NEW YORK (Bloomberg) — Microsoft Corp., the world's largest software maker, plans to sell $3.75 billion of debt in its first bond offering, taking advantage of its top credit ratings to help fund a share buyback and technology investments.

The sale will be split among $2 billion of five-year notes, $1 billion of 10-year debt, and $750 million 30-year bonds, according to a person familiar with the offering who declined to be identified because terms aren't set. The debt was expected to price as soon as yesterday, the person said.

Microsoft, whose shares have declined 34 percent in the past year, is seizing on a credit-market rally to help fund a $40 billion stock repurchase programme. The company, which for years resisted shareholder demands to add debt, is also investing in data centers to compete against Google Inc. in Internet search.

"As a corporation you should be buying back your stock when it's cheap," said Guy Lebas, chief economist at Janney Montgomery Scott LLC in Philadelphia. "It's probably the cheapest time for them to do it."

The five-year notes may pay a yield of 0.95 percentage point more than similar-maturity US Treasuries, and both the 10- and 30-year bonds may pay a spread of 1.05 percentage point, the person said.

The Redmond, Washington-based software maker became the first company in a decade to receive the top AAA rating from Standard & Poor's when it initially filed in September to tap debt markets. Moody's Investors Service also assigned its highest rating to Microsoft's debt.

"The company is not in need of financing," Microsoft said yesterday in an e-mailed statement. It's "taking advantage of good market conditions, and Microsoft's great credit rating."

Microsoft is offering bonds on the busiest day for corporate debt sales since June 16, 2008, joining 12 other borrowers. Investment-grade companies have sold $498.9 billion of US corporate bonds this year, 25 percent more than in the same period of 2007, when the debt sold at a record pace, according to data compiled by Bloomberg.

Yields on investment-grade debt relative to benchmark rates have dropped 159 basis points from their 2009 high on January 2, to 444 basis points as of May 8, according to Merrill Lynch & Co.'s US Corporate Master index. The average yield on AAA rated debt has plummeted 396 basis points from its high this year on March 9 to 211 basis points as of May 8, according to Merrill Lynch data. A basis point is 0.01 percentage point.

The Microsoft offering shows "companies are able to access the capital markets here, and that's good news", John Calamos, chief executive officer of Calamos Asset Management Inc. in Naperville, Illinois, said yesterday in a Bloomberg Television interview. "The credit markets are alive and well."

Soaring default rates and the deterioration of corporate credit give Microsoft's debt a unique appeal to bond investors, Lebas said.

"I don't know the last time you ever thought of tech as a defensive play, but in that sense of the word Microsoft, with its impeccable credit rating, is that defensive play," Lebas said.

S&P has downgraded 269 speculative-grade companies this year and upgraded 16, analysts led by Diane Vazza at New York- based S&P wrote in a May 6 report.

Microsoft has cut about 5,000 jobs since January to reduce expenses as the recession cools demand for software. The company had $25.3 billion in cash and short-term investments as of March 31, according to regulatory filings.

Much of Microsoft's cash is generated outside the US, while Microsoft needs to pay its dividend, stock repurchases and some of its capital expenditures in dollars, chief financial officer Chris Liddell said in a February speech. Microsoft may borrow in the US to fund some of those activities, and shareholders won't see Microsoft "significantly leveraging the company in the foreseeable future", he said at the time.