What is the Necessary qualification qualification to become a Good Design Engineer ?
The key word here is "Engineer". Whatever your age and/or discipline there is a minimum body of knowledge you must learn and become proficient in the application of that knowledge. Experience and exposure to multiple products and multiple companies will hone your skills to fine degree. No matter how good you are or how good you think you are, you absolutely must master at least that minimum body of knowledge. I recommend you accomplish this at an accredited university regardless of how long it takes. Remember this, when engineering screws up, sometimes people die. Take your discipline seriously. I am a senior opto-mechanical designer and I am enrolled in school in order to complete my BSME.