In recent years, prominent software companies have succeeded to grow by creating ecosystems of third-party providers and users around their software platforms. The overall well-functioning of such ecosystems is referred to as ecosystem health that is the result of a complex range of architectural design decisions at business and technical levels. Despite a body of work considering the architecture of software ecosystems, there is still a lack of solid methods with a precise foundation to facilitate architectural decision-making by providing automated techniques and tools. To fill this gap, we present SecoArc, which is a pattern-centric ecosystem modeling framework for architecting healthy software ecosystems. In this paper, we focus on the SecoArc modeling language, analysis technique, and a ready-to-use tool. SecoArc makes the architectural knowledge of well-established existing ecosystems available that enables platform providers to assess architectural suitability with respect to the quality attributes of ecosystem health and to deeply compare several competing architectures. We show the application of the SecoArc framework in a case study.