SMTA International Conference Proceedings


Authors: Wendy Chet Ming, Ngoh et al.
Company: Intel Products (M) Sdn Bhd
Date Published: 9/25/2005   Conference: SMTA International

Abstract: Microvia-in-pad technology plays an important role to accommodate high I/O density. However, the use of microvia-in-pad has been linked to solder joint reliability (SJR) issues at second-level interconnects (SLI). A study of the SJR issues created by the use of microvia-in-pad technology is presented in this paper. Factors included in the study are the effects of solder alloy system, pad design, solder voiding and board configuration/stiffness.

The fracture modes associated with these factors after temperature cycling were characterized and presented in the paper. Lead-free solder joints on pads with microvia at high stress locations of the BGAs were noted to show poorer thermo-mechanical performance compared to leaded solder joints at similar locations. Crack initiation locations and crack propagation directions were also noted to be different for solder joints on pads with and without microvia.

For MD pads without microvia and SMD pads with microvia, cracks were noted to initiate at the pad edge furthest from the BGA centre on the outermost corner pads and propagate inwards towards the centre of the BGAs, while for MD pads with microvia, cracks initiate at the microvia-in-pad region and propagate outwards from the microvia-in-pad region. Voids in solder joints on pads with microvia were generally associated with large cracks although there were also a number of solder joints with voids but cracks were not observed.

In terms of the effects of board configuration/stiffness, fracture modes of solder joints at pads with microvia were noted to be similar for single-sided and double-sided configuration, and also for test units with and without RF shield. The fracture modes characterized are useful to give an insight into the effects of design and process parameters of microvia-in-pad technology towards SJR, and hence enable development of design guidelines for safe adoption of microvia-in-pad technology.

Key words: Microvia-in-pad, BGA package, fracture mode, lead-free, thermo-mechanical.

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