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I have been working as an R&D Engineer at Company A for a year now and have decided to start looking for new employment opportunities due to financial reasons. Company A has given me a ton of practical engineering experience but has a bad reputation for having lower average salaries and being slow with performance reviews/advancement opportunities. During my one year review, I expressed my desire to continue to develop my technical skills at Company A as long as I could be assured of appropriate compensation and title advancement. Lately the company has been late to issue it's bi-annual bonus and tensions are slowly rising around the department. After re-evaluating my future with this company, I have decided to seek out new employment opportunities in hopes that my financial and career progress do not stagnate. Once I have found and accepted a position, I am unsure about how to bring this up to my manager.
Question: How do I explain to my manager that I have accepted a new position at a different company and that my reason for leaving was my lack of faith that Company A would have allowed me to advance within my career? We have a healthy work relationship and are open to talking about work-related topics. I am worried that bringing up this topic will negatively affect the rest of my time here as it would appear to my manager that I have already decided that this company is not where I want to be working at.