Reading time ( words)
IPC’s 3-11 Laminate/Prepreg Materials Subcommittee has announced the publication of the latest revision of the test method for the determination of glass transition temperature (Tg) of copper-clad laminates (CCL). IPC-TM-650 Method 2.4.25 utilizes Differential Scanning Calorimetry (DSC) to measure the Tg of the base materials for rigid printed circuit boards. The Tg is one of the characteristics used to estimate the thermal reliability of materials used in electronics.
The key change in the document clarifies what is and what is not a phase transition as the temperature approaches the Tg for the first time. In the past, the unusual exotherms or endotherms could be interpreted as being a Tg. There are a number of reasons cited for these spurious events including stress relief, release of moisture and evaporation of residual solvent. The Pre-Scan step is needed to make the baseline more stable for the calculation of Tg1 and Tg2. The Pre-Scan takes the temperature to about 10°C above the predicted Tg1, then the sample is cooled immediately back to the starting point temperature. The determination of Tg1 is then carried out by a second scan which terminates at either 175°C or 190°C depending of the type of epoxy resin system. At the termination temperature, the specimen is held at isothermal conditions for 15 minutes before cooling again to the starting point temperature. Finally, a third scan is conducted to determine Tg2. It is Tg1 and Tg2 that are used to calculate either the cure factor or the delta Tg.
All of the IPC-TM-650 test methods can be found free of charge at the IPC website, www.IPC.org under the “Knowledge” tab.
