Some issues have been raised regarding silicone in electronics. Consequently, in spite of their often-disadvantageous aging, thermal stability, and stiffness properties, PLP TIM products with non-silicone-based matrices have been gaining in popularity. This article by the R&D department of Nolato Silikonteknik addresses these concerns and underlines the benefits offered by silicone.
With few exceptions, current Thermal Interface Materials (TIMs) are “PLP type” materials – Particle Laden Polymers. These are composite materials with functional (in the case of TIM, thermally conductive) particles suspended in a polymer matrix.
This polymer matrix may be an elastomer, gel, oil, or wax – as needed to create an insulating film, gap filling pad or filler, paste, or phase-change material (PCM).
And in the vast majority of materials on the market, the polymer matrix in question is silicone based.
In silicone as in many other synthetic materials, molecules that have not been crosslinked and thus permanently tied into the polymer matrix can escape the compound.
Volatile molecules – often called low-molecular-weight siloxanes, or LMWs – can escape through evaporation, a process called outgassing. Longer molecules, too large to turn gaseous, can escape as oil leakage, called bleeding.
In TIMs, there will always be a portion of non-crosslinked molecules in the matrix, as TIMs need to be soft and pliable – highly crosslinked compounds are hard. Volatile siloxanes can also be added into a matrix as solvent, intended to evaporate after the material has been dispensed in an assembly, allowing one set of mechanical properties pre-application and a radically different one post. The escaped silicone may migrate by different paths. Liquid silicone can creep along surfaces as an oily film. Commonly employed as a lubricant, silicone oil can creep through narrow gaps.
Outgassed silicone migrates through condensation: molecules evaporating from a compound in one place may recondense onto surfaces elsewhere in the assembly.
What then are the risks with contamination from migrating silicone in electronics?
Well, there are some very specific ones:
The first and foremost hazard stems from sparking. This may affect devices where repeated flashes of hot plasma – electrical sparks or arcs – can form with any frequency on surfaces, such as in open relays, connectors, switches, motors – devices that allow (a) sparks to occur regularly, and (b) contaminants to reach active surfaces.
Silicone contaminating a surface where it gets subjected to the heat of an electrical spark may gradually form a glass coating. Over time, this coating may grow in thickness, until the point where the resulting insulation causes the electrical connection to fail.
It bears noting however that modern devices seldom allow open sparks, effectively eliminating the issue.
The second notable potential issue is optical surfaces. In devices with precision optics, silicone condensing or creeping onto lenses, mirrors, or optical sensors may affect refraction, reflection, or the quality of sampled images.
Hence, in designing optical devices with open optical surfaces, it is advisable to monitor potential silicone contamination issues.
Again however, it bears noting that optical assemblies are usually enclosed, blocking contamination. We have experience designing in silicon-based TIMs into a variety of advanced optical devices, including ADAS vision systems, high-end professional DSLR cameras, thermal-vision systems, CCTV cameras, and more. This particular issue, although a theoretical risk, has yet to manifest a practical problem.
In the 1960s, a major car manufacturer experienced problems with blotchy paint finish on their cars.
The issue was traced to the paint shop’s ventilation drawing air through ducts that also supplied the workers’ locker rooms. The 1960s loved hair spray – products containing volatile silicones. A lot of it found its way into the paint shop and condensed onto the car bodies, causing uneven paint adhesion.
This demonstrates that silicone migration can affect coatings. In most applications, TIM silicone contamination is so minute, and electronics so well enclosed, and/or assembled so far away from exterior paint processes, that any contamination from TIMs is unable to affect cosmetic finishes.
However, the real potential issue concerns conformal coating. It is therefore recommended to ensure that boards being coated don’t have silicone in the wrong places.
There are a couple of other potential issues:
One is cosmetics. Silicone oil leaking onto the exterior of a finished product may look unsightly. For some brands, it’s also important that the interior of opened products looks clean and neat. For such applications, possible silicone leakage from TIMs should be considered when designing assemblies and choosing materials.
Another is re-work: when components are de-soldered and then re-soldered, silicone contamination on solder pads may affect solderability. Silicone oil is difficult to clean off completely. It is therefore recommended, when possible, to test assemblies where repair and re-work are frequent issues to a point where the need for such operations is identified prior to mounting TIMs.
As noted, silicone is chemically very inert. There are thus no known issues of silicone reacting with or degrading electronic components, devices, conductors, PCBs, gaskets, solder joints, or other elements in electronics.
It has been suggested that migrated silicone can cause short circuits on PCBs. However, we have not been able to study any such occurrences; nor is such a failure mode possible to explain with physical phenomena, as silicone is a powerful dielectric – its breakdown resistance is one order of magnitude higher than air.
It can therefore – until we are able to study it or can formulate a physical explanation for the failure mode – only be regarded as a spurious phenomenon.
So, there are some potential issues with silicone migrating from TIMs. It is also clear that most electronic designs won’t be affected by them. In many of remaining cases, potential issues are also relatively easy to work around by proper design and/or processes.
But for those designs that may be affected, and where practical workarounds aren’t available – what should be done about it? How are silicone-related issues best mitigated?
The simplistic approach is of course to ban silicone outright, only using non-silicone based, or ‘silicone-free’ products.
However, typical silicone-free alternatives are still based on a polymer matrix of some other chemistry. While the properties of silicone are well known, the exact properties of these alternative chemistries may not always be.
More important, however, are the reasons why silicone is used in the first place; no other polymer matrix can boast a similar width of properties. This means that most alternative matrices provide materials that are either much stiffer, have lower thermal stability, or worse aging characteristics than silicone – most likely, all three.
Better then, to go the NASA route and determine limits to how much silicone can be tolerated in an application. While there are some silicone-based TIMs that extract relatively high amounts of silicone, there are also many that have basically no extraction at all, with a wide range in between.
The available selection of silicone-based materials is so wide that beyond certain very specific space-based applications, there will as a rule always be a superior silicone-based alternative.
Despite what has been implied in certain contexts, there is no foundation for any claims that silicone would somehow be dangerous to electronics.
Silicone is a highly useful material with a multitude of remarkable properties that make it singularly attractive for use in thermal interface materials. However – like everything else – it does have its own drawbacks and limitations, the properties of which any user is advised to be aware.
Given that awareness – which this article aims to promote – those drawbacks are surmountable, often with remarkable ease.