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PCBD-Aug2015

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56 The PCB Design Magazine • August 2015 Other issues that are critical to many high- frequency applications and also environment- related is TCDk and TCDf. All circuit materials have these attributes: TCDk is the temperature coefficient of dielectric constant (Dk) and TCDf is analogous in regards to dissipation factor (Df). Basically, this is the property of material which will change Dk or Df with a change in temperature. Many times this becomes an issue when an application is fine-tuned in a lab envi- ronment and it then performs different in the field as the temperature fluctuates. The amount that TCDk or TCDf impacts circuit performance is very design and application specific. Some ap- plications will be unaffected by these changes and others may see dramatic shifts of electrical performance. An ideal TCDk or TCDf would be 0 ppm/°C, which gives no change in Dk or Df with a change in temperature, but this is typi- cally not realistic. A general rule for a good TCDk is an absolute value of 70 ppm/°C; how- ever, there are many applications which could tolerate 200 ppm/°C or even more. Other thermal properties related to lami- nates are pretty well known in the PCB indus- try such as coefficient of thermal expansion (CTE), glass transition (T g ) and decomposition temperature (T d ). These properties are related to high-frequency applications in a manner similar to most other applications, where these issues are typically a concern for assembly and long-term reliability. However, some high-fre- quency laminates, due to the nature of their formulation, have very good CTE, T g or T d . Some of the ceramic-filled hydrocarbon lami- nates, such as RO4350B™ substrates used at high frequency, have good CTE and extremely good T g and T d . Another material property which is com- mon for all laminates is thickness control and high-frequency laminates excel with this con- cern. Because impedance and other circuit prop- erties are sensitive to the substrate thickness, high-frequency laminates generally have better thickness control than FR-4 type laminates. It is common to have high-frequency laminates with a thickness control of ±7% or better. The tight- ly controlled thickness combined with the fact that high-frequency laminates also control the dielectric constant strictly, allows the designer to have much tighter control over impedance and other electrical properties of a circuit. This combination of laminate properties control is why high-frequency laminates are sometimes used in applications where impedance control is critical, but it may not be a high-frequency application. Lastly, many high-frequency laminates that use special filler to achieve good CTE values and stable electrical properties often have much higher thermal conductivity. In general, PTFE based substrates have a relatively low thermal conductivity and it is typically in the range of 0.25 W/m∙K. However when the proper filler is added to PTFE, to formulate a high-frequency laminate, the thermal conductivity can in- crease significantly and values of 0.5 W/m∙K or more, are common. The ceramic-filled hy- drocarbon high-frequency laminate mentioned previously also has the benefit of high thermal conductivity filler and that substrate has a ther- mal conductivity value of 0.64 W/m∙K. A spe- cial formulation of a high-frequency laminate, RT/duroid® 6035HTC material, is a PTFE based substrate with unique formulation to achieve a thermal conductivity figure of 1.44 W/m∙K. Again, these laminates are sometimes employed in applications where high-frequency is not a concern, but where thermal management is a critical issue. It has been shown that high-frequency lam- inates have many attributes which make them attractive for applications that are not neces- sarily high-frequency, but can be used when other concerns are important. Obviously, when an application is high-frequency or high-speed digital, then these specially formulated lami- nates are the proper choice. However, when a designer or PCB fabricator is working with a new design, which may or may not be high- frequency related, they should consider all material options. PCBDESIGN A BRIEF OVERVIEW OF HIGH-FREQUENCy LAMINATES continues lightning speed laminates John Coonrod is a senior market development engineer for Rogers Corporation. To read past columns, or to reach Coonrod, click here.

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