Thermal management: Problems and Solutions
In my last piece I expressed what I hoped would prove a good starting point for my series of articles on the all-important subject of thermal management. This was a five-point guide, which I was able to compile based on the many questions that our customer support teams field every day on the telephone, at exhibitions and when visiting customers’ premises. I do hope readers have found it useful.
This month I’d like to progress matters further and concentrate on some of the problems you are likely to encounter when applying a thermal interface material, and how to select materials that are appropriate for your electronic assembly and its operating conditions. I’ll also be looking a little more closely at bond line thickness and its effects on performance, as well as the alternatives to non-curing thermal pastes. But let’s start with a problem that, I’m afraid, most of you will encounter at some stage in your product development work – pump out.
Pump out can occur when a device – an IGBT, for example – is subject to temperature changes resulting in relative motion between the conductor and its heatsink between which a non-curing thermal paste has been applied. This motion can cause such pastes to be squeezed or pumped out from the interface gap, reducing the thermal transfer performance.
In order to tackle issues with pump out, it is first important to understand the conditions and materials involved. Temperature extremes and rate of change of temperature are important factors that will determine the choice of thermal interface material; for example, if operating temperatures are likely to range between -50°C and 200°C, a silicone based thermal paste would be the preferred option.
In addition to these temperature considerations, the materials that are being used may have an effect on the interface material, particularly with regards to the spacing between the device and its heatsink – otherwise known as the bond line thickness.
Thermal pastes are often designed to be applied in as thin a layer as possible. They improve the contact between the device and its heatsink by eliminating air gaps and ensuring that the full surface contact area is available for heat transfer. There is, however, a critical thickness which determines maximum thermal transfer with minimal thermal resistance, and while this will depend on the ‘roughness’ of the substrates and required spacing, it is generally between 30 and 100 microns.
A thermally conductive heat transfer material will naturally have a lower thermal conductivity than the metallic heat sink material, so keeping the thickness of the film at the interface as low as possible will decrease the thermal resistance and, in turn, lower the operating temperature of the device. While interface materials should be applied at minimum thickness to achieve low thermal resistance, the resulting bond line may also be affected by the substrate’s ‘smoothness’ and spacing; i.e. components and heatsink surfaces at the interface. If a non- curing thermal interface material is applied more thickly due to spacing/materials considerations, the greater the pump out effect will be.
So, while the general rule is to minimise the thickness of non-curing thermal interface materials, this must not be to the detriment of their stability in use. For example, it is particularly important to ensure that a lower film thickness does not result in air gaps forming in the film between the device and its heatsink as this will increase thermal resistance and the device will not be cooled as efficiently as desired.
Consider using alternatives to non-curing thermal interface pastes such as the new phase change materials that are now coming on to the market or – for those wide operating temperature range applications – a surface-cure silicone thermal paste; both of these alternatives will minimise pump out whilst keeping thermal resistance low.
The low phase change temperature of phase change materials allows low thermal resistance over a wide temperature range, ensuring minimal bond line thickness with improved stability and pump out resistance. And while a surface cure thermal paste doesn’t set entirely (thus allowing for easy rework), it is specifically formulated to resist pump-out, particularly for those applications that are exposed to rapid and frequent changes in temperature.
As well as phase change materials and surface curing silicone thermal pastes, other alternatives to non-curing thermal pastes include thermal gap filler pads, which are available as silicone and non-silicone based sheet materials that can be cut to size and applied by hand. These are highly thermally conductive, but they do have a higher thermal resistance than thermal pastes.
Another approach is to use a room temperature vulcanized (RTV) product, which becomes a flexible rubbery material on curing, combining the properties of silicone gap filler pads with those of a conventional heat transfer paste. RTVs can be used to bond the heatsink to the component while also offering a flexible heat transfer medium. A much less flexible bond can be achieved between the device and its heatsink by using a two component epoxy resin which cures to a tough solid bond.
A promising new material is the thermal gel which is a silicone based formulation that offers the low thermal resistance of a non-curing thermal paste minus the latter’s pump out problems. Gels are highly conformable and, even better than the softest of silicone gap filling pads, they impart minimal mechanical forces on delicate components during application and in use.
For certain types and designs of heat generating circuitry, it may be more beneficial to encapsulate the device in a heatsink enclosure using a thermally conductive encapsulation compound. Silicone, polyurethane and epoxy resins provide both heat dissipation and environmental protection all in one.
Electrolube has a large portfolio of thermal management products but as I hope I’ve made clear in the foregoing, it’s complicated deciding on the right choice of material and/or application technique. Companies like Electrolube have years of experience resolving customer thermal management issues and, as always, I strongly recommend you get some expert advice before you settle on any particular material or method. I hope readers have found this article useful; look out for my next blog, which will feature soon.
