environmental simulation of circuit board epoxy group members greg kurtz brad walsh brad grimmel...
TRANSCRIPT
Environmental Simulation of Circuit Board Epoxy
Group MembersGreg KurtzBrad WalshBrad GrimmelJeff Baudek
Project Overview Lockheed/Martin is having a problem
with cracking in circuit boards and circuit board epoxy
Project involves Repairing an environmental chamber Duplicating the cracking through physical
testing Using FEA/FEM to model circuit boards Proposing a solution
Repairs Made to Chamber Replaced Blower motor Selected and
Purchased new controller
Rewired LN2 solenoid and cables between chamber and controller
Made new wiring diagrams for operator references
Had new 208V 40A power outlet installed in NHMFL
Testing of Repaired Chamber Chamber was
cycled through maximum range of –73°C to 316°C
Chamber is capable of completing diurnal cycle from –50°C to 80°C in less than one hour
Circuit Board Testing Purpose of testing
was to simulate the effect of the standard military diurnal cycle on a UR-329 encapsulated circuit board
Preparation Procedure
1. Heat mold to 100°C and apply releasing agent
2. Mix resin and hardener and insert into vacuum chamber (samples were also prepared without vacuum)
3. Curing period (either normal or accelerated cure)
Physical Testing Samples were then
cycled between –50 deg C and 80 deg C each hour
Samples were checked approximately every 25 cycles
Samples were exposed to approximately 550 cycles during testing
Finite Element Analysis (FEA) 2-D and 3-D models made using
FEA programs Ansys and Algor Assumptions
Uniform Temperature Distribution Constraint of zero displacement at
two corners Constant Material Properties
Results of FEA 2-D Ansys model showed stresses
that were below the yield point of the circuit board
3-D Ansys model was inaccurate due to constraints
3-D Algor model showed that the stress present in the circuit board was below the yield point
Thermal Stress Calculations Axial and Bending Stress due to
temperature variations in the circuit board and epoxy were done to supplement the FEA
Fe
Fb Fb
Fe
M M
Model Setup
Assumed: Constant value of
E and CTE Epoxy
delaminated from the circuit board
Uniform temperature distribution
M y
I
therefore L = LoT
Where LF LA E
Basic Equations:
Results of Calculations Calculated Axial stresses were below the
yield stress of the ceramic throughout the entire temperature range
Calculations determined that with a constant CTE of 191.7*10-6/°C an Elastic Modulus of 4230 psi for the epoxy is needed to crack the circuit board due to bending stress which occurs within the diurnal cycle specified by Lockheed/Martin for testing
Summary Repair of Environmental Chamber Recreation of circuit board and
epoxy cracking through physical testing and observed using SEM
FEA was completed Thermal stress calculations
revealed that circuit board should crack
Recommendations Use epoxy with
A modulus of elasticity with a lower glass transition temperature
A decreasing coefficient of thermal expansion with decreasing temperature
Further investigation of: Fatigue properties of circuit board Adhesive properties of the epoxy Examine use of different epoxy fillers