CONTRACT MANUFACTURING
The Advantages & Considerations of Dispensing Thermal Interface Materials
Tim Meyer, systems design engineer at Precision Valve & Automation (PVA) explores primary considerations when selecting a TIM material
extreme, excess compression on sensitive electronics.
A
s powerful electronics shrink in size and increase in variety, thermal interface materials [TIM] for heat dissipation continue to gain importance in the manufacturing industry. A variety of methods exist for aiding this heat transfer. Gap filling pads, adhesive tapes, and dispensable gels are the most common. The primary considerations when selecting a TIM material is the thermal conductivity and dielectric strength. However, there are several considerations that should be weighed to create an efficient assembly process while still achieving optimal product performance; considerations such as assembly tolerance, compression force, cycle time, waste generation, and equipment maintenance are among the determining factors. Dispensable gels, thermal pads, and tapes all provide a variety of thermal properties but differ significantly in application techniques and mechanical performance. Pads and tapes are cut from flat sheets of raw material. Applying shapes other than square or rectangular patterns to your design often results in waste from the cutting or stamping process (Figure 2). Pads cut with uniform thickness can make it difficult to achieve a TIM bond between components of differing height. Stacked tolerances inside an electronic assembly can create issues with creating contact of the pads or, in the other
50 MAY 2021 | ELECTRONICS TODAY
Dispensable gels offer more flexibility in design geometry and tolerancing. A gel will give and flow into cavities while still retaining enough body to hold shape and maintain contact with critical components, while a precut pad, when compressed, can cause continuous stress. The nature of dispensing allows for complex geometries to be applied. Multiple layers of material may also be applied in varying heights for a single TIM section without creating excessive waste. An automated dispensing solution simplifies the application of TIM by eliminating the stamping or cutting process entirely. A variety of options are available for dispensable materials besides the obvious variance in dielectric strength and thermal conductivity. Differences in viscosity, filler density, filler particle, curing mechanism, and chemistry are all things that provide for different applications. Pre-cured, single component moisture or heat cure, and two- component materials are all available. Nearly all dispensable options are filled with aluminum oxide, otherwise known as alumina.(Al)_2 O_3 is a relatively good thermal conductor and is electrically insulating. Glass beads are also commonly used for electrical insulation. The overall conductance of the TIM is usually dependent
on the density of the Al)_2 O_3 in the mixture. (Al)_2 O_3 happens to be a particularly hard
and abrasive filler, with a Mohs hardness of ~9.0 [Diamond being 10.0]. Because of this, pumping equipment and plumbing must be carefully selected to minimise settling of this filler and to prevent abrasion to seals and moving parts. PVA designs dispensing equipment for handling material with these specific properties in mind. Pumping and dispensing equipment are carefully designed to maximise life. Moving components are constructed of materials selected for their resilience, such as tungsten carbide. Soft seals are limited in quantity and designed in such a way that they can be readily replaced without introducing significant downtime. Fluid routing and plumbing is designed to minimise back pressure to the system and reduce dead space. For example, instead of using a 90-degree fitting, which completely changes flow velocity, a sweep or bent tube fitting would be used to prevent packing of fillers.
Figure 2
Curing & handling properties Pre-cured gels remove the need for mixing or post-application curing operations. Once the material has been dispensed onto the component, it is then ready for assembly. Pre-cured materials are packaged in the same form as they will be dispensed on the product. The consistency must be flowable enough to be dispensed, but thick enough to maintain its shape on the electrical component. PVA has designed equipment for extruding the precured material at high pressure through a flexible hose to the point of dispense with a single moving ram. The 1- Gallon Pail Unloader (1GPU) uses a hydraulically driven ram to extrude the material at pressures up to 800 psi. The design takes advantage of the original packaging of the TIM gel and is also available in a 5-Gallon configuration. The ram’s only wearable component is a gasket which can be easily replaced between pails. Alternative
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