Fatigue Behaviour of Reflowed Joints
This paper explores a copper test vehicle with multiple reflowed solder joints, which has direct relevance to ball grid arrays and high density interconnect structures.
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Authored By:
C.P. Hunt, O. Thomas, D. Di Maio,
National Physical Laboratory, Teddington, Middlesex, UK
E. Kamara, H. Lu
University of Greenwich, Greenwich, London,UK
Summary
This paper explores the behaviour of a copper test vehicle with multiple reflowed solder joints, which has direct relevance to ball grid arrays (BGA) and high density interconnect structures. The paper explores the relative stress conditions on the distributed joints and the sensitivity to ball joint shape. The joints were exposed to isothermal fatigue, which was produced by a mechanical load that induced a cyclic shear stress across all the joints. The same structures were modelled using finite element analysis.
The loading response distribution profile through the joints was analysed. The regions of likely failure were identified to be along the shear band and at the stress concentration areas in the corners of the joints. Failure of the individual joints was analysed by quantifying the accumulated creep strain per cycle. Solder joint models of three different shapes were investigated: rectangular, convex and concave shapes.
This analysis has shown that less damage is found in concave shaped joints, indicating that BGAs would have more damage than the rectangular joints tested here. Results have also shown that more damage occurs in the outer joints as a vertical component appears due to a turning moment on the copper test vehicle. This behaviour could affect the external joints of large components, where the same vertical stress component may arise due to the differential CTE of the PCB and BGA component.
Conclusions
Fabrication and testing of a multi-joint sample has been demonstrated.
Modelling has predicted that joint shape will have an effect on fatigue, with the natural convex shape having the superior performance.
Experiments and modelling have shown that damage occurs more in joints as they become further from the centre point.
Modelling has shown there is a turning moment, and that a vertical component force appears of increasing amplitude with distance from the centre of the component. This force, normal to the shear direction, is consistent with these joints failing more rapidly.
This behaviour could affect the external joints of large BGA components, where the same vertical stress component may arise due to the differential CTE of the PCB and component.
Initially Published in the IPC Proceedings
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