S-Bond Joined Phase Change Materials (PCM’s) Heat Sinks

S-Bond® active solders are being used extensively as a high conductivity bonding solution for foam cored phase change materials foam cored heat sinks. Increasingly, thermal management in electronics is the limiting factor in performance and/or life of electronics as higher power and higher speed in electronic devices generate more intense heat. High brightness LED’s, high speed/high bandwidth telecommunications, avionics, satellites and solid state conversion devices all have transient and steady power states where intense heat is generated and needs to be channeled away from the electronic device to prevent performance loss or permanent damage. The electronic industry is relying on a host of devices from conventional heat sinks with fins and fans to heat pipes and vapor chambers to more exotic materials and composites that include pyrolytic graphite or diamond. S-Bond materials and processes have been proven to be a good solution when bonding these various components and materials with a metallic “thermal interface material (TIM)” rather than filled polymeric bonding agents.

When electronics have a high transient heat output thermal engineers are using “phase change” heat sinks. Such heat sinks utilize “phase change materials (PCM’s)” that when exposed to heat absorb it very quickly and effectively as the material “changes phase”… either going from solid to liquid or liquid to vapor. Materials with high latent heats of fusion or latent heat of vaporization at or near the maximum temperatures that electronics are being used in the core of such heat sinks. In PCM heat sinks during the phase change there is the potential to rapidly absorb a high heat load… that can later be more slowly released to the atmosphere with cooling fins as the phase change is reversed and the heat is released away from the electronic device.

Two of the most used PCM’s are paraffin and water… each has a high latent heat of fusion or heat of vaporization, respectively. The challenge in the use of PCM’s is to overcome their relatively low thermal conductivities. For example, in heat sinks with paraffin as the PCM, when the heat transfers from the electronic device into the heat sink package, the outer layer of paraffin melts and then slows the transfer of heat into the solid paraffin core. To offset this heat flow limitation, designers are incorporating metallic or graphitic foams into the core of the heat sinks. The foams’ cells separate the PCM’s into small reservoirs that are surrounded by high thermal conductivity cell walls that then transfer the heat to a small PCM filled pore in the foam and therefore quickly melts the paraffin or vaporizes the water. Later in the “reverse” cycle, the conductive foam “cell walls” transfer the heat out of the PCM filled pores to solidify or condense the full volume PCM in the heat sink.

S-Bond joining has found excellent application in paraffin based PCM heat sinks in combination with graphitic foams (PocoFoam® or K-Foam®). S-Bond can effectively bond to graphite and graphite foams to heat sink package materials such as aluminum, copper and many heat sinks composites such Al-SiC, Al-Gr, Cu-W or Cu-Gr. In such graphite core/paraffin heat sinks. S-Bond Technologies has S-Bond metallized the Gr-Foam preparing for it to be soldered directly to the heat sink package. After S-Bond metallization, various S-Bond solders and processing can be used to bond the graphite foam to the components of the heat sink.

Figure 1 shows a PCM core finned Aluminum heat sink box used to cool high power laser diode packages mounted on the flat side opposite the side with the fins. The aluminum box contains a core of graphite foam bonded to the walls and base of the aluminum enclosure. The aluminum enclosure is then heated to 100˚C and filled and infiltrated with paraffin PCM’s. After filling the enclosure is sealed and the assembly is a PCM heat sink. When the laser diodes are on for intermittent periods of time the graphite foams thermally bonded to the wall of the enclosure heat the PCM… later when the heat load from the diodes are off, the bonded fins with air convection assist, cool and solidify the paraffin PCM to get the heat sink ready for the next thermal cycle. S-Bond active solder joining enabled the graphite foam to have an excellent thermal interface to the enclosure without filling the graphite foam and compromising the graphite foam’s ability to hold and transfer heat quickly to the paraffin PCM.

Figure 2 shows another type of PCM cooling. The alternating bonded fluid circulating aluminum tubes bonded with S-Bond sandwiched between S-Bond metallized and bonded graphite foam plates. The stack is later encased in an enclosure and paraffin PCM is infiltrated into the foam to make a large ~ 24” x 24” x 12” PCM heat sink. When heated fluids circulate in the aluminum tubes the PCM filled graphite foam core rapidly absorbs the heat from the fluid.

Figure 3 illustrates another style of PCM heat sinks that are mounted around a central heat pipe. In this design, S-Bond metallization of the faces and ID’s of the graphite annular rings permitted a graphite foam outer core to be become an effective PCM heat sink for a heat pipe cored thermal management device.

Contact us to evaluate how S-Bond can be used to enable your thermal management components to be made and how phase change material (PCM) heat sinks can be effectively incorporated into your designs.