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48 The PCB Design Magazine • October 2014 connection between the PCB, so the heat sink has to be very good for most RF applications. The obvious critical material properties for TECA are thermal conductivity and electrical conductivity, but there are other concerns. As general statements, a TECA material with ther- mal conductivity of 3 W/m·K or more is con- sidered good, and electrical conductivity with a volume resistance value of 0.0005 ohm·cm or less is considered good. Regarding other prop- erties, TECA with good bond strength to differ- ent metals and robustness to lead-free solder reflow may be important. Having TECA bond well to different met- als is beneficial for bonding to heat sinks made with different metals and/or the different final plated finishes on a PCB, which will be bond- ed to the heat sink with the TECA. There have been TECA materials in the PCB industry which were not robust with lead-free soldering; they were still useful for heat sink attachment, but had special processing considerations. Lastly, electrical performance will be more challenging for RF applications. The electri- cal conductivity of TECA will be important for DC or low-frequency applications, but is even more critical for RF applications. With RF ap- plications, the ground plane of the PCB must be extremely well connected to the ground of the system, which can be through the heat sink. In these cases, at high frequencies, small anoma- lies in the grounding between the PCB and the heat sink can cause an increase in system noise and insertion loss. These small electrical anom- alies may not be detected at lower frequencies or DC, but they can be very problematic for high-frequency systems. When using TECA in this manner, the surface treatment of the bond surfaces should be considered. A gold surface will provide the best electrical bond interface. Another concern for TECA used in high-fre- quency applications is the possible variance of the ground return path due to the conductivity changing with a change in temperature. TECA uses conductive fillers to make the electrical con- nection through the volume of the material. When TECA is heated, it will expand and the fill- ers may move farther apart, causing less connec- tion or less conductivity. This generally chang- es little with electrical conductivity, but with high-frequency applications it can be detectable. Knowing how much the electrical conductivity will change for TECA, a change in temperature should be understood for RF applications. Laminate providers now offer TECA films. Rogers Corporation's COOLSPAN TECA Film has been tested for all of the concerns men- tioned in this column. It is robust for lead-free soldering and bonds well to any metal evalu- ated thus far. High-frequency testing has been conducted using 50 ohm microstrip transmis- sion line circuits and across a range of micro- wave frequencies. The table in Figure 1 shows a summary of these circuits, and it can be seen that there is no significant difference for inser- tion loss from room temperature to 65°C across the range of frequencies tested. There are several options to attach heat sinks to PCBs, and TECA materials are being used more often. Even though the other op- tions are usually higher in thermal conductive because they use metal or direct metal-to-metal interface, TECA is generally more consistent for heat flow path, relatively easy to apply, and comparable to the RF electrical performance of other options. PCBDESIGN ChILLING OUT WITh CONDUCTIVE ADhESIVES continues Figure 1: Test results for TECa films with 50 ohm microstrip transmission line circuits. John Coonrod is a market de- velopment engineer for rogers Corporation, advanced Circuit Materials Division. To read past columns, or to reach Coonrod, click here. lightning speed laminates