A major reason to do it is that in the US debt is massive tax advantaged compared to things like stock, and a major reason for entities to lend out money for something like this is how poorly other less risky vehicles are paying in this era of financial repression (http://lexicon.ft.com/Term?term=financial-repression or https://en.wikipedia.org/wiki/Financial_repression).
And it's very common for a company with a great cash cow to bankrupt themselves by going out of their area(s) of core competence ... which this acquisition might be a doubling down on. I.e. isn't Qualcomm fabless, while NXP has 8 sites according to this list: https://en.wikipedia.org/wiki/List_of_semiconductor_fabricat...
"they don't reveal all the details of achieving good power consumption in their patents"
Patents are essentially an award of a short-term monopoly, as recompense for publicly disclosing an invention. If the invention is not sufficiently explained, then it's merely a trade secret, and can be copied without challenge.
I think this is pretty clearly a consequence of dirt cheap debt, which may lead to another economy crisis in 5-10 years, because not all of these multi-tens of billions of dollars debt deals for acquisitions are going to be successful, and when they'll crash, they'll crash hard.
As a layman, I'm surprised a company can simply raise more than a third of their market cap. At this level, isn't debt almost an investment in the company? If revenue falters, how can the lenders make sure they're made whole?