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Phase Change Materials

Phase change materials (PCM) are often used as matrix materials for thermal interface applications, because they are solid at room temperature, and soften when heated. They can fully fill the gaps of contact surfaces, therefore providing a thin bondline and high reliability without pump-out issues.  This is achieved by adding a TC filler to the PCM matrix which forms a phase change composite with high heat transfer performance.

We offer silicone free, thermally conductive Phase Change Materials in both pad and paste formats. These products are designed to minimize thermal resistance at interfaces and maintain stable performance through the rigorous reliability testing required for long product life applications. Pastes are exactly the same as the pad versions with the addition of solvent that makes them screen printable and ideal for IGBT.

Based on a robust polymer PCM structure, these materials 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.

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Product Selector Guide

Thermal Phase Change Materials
Product name Specific Gravity Thickness Range (mm) Thermal Conductivity (W/m·K) Thermal Impedance @ no shim (˚C·cm2/W) Volume Resistivity (ohm·cm)
LTM 6300-SP 1.8 0.20-1.00 1.8-2.4 0.12-0.14 3.0×1015
PCM 45F-SP 2.2 0.20-1.00 2.0-2.5 0.09-0.12 8.2×1014
PTM 5000 2.3 0.20-1.00 3.5-4.5 0.06-0.08 2.1×1014
PTM 5000-SP 2.3 0.20-1.00 3.5-4.5 0.06-0.08 2.1x1014
PTM 6000 2.3 0.20-1.00 3.5-4.5 0.06-0.08 2.1×1014
PTM 6000HV-SP 2.6 0.20-1.00 5.2 0.09 2.1×1014
PTM 7000 2.7 0.20-1.00 6.5 0.06 2.1×1014
PTM 7000-SP 2.7 0.20-1.00 6.5 0.06 2.1x1014
PTM 7000-SPS 2.7 0.20-1.00 6.5 0.06 2.1x1014
PTM 7900 2.8 0.20-1.00 8.0 0.045 2.1×1014
PTM 7900-SP 2.8

0.20-1.00

 8.0 0.045 2.1x1014
PTM 7950 2.8 0.25 8.5 0.04 2.1×1014
PTM 7950-SPS 2.8 0.25 8.5 0.04 2.1x1014

Frequently Asked Questions

What are some typical PCM applications?


Typical phase change material Applications include:

  • Power control unit, inverter, onboard electronics, IGBT
  • Auto electronics, traction power inverters, transmission, rechargeable energy storage systems, cockpit electronics, engine control systems, on board charging modules
  • 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

What happens to Phase Change Materials after Shelf Life?


Past the shelf life, the most readily observed response is precut pads (kiss-cut) tend to stick to each other and customers may have difficulty peeling out one without damaging adjacent one. There will be some viscosity change that may or may not affect thermal performance and we will not test or recertify material. We will not be liable for product quality or performance if expired parts are used in manufacturing.

Why choose PCM over Thermal grease?


One can say that it comes down to preference, but it really doesn't. 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.

Additionally they come into both paste (stencil print and dispence) and pad form (0.2~1mm thickness) and their Silicon free chemistry can achieve a bondline of 20~30μm making it the thinnest, cleanest and most reliable bondline in the market. Does this make thermal frease bad? Definitely not. Grease is great for all the other application types that PCM can't cover due to the temperature restrictions.

How should I clean Stencil printable PTM?


There are several ways to clean the stencils and printing tools while using PTM pastes or to clean heatsinks and modulus boards during rework. These solvents need to be carefully cleaned with a Non-woven antistatic microfibre (lint-free) cloth.

From "safe" options, the best solvents are IPA (isopropyl alcohol) and Isopar M (brand solvent containing small amounts of Naphtha). These are the most suitable for cleaning.
Naphtha and toluene are the best and most efficient solvents for removing phase change materials during reworking, but they have the downside of being carcinogenic and should be handled with care. You could also try Acetone and ethanol but they will not be as good as IPA and Naphtha.

Why are some properties a range?

 

Phase change materials come in thicknesses ranging from 0.2-1mm. Those are the initial values that are adjusted after you heat up (>45°C) and pressure the material. The installation pressure will determine the final thickness and thermal properties that those materials will keep throughout their lifetime. Thermal conductivity and bond line are directly affected by the installation pressure and no matter the stated properties, the final bondline is what is going to determine the material's thermal fate.

What are the differences between Pads and Pastes?


Phase change Pastes have the exact same chemistry as the phase change pads, with added solvent to make them printable. Stencil print is highly sought after for applications with high throughput such as IGBT screen printing.

Solvents can be adjusted so that the drying process is faster or slower, depending on your application requirements.

Learn More

Phase Change Application Notes

Phase Change Materials

Phase change materials are made for post reflow operations. They activate at 45°C and are not suitable for 245 - 260°C. Those Silicone free chemistries rely on thermally conductive fillers such as ZnO, Aluminium and Alumina to achieve their thermal properties. Their pad form is clean and efficient and allows for a very thin bondline, down to 20-30μm.

Precise and reliable application requires spring pressure or initial heat to be applied to reduce the bond line thickness.Their nature doesn't work for high temperature applications but if the temperatures allow, it is a much better alternative for thermal grease since they are not prone to pump or dry out and are not "messy" at all. They definitely tick the boxes for conformable and reliable.

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.

On the other hand, the tried and proven thermal grease has good flowability and low contact resistance but its short chains and Si-O-Si structures create a less rigid structure that has potential for migration, dry out and pump out issues.

To sum up, phase change materials exhibit the following characteristics:

  • Optimal surface wetting
  • Low contanct resistance
  • Low thermal impedance
  • Stable thermal impedance across accelerated aging tests such as HTB, thermal cycling, HAST
  • No bleed out, pump out and flow out

These silicone free materials that come in both pad and paste form and are filled with thermally conductive fillers, can achieve very (very) low bond lines. These results come with the assistance of spring loading screws or preheating processes that assist thermal interface materials to reduce their bond line thickness to a minimum. Next up, we are explaining how to apply phase change materials with the stencil printing method.

Stencil Printing Process (SP)

Stencil printable Phase change materials require the following steps for a successful application:

  1. Stencil print. “SP” or “SPS” versions of Phase Change Materials contain solvents which allow the paste to be stencil printed.
  2. Dry. Stencil-print Phase Change Materials require a drying step to remove the solvent prior to application. This drying time depends on the selected temperature.
  3. Apply Pressure. Phase Change Materials require pressure to reduce the thermal impedance. A minimum pressure of 70kPa (10psi) is required and must be applied after the drying phase and during assembly.

Step 0 - Storage & Jar Rolling

PCM paste does not need to be refrigerated at 5˚C, it needs to be stored at RT (room temperature) with the lid tightly sealed. If stored at 5 ˚C, it would need 4~5 hrs thawing and mixing at RT before it can be used. Otherwise, the bad homogenization could cause spreading problems.​ Its shelf life is 12 months at RT.

You should Jar-roll PCM paste jar for 30 minutes before use .​After jar-rolling and just before printing, maintain thorough mixing of the SP material utilising a glass rod. ​ 

 

Step 1 - Stencil Printing

The basic printing guidelines can be borrowed from the SMT (Surface mounting Technology) processes of printing solder paste:

  • The ideal aspect ratio should be greater than 1.5
  • The ideal area ratio should be greater than 0.66
  • PHASE CHANGE MATERIALS (PCM) WILL SHRINK 20% AFTER DRYING STEP
  • Plan the print height to be 20% higher than the final dried height

For example, for a PCM print of 46 x 40 x 0.2mm 

  • Stencil Thickness should be 0.25mm (0.2mm/80% = 0.25mm)
  • Aspect Ratio = 40 / 0.2 = 200 (OK)
  • Area Ratio = (46 x 40)/ (2 x (46+40) x 0.25) = 1840/43 = 42.8 (OK)

Step 2 - Solvent Drying

Approximate recommended drying times for PTM-Series Stencil Print (SP) type Phase Change Material (based on 0.25mm – 0.3mm thickness):

  • 15 hours @ 25°C
  • 3.5 hours @ 50°C
  • 20 mins @ 80°C
  • 2-3 mins @ 100°C
Solvent evaporation vs Time in open air

Please note that times are based on the part's temperature and are best measured using a thermocouple. As a pro tip, it is also possible to determine the time required based on the 20-22% height reduction of the Phase Change Material

Different types of Stencil Print materials will dry in different times and temperatures. We generally offer SP, SPM and SPS version. You can imagine them as Normal, Medium and Slow dry. In Room temperature, normal pastes dry pretty fast, at ~2.5 hours. Medium pastes can take up to two days while Slow versions can dry for more than two days... Of course this changes into minutes at 100°C. At 100°C, Normal ones take ~2.5minutes, Medium ones ~7 minutes and Slow ones can take more than 10 minutes to dry. 

 

Thickness Shrinkage

All of our PCM materials come in both Pad and Paste versions (SP). The SP versions are identical to the pads, with the main difference being the added solvent that makes them more viscous. With this form come a couple of questions, first one being, "How much does the solvent shrink and how does this affect the final bondline thickness?".

To answer this we printed in a few different target thicknesses, namely 0.2mm, 0.25mm and 0.3mm. Then we put these samples in the oven for 20 minutes at 80°C for the solvent to dry and measured the outcome. From the resulting thicknesses, we concluded that, regardless of the print thickness, the dry version is ±20% thinner than the thickness right after print. So, for example, to get a 0.25mm final (dry) bondline we need a 0.3mm initial thickness.

Sample # Thickness right after print (mm) Thickness after Solvent dry (mm) Height reduction
1 0.299 0.239 20.07%
2 0.247 0.195 21.05%
3 0.194 0.154 20.62%

Step 3 - Mounting and Assembly

Phase Change Materials require pressure for the lowest possible Thermal Impedance. Some application parameters that we should take into account are the following:

  • We require clamp pressure of at least 10 psi (70 kPa) to compress the PCM and achieve the lowest thermal impedance and thus the best thermal performance.
    The modules with printed and dried PCM layers can be mounted to the heatsink after it has been soldered or pressed in. Spring loaded screws to the heatsink can be fastened and tightened in a single step. ​
    The PCM is solid at room temperature. Screws can be tightened immediately without any relaxation time.

  • PCM is not compressible at room temperature, but gel like above 45°C and thins down with temperature and pressure increasing. 
    To achieve the lowest thermal impedance and thus the best thermal performance, check the pressure and thermal impedance curve below.

  • Once the module has been mounted, the system should be heated up while leaving enough time for PCM to melt.​
    Be sure the material sees at least 60°C (about 15°C above transition temperature of 45°C ) for at least 30 mins after installation.

Phase change materials will melt, spread, fill gaps and  provide an optimal thermal connection between module and heatsink. Spring loaded screws ensure that pressure is sustained. ​

Applied pressure vs Thermal impedance
 
If we observe the results of applied pressure on a PTM7000 phase change material we can see that when a PCM pad is installed and compressed under pressure, temperature and time, it will become thinner and spread out.

Size change
Typically, most customers use an initial pad size that is 70-80% of the target area to accommodate this spread, when the initial thickness is 0.2mm. You will have to determine the ideal size for your application in a way that fits the equipment in your assembly line.