Thermal Interface Materials – Frequently Asked Questions
SIL-PAD A2000 utilizes a different filler package than SIL-PAD 2000. This change results in a more compliant SIL-PAD A2000 material that inherently lowers interfacial resistance losses. This reduction in interfacial resistance with SIL-PAD A2000 results in improved overall thermal performance when measured at lower pressures in standard ASTM D5470 and TO-220 testing.
The answer is based on the assumption that the primary design intent is to increase thermal performance. If your application utilizes lower clamping pressures (i.e. 10 to 75 psi) you will find the SIL-PAD A2000 to provide excellent thermal performance. In contrast, if you are designing for higher clamping pressures (i.e. 100 psi or greater), it is likely that you will require the thermal performance characteristics of the SIL-PAD 2000.
Yes. Henkel evaluates and publishes voltage breakdown, dielectric constant, and volume resistivity data per ASTM standards for these materials. Due to differences between ASTM lab testing and actual application performance, for best results, these characteristics should be evaluated within the actual customer system.
Yes. With the new environmentally “green” process improvements added with the introduction of SIL-PAD A2000 products, the materials are now available in roll form. The original SIL-PAD 2000 material cannot be produced in continuous roll form.
SIL-PAD 800 is specifically formulated to provide excellent thermal performance for discrete semiconductor applications that utilize low clamping pressures (i.e. spring clips at 10 to 50 psi). In contrast, if you are designing for higher clamping pressure applications using discrete semi-conductors (i.e. 50 to 100 psi), it is likely that you will prefer the combination of high thermal performance and cut-thru resistance inherent in our SIL-PAD 900S material.
SIL-PAD 980 is specifically formulated to provide superior cut-through and crush resistance in combination with excellent heat transfer and dielectric properties. SIL-PAD 980 has a proven history of reliability in high-pressure applications where surface imperfections such as burrs and dents are inherently common. These applications often include heavily machined metal surfaces manufactured from extrusions or castings. SIL-PAD 900S carries a high level of crush resistance and is more likely to be used in burr-free or controlled surface finish applications.
SIL-PAD 1500ST has an inherent tack on both sides of the material. This inherent tack is used instead of an adhesive. The tack provides sufficient adhesive for dispensing from the carrier liner and placement on the component. SIL-PAD 1500ST can be repositioned after the initial placement.
Each application has specific characteristics (e.g. surface finish, flatness tolerances, high pressure requirements, potential burrs, ect.) that determine which SIL-PAD will optimize thermal performance. Select a minimum of two pads that best fit the application, then conduct testing to determine which material performs the best.
SIL-PAD 1500ST wets-out the applicaton surfaces at a very low pressure. Optimal thermal performance is achieved at pressure as low as 50 psi.
The ISO certification is the adoption of a quality management system that is a strategic decision of the organization. This International Standard specifies requirements for a quality management system where an organization: a) needs to demonstrate its ability to consistently provide product that meets customer and applicable regulatory requirements, and b) aims to enhance customer satisfaction through the effective application if the system, including processes for continual improvement of the system and the assurance of conformity to customer and regulatory requirements.
The Anter Quickline 10 was used to run this test. Henkel has published an application note about the modifications to the ASTM D5470 test method to appropriately test GAP PAD materials at low 10 psi pressure; see Henkel Application Note #112.
Currently, GAP PAD VO, GAP PAD VO Soft and GAP PAD VO Ultra Soft are offered with or without an adhesive on the SIL-PAD 800/900 carrier-side of the material. The remaining surface has natural inherent tack. All other GAP PADS have inherent tack.
Depending on the surface being applied to, if care is taken, the pad may be repositioned. Special care should be taken when removing the pad from aluminum or anodized surfaces to avoid tearing or delamination..
The characteristic of the rubber itself has a natural inherent tack, without the addition of an adhesive. As with adhesive-backed products, the surfaces with natural tack may help in the assembly process to temporarily hold the pad in place while the application is being assembled. Unlike adhesive-backed products, inherent tack does not have a thermal penalty since the rubber itself has the tack. Tack strength varies from one GAP PAD product to the next.
Again, depending on the material that the pad is applied to, in most cases they are repositionable. Again, care should be taken when removing the pad from aluminum or anodized surfaces as to avoid tearing or delaminating the pad. The side with natural tack is always easier to reposition then an adhesive side.
Depending on the application and the pad being used, GAP PAD has been reworked in the past. Henkel has customers that are currently using the same pad for reassembling their applications after burn-in processes and after fieldwork repairs. However, this is left up to the design engineer’s judgment as to whether or not the GAP PAD will withstand reuse.
It is highly dependent on the application and its surface topography. Liquid GAP FILLER will cure with low adhesive strength to the application surfaces.
In the temperature range of -60°C to 200°C, there is no significant variance in hardness for silicone GAP PADS and GAP FILLERS.
Shelf life for most GAP PADS is one (1) year after date of manufacture. For GAP PAD with adhesive, the shelf life is six (6) months after the date of manufacture. After these dates, inherent tack and adhesive properties should be recharacterized.
GAP PAD VO materials and GAP PAD A3000 are more stable at elevated temperatures. GAP PAD in general can be exposed to temporary processing temperatures of 250°C for five minutes and 300°C for one minute.
Yes, all GAP PAD materials are electrically isolating. However, keep in mind that GAP PAD is designed to FILL gaps and is not recommended for applications where high mounting pressure is exerted on the GAP PAD.
Refer to the Pressure vs. Deflection charts in Henkel Application Note #116.
GAP PAD and GAP FILLER can be used wherever air can be replaced, such as between a heat generating device and a heat sink, heat spreader or housing. This can be done by using one sheet of GAP PAD or individual pieces of appropriate thicknesses if stack-up tolerances and height variations are significant.
The better a GAP PAD complies and conforms to a rough or stepped surface, the less interfacial resistance will be present due to air voids and air gaps. This leads to a lower overall thermal resistance of the pad between the two interfacest.
Primarily for aerospace applications, outgassing data is available by contacting a Henkel representative.
Bond-Ply 660B utilizes a dielectric film, replacing the fiberglass inherent in our Bond-Ply 100 series products. The addition of the film allows for high dielectric performance without additional product thickness.
Bond-Ply product testing has been completed on various interface materials. These tests have demonstrated that improper surface wet-out is the single largest variable associated with maximizing bond strength and heat transfer. Henkel has found that reducing the size of the interface pad to roughly 80% of the total interface area actually improves the overall bonding performance while offering significant improvements in total package cooling. Henkel offers three standard thicknesses for Bond-Ply 100 allowing each application to be optimized in three dimensions.
The answer to this varies from application to application, depending upon surface roughness and flatness. In general, pressure, temperature, and time are the primary variables associated with increasing surface contact or wet-out. Increasing the application time and/or pressure will significantly increase surface contact. Natural wet-out will continue to occur with Bond-Ply materials. This inherent action often increases bond strength by more than 2x within the first 24 hours.
Adhesive performance on plastic packages is primarily a function of surface contact or wet-out. If surface contaminants such as plastic mold release oils are present, this will prevent contact and/or bonding to the surface. Make sure all surfaces are clean and dry prior to applying Bond-Ply materials.
Liqui-Bond SA 2000 requires heat to cure and bond in the application. Altering the bond line temperature and time can control the cure schedule. The components should not be moved during the curing process.
Henkel uses the Anter Quickline 10 to characterize our ASTM D5470 test results. The method is modified to condition the phase change material to 5°C over the stated phase change temperature. Understanding that time is also a key variable for material displacement or flow, the over-temperature conditioning is limited to 10 minutes and then allowed to cool, prior to initiating the actual test at the given pressure. The 10-minute time period has been demonstrated to be an acceptable time period for the thermal mass inherent in the Anter setup. Note: Actual application testing may require more or less time to condition, depending upon the heat transfer and thermal mass associated. The performance values are recorded and published at 10, 25, 50, 100, and 200 psi to give the designer a broad-based understanding of Hi-Flow's performance.
Upon achieving phase change temperature (i.e. pre-conditioning), Henkel has demonstrated that 10 psi provides adequate pressure to achieve exceptional thermal performance. Henkel continues to research lower pressure wet-out characteristics in an effort to minimize interfacial losses associated with ultra-thin material interfaces.
Mechanical fasteners are required. Henkel recommends the use of spring clips to maintain consistent pressure over time.
Hi-Flow works best with a clip or spring washer mounted assembly. The continuous force applied by these devices allows the Hi-Flow material to flow and reduce the cross sectional gap. Henkel suggests that design engineers evaluate whether a screw mount assembly will have acceptable performance. See TO-220 Technical Note.
The adhesive in the current construction does adhere more to the heat sink aluminum than to the Hi-Flow material. There is the potential that the adhesive will be removed by the heat sink surface when it is removed to reposition on the heat sink. Time and/or pressure will increase the bond to the aluminum increasing the potential for the adhesive to adhere to the heat sink.
Standard electronics industry cleaning procedures apply. Remove dirt or other debris. Best results are attained when the Hi-Flow material is applied to heat sink at a temperature of 25° +/- 10°C. If the heat sink has been surface treated (i.e. anodized or chromated), it is typically ready for assembly. For bare aluminum, mild soap and water wash cleaning processes are typically used to eliminate machine oils and debris.
If the material has not gone through phase change, the material will readily release from the device surface. For this situation, the Hi-Flow material will not likely have to be replaced. If the material has gone through the phase change, it will adhere very well to both surfaces. In this case, Henkel suggests warming the heat sink to soften the Hi-Flow compound for easier removal from the processor. Replace with a new piece of Hi-Flow material.
Insulated Hi-Flow products are manufactured with inner film support. This film stiffens the material, allowing parts to be more readily die-cut as well as making the material easier to handle in manual or automated assembly.
Many Hi-Flow materials have no surface tack at room temperature. The softer materials will pick up dirt more readily. Softer resins are more difficult to clean if any dirt is on the surface. If you try to rub the dirt away, the dirt is easily pushed into the soft phase change materials. Hi-Flow coatings are typically hard at room temperature rendering them easier to clean off without embedding dirt.
Hi-Flow 625 does not require a protective film during shipment. There are two issues with competitors’ materials: 1. Melt point of the material is low enough that it can go through phase change in shipment and be very tacky. Hi-Flow has a higher phase change temperature and remains hard to a higher temperature. 2. The Hi-Flow material is harder and is not as easy to scratch or dent in shipping and handling.
The 65°C phase change temperature was selected for two reasons. First, it was a low enough temperature for the phase change to occur in applications. Second, it would not phase change in transport. Henkel studies show that shipping containers can reach 60°C in domestic and international shipments. The higher phase change temperature eliminates the possibility of a product being ruined in shipment. We offer a standard line of Hi-Flow 225 and 300 series products with 55°C phase change for those customers wanting the lower phase change temperature.
Avoid using Hi-Flow in applications in which the device will not reach operation at or above phase change temperature. Also avoid applications in which the operating temperature exceeds the maximum recommended operating temperature of the compound.
Fundamentally, Electromagnetic Interference (EMI) is any kind of unintended transfer of electrical energy from one circuit or system to another. (And it isn’t limited to switching regulators.) Whatever its source, this energy has the ability to interfere, corrupt or damage the operation of any nearby circuit. The interference may be coupled by conduction or by radiation. (Fig.1)
Most circuit designers are familiar with gap fillers in the form of pastes or pads that are used to enhance heat exchange between a heat-generating electronic device and its associated heat sink. Paste-type materials are generally used for prototyping and short production runs. Pad-type materials are shaped to various types of heat-generating devices (e.g., a power transistor in the familiar TO-220 package). They avoid the mess of paste-type materials, they’re inherently gap-free for consistent heat-transfer characteristics, and they lend themselves to automated assembly.
The thermal path in certain gap filling pads employs electrically conductive materials like graphite in the elastomeric matrix. This makes it possible to use the same types of material in building Faraday cages that either shield EMI-generating or EMI-sensitive circuits from external sources of EMI or to isolate those sources within the product being protected.
A common instance would be switching semiconductors in voltage-regulator circuits. They typically switch large DC currents at high rates in the course of performing their regulation function, creating harmonics of the switching frequency.
From a product-design standpoint, EMI is a potential contributor to product field failures. EMI is very often a “could not duplicate” root cause for field problems. EMI problems appear during product introduction (insufficient noise margin), minor changes (board modifications, wire harness changes, etc.). In addition, because EMI can cause various types of field-failure, governments around the world have established procedures for measuring EMI, and electromagnetic compatibility (EMC) standards that must be met before a product can be sold. Government agencies, like the FCC, approve and monitor product EMI emissions for EMC.
A: A conductive shield (Faraday cage) is typically used to keep EMI from leaving or entering a defined space. A Faraday cage is ~100% reflective. The reflected EM energy can easily interact with other components in the shielded space. An absorber shield is sometimes added to the Faraday cage to mitigate reflections.
If you’re not sure after reading that explanation, check out any Web article on Faraday Shields, for example, the Radiative Wikipedia article on Faraday Cages at: https://en.wikipedia.org/wiki/Faraday_cage. Essentially, the conductive “cage” around the circuit “captures” incident electromagnetic radiation because that radiation causes currents to flow in the conductive material of the “cage.” Ohmic losses in the cage convert the incident RF energy and turn it into heat, which can be conducted or radiated away.
First they can carry away the heat generated by those ohmic losses, transferring it to whatever heat sink has been designed into the product; second, they simplify assembly (they are adhesive). In addition, they make it easier to physically assemble the Faraday cage; and, can make the cage more mechanically robust.
The shielding effectiveness (SE) of a material is based on comparing measured field strength has been defined as: SE (dB) = 20 log (FM/F0). By comparing the measured field (FM) and reference field (F0). Individual absorption, reflection, and transmission terms can be determined.
Material supplier Bergquist recently introduced GAP PAD™ EMI 1.0. This combination gap filling material offers both thermal conductivity performance and electromagnetic energy absorption (cavity resonances and/or cross-talk causing electromagnetic Interference) at frequencies of 1GHz and higher. The material offers EMI suppression and 1.0 W/m-K thermal conductivity performance with low assembly stress. The soft nature of the material enhances wet-out at the interface resulting in better thermal performance than harder materials with a similar performance rating. GAP PAD™ EMI 1.0 has an inherent, natural tack on one side of the material eliminating the need for thermally-impeding adhesive layers and allowing improved handling during placement and assembly. The other side is tack-free, again enhancing handling and rework, if required.
The term refers to visco-elastic materials installed between a power transistor or other heat-generating device in an electronic circuit and a heatsink or chassis to facilitate heat transfer. It may take the form of a compliant die-cut pad or dispensable liquid. For pre-production, the material may be removable. In production, it may cure into a resilient adhesive. In either case, the objective is to maximize thermal conductivity and reduce operating temperatures.
Die-cut pads enable the use of tight dimensional control and manual assembly. Compressing them by tightening the semiconductor device’s mounting screws increases contact area, and therefore heat transfer, but not as much as a well-dispensed curable liquid gap filler. Dispensable gap fillers best suit situations in which high production is important, such as automotive electronics, telecom equipment, and low-energy lighting. Secondary advantages include thinner bond lines than pads and less mechanical stress on mechanical components.
Two-part liquid TIMs are cured in application and provide several advantages over pad-form solutions. One key differentiator favoring liquids includes significantly reduced stress during assembly. In many electronics applications, select components are highly sensitive to mechanical stress. To improve long-term reliability, stress induced through assembly should be eliminated, managed, or minimized. Because two-part liquid TIMs are dispensed and cured following assembly, these materials provide significantly reduced modulus, virtually eliminating stress in application.
Liquid TIMs are available in one-part and two-part designs in pre-cure or post-cure formats (Fig. 1). In liquid TIMs requiring the highest thermal performance, one-part post-cure designs require expensive packaging and cold storage to ensure the shelf life stability of the cure chemistry prior to dispense. To prepare for dispense, one-part post-cure designs are typically required to stabilize at room temperature to ensure the lowest possible viscosity. Single-part pre-cured designs (e.g., gels) inherently carry much higher viscosities for a given thermal performance when compared to two-part designs. This lower viscosity advantage inherent with two-part TIMs offers greatly improved (shear thinning) flow for ease of dispensing in application, while minimizing stress to the final assembly.
Liquid TIMs are designed with a broad range of rheological characteristics and can be tailored to meet customer-specific flow or shear requirements, from ultra-low viscosities required for self-leveling applications to highly thixotropic properties needed to maintain form or shape once dispensed in an application. This broad range of rheological characteristics enables the use of a wide range of dispensing solutions available in the market today. As liquid TIM technologies often utilize abrasive fillers, innovative equipment designers and integrators are available to manage your most demanding needs, ensuring consistent performance in production dispensing.
Liquid TIM can be applied with manual or automated dispensing equipment. Simple handheld manual or pneumatic dispensing guns are available for smaller volume cartridges. Mid-range dispensing equipment is available for larger, quick-change cartridge sizes. Fully automated dispensing lines are often used with pail sizes for optimal material utilization and throughput. Equipment options, performance levels, and pricing all range considerably with the numerous solutions available on the market. Yet numerous financial studies have demonstrated that liquids offer a relatively quick return on investment (ROI) for even the most expensive automated systems available (Fig. 2). These advantages favor high-performance liquids and offer a significant total cost advantage compared to pad-form solutions.
The resin and filler systems used in liquids and pads are essentially identical. Both provide excellent long-term thermal stability. However, once assembled in application, the near-zero stress, cure-in-place liquids provide greater consistency in long-term TIM performance compared to pads (Figures 3 and 4). For pad-form TIMs where many stress-related variables include initial thickness, deflection, part size, and interface contact area, the most effective means to evaluate long-term performance is to complete application-specific reliability testing. Industry expectations continue to grow with typical baseline material qualification testing including room temperature (25°C), high ambient temperature (150°C), high ambient/high RH (85°C/85 RH), and thermal shock (50°C/150°C) exposure for 1000 hours (min).
A GAP PAD is a thermally conductive elastomer material that sits between two surfaces to conduct heat away from devices such as power semiconductors or high-power LEDs.
A GAP PAD minimizes the thermal resistance of the joint between two surfaces and thus can help keep the junction temperature of a power device within its operating limits. Moreover, it conforms to surfaces that do not align well, eliminating air gaps at the interface. As an electrical insulator, it resists punctures, shears and tears.
A GAP PAD can be characterized by its thermal, electrical, and elastomeric qualities. Thermal qualities refer to thermal conductivity, thermal resistance, and thermal impedance. Heat transfers faster across materials of high thermal conductivity. Thermal conductivity depends on temperature. Typically, the thicker the gap to fill, higher thermal conductivity becomes more necessary. Thermal resistance measures the degree to which a material resists a heat flow. It is the reciprocal of thermal conductance. Thermal impedance is a measure of total resistance to heat flow between a hot and cold surface via the thermal interface materials (GAP PAD). It includes area and interfacial resistance whereas thermal resistance does not.
Thermal performance of a GAP PAD can be calculated from the thermal impedance. The thermal impedance is typically the sum of the GAP PAD thermal resistance plus the thermal resistance of its interfaces with the heat sink and heat source.
The dielectric breakdown voltage defines the maximum voltage difference a GAP PAD can stand before it collapses and conducts. Breakdown voltage can also be converted to dielectric strength by dividing it by pad thickness (V/mm).
The volume resistivity refers to how well a GAP PAD reduces the flow of electric leakage current. A low resistivity indicates a material that readily allows the movement of electric charge. The dielectric constant is the relative permittivity of a dielectric material and reflects the extent to which it concentrates electrostatic lines of flux. It is the ratio of the electrical energy a material stores at an applied voltage, relative to that stored in a vacuum. It is given for a specific frequency.
Compression and deflection are among the most important. GAP PAD materials behave like highly filled elastomers. Variables that contribute to compression/deflection can be classified as stress (pressure) or strain (deflection). Compression/deflection qualities depend on various parameters such as pad thickness, surface area, rate of deflection, etc.
Because they are viscoelastic, GAP PADS behave in a nonlinear, non-Hookean fashion under a constant strain. Application of a compressive load causes an initial deflection followed by a slow relaxation. The process continues until the compressive load is balanced by the cohesive strength of the GAP PAD. Compression set refers to the permanent deformation remaining in the GAP PAD after a compressed force is removed. It’s the result of the stress relaxation. After an extended exposure to compressive force, part of the GAP PAD deflection becomes permanent and the pad will not recover after the compressing load disappears.
Mainly, the user must account for growth in x-y direction as the pad deflects from the original thickness. This quality may be important if an obstacle prevents the pad from expanding.
The mounting system determines the pressure needed to compress the GAP PAD with minimum contact resistance and without damage to the components. Hence, the GAP PAD must be able to accommodate the amount of pressure for compression in high-stress applications. Rate of compression is another consideration. The force/pressure to compress the GAP PAD is rate dependent (example: faster rate requires higher force/pressure).
Other considerations include the necessity for a tack or non-tack or adhesive surface, high durability for cut-through resistance, electrical isolation, compliance with UL and RoHS, and whether color or shelf life is important. Also, liquid thermal interface materials may have outgassing issues that are absent when using GAP PADS.
Phase-change material compounds are designed using a mixture of materials specifically designed to change state through temperature shifts. The idea is for the material to change states – from a solid to a liquid – at certain temperatures. In a liquid state, the PCM creates a low interfacial resistance – low amount of air between the heat sink and the material – for better thermal response.
PCMs are typically composed of waxes or thermal plastics that change state at particular temperatures so that they can be handled, or even die-cut at room temperature. Only at higher temperatures will the PCM wet out between the two interfaces. Additional fillers, such as alumina, graphite, or aluminum can be incorporated into the compound to provide electrical functionality dependent on the application needs.
Phase-change materials are an excellent replacement for grease as a thermal interface between a CPU or power device and a heat sink.
PCMs are used between -40°C and +125°C (specialty materials are available to 150°C). The standard temperature range is optimal for most applications, but for high-heat areas such as underthehood applications in the automotive field, the higher 150°C temperature material may be required.
PCMs are used between -40°C and +125°C (specialty materials are available to 150°C). The standard temperature range is optimal for most applications, but for high-heat areas such as underthehood applications in the automotive field, the higher 150°C temperature material may be required.
The materials present no mess (thixotropic characteristics keep them in a semi-solid state), are easier to handle, are low tack at room temperature, and are simpler to apply. Most importantly, phase-change materials save time and money without sacrificing thermal performance.
The materials can be designed for use between a high-power electrical device requiring electrical isolation from the heat sink, which makes it ideal for consumer electronics and power electronics, most notably in the automotive field. It is recommended that the user employ spring clips to assure constant pressure with the component interface and the heat sink, for this will increase the thermal performance of the mating surfaces.
Yes, in fact PCMs are available as a paste compound specifically for use through an automated dispenser system. This is especially useful in high-volume automation or where complex or multiple shapes are needed. The paste provides an ease of dispensing and customization aspect to the PCM. Other methods of quickly and easily applying thermal interface materials include the use of automated dispensers for padform PCMs.
Yes, there are materials that are electrically isolating and non-isolating, as well as materials with fillers such as alumina, aluminum, and graphite. Depending on the customer’s production process and application needs, the materials are reinforced using different materials, such as polyimide, fiberglass, and aluminum foil.
Be sure that your PCM temperature is in your desired range, that the material is non-toxic (to humans or animals) and non-carcinogenic; that it is commercially available at low cost; does not react with or act as a solvent for packaging materials; that it is biodegradable, low or non-flammable, and noncorrosive; and that it provides a limited volumetric expansion/contraction upon freeze/thaw.
In regards to disposal, PCM materials are land-fillable (subject to federal, state, and local regulations), and are not considered a hazardous waste or hazardous material. However, given their high energy content, incineration (i.e., cement kiln) is preferred over landfill disposal.
TIMs are used on circuit boards to eliminate air gaps between heat sources, such as chips, and heat-dissipating devices, usually heat sinks. By minimizing air gaps between the two, they maximize thermal flow between the components and heat sink’s performance. This lets the heat sink and chip perform consistently over time and in various operating conditions.
TIMs can be pads or liquids. As polymeric pads, they are die-cut to fit in the gaps. TIMs can also be one- or two-part liquids. Two-part versions consist of a resin or base and a catalyst that must be mixed prior to application and then cured. One-part liquids require no mixing. Both versions fill gaps and contain fillers. These fillers, ceramic or metal, improve thermal conduction. Ceramic fillers also make the liquid a dielectric, so it provides electrical isolation.
Pads need to be sized for the specific jobs they do. This means there could be tens or hundreds of die-cut parts for each circuit board. This increases the parts count and inventory headaches for procurement. The chips and heat sink around the pads also need to have pressure put on them to compress the pads and make them more efficient at transferring heat. With liquids, one type fills any shaped gap, and there’s no need for compression, which could damage the chip.
Two-part liquids need to be mixed in correct ratios or they will not perform as advertised. They also have curing times. One-part TIMs are already mixed, and some, like Liqui-Form 3500 from Bergquist, a Henkel company, are precured, so there’s no mixing, no exothermic reactions, and the product retains the same viscosity over its entire life. Pre-cured TIMs also have no pot life or working times. Two-part liquid TIMs, on the other hand, have to be used within a certain period of time after they are mixed. Any left over must be thrown away.
Precured TIMs, because they always remain liquid or a gel, can be easily removed from components if necessary. This is crucial for some applications in which the chips cost thousands of dollars. If the circuit board needs to be worked, technicians can easily remove the heat sink and chip without putting potentially damaging stress on them, and simply wipe off the TIM.
They can be applied manually or application can be automated, a more common approach. Companies can use currently available dispensing equipment to automate the application process. There is one caveat: The fillers in liquid TIMs can be abrasive on the moving parts in dispensing equipment and accelerate wear. Proper selection of automated equipment is required to ensure long and reliable service.
They can be stored for up to 12 months and require no special arrangements other than avoiding temperatures above 35°C. Once applied, the silicone-based liquids have consistent reliability and performance that does not significantly degrade over time. They usually outlast the device in which they are used.
One-part liquid TIMs are safe in that they do not emit any VOCs or volatiles. They are not considered hazardous waste, so when necessary, they can be disposed of normally.
They are used to fill gaps ranging from about 0.1 to 1 mm. This makes them well-suited for use in mobile devices and tables, handheld devices, and telecommunications.
There are at least three. It should contain as few ionic compounds as possible. Ionic particles, those that can hold a charge, can be problematic in applications involving humidity or a dielectric bias between components. This can lead to dendrite growth, small electrically conductive formations that can cause a short if they grow through the material and connect the components.
It should have a high viscosity and be shear thinning. This makes it form stable after dispensed, so it retains its shape until a force acts on it. And being shear thinning means it loses some viscosity when under shear. Hence, it is easier to pump while dispensing it.
It should also have low volumetric expansion so it does not expand too much when heated. This is especially critical for those that use it on PC boards they want to send through a solder reflow process. The temperatures, up to 265°C, make the TIM expand, pushing on the heat sink and chip. And during reflow, the solder is liquid, so the TIM could push components out of place.
They are a great way to simplify design, purchasing, inventory, and assembly, as well as increase the thermal performance of the finished product. But they are not right for every application. That’s where reliable TIM suppliers are helpful in guiding customers to the proper thermal solution.
Electronic designs need thermal interfaces or gap fillers to thermally connect heat-generating components like power transistors with heat sinks. Keeping the assembly from overheating is key to consistent, long-term performance. Thermal interface management has become both more important and more challenging as electronics pack more function into smaller packages. TIM include gap-filling pads of cured, pre-cut elastomer and cure-in-place gap-filling liquids. Liquid gap filler comes in one and two-part formulations.
Liquid TIMs improve thermal management when compared to elastomeric gap fillers. For instance, a liquid TIM has a thermal conductivity of 1.8 W/m-K and a thermal resistance of 2.05°C/W as compared to 2 W/m-K and 3.03°C/W for the gap pad. This is due to improved interfacial coupling.
Where gap pads and liquids differ, however, is in their viscosity. Liquid TIMs have a typical viscosity on the order of 10 Pa-s while gap pads’ viscosity is two to five orders of magnitude greater. Lower viscosity lets liquid gap fillers wet out intricate surfaces readily, providing more surface area for heat conduction and supporting a wide variety of assembly tolerances.
To achieve the greatest contact area with an elastomeric gap pad, manufacturers have to squeeze it, introducing stress during assembly. These pads have moduli on the order of 104 to 106 Pa, therefore squeezing them to 20 to 30% compression introduces significant stresses in three axes that can compromise the longevity of the assembly. Liquid gap fillers’ moduli are on the order of 102 Pa and they can thin out to about 0.1 mm, the size of the filler material, with minimal stress on components, substrates, and assemblies.
Suppliers of liquid TIMs have a variety of compounds available to meet the needs of each design and its assembly process. Liquid gap-fillers can be formulated to be thixotropic, a property also know as slump resistance, so they stay where dispensed until displaced by an external force. Or the materials can be self-leveling; the liquid flows to fill the available surface area.
Other properties to consider when choosing a liquid TIM material include the range of dispensing viscosities, the degree to which the liquid is filled, and the pot life or the amount of time the material can be dispensed and worked at ambient temperature.
Pre-cured elastomeric gap pads must be cut to fit the areas in which they will be installed. That means each pad in an assembly has to have its own part number and inventory and its own tooling. Each pad is also cut from a sheet, incurring material waste. And overall assembly tolerances need to be tight to ensure sufficient contact for good thermal management.
By contrast, liquid gap fillers dispense just where they are needed with a single part number and one-time qualification. Their conformable nature is tolerant of dimensional variability. A single manufacturing cell dedicated to dispensing liquid TIMs can serve multiple programs or platforms.
For mid-range and high-volume applications, automating the process of dispensing liquid TIM can further bring costs down and boost throughput. Although automation requires a high initial investment – $50k for desktop units and $150k for automated pass through systems – faster throughput, higher precision, and placement accuracy of the dispensed material provide a rapid return on the investment.
Depending on the volume of gap filler required, assemblers choose to apply liquid TIMs with a manual or pneumatic dispensing gun or use partially or fully automated dispensers. Two-part materials need a static mixing nozzle to combine parts A and B in equal volume, but they can be stored at room temperature. One part-materials, on the other hand, may require refrigeration and have a higher dispensing viscosity.
One-component materials are commonly available in 600-cc cartridges or 1- and 5-gallon pails. For twocomponent materials, typical kits are 50 cc, 400 cc, 1,200 cc, or 10 gallons, each divided into two equal-volume containers for parts A and B. For both one and two-part materials, it is important to minimize out-of-shelf life waste with first in-first out material management and compliance with required storage conditions.
Dispensing liquid TIMs differs from applying other polymers. TIMs have a high concentration of abrasive fillers and can separate under pressure. Automated dispensing systems must minimize constrictions and other flow inhibitors like long pipe runs or elbows. Material should also be dispensed at the lowest possible pressure. Components that contact the material must be abrasion resistant and examined or replaced according to a preventive maintenance plan.
Liquid TIMs have shear-dependent viscosity, so they must be metered with constant volume displacement methods instead of the pressure-time methods employed for other liquid polymers. Dispensers must also take steps to prevent air entrainment.
When automating your gap-filling operation, remember that the choice of material and dispensing method go hand-in-hand; validating the dispensing process is just as important as verifying the performance of the material. Liquid TIM suppliers and dispensing equipment manufacturers can help with the finer points of choosing and dispensing the right gap filler.
A knowledgeable liquid TIM supplier can recommend automation partners. For instance, material supplier Bergquist partners with Rampf, Scheugenpflug, and Bdtronic to help customers worldwide boost throughput and device reliability by automating the application of liquid TIMs.