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Emerald Group Publishing Limited
Copyright © 2004, Emerald Group Publishing Limited
I am delighted to welcome you to this themed edition of Circuit World. Much has been written and spoken about the changing market for printed circuit boards and it would have been easy to miss the exciting developments to the basic building block for printed circuit boards – the substrates. The longevity and resilience of printed circuit board substrates is a testament to the committed researchers around the world who have continued to develop products meeting the increasing performance demands of the end-user whilst maintaining the essential attribute of processability. If we were to compare the basic properties of a modern day FR4 with that defined by NEMA L1-1 in 1965 there would be little similarity in performance. In addition to the basic chemistry and raw material improvements the laminate industry has made great gains in reducing product variability in order to deliver known and consistent performance to their customers.
The evolution of printed circuit board substrates has necessitated a changed view of how performance is classified. Traditionally, substrates have been classified according to the physical properties of their basic chemistry. By way of example, we are used to classifying substrates according to their glass transition temperature Tg. Accordingly we refer to “standard” or “high” Tg substrates. It is no surprise therefore that many have assumed that the Tg value has a direct relationship to the thermal endurance of the substrate and have specified a “high Tg” product for an application requiring high thermal resistance. In reality, the Tg value merely marks a phase change in the resin chemistry whereby enough energy (temperature) is available to increase the rotational freedom of the polymer to allow it to transition from a glassy to a rubbery state. If further energy is applied then the next transition that would conventionally be expected is the Tm or melting temperature. Here is where thermoset plastics spring a surprise. In theory, there is indeed a Tm, however, the temperature required to melt a thermoset is in excess of the thermal decomposition temperature. The consequence of this is that a thermosetting material never reaches the Tm, it decomposes into carbon and a variety of gases long before it can ever get there.
Selection of a substrate for an application or process requiring high thermal resistance should therefore be based on decomposition temperature instead of Tg. This marks a shift in the classification, it is not sufficient to specify a “high Tg” substrate, instead performance should be specified according to the application. Fortunately, the industry has been quick to recognize this and has developed the T260 and T288 tests. These tests define the time to delamination (a measurable physical effect of thermal decomposition) at 260 and 288°C, respectively. These temperatures fall quite neatly into the range for that of lead containing and lead-free soldering thereby allowing the user to specify the substrate for current and future soldering practice.
The main performance benefit of using a high Tg substrate is that of reducing the overall thermal expansion. A substrate above its Tg occupies more volume than one beneath for the reasons explained above. Therefore, the higher the Tg the lower will be the overall thermal expansion and consequently, the lower the stress on plated through holes or other circuit board features with a Z-axis dimension. Increasing the Tg is however only one of the ways to reduce the Z-axis expansion. Many new products have become available that achieve this performance by a new generation of finely dispersed fillers, for example, Isola's IS400 range. To complete the group we could add in substrates providing enhanced thermal conductivity and consider these all belong to the group of “substrates for thermal management”. The underlying properties defining this group would therefore be Tg, time to delamination, coefficient of thermal expansion in the Z-axis and thermal conductivity.
Following the same theme of product classification by application leads us to the exciting and active development area of high frequency substrates. The traditionally specified property in this area is dielectric constant measured at 1 MHz. We are all now used to even consumer electronic products operating well in excess of 1GHz and communications devices in the range of up to 10 GHz, thus highlighting the inadequacy of measurements at 1 MHz. The high frequency spectrum in the past was divided into two bands – one where FR4 worked and one where it did not. Where it did not work PTFE was needed. We are indeed fortunate that the gap between FR4 and PTFE performance has been filled by entirely new ranges of substrate materials based on their dissipation factors. This allows the designer the flexibility to select the appropriate substrate for the application without over engineering performance that is neither required nor can be afforded.
We can feel confident that even through the last years of tremendous upheaval, the industry has continued to aggressively develop innovative material solutions to satisfy current and future demand. I hope that you enjoy reading this issue and share our enthusiasm and excitement for the continued development of the material solutions of tomorrow.