Fatigue Delamination Crack Growth of Potting Compounds in PCB/Epoxy Interfaces Under Flexure Loading

2019 ◽  
Author(s):  
Pradeep Lall ◽  
Kalyan Dornala ◽  
Jeff Suhling ◽  
John Deep ◽  
Ryan Lowe
Keyword(s):  
Crack Growth ◽  
Shock Loading ◽  
Interfacial Crack ◽  
Mechanical Shock ◽  
Interfacial Delamination ◽  
Ultimate Elongation ◽  
Compound C ◽  
Stiff Material ◽  
Number Of Cycles ◽  
Potting Compound

Abstract Electronics components operating under extreme thermo-mechanical stresses are often protected with underfills and potting encapsulation to isolate the severe stresses. By encapsulating the entire PCB, the resin provides complete insulation for the unit thereby combining good electrical properties with excellent mechanical protection. In military and defense applications these components are often subjected to mechanical shock loads of 50,000g and are expected to perform with reliability. Due to the bulk of material surrounding the PCB, potting and encapsulation resins are commonly two-part systems which when mixed together form a solid, fully-cured material, with no by-products. The cured potting materials are prone to interfacial delamination under dynamic shock loading which in turn potentially cause failures in the package interconnects. The study of interfacial fracture resistance in PCB/epoxy potting systems under dynamic shock loading is important in mitigating the risk of system failure in mission critical applications. In this paper, three types of epoxy potting compounds were used as an encapsulation on PCB samples. The potting compounds were selected based on their ultimate elongation under quasi-static loading. Potting compound, A is a stiffer material with 5% of ultimate elongation before failure. Potting compound, B is a moderately stiff material with 12% ultimate elongation. Finally, potting compound C is a softer material with 90% ultimate elongation before failure. The fracture properties and interfacial crack delamination of the PCB/epoxy interface were determined using three-point bend loading with a pre-crack at the interface. The fatigue crack growth of the interfacial delamination was characterized for the three epoxy systems. A prediction of number of cycles to failure and the performance of different epoxy system resistance under cyclic bending loading was assessed.

Interfacial Delamination and Fracture Properties of Potting Compounds and PCB/Epoxy Interfaces Under Flexure Loading After Exposure to Multiple Cure Temperatures

2017 ◽  
Author(s):  
Pradeep Lall ◽  
Kalyan Dornala ◽  
Jeff Suhling ◽  
John Deep
Keyword(s):  
Finite Element ◽  
Shock Loading ◽  
Printed Circuit Board ◽  
Circuit Board ◽  
Fracture Properties ◽  
Printed Circuit ◽  
Interfacial Delamination ◽  
Ultimate Elongation ◽  
Point Bend ◽  
Potting Compound

Electronics components operating under extreme thermo-mechanical stresses are often protected with conformal coating and potting encapsulation to isolate the thermal and vibration shock loads. Development of predictive models for high-g shock survivability of electronics requires the measurement of the interface properties of the potting compounds with the printed circuit board materials. There is scarcity of interface fracture properties of porting compounds with printed circuit board materials. Potting and encapsulation resins are commonly two-part systems which when mixed together form a solid, fully cured material, with no by-products. The cured potting materials are prone to interfacial delamination under dynamic shock loading which in turn potentially cause failures in the package interconnects. The study of interfacial fracture resistance in PCB/epoxy potting systems under dynamic shock loading is important in mitigating the risk of system failure in mission critical applications. In this paper three types of epoxy potting compounds were used as an encapsulation on PCB samples. The potting compounds were selected on the basis of their ultimate elongation under quasi-static loading. Potting compound, A is stiffer material with 5% of ultimate elongation before failure. Potting compound, B is a moderately stiff material with 12% ultimate elongation. Finally potting compound C is a softer material with 90% ultimate elongation before failure. The fracture properties and interfacial crack delamination of the PCB/epoxy interface was determined using three-point bend loading with a pre-crack in the epoxy near the interface. The fracture toughness and crack initiation of the three epoxy systems was compared with the cure schedule and temperature. Fracture modeling was performed with crack tip elements in ABAQUS finite element models to determine the crack initiation and interfacial stresses. A comparison of the fracture properties and the performance of epoxy system resistance to delamination was shown through the three-point bend tests. The finite element model results were correlated with the experimental findings.

Unification proposals for fatigue crack propagation laws

2017 ◽  
Vol 13 (2) ◽  
pp. 262-283 ◽  
Author(s):  
Vladimir Kobelev
Keyword(s):  
Differential Equation ◽  
Crack Propagation ◽  
Crack Length ◽  
Closed Form ◽  
Crack Growth ◽  
Stress Ratio ◽  
Elementary Functions ◽  
Content Type ◽  
Number Of Cycles ◽  
Cycles To Failure

Purpose The purpose of this paper is to propose the new dependences of cycles to failure for a given initial crack length upon the stress amplitude in the linear fracture approach. The anticipated unified propagation function describes the infinitesimal crack-length growths per increasing number of load cycles, supposing that the load ratio remains constant over the load history. Two unification functions with different number of fitting parameters are proposed. On one hand, the closed-form analytical solutions facilitate the universal fitting of the constants of the fatigue law over all stages of fatigue. On the other hand, the closed-form solution eases the application of the fatigue law, because the solution of nonlinear differential equation turns out to be dispensable. The main advantage of the proposed functions is the possibility of having closed-form analytical solutions for the unified crack growth law. Moreover, the mean stress dependence is the immediate consequence of the proposed law. The corresponding formulas for crack length over the number of cycles are derived. Design/methodology/approach In this paper, the method of representation of crack propagation functions through appropriate elementary functions is employed. The choice of the elementary functions is motivated by the phenomenological data and covers a broad region of possible parameters. With the introduced crack propagation functions, differential equations describing the crack propagation are solved rigorously. Findings The resulting closed-form solutions allow the evaluation of crack propagation histories on one hand, and the effects of stress ratio on crack propagation on the other hand. The explicit formulas for crack length over the number of cycles are derived. Research limitations/implications In this paper, linear fracture mechanics approach is assumed. Practical implications Shortening of evaluation time for fatigue crack growth. Simplification of the computer codes due to the elimination of solution of differential equation. Standardization of experiments for crack growth. Originality/value This paper introduces the closed-form analytical expression for crack length over number of cycles. The new function that expresses the damage growth per cycle is also introduced. This function allows closed-form analytical solution for crack length. The solution expresses the number of cycles to failure as the function of the initial size of the crack and eliminates the solution of the nonlinear ordinary differential equation of the first order. The different common expressions, which account for the influence of the stress ratio, are immediately applicable.

The Reliability of Microalloyed Sn-Ag-Cu Solder Interconnections Under Cyclic Thermal and Mechanical Shock Loading

2014 ◽  
Vol 43 (11) ◽  
pp. 4090-4102 ◽  
Author(s):  
Toni T. Mattila ◽  
Jussi Hokka ◽  
Mervi Paulasto-Kröckel
Keyword(s):  
Shock Loading ◽  
Mechanical Shock

scholarly journals Low-Cycle Fatigue Crack Growth in Ti-6242 at Elevated Temperature

2014 ◽  
Vol 891-892 ◽  
pp. 422-427 ◽  
Author(s):  
Rebecka Brommesson ◽  
Magnus Hörnqvist ◽  
Magnus Ekh
Keyword(s):  
Crack Length ◽  
Crack Growth ◽  
Low Cycle Fatigue ◽  
High Strength ◽  
Cycle Fatigue ◽  
Load Drop ◽  
Actual Measure ◽  
Smooth Bars ◽  
Number Of Cycles ◽  
The Relationship

During low-cycle fatigue test with smooth bars the number of cycles to initiation is commonly defined from a measured relative drop in aximum load. This criterion cannot be directly related to the actual measure of interest - the crack length. By relating data from controlled crack growth tests under low-cycle fatigue conditions of a high strength Titanium alloy at 350°C and numerical simulation of these tests, it is shown that it is possible to determine the relationship between load drop and crack length, provided that care is taken to consider all relevant aspects of the materials stress-strain response.

Bimaterial Interfacial Crack Growth With Strain Gradient Theory

1999 ◽  
Vol 121 (4) ◽  
pp. 413-421 ◽  
Author(s):  
Su Hao ◽  
Wing Kam Liu
Keyword(s):  
Crack Growth ◽  
Strain Gradient ◽  
High Stress ◽  
Stress Triaxiality ◽  
Interfacial Crack ◽  
Strain Gradient Theory ◽  
Gradient Theory ◽  
Theory Analysis ◽  
Theoretical And Numerical Analysis ◽  
Material Mismatch

The purpose of this paper is to investigate the effect of material heterogeneity on damage evolution and subsequent crack propagation in bimaterial systems. Strain gradient theory analysis reveals that a higher stress triaxiality always occurs on the softer material side due to the material mismatch in yield capacity and the corresponding strain gradient along the interface. High stress triaxiality is a major condition which promotes ductile damage and facilitates crack growth. To investigate this link, numerical simulations of ductile interface crack growth are performed using a damage based constitutive model. Both the numerical and experimental results show that a crack may grow along the interface or deviate into the softer material, but never turn into the harder material. The theoretical and numerical analysis reveal three factors which strongly affect the direction of crack growth and the resistance capacity of the bimaterial system against fracture. These are the boundary conditions which determine the global kinematically admissible displacement field, the stress/strain gradient near the interface due to the material mismatch, and the distance from the crack tip to the interface.

scholarly journals New CZM for Interfacial Crack Growth

2017 ◽  
Vol 2017 ◽  
pp. 1-8 ◽  
Author(s):  
Huifen Peng ◽  
Yujie Song ◽  
Ye Xia
Keyword(s):  
Finite Element Method ◽  
Finite Element ◽  
Crack Growth ◽  
Interface Crack ◽  
Interfacial Crack ◽  
Structure Design ◽  
Zone Model ◽  
Simulation Results ◽  
The Impact ◽  
Element Method

The cohesive zone model (CZM) has been widely used for numerical simulations of interface crack growth. However, geometrical and material discontinuities decrease the accuracy and efficiency of the CZM when based on the conventional finite element method (CFEM). In order to promote the development of numerical simulation of interfacial crack growth, a new CZM, based on the wavelet finite element method (WFEM), is presented. Some fundamental issues regarding CZM of interface crack growth of double cantilever beam (DCB) testing were studied. The simulation results were compared with the experimental and simulation results of CFEM. It was found that the new CZM had higher accuracy and efficiency in the simulation of interface crack growth. At last, the impact of crack initiation length and elastic constants of material on interface crack growth was studied based on the new CZM. These results provided a basis for reasonable structure design of composite material in engineering.

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