thermal pad bleed paper
Understanding the Risks of D3 – D10 Silicone Molecules in Thermal Pads: A Guide to Silicone Bleeding and Material Awareness
Thermal interface materials (TIMs), particularly silicone-based thermal pads, are widely used in electronics to bridge gaps between heat-generating components and heat sinks. While silicone materials offer excellent thermal conductivity and conformability, not all silicone formulations are created equal. A critical but often overlooked concern in the industry is the presence of low molecular weight silicones, specifically D3 – D10 siloxane molecules, within these pads.
What Are D3 – D10 Silicones?
D3 to D10 silicones refer to cyclic siloxanes with three to ten silicon-oxygen bonds forming a ring structure. Due to their low molecular weight, and volatile nature, these molecules can migrate, or ‘bleed’ out of silicone-based materials over time.
Distinguishing D3 – D10 From D11 – D20 Silicones
While D3 – D10 silicones are highly volatile and mobile, larger cyclic siloxanes like D11 to D20 have significantly higher molecular weights and lower volatility. These higher-weight siloxanes exhibit:
- Lower Vapor Pressure: Making them less prone to evaporation, and outgassing.
- Reduced Mobility: They are less likely to migrate under heat and pressure.
- Improved Stability: Their larger ring structure provides better integration within the silicone matrix, reducing the risk of contamination and surface bleeding.
Thus, while D3 – D10 content is problematic, D11 – D20 silicones pose far less risk in terms of bleeding and outgassing, making them a more acceptable component in thermal pad formulations. Laird, and other thermal pad manufacturers often show D3-D10 counts in their thermal datasheets.
Why Is This Detrimental in Thermal Pads?
- Contamination of Adjacent Components:
- When D3 – D10 molecules bleed from the pad, they can deposit on surrounding components, circuit boards, and connectors.
- This contamination can disrupt electrical contacts, particularly in sensitive high-voltage or RF circuits.
- Outgassing in Vacuum or Enclosed Systems:
- In sealed environments or vacuum applications, outgassed siloxanes can condense on optical surfaces, sensors, or other critical components, leading to performance degradation or even failure.
- Long-term Reliability Risks:
- The loss of low molecular weight silicones can alter the mechanical properties of the pad over time, reducing its conformability and effectiveness.
- Regulatory and Industry Restrictions:
- Many industries, such as aerospace, automotive, and medical devices, have strict limits on volatile organic compounds (VOCs) and low molecular weight siloxanes.
In a video we did on thermal pad silicone bleeding, we showed that are thermal gap filler pads bleed, even ones that are “silicone-free”. This is why its important to keep in mind that while thermal gap filler pads bleed, there is some nuanced involved in the types of silicone bleeding out of the pads themselves.
Types of Silicone Bleeding and Migration
Understanding the forms of bleeding is critical to selecting the right thermal gap filler:
- Oil Bleed:
- A visible exudation of silicone oil on the surface of the pad.
- More common in low-durometer or highly-filled thermal pads.
- Volatile Outgassing:
- Invisible to the naked eye but measurable via techniques like TGA (Thermogravimetric Analysis) and GC-MS (Gas Chromatography-Mass Spectrometry).
- Particularly problematic in vacuum environments.
- Migration Under Heat and Pressure:
- Silicones can migrate laterally into adjacent areas under thermal cycling, which is especially damaging in densely populated PCBs.
What Should Engineers and Buyers Look For?
- TML (Total Mass Loss) and CVCM (Collected Volatile Condensable Material) Testing: Ensure the supplier provides data that reflects low outgassing properties.
- Certified Low-Molecular Weight Content: Reputable manufacturers control D3 – D10 content through purification steps.
- Alternatives to Silicone: In applications where even trace levels of bleed are unacceptable, consider non-silicone-based thermal pads or phase change materials. Laird for example offers TFLEX SF10, SF7, and SF4 for non-silicone applications while Henkel/Bergquist offers GP2200SF, and GP3004SF offer low free silicone formulations.
More Information on Thermal Pads
D3 – D10 silicone molecules might seem like a subtle detail, but their presence can have significant implications for the longevity, reliability, and safety of electronic systems. Engineers, designers, and procurement specialists must be vigilant about understanding not just the thermal conductivity of a pad but its chemical stability under operational conditions.