The Impact of Circuit Material Properties on Microwave PCB s RF Heating Patterns

Transcription

1 The Impact of Circuit Material Properties on Microwave PCB s RF Heating Patterns

2 The Impact of Circuit Material Properties on Microwave PCB s RF Heating Patterns Agenda Basic heat flow theory applicable to Printed Circuit Boards (PCB s) Circuit material properties related to RF heating RF heating differences of circuits using dissimilar materials Thermal images of PCB s during RF heating and pattern explanations 1

3 H k A T H k is thermal conductivity (W/m/K) A is the area (m 2 ) between the reservoir and thermal conductor L T C Heat flow model of a microstrip PCB The dielectric or substrate is acting like the thermal conductor but the properties are more of a insulator A thinner circuit has a shorter heat flow path 2

4 Key material properties for thermal management issues related to RF heating Thickness of the substrate (thinner substrates have a shorter heat flow path) Thermal conductivity (higher thermal conductivity increases heat flow) Insertion loss (insertion loss generates heat when RF power is applied) Dissipation factor (lower dissipation factor yields lower dielectric loss) Smooth copper surface (smooth surface enables lower conductor loss) Dielectric constant (lower ε r allows wider conductors and lower conductor loss) Typical Thermal Conductivity values Copper 400 W/m/K High Tg FR W/m/K Nearly pure PTFE laminate 0.20 W/m/K Ceramic filled PTFE laminate 0.50 W/m/K RO4350B TM Laminate 0.62 W/m/K RO4360G2 TM Laminate 0.80 W/m/K RT/duroid 6035HTC laminate 1.44 W/m/K 3

5 Heat rise testing, using 50 ohm microstrip transmission lines, given an applied RF power Comparing ID: 1 to 2 is showing heat rise difference due to thickness difference 2 to 3 is showing thermal difference due to copper surface roughness ID 4 is a worst case example regarding circuit material properties ID 5 is a best case scenario For consistent thermal imaging a flat black paint was applied to all circuits 4

6 The RF heating patterns will be related to the current density patterns However significant differences in heat patterns will be due to thermal conductivity The copper will spread the heat on the conductor very effectively The edges of the conductor have low thermally conductive material (thermal insulator), additionally the heat flow path is going down to the heat sink blow Top view of current density pattern of transmission with highest density at the edges Top view of thermal image for transmission line with 85 watts at 3.4 GHz RF power applied; highest temperature is in the middle of the conductor, not at the edges 5

7 Thermal image of a microstrip bandpass filter with 30 watts at 2 GHz applied power The filter has 6 db loss in the passband and this is a partial reason why there is diminishing heat in the filter segments. Also at different frequencies within the passband, there are different current density patterns. When power is applied in the stopband, there is still some heat generated in the first elements 6

8 Page The information in this presentation is intended to assist you in working with Rogers' High-Frequency Materials. It is not intended to and does not create any warranties, express or implied, including any warranty of merchantability or fitness for a particular purpose or that any results show in this presentation will be achieved by a user for a particular purpose. The user is responsible for determining the suitability of Rogers' High Frequency Materials for each application. Prolonged exposure in an oxidative environment may cause changes to the dielectric properties of hydrocarbon based materials. The rate of change increases at higher temperatures and is highly dependent on the circuit design. Although Rogers high frequency materials have been used successfully in innumerable applications and reports of oxidation resulting in performance problems are extremely rare, Rogers recommends that the customer evaluate each material and design combination to determine fitness for use over the entire life of the end product. RO4350B, RO4360G2, RT/duroid and LoPro are licensed trademarks of Rogers Corporation Rogers Corporation. All rights reserved