BANK of Communications, a Shanghai-based institution, will sign an agreement today on issuing $260 million in floating rate certificates of deposit (CDs). The issue, launched in the name of the Hong Kong branch, should be boosted by ratings which are expected from Moody's and Standard and Poor's by the end of the year. ''The ratings on the bank will help promote liquidity in the secondary market since there aren't many Chinese issuers with ratings,'' said Tony Tam, senior manager at DKB Asia, one of the three firms arranging the issue. The other two are Bayerische Landesbank Girozentrale and Westpac Finance Asia. The CD issue is divided into two tranches, of different maturities and interest coupons. The first is over three years, paying the three-month Hong Kong Inter-bank Offered Rate (HIBOR) plus 47.5 basis points. The second, which amounts to $100 million, matures in 21/2 years with coupons priced on the three-month HIBOR plus 42.5 basis points. The whole syndicate includes China Development Finance Co as lead manager, plus Commonwealth Bank of Australia, Development Bank of Singapore, Fuji International Finance and Sumitomo Finance (Asia) as co-lead managers.