Considerations for Dispensing Thermal Interface Materials
By John Urquhart and Tim Meyer
What is Thermal Interface Material?
Electronic assemblies can get hot, and if they get too hot, they fail. Examples of heat-generating devices include LED lighting arrays, engine control modules, electric vehicle battery assemblies, and telecommunication switching modules. To prevent overheating, a thermally conductive pathway such as a processor or capacitor must be created between the heat-generating device and the outside atmosphere. Simply placing a heat sink on top of a heat source will allow a percentage of the thermal mass to decrease but, the thermal transfer efficiency will be quite low due to variations in surface finish or assembly tolerances. This is where thermal interface material (TIM) can be used as a conductive bridge between the heat-generating device and a heat dissipating device.
A variety of dispensable liquid TIMs exist for aiding with heat transfer and relieving stress caused by coefficient of thermal expansion (CTE) mismatch. Thermal grease, gap fillers, dispensable “gels,” and conductive potting are the most common forms of this technology. The primary considerations when selecting a TIM material are thermal conductivity and dielectric strength but other features must be considered to establish a proper dispensing process such as viscosity, filler density, particle size of filler, curing mechanism, and chemistry. These should be weighed alongside a manufacturing technique that creates an efficient assembly process while still achieving optimal product performance. Assembly tolerance, compression force, target cycle time, waste generation, and equipment maintenance are all points to be considered.
Compared to pre-cut thermal pads, which can be placed either manually or via machine, dispensing offers much more design flexibility. When a revision is needed in the assembly, the pattern and amounts dispensed can be changed virtually on-the-fly in production with minimal downtime via a simple program edit in the dispensing system. If pads are used, placement machine re-tooling may be required for new pad dimensions. Pads can create issues inside an electronic assembly such as stacked tolerances due to inconsistent contact on the mating surfaces and in extreme cases, excess compression on sensitive electronics. While a gel will submit and flow into cavities while still retaining enough body to hold shape and maintain contact with critical components, a precut pad can cause continuous stress when compressed. Designs that have multiple interface locations with various size pads can lead to an increase of bill of materials, potential sourcing, and inventory headaches.
A common conductive ingredient used as a filler is aluminum oxide or alumina, chemical formula Al2O3. Alumina is a relatively good thermal conductor and is electrically insulating. To help set compression limits during assembly, glass or ceramic beads are commonly used and also offer electrical insulation. The overall conductance of the TIM is usually dependent on the density of the Al2O3 in the mixture. Here arises one of the challenges of handling this kind of material. Al2O3 happens to be a particularly hard and abrasive filler, with a Mohs hardness of ~9.0 [Diamond being 10.0]. Because of these features, Pumping equipment and plumbing must be carefully selected to minimize settling or packing of this filler and to prevent abrasion to seals and moving parts.
As with meter mix applications, there are multiple pumping technologies available. However, dispensing TIM requires focus on careful fluid system design in order to maximize device life and maintain the integrity of the TIM during dispensing. Fluid routing and plumbing to the point of dispense is designed to minimize back pressure to the system and reduce dead space. For example, when routing a fluid system through a section that requires a 90-degree direction change, using a sweep or bent tube fitting will maintain flow velocity and prevent packing or separation of fillers compared to using a typical elbow fitting that contains tighter geometries. Small touches like this have a beneficial impact on the manufacturing floor where minimizing downtime for maintenance is critical.
Moving components that are in contact with the fluids are made from materials selected for their resilience, such as tungsten carbide. Dispense valves may utilize carbide sealing rods to maximize time between rebuilds. High accuracy progressive cavity pumps may use a combination of a carbide rotor for wear resistance with a soft rubber stator to allow fillers to pass through the unit without generating any shear effect on the fluid. Applying high shear forces on filled fluids can lead to grinding of any solid particles which may change the thermal properties and lower performance of the electronic module. Pumps are sized to keep rpm to a minimum and prolong rotor wear while maintaining target flow rates. Soft seals are limited in quantity and designed in such a way that they can be readily replaced without introducing significant downtime.
Feed reservoir package sizes are important to note when planning production. Smaller containers such as syringes and cartridges may be appealing for their size and ease of implementation in manual or low volume automated production but have limitations on the amount of pressure that can be applied. Dispense volume, production throughput, and frequency of changing material containers are often the determining factors for selecting the best dispensing package.
Thermal grease is one type of TIM that is very smooth and flowable. Often silicone-based and containing fine fillers, thermal grease plays well with cartridge dispensing due to its highly thixotropic nature. Assemblies such as memory chips and high-power LED arrays often use thermal grease for its ability to wet into cast or machined surface textures and provide for thin bond lines, some as low as 50 microns. The required amount to dispense is often quite small so syringes and cartridges are acceptable packages which can hold enough volume for at least a single production shift. Volumetric dispensers such as servo-driven auger valves or progressive cavity pumps work well for controlling the small dispense amounts required while the soft grease composition typically generates very little wear on the fluid components. Some highly engineered TIM products mainly used with computer processors are composed of Indium and/or Gallium. These products look and act like a “liquid metal” and are chosen for their superior thermal transfer efficiency and wetting ability while achieving ultra-thin bond lines.
Designs that require higher thermal conductivity within larger assembly spaces such as automotive control modules and telecom devices may lean toward using higher density products that contain heavier and often more abrasive fillers. 1-part room temperature cure and pre-cured gel products are chosen for their ability to conform to larger variable thickness gaps but also require specifically designed dispensing equipment. A range of dispensing options exist depending on the package size, dispense amounts, and tolerances required. Pneumatic and servo-driven ram systems are available to feed a dispense valve remotely from larger cartridges such as a 20 oz. Semco™ style package. A reinforced clamshell or retainer is used around the cartridge to prevent it from bursting due to increased pressure from the linear drive. If using a semi-flowable TIM for larger dispense volumes coupled with high throughput, 5-gallon packaging is an option, but the proper feed mechanism must be used. Specific pail pumps are used with a special hard coating applied to internal surfaces for abrasion resistance combined with minimal friction points to allow smooth flow and maintain fluid consistency.
Pre-cured TIM gel is desirable for its high thermal conductivity and physical properties. Gel products are packaged in the same form as they will be dispensed on the product and eliminate the need for mixing or post-application heating and curing operations. Once the material has been dispensed onto the component, it is ready for assembly. The consistency must be flowable enough to be dispensed but thick enough to maintain its shape on the substrate. The high viscosity of these gels can lead to a tradeoff of dispense rate with cycle time. Thicker material may hold its shape and resist flowing after application. However, thicker materials limit the rate at which they can be dispensed. Pre-cured gel products often come in larger packaging such as 1-gallon and 5-gallon pails which are necessary due to the high pressures required for moving these dense and often tacky compounds. Hydraulic pail unloaders take advantage of the original packaging by using a single moving ram to extrude the TIM gel at up to 800 psi out to the point of dispense. The simple design utilizes a single seal and does not require check valves or other moving components that can wear out or alter the TIM properties.
Two-component gap fillers may be used for large area coverage or designs with multiple component heights. Additional volume may be dispensed beyond only the heat sink area to encapsulate an additional percentage of nearby components in order to channel as much heat as possible away from the assembly. Two-component materials pose a unique challenge in that potentially high-density fluids must be held at a fairly tight mix ratio while accurately controlling dispense volumes. Equipment must be precise, powerful, and durable enough to deliver these fluids at the proper ratio and rate to ensure repeatable curing of the material. Fortunately, most two-component TIM chemistries are provided in a convenient 1:1 ratio that provide for good mixing in a typical static mixing nozzle.
For lower production volume, cartridges are often a more economical solution for manufacturers but require a metering system to maintain proper ratio and dispense rate. As mentioned above, servo drive units can be configured for a two-cartridge feed with programmable linear actuators. This configuration will drive material directly out of the cartridge to a dispense valve with static mixer. A precise proportioning of material will be achieved while eliminating wearable components in the delivery unit.
For the highest accuracy combined with rapid production rates, volumetric dispensing heads are an optimal choice. These provide point of dispense metering while eliminating common issues seen with remote metering systems such as an excessive pressure drop through long feed lines or inconsistent dispense volumes due to viscosity variation from batch to batch. Highest volume dispensing from pails is easily performed using hardened high flow pumps to remotely feed the metering head which maintains ratio and dispense rate on a robot end effector regardless of variation in feed pressures from the bulk supply. Both one and two-component servo-drive heads are available.
Dispense patterns can be critical in achieving optimal thermal contact and should not be overlooked. For example, using a square 20×20 mm plateau as a dispense site, a simple dot pattern can be used to apply TIM to one side of the assembly then the dot will spread when the mating piece is assembled. Too much volume can lead to squeeze out and possible wetting into undesired areas; too little volume can lead to insufficient coverage and lower thermal efficiency. Changing the dispense program from a dot to a square spiral or even an X shape can optimize spread of the TIM with little to no squeeze out during assembly. The final dispense shape may need to be optimized in order to provide adequate coverage within assembly tolerances and without voids.
The dispensing nozzle is another important parameter to note. It is recommended to start with the largest diameter that is suitable for the application to provide that maximum flow rate. Nozzles that are too small can create shear forces on the TIM and alter its properties or even generate excess back pressure on the fluid system during the dispense cycle. One condition where smaller nozzles prove beneficial is when the surface finish if the substrate is very smooth and the TIM tends to pull away from the substrate when the nozzle retracts after dispensing. This tends to happen with TIM that is very dense and has a very sticky or spongy consistency and does not want to separate from itself easily. Using a smaller nozzle diameter will reduce the cross section at the dispense point and help “break” the fluid when dispensing stops. In some cases, the robot may also need to be programmed to perform a “dance” with the nozzle after dispensing in order for the fluid to break cleanly without pulling the last deposit away from the surface or leave a tail of TIM in an undesired area of the substrate.
If using a two-component TIM, the dispensing system should be configured with a pot life timer that will perform an automatic purge sequence if the system sits idle for a given period. This prevents the volume inside the mixer from curing or allowing the viscosity to increase to a point where the dispensed results are not as expected from reduced volume or flow. Some TIM products may have a sensitivity to filler separation if left under pressure for extended periods. The pot life timer may also have an additional step where the fluid system may relieve pressure in the hoses and delivery system if idle for too long.
As electronic modules continue to be developed with greater component density and increased processing power, liquid TIM application is becoming a standard process for many new devices. The diverse nature of dispensing provides a wide variety of solutions for every application. As new applications emerge, innovative and adaptive technologies are developed that enable increased manufacturing efficiencies for an ever-evolving future.