal for this analysis case.
These REB3 connections are created when the solution is solved in NASTRAN rather
than PATRAN. The tolerances of these connections are set within PATRAN and are
usually a percentage of the smallest shell or solid element. More detail will be discussed
in the results chapter about these boundary conditions.
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Lastly the elements have all been chosen for a reason. The hex-8 shell elements are used
for all shell elements. The hex-8 elements allow the mesh on the PWB and the BGA
body to be much more course due to the larger number of nodes. This is necessary to
keep the model size down. For the 3-d solid solder balls, the mesh was done by taking a
section view of the solder ball and doing a surface mesh seed. This 2-d mesh seed was
then revolved to get the spherical shape of the solder ball. Hex-8 elements were used for
the 2-d mesh seed so a more course mesh could be used and the element edges that are
along the spherical edge actually resemble a sphere and map to the surface much better.
A very fine hex-4 can be used to get the same result, but is much more heavy in the
model. A detailed description of the FEM will be described in the results section.
2.3 FEA Model Optimization
FEA model optimization is crucial for this problem as the model size is a few hundred
thousand elements and many more nodes. Since there are only three different bodies in
this analysis, the PWB, BGA solder balls, and the BGA body, there are a few different
mesh optimization ideas. Firstly the solder balls must be detailed and accurate, but
cannot slow down the model and have such a fine mesh that the model will not even
converge in a reasonable time. This will be done by looking at static cases of single
solder balls under static loading conditions. Displacement and stress gradients are
observed to get the most course mesh yet highly accurate results. The second option is to
refine the mesh size of the PWB and the BGA body that best connect to the BGA balls
with the very small tolerances applied to the Glue boundary condition.
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Literature Cited
[1] RTCA, Incorporated. “Environmental Conditions and Test Procedures for Airborne
Equipment.” RTCA, Incorporated., Washington, DC. SC-135, Dec. 2007.
[2] Pierce, David M., and Sheri D. Sheppard. Fatigue Life Prediction Methodology for
Lead-Free Solder Alloy Interconnects: Development and Validation. Tech. Stanford, CA: Stanford University. Print.
[3] Steinberg, Dave S. Vibration Analysis for Electronic Equipment. New York: John
Wiley & Sons, 2000. Print.
[4] Chen, Y. S. "Combining Vibration Test with Finite Element Analysis for the
Fatigue Life Estimation of PBGA Components." (2007): 638-644. Science Direct. Web. Aug.-Sept. 2010.
[5] Amy, Robin A. “Accuracy of Simplified Printed Circuit Board Finite Element
Models.” (2009): 1-12. Science Direct. Web. Aug-Sept. 2010.
[6] Bieler, T. R. “Lead Free Solder.” (2010): 1-12. Science Direct. Web. Aug-Sept
2010.
[7] Arulvanan, P., Zhong, Z. W. “Assembly and reliability of PBGA packages on FR-4
PCBs with SnAgCu solder.” (2006): 2462-2468. Science Direct. Web. AugSept. 2010.
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