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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.
Typical phase change material Applications include:
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.
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.
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.
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.
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.
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:
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 printable Phase change materials require the following steps for a successful application:
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.
The basic printing guidelines can be borrowed from the SMT (Surface mounting Technology) processes of printing solder paste:
For example, for a PCM print of 46 x 40 x 0.2mm
Approximate recommended drying times for PTM-Series Stencil Print (SP) type Phase Change Material (based on 0.25mm – 0.3mm thickness):
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.
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.
Phase Change Materials require pressure for the lowest possible Thermal Impedance. Some application parameters that we should take into account are the following:
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.