SMTA International Conference Proceedings


Authors: Jiachun Zhou (Frank), K. Elmadbouly, and O. Sorian
Company: Kulicke & Soffa
Date Published: 9/22/2002   Conference: SMTA International

Abstract: Pin soldering is widely used in contactor board or interposer manufacture. The quality of the solder joint largely affects the performance and service life of the board. But few methods have been developed to inspect its quality at production sites due to difficulties in operation.

Microstructure observation, such as X-ray, is usually used for such inspection, but this technique can only find structure defects in the solder. The failure of solder joints is primarily caused by external forces, especially external shear force on the pin. The solder joint strength actually affects this failure.

A strength test method has been developed and applied by Kulicke & Soffa to inspect the pin soldering quality during board manufacturing and in development of soldering technology. This method applies an external force on the pin, and the relationship between pin deflection and applied force is recorded.

The force-deflection curves show an initial linear elastic deformation region and non-linear plastic deformation region. The proportional elastic deformation is mostly time-independent and the plastic deformation is mainly time-dependent. Experimental results indicate that external load significantly decrease at the crack point of the solder joint.

Large quantities of such measurements were taken to establish fracture criteria for qualified solder joints. For example, a force of >250gf must be applied at the middle of the pin to crack qualified solder joints on a specific board.

In mass production of this board, one pin on each board is inspected using this test, and the board is accepted if the fracture force is larger than the criterion value. This method is also applied in development of new boards. A pin/soldering structure was developed that increases the fracture force by about 50%.

There are four types of fracture modes, which explain the root-cause of force-deformation curves and provide help in improving soldering quality. The test method, results, and pictures of soldering fractures are presented in this paper.

Key words: solder joints, PCB, soldering strength.

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