Have you ever considered the long-term reliability of circuit boards in electronics? Failures can lead to downtime, product recalls, and reputation damage. Traditional product development often involves building and testing prototypes—a process that can be costly and time-consuming. Our client faced this challenge, seeking to validate the structural integrity of a circuit card assembly (CCA) design before committing to physical prototyping.

Finite Element Analysis (FEA) offers a powerful solution. It allows us to simulate and analyze component behavior under various conditions, reducing the need for extensive physical prototypes.

A CCA is the complete circuit board containing electronic components, connectors, and supporting structures. Modern CCAs can contain hundreds or even thousands of components in a compact space. This increasing density presents significant challenges to structural integrity. Understanding how stresses are distributed and how potential failure modes arise is crucial to ensuring the reliability of electronic products.

Imagine a cracked printed circuit board (PCB) trace, a loose component, or a fatigued solder joint—each can trigger a chain reaction leading to failure. As devices become smaller and components are packed closer together, thermal stresses and mechanical loads intensify. Structural analysis isn’t simply a “nice-to-have”—it’s a necessity for ensuring product reliability and minimizing risk.

The Challenge: Limiting Prototype Iterations Through Simulation

Our client, a leading manufacturer of aerospace equipment, faced a design challenge with a new, high-density CCA for an advanced aircraft system. The primary objective was to ensure the prototype could withstand operational loads without failure.

Key Structural Analyses for CCAs

Modal analysis helps determine the natural frequencies and mode shapes of a CCA. Understanding these modes is essential for preventing resonance. If those natural frequencies coincide with frequencies experienced during operation, resonance can occur, leading to excessive vibration and potential failure.

Unlike harmonic vibration, random vibration is unpredictable. It’s often encountered in environments like a vehicle on a roadway or an aircraft during flight. This analysis determines how the CCA will respond to those unpredictable vibrations. The primary input is a Power Spectral Density (PSD) curve, which describes the distribution of power across a range of frequencies. It’s usually derived from physical testing. Random vibration analysis highlights areas of high stress concentration, guiding design modifications to limit vibration-induced stresses.

Shock response analysis evaluates CCA performance under sudden, transient impacts like drops or collisions. It helps identify stress concentrations and potential failure points during a shock event.

Bolted joints are critical connections in CCAs, securing components and the CCA to its mounting structure. This analysis evaluates the performance of these connections under various loads, including axial, shear, and cyclic loads. It assesses potential loosening and establishes proper torque requirements.

Typical Failure Modes

Solder joints are often the weakest link in CCAs, susceptible to fatigue from thermal cycling and mechanical stress. This analysis evaluates their reliability under mechanical loading, assessing fatigue life and potential failure mechanisms. It provides an estimate of how many vibration-induced cycles a solder joint can withstand before failure.

Objectives

#ObjectiveSuccess Criterion
1Determine natural frequencies and mode shapes.All frequencies > 2X dominant aircraft vibration bands (≤ 800 Hz)
2Quantify RMS stresses and displacements under worst-case (launch) environment PSD.Frame RMS 3σ stress margin of safety (MS) > 0 for based on yield strength.
CCA displacement < allowable deflection limits (Steinberg)
3Evaluate peak stresses from a terminal-peak sawtooth shock worst-case (Qualification) condition.Peak stress < allowable (0.8 × yield)
MOS > 0.00
4Verify bolted‑joint performance (clamp force, bearing stress, loosening risk), and determine margins of safety (MOS).Bearing stress < allowable (0.7 × bolt yield)
Preload retained within ±10 %.
MOS > 0.00
5Predict solder‑joint fatigue life under shock and vibration environments.Predicted Cumulative Damage Index (CDI) using Steinberg methods
CDI < 0.25, for 4X life based on 20 x 106 cycles‑to‑failure.

Typical conservative assumptions

  • Component position factor r = 1.00 assumes worst case component location for all components
  • 3σ displacements used for CDI calculation
  • Worst case 2.5% damping assumed for vibration analysis
  • Safety factor (S.F.) = 1.20
  • SRS analysis assumes Q = 10
  • Safety factor or design factor (S.F.) of 1.40 for critical fastener loads retrieved from FEA
  • Blend radius transition between the fastener head and threads is smooth enough to avoid stress concentrations
  • Minimum preload reduced by 15% to account for possible joint relaxation due to creep in PCB epoxy material
  • No thermal loading at fastener
  • Analysis does not account for:
    • Joint fatigue due to cyclic environment loading
    • Torsional loading experienced by the fastener due to higher initial torque required to overcome fastener and insert self-locking mechanisms

Our Solution: Predictive Analysis for Confident Design Validation

Understanding critical operating conditions, and gathering necessary data are essential for delivering accurate results. This includes CAD models, material properties, and environmental specifications.

FE Model

For effective FEA, the CAD geometry is typically simplified where possible to reduce computational burden while preserving critical features. Connections, like fasteners and connectors, are modeled as rigid or spring elements.

PBA 1, Albus Engineering

Operational Loads

For effective FEA, the CAD geometry is typically simplified where possible to reduce computational burden while preserving critical features. Connections, like fasteners and connectors, are modeled as rigid or spring elements.

AnalysisDefinitionApplication
ModalNo external loads; only the mounting constraints.Determines natural frequencies.
Random VibrationEndurance profile PSD: 10 Hz – 2000 Hz, 0.04 – 0.06 g²/Hz. Applied as a base‑excitation at CCA mounting points.Base‑excitation at bracket nodes (spectral analysis).
ShockTerminal-peak sawtooth, 40 g, 11 ms duration, board-normal axis.Transient dynamics (explicit integration).
Bolted JointEach M3×0.5 mm SS bolt pretension set to 2.5 N·m torque.Evaluates clamp force variation and bearing stress.
Frequency (Hz)Endurance PSD (g2/Hz)
100.04
800.04
3000.25
10000.25
20000.06
gRMS = 17.9
Duration = 1 hour
Launch – Endurance Profile PSD
PBA 1, Albus Engineering

Material Properties

Material properties for the PCB, components, fasteners, and adhesives were carefully defined based on manufacturer data and previous testing.

PartMaterialElastic Modulus (E)Density (ρ)Poisson’s Ratio (μ)Yield Strength (σY)Ultimate Strength (σU)
PCB SubstrateFR-4
(epoxy-glass)		3,000 ksi0.067 lb/in30.13—50 ksi
Copper PlanesCu16,000 ksi0.323 lb/in30.344.8 ksi30 ksi
Aluminum FramesAl-6061-T610,011 ksi0.098 lb/in30.3335 ksi42 ksi
SS Fasteners
(NAS1352)		A286
(AMS5731L)		29,000 ksi0.286 lb/in30.3085 ksi130 ksi
SS Thread InsertsCRES 30428,000 ksi0.289 lb/in30.2931 ksi73 ksi
PCB Effective Mechanical Properties
Composite Density (lb/in3)Composite Elastic Modulus (ksi)Composite Poisson Ratio (μ)
0.09435,2130.27

PCB Traces

PCB Layer Stack-Up

The total weight of the electrical components was considered to be 0.50 lb. This weight was modeled as a non-structural mass evenly distributed across the entire PCB.

Boundary Conditions

Simply supported at mounting points – Fixed translation degrees of freedom (DOF).

PBA 1, Albus Engineering

Results and Impact: Validating CCA Design

The analyses provided valuable insights into the CCA’s structural integrity.

Modal Analysis

The first bending mode was at 1130.3 Hz, which sat comfortably above the defined 2X aircraft dominant frequency of 800 Hz, mitigating resonance risks.

Random Vibration Analysis

Results indicated that certain components experienced higher stress levels than initially anticipated. Areas around the unsupported center areas of the PCB were vulnerable. RMS max. displacement ≈ 0.006 in. @ PCB, and ≈ 0.011 in. @ Secondary Frame.

Max. RMS 3σ von Mises stress in secondary frame ≈ 51.9 ksi.

Shock Response Analysis

The shock response analysis revealed minimal displacements. Max ≈ 0.001 in.

Peak Von Mises stress ≈ 55.7 lb.

Bolted Joint Analysis

Current fasteners meet performance criteria. MOS > 0

Failure ModeMOS
Joint Separation0.34
Fastener Thread Shear7.43
Insert Inner Thread Shear6.64
Insert Outer Thread Shear5.62
Parent Material Thread Shear2.11
Fastener Strength (Tension + Shear) (Ult.)1.07
Joint Bearing (FR4)4.01
Joint Tension (hoop stress) (FR4)3.86
Shear Tear-Out (FR4)5.25
Pull-Through Shear (FR4)12.76

Solder Joint Reliability Analysis

Max. desired PCB displacement for an expected component life of 20 million stress reversals in random vibration and shock operational environments.

 

Z = 0.00022 ⋅ B C ⋅ h ⋅ r ⋅ L

 

n 1 = f n ⋅ t N 1 = 20 × 10 6 ( d Z ) b

 

Z = allowable 3 σ CCA displacement for 20 million stress reversals B = length of CCA edge parallel to component (in) L = length of electronic component (in) h = height of thickness of CCA (in) C = constant for different types of components (Steinberg) r = relative position factor for location of components in CCA fn = number of positive crossings = 1103.1 Hz b = component vibration fatigue exponent = 6.4 d = CCA peak deflection (3 σ )

* Adapted from: Steinberg, D. S. (1988). Vibration analysis for electronic equipment (2nd ed.). John Wiley & Sons.

 

Cumulative damage index (CDI) is based on deflection using Miner’s cumulative damage ratio.

C D I = n 1 N 1 + n 2 N 2 + n 3 N 3

 

The worst case predicted CDI exceeded fatigue life requirement.
Max. CDI ≈ 0.22 was predicted for the worst case J1 component, which is below the requirement limit of 0.25.

Critical Components
Ref. DesignatorComponent TypeCDI
U5SMT0.11
U30BGA0.12
U41FP0.10
J1FP0.22

Results Summary

The analysis revealed a potential structural weakness in the component’s secondary side cover. Peak 3σ stresses (55.7 ksi) exceeded the material’s ultimate strength (42 ksi), indicating a need for design adjustments, such as incorporating proper blend radii.

CCA Displacement (0.0056 in) remained within allowable deflection limits for a 20 million cycle component life in leaded and leadless components, with a minimum cumulative damage index below 0.25 for all critical components. 20 million cycles is roughly equivalent to 4.9 hours of vibration at the CCA natural frequency and critical operational load level. The predicted displacement remained below sway space limits, and a safety margin of 0.34 was projected for the most critical bolted joint under peak board-normal vibration.

Overall, the CCA demonstrated strong performance against structural and reliability requirements, achieving acceptable margins across key metrics; however, targeted design adjustments to the secondary side cover are necessary to ensure long-term structural integrity and consistent component reliability.

  • Limiting pin extension on through-hole connectors to a maximum of 0.05 inches beyond the PCB is advised to prevent potential electrical shorts with the secondary cover.
  • Incorporating adequate blend radii around main connectors in the secondary frame cover is advised to avoid stress concentrations.
  • Maintaining a minimum sway space of 0.006 inches is advised to accommodate displacements caused by shock and vibration.
  • For increased safety factor in bolted joints, consideration should be given to using slightly larger fasteners to increase the bearing area by approximately 20%.
  • Future work should incorporate thermal analysis and consider more non-linear material behavior for a more comprehensive assessment.

Expertise in Action

The successful outcome of this engagement demonstrates our ability to provide end-to-end FEA solutions specifically tailored to the unique mechanical design challenges of each product.

Our team’s expertise in mechanical analysis provided a critical layer of risk mitigation for this project, not only ensuring the structural integrity of the CCA but also accelerating the design and qualification process.

Key Benefits of Using FEA

  • Accelerated Time to Market

    Accelerates design iteration cycles, shortening the development process.

  • Improved Product Reliability

    Proactively identifies and addresses potential weaknesses.

  • Identification of Potential Failure Modes

    Provides a deeper understanding of the structural behavior under operating conditions; uncovering potential hidden stress concentrations.

  • Improved Reliability

    Ensures that designs can withstand operational stresses and environmental conditions.

  • Reduced Development Costs

    Identifies potential issues early in the design process, reducing costly redesigns and failures.

  • Performance Optimization

    Allows optimization of a design before building a prototype.

We offer a comprehensive range of services, from initial design concept evaluation to detailed analysis and validation. We utilize advanced simulation tools and methodologies to provide accurate, reliable results that empower our clients to make informed decisions and optimize product performance.

Ready to enhance your product development process and mitigate risk?

Contact us today to discuss your specific needs and discover how our FEA consulting services can drive innovation and deliver tangible business value.

Let us help you navigate the complexities of advanced simulation and unlock the full potential of your designs.

Make the next move. Partner with Albus Engineering.

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