Question

You are given a sample of polymer; it is not obvious if it is cross-linked, filled or not filled.
Explain how you would determine the Tg (glass transition temperature).Regarding the polymer in question, if your manager asks you to use it for a product that would
typically be used above Tg and in a humid environment. What technical concerns do you need to
explain to your manager so that they fully understand any possible risks?

Answer 1

The simplest way to define Glass Transition Temperature is to call it "The temperature at which the mechanical properties of a laminate begin to change rapidly." Glass Transition Temperature is just what its name suggests:it is that temperature at which a material changes from a hard,brittle "glass-like" form to a softer, rubberlike consistency. Technically, we can relate this phenomenon to changes in "free volume" and the relative mobility of individual molecular segments within the crosslinked lattice. With a highly crosslinked thermoset polymer the transition is not dramatic like the sharp melting point of a pure crystalline compound or the extreme softening of a thermoplastic.Nevertheless at the Tg there are significant changes in the physical properties of many resin systems which can, and will, affect its performance as part of a PWB.There are a number of ways to measure Tg (Thermo-Mechanical Analyzer, Quartz Tube Dilatometer, Dynamic Mechanical Analyzer, Surface Strain Gauges, Differential Scanning Calorimetry and maybe others). The values obtained with each will be different, because each measures a slightly different property or combination of properties. There is no "right way" or "wrong way" to measure Tg, but because of the wide availability of Thermomechanical Analysis (TMA) equipment within the PWB industry, it has become the method of choice, and gives good and reliable results when performed by a well-trained technician using careful sample preparation techniques.The reason TMA is our method of choice is that in the process of determining Tg, the test method measures CTE and expansion properties which are a direct indicator of the stresses on a plated through hole (PTH).when we heat a resin above its Tg, either in use or during processing, it may cause substantial reduction in bond between the resin of the laminate and its copper foil cladding, resulting in pad or line lifting and other problems with surface features. Materials such as polyimide e with high Tg’s are much less affected by phenomena such as smearing and pad lifting.