Commonwealth Bank will pay a margin of 90 basis points over the bank bill swap rate on the top tranche of its latest issue of residential mortgage-backed securities. When it last issued in February it paid a margin of 80 basis points.CBA raised A$2 billion - up from the $1 billion it was seeking when the issue was launched early last week. Medallion Trust Series 2015-2 is backed by a pool of loans with an average loan size of $257,400 and an average loan-to-valuation ratio of 58.4 per cent. Investment loans make up 24 per cent of the portfolio and interest-only loans make up 19.2 per cent.Pricing on the $1.84 billion of A1 notes, which have a weighted average life of 2.8 years, was 90 bps over the one-month bank bill swap rate.Pricing on the B and C notes was not disclosed.The A1 notes are subject to a refinance option from September 2020. The weighted average life of the refinanced A1 notes would be 2.7 years.