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Lightning Speed Laminates
By John Coonrod
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Chilling Out with Conductive Adhesives
Conductive adhesives have been around for many years. Some are electrically conductive, others are thermally conductive, and some have both properties. Additionally, the conductive adhesives can be a pressure-sensitive adhesive (PSA) or a thermoset adhesive. To narrow the scope of this month’s column, only thermoset thermally and electrically conductive adhesive (TECA) will be discussed.
TECA is often used for heat sink attachment of PCB assemblies when thermal management is a concern. In the RF industry, power amplifiers often generate a lot of heat, and the PCB supporting the power amp is generally attached to a large metal heat sink. The attachment between the PCB and the heat sink is usually done by mechanical attachment with screws, sweat soldering or TECA. Each attachment method has its own set of capabilities and limits.
The mechanical attachment may have air gaps between the circuit and the heat sink, and if these are in critical areas they can make a less efficient heat flow path from the PCB to the heat sink. A less efficient heat flow path can cause the PCB assembly to have a higher temperature than desired and sweat soldering can have similar issues due to voiding. Air gaps are not an issue with TECA when parts are properly bonded using vendor supplied parameters. The drawback to TECA, compared to the other two technologies, is that TECA is usually not as thermally conductive as a metal-to-metal contact. And for RF applications, the heat sink is often used as the system ground and the electrical 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 thermal conductivity of 3 W/m·K or more is considered good, and electrical conductivity with a volume resistance value of 0.0005 ohm·cm or less is considered good. Regarding other properties, TECA with good bond strength to different metals and robustness to lead-free solder reflow may be important.
Having TECA bond well to different metals is beneficial for bonding to heat sinks made with different metals and/or the different final plated finishes on a PCB, which will be bonded 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.
Read the full column here.
Editor's Note: This article originally appeared in the October 2014 issue of The PCB Design Magazine.
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