Patents are well-recognized as valuable data source for product innovation, design and development. They contain rich information about design rationale (DR), technology development and so on. In this paper, we intend to discover design focuses, which refer to types of design that a corporation emphasizes on in innovations and R&D approaches. By analyzing invention features, we attempt to reveal distinct design focuses that corporations have targeted at. In our previous study, we have explored text mining to discover DR based on the proposed ISAL model (issue, solution and artifact layer) from patents. In this paper, we aim to design a rationale-based patent analysis approach to analyze technical focuses of corporations from a large volume of patents. In our proposal, DR information of each patent is firstly discovered and each patent is represented by a structured manner of issue, solution and artifact layers. A feature association-based approach is designed to measure connections between artifact features and highly related features are clustered into groups to represent the categories of invention and design focuses from a corporation perspective. A scoring function is then defined to categorize patents based on artifact feature groups. In experimental study, a case study using patents filed by Apple Inc. and RIM (Research In Motion) over a period of time was conducted to demonstrate the approach proposed. We reported our findings on issues like design topic clusters and design focuses involved in the inventions over time.