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Epoxy molding compounds (EMC or duroplast) with high thermal conductivity are used to dissipate heat from the encapsulated substrate to the outside environment. Standard epoxy molding compounds are filled with Silion Dioxide (or more commonly referred to as silica) has a bulk thermal conductivity of 1.3 W/mK. Because epoxy mold compounds can be loaded from 70% to 92% by weight with silica, the resulting thermal conductivity of a silica-filled EMC ranges from 0.3 W/mK to 1 W/mK for the most highly silica-filled duroplasts.
In order to obtain EMC with superior heat dissipation, more thermally conductive fillers are used. Though there are truly a dizzying array of possible fillers that can be used to increase thermal condutivity, the choice of filler must be balanced between thermal conductivity and other desirable mold compound fucntionality including flex strength, flow, packing density and price.
The most common fillers (and their respective thermal conductivities) used for maximum heat dissipation are as follows:
Please continue to our Learn More section to learn more about what makes epoxy mold compounds thermally conductive, which filler types can be used to provide the highest heat dissipation, which applications require high thermal conductivity and how epoxy molding compounds can be formulated for the best balance of eletcrical insulation and thermal conductivity.
Yes, they would. However, there is an implicit understanding when epoxy mold compound manuafcturers refer to "thermally conductive" EMCs. What they are actually saying is "thermally conductive and yet still electrically insulating". The reason is because functional epoxy mold compounds used for semiconductor encapsulation have a first, basic function - and that is to electrically insulate. Once this basic need is provided, we can look at providing thermal conductivity, which rules out electrically conductive fillers.
If price per not an option, diamond woudl be the best thermally conductive filler to use. It has a bulk thermal conductivity of 2200W/mK and mixes well with epoxy resins to give excellent flow properties. Furthermore, it is also an excellent electrical insulator. Unfortunately, the price for such a compound woudl be upwards of $7500/kg which makes it far too expensive for practical applications.
As package design applications evolve and the chips being encapsulated become higher and higher power, many customers are starting to demand higher thermally conductive epoxy mold compounds for better heat dissipation. This section will explain in more detail what makes epoxies thermally conductive, which filler types provide the best heat dissipation, and how you should evaluate products to make the best selection.
Before we get too far down the path of thermal conductivity, it's important in the context of epoxy molding compounds to specify that in most cases, "thermally conductive" actually means "thermally conductive while still being electrically insulating". While typically the most thermally conductive materials are those that are also electrically conductive, these fillers cannot be considered for semiconductor encapsulation because they would short out the device, rendering it useless. Therefore it is important that we only consider fillers that are electrically insulating.
There are three factors that must be considered when making EMC's thermally conductive. They are listed in order of priority:
When it comes to selecting thermally conductive fillers, the first property that one looks at is the bulk thermal conductivity. This is important as it is the base from which the epoxy will ultimately get its thermal conductivity. By definition, if an epoxy is 100% filled with a filler then its thermal conductivity will be equal to that of the bulk thermal conductivity of the filler.
Below is a table listing the most common fillers used to make thermally conductive epoxy molding compounds.
As you can see in the preceding table, it is important to select a material that has a very high thermal conductivity. In addition to this, it must be possible to load the thermally conductive filler into an epoxy matrix in high enough percentage that the filler is in close enough contact with itself to translate its bulk thermal conductivity to the thermal conductivity of the entire system.
As can be seen in the image below, the filler percentage by weight has to be above 70% before even a thermal conductivity of 1 W/mK can be achieved. it is only once we start going higher than 70% that we start getting thermal conductivities for the entire system high enough to have the system characterized as "highly conductive"
The challenge for formulators then becomes how to load the epoxy resin system as high as possible and still retain the properties required for epoxy mold compound encapsulation. Keep in mind that adding the thermally conductive fillers means removing the other less thermally conductive fillers that are used to give the EMC duroplast its properties other than thermal conductivity such as flow, modulus, and flexural strength. Getting the very high filler loadings required to get the desired thermal conductivity while maintaining the epoxy mold compound's other properties is the challenge each formulator faces.
The Bruggeman equation is one each formulator should know when experimenting with thermally conductive fillers. It is shown here below.
Bruggeman's Equation Figure 2: Bruggeman equation to optimize high thermal conductivity in epoxy molding compound - Where ψ is volume fraction of filler
- And λEMC, λFiller, λResin are the thermal conductivity of the epoxy mold compound, filler, and resin respectively.
From this equation it is clear that the choice of the base epoxy resin also an important consideration for formulating highly conductive duroplasts. As with the filler choice, the choice of epoxy resin will mean selecting the tradeoffs that are acceptable for the application.
Formulating epoxy mold compounds to be very thermally conductive is as much an art as it is a science. The choice of fillers, resins, and hardeners needs to be balanced with the requirements of the application. Experience in formulating epoxies and working with customers to qualify applications that use thermally conductive applications goes a long way in helping you make your product choice. CAPLINQ has more than 25 years working with epoxy molding compounds and customers and can be a powerful tool in your arsenal.
If you have any other questions about thermally conductive epoxy molding compounds, please feel free to contact us.