Inter-enterprise collaboration is one of key important issues that affect the successful of a PSS (Product- Service System) project. This paper sets the bases of a global model that help characterizing and assessing collaboration processes along the PSS lifecycle. Based on a critical literature review of main papers dealing with collaboration issues, a list of ten key factors has been identified. Social capital theory is used to clarify the definitions and impacts of some of these factors. A UML class diagram of PSS collaboration processes is then proposed to connect these factors with other characteristics of PSS.