What are Phase change materials?
Phase change materials (PCM) are silicone free pads and stencil printable pastes that are typically used as matrix materials for thermal interface applications.
The magic of Phase change materials is that they change phase (form) at 45°C from solid to gel like. In this state and with the help of some pressure, they can squeeze into any gap and severely warped design and fill it in with their thermally conductive goodness. This flexibility also allows for very thin bond lines that after this one time application step ensure a low Thermal impedance and amazing thermal properties.
They exhibit effective wetting properties during typical operating temperature ranges, resulting in very low surface contact resistance. With a breakdown temperature of ~180°C they provide superior reliability and maintain low thermal impedance, making PCM desirable for high-performance integrated circuit devices. Typical operating temperature range is ‑40 to 125°C that can go much higher depending on the specific product. PTM6000-HV for example that has been designed for IGBT Honeycomb structures offers great thermal stability after various long-term reliability tests including HAST 192hrs, T/C‑B 4000x and High Temperature Baking at 3000hrs.
Thermal impedance remains stable across accelerated aging tests without any bleed out, pump out or slow out thanks to the steric hindrance and the branching chemical bond structure of PCM.
Installing Phase change materials
PCM are TIM2 products that require a setup stage before they exhibit their final properties. Their initial thicknesseses are 0.2, 0.25 or 0.3mm and these should cover even the most warped designs. 10–20psi are enough to achieve the 30–40um sweet spot. But how do we get to that?
These materials need to be clamped down under pressure. Spring loaded screws are typically used to maintain the pressure. Once the material undergoes the phase change phase and spreads out, it needs sustained pressure to achieve the final properties.
At time zero, Phase change materials are solid. They start softening above 45°C and our suggested temperature is 60°C so that the materials are soft enough.
At this point you clamp the materials with 20–30psi pressure for around 30 minutes. The soft material melts, spreads and wets on the heat sink and on the heat component, reducing the interfacial resistance. The initial 0.2mm slowly goes down to the sweet spot of 30–40um where the product sees the optimal and lowest Thermal impedance.
The setup phase (change) needs to be done for at least one cycle. It can happen after mounting or even as part of a functional test and even though it is recommended (especially if you want to heat cycle immediately), it doesn’t have to be a separate step. For more information about stencil printing and the installation process, take a look at our Phase change Application notes.
Typical Properties and limitations
We already mentioned that steric hindrance and the branching chemical bond structure help with eliminating bleed out and pump issues.
Slumping and dripping on the other hand can be a problem for vertical applications that see high operating temperatures. Normally we do not expect such issues for 70–80°C but they are to be expected when we use PCM in i.e. 120°C. In these high temp vertical applications, thermal gap pads, thermal putty pads and two part hybrids are much more suitable solutions. Let’s stress this a third time. Vertical applications. This issue does not apply in horizontal applications.
Another common misconception is adhesion. Phase change materials are not designed for structural strength. Period. They are there to provide flexibility and thermal conductivity. They are reworkable by design so it goes without saying that they don’t exhibit good adhesion strength. If you run them at high bond line thicknesses for adhesion and reliability, you are doing it wrong.
Why should I choose PCM instead of Thermal Grease?
Phase change materials are hands down the best materials you can use as thermal interface if the application parameters allow it. They are clean, efficient,can achieve the thinnest bondlines out of any other materials and are silicone free. Power cycling? Phase change can easily handle it while thermal grease can be pushed under the chip after a few hundred cycles. Grease doesn’t work for high power cycling and we have seen that 300–500W chips can’t work with Grease. At these power ranges nothing works properly except for Phase change materials.
On this note we should say that Phase change materials are designed for post reflow operations. They are not meant to be used at 245–260°C.
Typical phase change material Applications include and are not limited to:
- Power control units, inverters, onboard electronics, IGBT
- Servers, supercomputing, video graphic array (VGA) cards, AI, GPU/CPU/Desktop, solid state drives (SSD)
- Switches, routers, base stations
- Tablets, gaming, notebooks, smartphones, action cameras & lighting
Compared to Silicone grease, PCM have longer molecular chains with high molecular weight and surface tension ensuring a robust polymer structure and a stable filler-polymer matrix. The H steric hindrance provides a Rigid structure that ensures low filler migration and separation that at the same time limits the material mobility and pump out.
Does this make thermal grease bad? Definitely not. Grease is great for all the other application types that PCM can’t cover due to the temperature restrictions. To be fair, TG 3000I boasts a 7 micron bond line thickness. That’s probably the thinnest BLT you’ll be able to get out of any thermal interface material.
Conclusions
To sum up, phase change materials should be your go to choice if your applications allows it. Thin BLT, low thermal impedance, great wetting and flexibility, no bleed out, pump out or blow out. The works.
Contact us with your application requirements and we’ll work together to find the best thermal interface material for your application.
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