Dispensed thermal gels: Reducing stress on components
05/01/2000
In selecting thermal solutions, packaging designers can easily agree that minimizing thermal resistance is critical, and numerous articles now focus on performance and improvements in thermal resistance of interface materials. Today's designers, however, must consider mechanical issues as well. One particular challenge is that smaller, higher density devices, some requiring the thermal solution to interface directly with bare die, impose relatively low limits on the allowable force that the thermal solution may impart. Examples include the latest high-speed microprocessors and Rambus RDRAM chip-scale packages.
 Figure 1. Dispensed vs. cure-in-place process flow. |
Solutions to minimize thermal resistance also must be reliable, stable over time, and allow for clean and easy rework. Ease of manufacture is another key issue because product life cycles are short, most assembly is done by contract manufacturers at multiple global locations, and special equipment and processes can be undesirable.
Finding Suitable Candidates
Today's designer finds various choices of interface material available, with each application requiring careful consideration of the advantages and disadvantages the individual solutions may carry. In performing the assessments, it may be helpful to rank key design requirements and then rate candidates against each.
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Table 1 lists common interface material choices and summarizes their attributes - positive and negative. This data was acquired through relative performance assessment from internal comparative measurements, evaluation of various suppliers' literature and customer feedback. In applications where stress on the component tops the list, it may be appropriate to select a material with higher thermal resistance, provided maximum junction temperature specifications are comfortably met. When low force, tolerance accommodation and rework are needed, soft materials (such as gels) can be the preferred choice.
Gels can be applied as a reinforced pad, dispensed, cured pad pre-applied on the substrate, or as a dispensable cure-in-place liquid. For maximum compressibility, a non-reinforced material would be desirable. In this case, dispensing of the gel would be the preferred processing route.
Dispensing Options
A dispensable gel solution can be delivered to the original equipment manufactuer (OEM) or contract manufacturer (CM) in two ways (Figure 1).
Dispensed Gel: In this option, the two-part material is mixed, dispensed and cured by the thermal solution provider. The final OEM assembly is done with the pre-cured gel already attached to the heat sink, a simple two-component process without heat.
Cure-in-Place: In this more conventional process, the two-part material is mixed and dispensed onto either the heat sink or printed circuit board (PCB) and the assembly is mated and cured in place, generally by the application of heat.
The cure-in-place process requires an OEM/CM to handle the liquid components, and to purchase and integrate dispensing equipment and curing process into the assembly line. The dispensed gel process, in contrast, allows the OEM/CM to handle a single purchased component, the fully cured thermal interface pad, pre-formed onto the heat sink, and then assemble it in a simple mechanical process.
While dispensing technology is well-known and used successfully across the electronics industry in such areas as chip bonding and thermal underfill, many start-up issues must be overcome to implement dispensing successfully. In the case of any filled two-part curable material, the following generic issues can be encountered:
- Filler segregation or separation
- Shelf life: viscosity and cure stability
- Pot life/cure rate balance
- Equipment clogging and maintenance
- Dimensional accuracy of dispensed material.
Properly selecting equipment and formulation components can minimize these issues in dispensable gels. A knowledgeable person must also oversee the mixing and dispensing portion of the process.
Methods for Applying Gels
 Figure 2. Dispensing thermal gel on RIMM heatspreader |
There are different processes used to apply dispensable thermally conductive gels, depending on the final dimensions of the thermal interface and the nature of the heat sink. Conventional dispensing equipment can be used to mix the gel and dispense it into a form or shape. The gel can be dispensed into beads, dots or ribbons where a thicker gap (0.5 mm and larger) is to be filled. Figure 2 shows the dispensing of a ribbon of 0.5 mm thick gel into a heat sink plate for Rambus RDRAM RIMM modules.
 Figure 3. Stencil printed thermal gel configurations. |
For thinner thermal interface pads (0.5 mm and under), printing techniques, such as screen printing and stencil printing, can be used to apply the mixed/dispensed two-part gel. Equipment and techniques used in the application of solder paste can be modified to apply thermally conductive gels. Figure 3 shows an array of shapes produced by stencil printing.
Performance of Dispensed Gels
Two-part thermally conductive materials that lend themselves to the cure-in-place process are commercially available. In the wet uncured state, these materials are soft enough to assemble components under low force. However, unlike dispensable gels, these materials cure to a harder state, which would not allow later assembly under low force.
Because gel materials are very soft, they can be difficult to handle in thin sections without some sort of physical support. A thermal interface pad can be formed from a soft rubber or gel by supporting it on a carrier such as fiberglass, fabric mesh or film; this supporting carrier restricts the movement of the soft material under compression and leads to higher assembly forces. By dispensing gel directly onto one of the assembly substrates, the carrier is eliminated and lower compression forces result.
 Figure 4. Pressure vs. deflection comparison. |
Figure 4 shows compression force vs. deflection for a dispensed gel vs. a fiberglass reinforced gel and a more conventional elastomeric pad. Additional compressibility can be achieved by dispensing or printing the gel in a pattern of dots or stripes where some space for material flow is allowed.
The dispensed cured gel described above is soft with a compression modulus typically under 100 psi, as compared to elastomers ranging from about 200-600 psi compression modulus. Gels have excellent wetting properties and can make complete surface contact with standard metal or plastic finishes, even at low applied pressures.
 Figure 5. Thermal conductivity comparison. |
One way to assess a thermal interface material's quality of performance is to measure the thermal resistance across the material as a function of applied pressure. If the area of contact and actual compressed thickness are known, an apparent conductivity can be calculated. Both the bulk thermal conductivity of the material and the interfacial or contact resistance at the heat sink and heat source contribute to the apparent conductivity. The closer the value of apparent conductivity is to the known bulk thermal conductivity, the lower the interfacial resistance. In many cases, a thermal interface material must be in the liquid state to provide low interfacial resistance. Dispensed cured gels are solids that are soft and conformable enough to provide this same liquid-like low interfacial resistance behavior. Figure 6 shows that the apparent conductivity measured directly on the thin dispensed gel closely matches the bulk conductivity and is not strongly affected by increasing pressure.
Summary
With many materials now available to achieve heat transfer for high-performance components, it is necessary to weigh many factors in making an interface choice that will lead to a reliable heat transfer system. Designers and manufacturing engineers must strike a balance between performance, reliability and manufacturability. Many materials that perform well in a laboratory test do not provide a reliable interface at low pressures when true flatness and surface roughness factors are taken into account. Some materials provide excellent thermal wetting yet cannot easily be re-entered and reworked or are not stable through thermal expansion cycling.
While cure-in-place materials overcome many of the issues described above, they require the OEM or CM to invest in extra equipment and process design to integrate the dispensing and curing process into the assembly line. Engineering time and effort are needed to ensure a robust process. Designers and engineers now have the option to select a pre-dispensed gel interface that retains most of the benefits of the cure-in-place process without some of the process drawbacks inherent in a multicomponent mixing process.
Because dispensed gels are very soft even in the cured state, they offer the unique capability to allow low force assembly without the need for a wet chemical process. The surface wetting characteristic of cured gels provides good interface mating capability. Dispensed gels that are pre-cured onto the final substrate (generally the heat sink) provide the performance reliability that most applications require, while minimizing the equipment and component storage needed at the final assembly site. These features are critical in designing a final assembly process that is reliable and robust, and can be carried out in various locations around the world.
References:
- C. Vogdes, "Predicting the Performance of Thermal Interface Materials," Advanced Packaging, Nov/Dec 1998, pp. 46-50.
- C. Vogdes and Felix Oseguera, "Thermal Testing Methods for Evaluating Thin Thermally Conductive Materials," Proceedings of Sixteenth Annual IEEE Semiconductor Thermal Measurement and Management Symposium, 2000.
GREG BISCHAK, director of business development for thermal products for the Raychem Interconnect Division of Tyco Electronics, can be contacted at M.S. 103/2A, 305 Constitution Drive, Menlo Park, CA 94025; 650-361-6300; Fax: 650-361-5836; E-mail: [email protected]
CHRISTINE VOGDES, product development manager for thermal products for the Raychem Interconnect Division of Tyco Electronics, can be contacted at M.S. 103/2A, 305 Constitution Drive, Menlo Park, CA 94025; 650-361-3549; Fax: 650-361-2559; E-mail: [email protected]