Thought the mortgage meltdown was just a subprime affair? Think again. There’s another time bomb waiting to explode, experts say: risky loans made to people with good credit.

So-called pay-option adjustable-rate mortgages, or option ARMs, were the easiest and most profitable home loans for lenders and brokers to make for much of this decade. Last year, they accounted for about 9 percent of the volume of all mortgages made in the U.S. and were especially popular in California, Florida and Nevada – states where home prices rose the most during the housing boom and are now falling most sharply.

An option ARM loan gives a borrower the option of paying less than the interest due, causing the loan balance to rise. If it rises too much – say, by 10 percent or 15 percent – the opportunity to make a low payment vanishes and the required payment skyrockets.

That scenario is becoming increasingly common. In fact, more than 75 percent of option ARM borrowers have been making only the minimum payments, analysts at Standard & Poor’s Corp. said. As a result, the delinquency rate on option ARMs already is jumping and is likely to keep rising sharply, S&P said. Because option ARMs went only to “prime” borrowers, they aren’t eligible for a much-publicized interest rate freeze that is part of a White House-backed plan to stem subprime foreclosures.

One upshot could be foreclosures growing more common in affluent neighborhoods.

“Whether it’s a wealthy community or a subprime community, it all comes down to how much equity the borrower has and how much home prices fall,” said Joshua Rosner, a managing director at investment research firm Graham Fisher & Co.

Option ARMs were originally offered in the 1980s by California savings and loans as a way to give flexibility to self-employed people and others with variable incomes. But as homes became more expensive this decade, the loans became increasingly desirable because of the ability to make low payments for a good period of time.

“The only reason for taking (an option ARM) was to use the minimum payment to get more house or a bigger refi than you otherwise could afford,” said Guy Cecala, editor of Inside Mortgage Finance.