Advances in the composition and properties of dispensable interface materials, coupled with accurate, economic application methods, have made them worthy of consideration for applications currently addressed by thermal pads, gap filling pads and greases.
Chomerics Europe applications engineer, Caroline Ricouard, explains. THENEW generation of dispensable materials typically comprise a single component silicone containing ceramic particles that promote heat transfer. The pre-cured nature of these materials allows the assembly of mating components immediately after the material has been applied to the target surface.
In contrast, many existing dispensed materials are two-component systems, with one element acting as a curing catalyst once the material is applied. The disadvantages of this approach are the post-application curing time and difficulty in guaranteeing the two elements are well-mixed in the correct ratio.
New formulations of one-part, pre-cured, dispensable thermal interface gap fillers, such as Chomerics T630, T635 and T636, are viscoelastic and form stable. This lets them fill large, uneven gaps with thicknesses of 0.25mm upwards. For assemblies that require a guaranteed separation between components to provide electrical isolation, a version is available with 0.25mm dia glass beads that act as a compression stop.
Different compounds provide a choice of thermal performance. However, thermally efficient fillers are inherently expensive. Furthermore, processing can be expensive because the particle size needs to be controlled to optimise thermal performance. It is important that a material with the appropriate thermal performance is chosen for a given application. Selecting a material with heat transfer properties beyond the applications needs may add unnecessary cost. Thermal conductivity of Chomerics materials ranges from 0.7 to 2.4W/m-k. These figures are a measure of the materials thermal efficiency alone. Overall thermal performance of an assembly depends on additional factors including: compressed thickness; contact surface area; and how much air has been excluded from the interface.
New one-part silicone-based materials deflect at extremely low pressures: lower even than die-cut, sheet gap fillers. The material flows to fill air voids. This characteristic makes them ideal for applications where it is impossible to apply significant assembly pressure or where high pressure is likely to stress solder joints and leads.
Sandwiching a soft gap filling material between a delicate electronic component and a chassis or heatsink can also bring important mechanical benefits by dampening vibration and protecting components from sudden mechanical shock. This characteristic has become more important in recent years with the continuing miniaturisation of electronics driving demand for portable electronics devices. This type of equipment is more likely to be dropped or experience rough treatment compared with office of laboratory based equipment.
Silicone-based materials also have a wide operating temperature range: typically -50 to 150C. High stability prevents sagging or slumping when the material is subject to sustained temperatures towards the top of its operating range. Unlike thermal greases, materials such as T630, T635 and T636 do not change consistency during their operating life, never becoming hard or fluid. Therefore, the thermal efficiency of the assembly as a whole remains constant. Traditional thermal greases tend to dry out over time, which can often be accelerated by harsh operating conditions or temperature cycling. Greases also tend to flow out of the interface with a negative impact on overall thermal performance. A reduction in thermal performance means an increase in device operating temperatures which may lead to a shorter product.
Pre-cured thermal gap fillers are usually supplied in a syringe, cartridge or pail. Syringes contain 30cc of material and are designed for small production volumes or field repair work. They may also be pneumatically operated, providing a semi-automatic application system.
Cartridges and pails are designed for use with pneumatic automated assembly equipment. Cartridges contain around 30 ounces of material, while straight walled pails may contain up to 25kg, making them suitable for mass production. Larger size containers help reduce production downtime. Metered pneumatic systems allow rapid placement with excellent repeatability.
There are clear benefits when comparing the economics of dispensed gap fillers versus thermal pads and gap fillers cut from sheet. Firstly, the material itself is generally lower cost. Secondly, the assembly of die-cut pads into an application can be difficult and expensive to automate. Finally, when using dispensed material, waste is minimised. Conversely, die-cut profiles result in significant and unavoidable waste material.
Thermal management solutions have evolved to keep pace with the electronics industrys changing needs. Until recently, thermal greases were good enough for most applications. However, with the downsizing of heat generating through-hole devices a need for thermal interface pads to fit between the component base and heatsink has arisen. The large scale adoption of surface mount designs created a market for die-cut gap filling pads to couple the top-side of hot devices, such as processors running at high clock speeds, to the equipment chassis that acts as a heat-spreader. Now the availability of pre-cured dispensed gap fillers offers a solution that is economic, provides good levels of performance and stability and, through automation, lends itself to low, medium and high volume applications.