pcba flexural fatigue life evaluation - cascade … flexural fatigue life evaluation reliability...

PCBA Flexural Fatigue Life Evaluation

www.cascade-eng.com

Reliability Engineering GroupCascade Engineering Services, Inc.

6640 185th Ave NE Redmond WA 98052(425) 895-8617 x 564

PCBA Flexural Fatigue Life Analysis

Test Objective• To evaluate PCB fatigue life under bending loads • To quantify the effect of board modulus and thickness on fatigue life

Methodology• Step 1: Geometry and Material Properties• Step 2: Loads and Boundary Conditions• Step 3: FEA Result• Step 4: Verification of FEA simulation with Test Data• Step 5: Fatigue Life Estimation Result• Step 6: Design Parametric Study • Case Study Benefits

Step 1: Geometric Model & Material Properties

• Structure Dimensions L : W : T = 142.7 : 87.9 : 1.64 (mm)

• Material Properties for elastic model Young’s Modulus: 3.5 GPa Poisson’s Ratio = 0.34

PCB Board Sample

Step 2: Loads & Boundary Conditions

Displacement

Boundary constraints: X=Y=Z=0

Displacement

Boundary constraints: X=Y=Z=0

Step 3: FEA Results

Displacement Plot at Z axis Strain Contour Plot

Von Mises Stress

Step 4: Veri�cation of FEA simulation with Test Data

Displacement (mm)

Test Setup Load Displacement Curve

Variation of PCBA Strain with Applied Force

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Force(N)

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ain

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Experimental Data Simulation FR4-E1 Simulation FR4-E2

Calibrating Experimental and FEA Strain Data

Step 5: Fatigue Life Estimation

bf

f NE

)(σ

ε =ε = elastic strain amplitude σf = fatigue strength coefficient Nf = Cycles to failure

E = Young’s modulusb = fatigue strength exponentε = elastic strain amplitude σf = fatigue strength coefficient Nf = Cycles to failure

E = Young’s modulusb = fatigue strength exponentFatigue Model :

Fatigue exponent and coefficient are material properties and related to flexural strength and endurance limit

Effect of PCBA Strain on Flexural Fatigue Life

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BA

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BA

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Microstrains Displacement

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Life Test:8mm Life Test:12 mm Use Condition:4 mmPCBA Displacement

PCBA Flexural Fatigue Life: Experiment vs Prediction

Prediction Experiment

Step 6: Design Parametric Study

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PCBA Displacement

Effect of PCBA Thickness on Flexural Fatigue

t=1.64mmt=0.82mm

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Use Condition:4mm Life Test:12mmPCBA Displacement

Effect of PCBA Young's Modulus on Flexural Fatigue

E=3.5GPaE=7GPa

Fatigue life change with thickness change Fatigue life change with Young’s Modulus change

• PCBA fatigue life deceased significantly when the thickness decreased to half. Reducing thickness was not acceptable for the current design.

• PCBA fatigue life increased with the increasing board modulus. Choosing stiffer PCBA board was a viable alternative for the next design revision.

Case Study Bene�ts

A similar quick turnaround test-analysis-prediction methodology can be useful for:

• Supplier qualification: Comparison of PCBA quality for multiple vendors• Specification testing: Industry PCBA standard test (JEDEC 3 Pt. bend, IPC etc.)• Design Limit Evaluation: Time independent step stress test to failure• Design Sensitivity Analysis: Virtually quantify effects of material and geometry changes on product durability• Develop Accelerated life tests: determine fatigue test limits for computing acceleration factors for relevant failure mechanisms