Quality system breakdown structures are created by quality design. Quality design relies upon:

• having a good definition and understanding of the problem to be solved
• having and applying deep knowledge of relevant solution technologies
• applying a degree of creativity influenced by the novelty, complexity and importance of the problem/solution to conceive how the technology knowledge can best be applied
• using a “divide and conquer” strategy to balance risk due to complexity against definite engineering overhead, trading system breakdown structure depth against system breakdown structure breadth, to achieve an optimum balance (and no, not the fiction of 7 +/- 2)
• employing sound design principles without qualification
• employing structured design processes selectively, commensurate with the novelty, complexity and importance of the problem/solution

How to achieve the above fills books, including some not yet written!