FEATURE EMC & THERMAL MANAGEMENT THE CONDUCTIVITY CHALLENGE
Riaz Ahmed, thermal interface materials R&D manager at Parker Chomerics explores how the latest advanced thermal gel is handling a range of thermal issues in next generation electronics products
T
hermal gel, a low compression-force thermal interface material (TIM), has
played an important, if unheralded, role in the electronics industry’s successful integration of advanced processors and power components into mass-market products.
The provision of effective pathways for waste heat from microprocessors, systems-on-chip (SoCs), FPGAs and FETs is a crucial part of the design of high- volume products such as mobile phones, laptop computers, televisions and set- top boxes (see Figure 1). A thermal gel which can be applied automatically by dispensing machines can provide an effective thermal and mechanical interface between these hot components and a rigid metal device such as a heat spreader or heat sink. Automated dispensing keeps assembly
unit costs low at annual production run- rates of higher than 100,000 units, and eliminates the risk of error inherent in the manual application of liquid or solid TIMs (see Figure 2). For these reasons, thermal gel was
welcomed enthusiastically by production engineers and thermal design specialists on its first introduction. But the increasing power density of new electronics designs, and the ever-rising frequencies at which today’s multi-core processors operate, mean that ever more thermal energy is being dissipated by densely populated PCBs. So how might thermal gel technology evolve to meet this growing challenge?
STEEP RISE IN THERMAL CONDUCTIVITY Users of thermal gels are today stepping up the pressure on suppliers, such as Parker Chomerics, to increase the thermal conductivity of their products. By reducing the thermal impedance to the flow of heat from hot components to the ambient air, system design engineers can ensure that high-performance SoCs and other heat-generating components can operate at full capacity even at high ambient temperatures without the risk of triggering over-temperature protection functions. It is important, however, for the
engineers who specify thermal gels to
make a detailed evaluation of all the attributes of the gels that they consider for use in a product design. The headline thermal conductivity value of a thermal gel is clearly of great interest. On this parameter, producers of thermal gels can show remarkable progress in recent years.
Parker Chomerics, for instance, entered the market with the first single- component, fully cured dispensable gel, the THERM-A-GAP T630. This featured a thermal conductivity rating of 0.7W/m-K. Over subsequent years, it introduced a series of new gels offering progressively higher thermal conductivity: T635 – thermal conductivity of 1.7W/m-K
T636 – thermal conductivity of 2.4W/m-K
GEL 30 – thermal conductivity of 3.5W/m-K
GEL45 – thermal conductivity of 4.5W/m-K (see Figure 2) This performance improvement of 640% is impressive, and has given thermal engineers valuable scope to reduce the size and weight of heat- spreading elements while handling higher thermal loads. Parker Chomerics has also introduced its THERM-A-GAP TC50, described as a thermal putty rather than a gel, but still suitable for dispensing by automated equipment: it offers even higher thermal conductivity of 5.0W/m-K. Further improvements in thermal conductivity will be reliant on discoveries in materials science, and on experiments with new combinations of base materials and conductive fillers. Parker Chomerics is constantly seeking to optimise the balance between improving thermal conductivity – achieved by increasing the proportion of conductive filler material – and desirable mechanical and electrical properties, which are provided by the base material. Besides thermal conductivity, other
important attributes of effective gels are these: Mechanical stability over time and temperature. A low thermal coefficient of expansion is desirable, to minimise mechanical strain on the housing and fasteners caused by thermal expansion
Figure 1:
A heat sink on an Intel microprocessor. The cooling system requires an effective thermal interface between the chip and the heat sink
Resistance to shock and vibration Dielectric strength Dispensability – a thermal gel should precisely maintain its specified flow rate to support use by automated dispensing equipment
Reliability – the gel’s nominal values for all parameters should be constant over all production batches of the gel, over time, and over temperature cycles In some product sectors, such as automotive, aerospace and military equipment, electronics systems are expected to operate over extended lifetimes of ten years or more, and so a gel’s material must resist degradation and contamination over long periods.
Figure. 2:
A typical assembly for a high-volume electronics product incorporating automatically dispensed thermal gels
MEETING THE THERMAL CONDUCTIVITY CHALLENGE The development roadmap which has seen the performance of the Parker Chomerics THERM-A-GAP family of gels increase thermal conductivity from 0.7W/m-K to a high of 4.5W/m-K in the latest product continues: new compounds under development today will enable the development of products with even higher thermal conductivity in future. But the search for higher thermal conductivity must not be allowed to compromise other important parameters of a thermal gel’s performance. Engineers should always look beyond the headline conductivity value and study the detailed characteristics published by manufacturers in datasheets. They should also satisfy themselves that the data is properly verified and underpinned by rigorous testing. Production engineers will also benefit
from a comparison of a single- component gel such as THERM-A-GAP products from Parker Chomerics with two-part cure-in-place gels. In a high- volume production environment, precise management of the dispensing of two- part gels to ensure that they are mixed in the correct proportions can be particularly difficult to achieve – a problem which is eliminated by the use of single-part gels.
Figure 3:
The THERM-A-GAP GEL45 product from Parker Chomerics
36 JULY/AUGUST 2018 | ELECTRONICS Parker Chomerics
www.parker.com/chomerics T: 00800 27 27 5374
/ ELECTRONICS
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