Feature: Thermal protection
mechanical and elastomeric stability during temperature ageing. Advanced formulations of these
Silicone gel with thermal conductivity
also decouple the stresses caused by the expansion and contraction of layers with different CTEs. Tese adhesives are available in one-part (1-part, 1K) formulations that require cold storage and two-part (2-part, 2K, A&B) formulations that require mixing. To support different dispensing
requirements, thermally-conductive adhesives come in a range of rheologies. Flowable adhesives support the fast filling of channels and cover larger surface areas. Low-flow products allow a controlled flow-out aſter dispensing to fill the desired area. Highly thixotropic products allow for quick dispensing with exact placement without overflow or sagging. Termally-conductive silicone
adhesives are available with three different curing different mechanisms: condensation, addition and Termal Radical Cure (TRC), a proprietary curing system from Dow. All provide long-lasting performance and bond lines that resist temperature and humidity. Fast-curing adhesives with short cycle times are available, but engineers need to consider all their requirements, such as demand for greater energy efficiency, before selecting a cure system. One-part condensation cure adhesives
are room temperature vulcanising (RTV) silicones that use the moisture in the air to cure. Tey are the simplest of adhesives because, once dispensed, they cure without additional action. Condensation cure adhesives maintain their properties with environmental ageing and are suitable for a range of electronic applications. For lighting, they can be optically clear with UL 94 V-0 recognition.
Cure time for one-part condensation
cure adhesives depends on many factors, including ambient temperature, relative humidity and the depth that moisture from the air needs to penetrate. Bond line thickness has very little impact on cure speed. Condensation cure adhesives may take longer to cure if they have limited surface area in contact with the air. Advanced formulations have a wide adhesion profile and provide reliable bonding to most substrates. Termally-conductive silicone
adhesives that require addition curing use heat from an oven to cure. Tey can be either one- or two-part materials. TRC adhesives have mild heat requirements for reduced energy costs and a controllable flow immediately aſter dispensing, to eliminate over-flow. Tey adhere to a wide variety of substrates and are not sensitive to traditional heat cure trace contaminants such as sulfur species and amines.
Gap fillers Termally-conductive silicone gap fillers provide a highly-conformable thermal path between heat source and heat spreader. Tey are replacing traditional elastomeric pads that can fail to protect solder joints or leads. Termally- conductive silicone gap fillers also conform to and completely fill uneven spacing or higher part-to-part tolerances that create challenges for dimensionally- constrained pads. Because thermally-conductive gap fillers
are soſt and compressible, they dampen vibrational energy and oscillations. For enhanced reliability, they provide excellent
28 December/January 2021
www.electronicsworld.co.uk
materials are optimised for dispensability and rheology. Tey have a controlled/ engineered flow that can range from flowable-aſter-dispensing to low-slump to non-slump/flow. Slump resistance is important during vertical, pre-curing assembly. Ticker bond lines need more thixotropic gap fillers, while thinner bond lines need materials that flow easily and thin out. Termally-conductive silicone gap
fillers adhere with tack adhesion instead of chemical bonding, a consideration for repairability. Tey can dissipate heat from critical automotive components, such as engine or transmission control units, or braking and stability controls and sensors. In applications where electrical isolation is required, products may contain glass beads for a compression stop.
Thermal greases Termal greases are one-part materials that provide an interface between a heat sink and a heat source. Tey eliminate air gaps to maximise heat transfer and dissipation. Termal greases are applied in thin layers, typically less than 100 microns. Given this limited thickness, thermal resistance (rather than thermal conductivity) is the dominant heat flow property. Notably, thermal greases have an especially low thermal resistance compared to other types of thermally- conductive silicones. Historically, thermal greases have been
used in microprocessors with flat chips and a relatively limited temperature range. Automotive applications have broadened the required temperature range and positioned electronics vertically. In turn, gravity has contributed to a problem called bleeding or bleed-out where the liquid polymer separates from the conductive filler particles. Compared with other grease chemistries, advanced silicones exhibit less bleed-out for greater assembly reliability. During PCB assembly, bleed-out can
happen when the liquid polymer migrates beyond the bond line and interferes with
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