Standard & Poor's yesterday assigned preliminary ratings to a $495 million prime floating-rate issue by the Illawarra Series 2004-1 RMBS Trust.This is the second issue under the residential mortgage-backed securities program established by Wollongong-based IMB Ltd last year. $482.5 million of 'A class' securities are rated AAA and $12.5 million of 'B class" are rated AA and all mature in 2035.The mortgage loans in the portfolio are originated and serviced by IMB both directly and via mortgage brokers. They are all to prime quality borrowers, secured by first-registered mortgages and are fully verified.One third of the portfolio is interest-only or line-of-credit loans and 37 per cent have loan-to-valuation ratios of over 80 per cent. All loans are covered by either individual or open lenders mortgage insurance policies.IMB has an 'above average' residential loan servicer ranking from S&P and in June became the first Australian building society or credit union to be given an investment grade rating when S&P rated it BBB long term and A-3 short term.This week, mortgage broker Mortgage Choice added IMB to its lender panel, noting its "strength in regional markets" and "outstanding service levels."