This document discusses a finite element analysis of extremely low cycle fatigue failure (less than 100 cycles) in an integrated circuit package solder joint. The goals are to evaluate the possibility of such failure during installation and extraction of a printed wiring board. The analysis predicts peak solder strain in the corner land of the component. It estimates that 25 flexes of the board at the connector edge could cause failure, with each flex accumulating enough strain energy in the solder joint. Specifically, a flexure of only 0.014 inches per cycle would result in failure after 25 cycles due to cumulative plastic strain energy in the solder.

1. LOW CYCLE SOLDER JOINT FEA
FATIGUE FAILURE ANALYSIS OF
AN INTEGRATED CIRCUIT
PACKAGE
using a strain energy method
06/18/2005

2. GOALS
 Evaluate the possibility of extremely
low cycle (<100 cycles) fatigue failure
in an integrated circuit package
 Such damage has been seen during
installation and extraction of a parent
printed wiring board assembly.

3. Typical failure
Solder joint fractures

4. Host pwb asy joint loading
Bending about this line
During insertion in
Back plane

5. FEA mesh
Hinge line / flex axis

6. Component detail

7. Strain energy contours
Peak solder strain
predicted in corner land

8. Deflection @ 700X mag

9. reference

10. hysterisis

11. Dwell time as a variable
The larger the area the higher the damage

12. Strain Energy testing see ref.

13. Estimate for 25 cycles

14. Assume a failure after 25
flexes of
the board at the
connector edge.
The accumulated strain
energy
density in a solder joint
must be at failure:
580,000 joules/cu-meter
or
23,200 joules/cu-meter
in each flex

15. Board handling insertion
flexure
 Plastic strain energy in solder has
shown to be cumulative in eutectic
solders.
 A flexure of only .014 inches per cycle
will result in a strain energy sufficient
to cause a fracture in the solder joint
after approximately 25 cycles.