Dear Yuan, Here is a physical chemists answer, which may or may not be what you want! The water absorption rate of a substance may be defined as the amount of moisture taken up (often expressed as a percentage the 'dry' weight of the material) as a function of time (e.g. 0.01%per hour), for a specified temperature and humidity condition (e.g. 25C 90%RH). Water aborption may also refer to immersion in the liquid rather than uptake from atmospheric moisture. Absorption rates are complex properties, as they are often time dependent, as well as being dependent upon the geometry of the sample. ie. a thin film will absorb moisture faster than a large block of material. In the context of your question , the absorption rate should be distinguished from the equilibrium moisture content of a material, which is usually expressed as a percentage of the dry mass of the material. Hence a substrate such as polyimide (used in flexi circuits for example) will have an equilibrium moisture content of about 2% if stored at 25C and 95% RH for some time. This may be the quanitity you are referring to in your posting rather than the absorption rate. If all this sounds like 'double dutch' then I apologise! Dr. Carl Levoguer CMMT National Physical Laboratory Queens Rd, Teddington, Middlesex. TW11 0LW Direct Line : +44 181 943 6263 Fax: +44 181 943 6453 ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################