Adhesion mechanisms of nano-particle silver to electronics packaging materials
Joo, Sung Chul
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To reduce electronics packaging lead time and potentially to reduce manufacturing cost, an innovative packaging process targeting rapid package prototyping (RPP) has been developed. The developed RPP process, which is based on a data-driven chip-first approach, provides electrical functionality as well as form factors for micro-systems packages. The key component of the RPP process is the nano-particle silver (NPS) interconnect. However, NPS has not yet been adequately proven for use in electronics packaging applications. Moreover, its adhesion to electronics packaging materials such as polyimide, benzocyclobutene (BCB), copper, and aluminum is found to be weak. Thus, improving the adhesion strength of NPS will be a key issue for reliable package prototypes with NPS interconnects. In this research, the adhesion of NPS to substrate materials is found to be attributed to particle adhesion and more specifically, van der Waals forces. An adhesion model based on the van der Waals force is suggested in order to predict NPS adhesion strength to packaging materials. A new adhesion test method that is based on a die shear test and a button shear test is developed to validate the NPS adhesion prediction model. The newly developed adhesion test method is generic in nature and can be extended to other thin films' adhesion tests. The NPS adhesion model provides a general and explicit relation between NPS tensile bond strength and adhesion factors such as substrate hardness, adhesion distance, van der Waals constant, and particle diameter. The NPS adhesion model is verified as a first order adhesion model using experimental data from seventeen packaging materials. Substrate hardness is identified as a primary factor in NPS adhesion. Adhesion distance and van der Waals constant are also significant in organic and inorganic materials. Diffusion or other interfacial reaction between NPS and metal substrates such as copper and silver seems to exist. Finally, guidelines to improve the adhesion strength of NPS are suggested based on the adhesion model and on external adhesion factors such as Silane coupling agents and plasma treatment.