Thermal Interface Materials

Technical Needs, Gaps and Solutions

This section covers technology issues surrounding Thermal Interface Materials (TIMs), the associated needs, technology status of those needs, as well as gaps and challenges to overcome, are summarized below in Table 1. The time period considered is from 2024 to 2034. Assessments of to address these gaps and challenges is presented in Table 2 as a set of potential solutions for each set of issues.

Technology Status Legend

For each need, the status of today’s technology is indicated by label and color as follows:

In-table color + label key	Description of Technology Status
Solutions not known	Solutions not known at this time
Solutions need optimization	Current solutions need optimization
Solutions deployed or known	Solutions deployed or known today
Not determined	TBD

Table 1. Thermal Interface Materials Gaps, and Today’s Technology Status with Respect to Current and Future Needs

TECHNOLOGY ISSUE	ROADMAP TIMEFRAME
Thermal Interface Materials
	TODAY (2024)	3 YEARS (2027)	5 YEARS (2029)	10 YEARS (2034)
TIM ISSUE #1: Addressing voiding to reduce thermal impedance/resistance in solder TIMs
NEED	Ways to reduce or make voiding less impactful on thermal resistance/impedance and board level reliability.	Ways to reduce or make voiding less impactful on thermal resistance/impedance and board level reliability for next generation packages.
CURRENT TECHNOLOGY STATUS	Solutions need optimization	Solutions need optimization
	TIM material heats up, melts and resolidifies- voiding issues. The voiding issue is a perceived future issue. This is specifically looking at the TIM1 inside of the package (1^st level assembly). Most of the solder TIM (STIM- solder TIM) (solder based or liquid metal (e.g., Ga alloys) used in the world is on large grid array (LGA) and pin grid array (PGA) packages so re-melting is not an issue. As thermal demands increase, there is a driving interest in using metal STIMs for BGA components. In this case, standard indium with a melting point (MP) of 157°C does not work well (due to re-melting during PCB assembly). In/Ag alloys have been developed to improve the voiding issue because the Ag addition increases the liquidus point of the alloy. However, with the addition of silver, the alloy becomes more rigid and mechanical reliability can suffer. InAg alloys being developed today will play a role. However, even higher MP alloys still needed for some automotive applications and are not fully developed yet. Voiding occurs during package assembly and increases/consolidates during 2^nd level SMT assembly, so low/no voiding is essential in the package assembly step to reduce sizes in SMT assembly and the impact on thermal resistance/impedance and board level reliability.
GAP	There are many innovations ongoing for TIM1.5 (bare die) and TIM2, which both occur at the PCB assembly level. As the TIM location changes (i.e., TIM1, TIM1.5, TIM2, etc.) the needs can be dramatically different. It is difficult to put all TIM’s into one bucket due to different requirements for different TIM types.	Thinner solder TIMs are needed (i.e., ≤50 um) which is a challenge for Indium TIMs or finding ways to increase the bulk thermal conductivity of point of record (POR) STIMs without negatively impacting the assembly or reliability.
GAP	Flux with low/no voiding. TIM alloys with lower melting point than SnAgCu with increased thermal conductivity (TC).
GAP	Carbon fibers coated with solder that can reduce the impact of voiding on thermal resistance (e.g., embedded carbon fiber solder TIMs).
CHALLENGES	Packaging issues rather than solder material issue. Component has some amount of voiding as received. Reflow makes voiding worse. Voiding affects the thermal performance in the field. Removal of oxides via flux causes outgassing. Alloys/solutions with higher TC require different assembly conditions/parameters (e.g., higher temp, different equipment, tighter Quality Assurance (QA)/Quality Control (QC), etc.). Other solutions (e.g., vertically aligned anisotropic carbon fibers coated with solder) require a large amount of data collection prior to adoption to insure they meet all necessary requirements.
TIM ISSUE #2: Voiding in other TIMs (non-metal TIMs) (thermal greases, gels, pads, TC adhesives) (Voiding is more of an issue with adhesives.)
NEED	Less voiding to reduce impedance/ thermal resistance
CURRENT TECHNOLOGY STATUS	Solutions not known
	Different materials have advantages and disadvantages.
GAP	No ideal material to meet all requirements.
CHALLENGE	Development of new materials.
TIM ISSUE #3: Bleeding of TIM (gel and pad type materials- silicones and urethanes) (TIM 2 and TIM 0 materials)
NEED	TIM that does not bleed from the interconnection during operation.
CURRENT TECHNOLOGY STATUS	Solutions not known
	TIM2 or TIM0 (non-solder- e.g. Thermal grease) : Bleed occurs during operation over time.
GAP	TIMs that can stay within the interconnection
CHALLENGE	No off-the-shelf options today for TIM2 or TIM0 that can prevent bleeding. Development of new materials needed.
TIM ISSUE #4: TIM Pump-out/bleed-out/dry-out
NEED	TIM that does not pump out from the interconnection during assembly or during operation.
CURRENT TECHNOLOGY STATUS	Solutions not known
	STIM/TIM1 (Solder): Pump out can occur during assembly/reflow.
GAP	TIM that can stay within the interconnection (e.g., solder preform embedded with vertically aligned anisotropic Ni plated carbon fiber).
CHALLENGE	No off-the-shelf options today for liquid metal that can prevent pump out.
NEED	TIM that does not bleed out from the interconnection during assembly or during operation.
CURRENT TECHNOLOGY STATUS	Solutions need optimization
	TIM2 or TIM0 (non-solder- e.g., thermal grease) : Bleed out occurs during operation over time. Currently dams are used to prevent bleed out.
GAP	TIM that can stay within the interconnection (e.g., solder preform embedded with vertically aligned anisotropic Ni plated carbon fiber).
CHALLENGE	No off-the shelf solutions for TIM2 or TIM0 that can prevent bleed out. There are some solutions but not off-the shelf at this time (e.g., vertically aligned anisotropic carbon fibers coated with solder), but they require a large amount of data collection prior to adoption to insure they meet all necessary requirements.
TIM ISSUE #5: Material Stability (Non-solder TIMs)
NEED	More stable materials or processes
CURRENT TECHNOLOGY STATUS	Solutions not known
	Filler separation and/or viscosity changes within pot-life
GAP	Stability for 1 year
CHALLENGE	No low cost or material solutions to date
TIM ISSUE #6: Inspection of TIMs
NEED	Suitable methodology for high-volume manufacturing
CURRENT TECHNOLOGY STATUS	Solutions need optimization
	No agreed upon inspection method (i.e., 2D, 3D, combination, etc...)
GAP	Standardization
CHALLENGE	No agreed upon inspection method
TIM ISSUE #7: Pick and Place (PnP)/Insertion process
NEED	PnP/Insertion process guidelines/standard
CURRENT TECHNOLOGY STATUS	Solutions need optimization
	No agreed upon PnP/insertion process standard. Insertion tooling and equipment for insertion process control, as you can have a good placement, but the overall application will still fail without a total placement, inspection, insertion and cure schedule (for non-solder TIMs)
GAP	Standardization
CHALLENGE	No agreed upon PnP/insertion process standard for basic or complex assembly designs.

Approaches to address Needs, Gaps and Challenges

Table 2 considers approaches to address the above needs and challenges. The evolution of these is projected out over a 10-year timeframe using technology readiness levels (TRLs).

In-table color key	Range of Technology Readiness Levels	Description
2	TRL: 1 to 4	Levels involving research
6	TRL: 5 to 7	Levels involving development
9	TRL: 8 to 9	Levels involving deployment

Table 2. Thermal Interface Materials Potential Solutions

TECHNOLOGY ISSUE		EXPECTED TRL LEVEL*
Thermal Interface Materials	POTENTIAL SOLUTIONS	TODAY (2024)	3 YEARS (2027)	5 YEARS (2029)	10 YEARS (2034)
TIMs ISSUE #1: Addressing voiding to reduce thermal impedance/ resistance in solder TIMs	Development of novel materials and processes	4	6	7	8
TIMs ISSUE #2: Voiding in other TIMs (non-metal TIMs) (e.g., thermal greases, gels, pads, TC adhesives) (Voiding is more of an issue with adhesives.)	Development of novel materials and processes	2	4	6	7
TIMs ISSUE #3: Bleeding of TIM (gel and pad type materials- silicones and urethanes) (TIM 2 and TIM 0 materials)	Development of novel materials and processes	2	4	6	7
TIMs ISSUE #4: TIM Pump-out/bleed-out/dry-out	Development of novel materials and processes	4	6	7	8
TIMs ISSUE #5: Material Stability (Non-solder TIMs)	Development of stable materials (no filler separation)	2	3	4	5
TIMs ISSUE #5: Material Stability (Non-solder TIMs)	Development of stable materials (No viscosity changes)	2	3	4	5
TIMs ISSUE #6: Inspection of TIMs	Development of agreed upon inspection methods	3	4	5	7
	Development of methodology for high-volume manufacturing	3	4	5	7
	Standardization of Inspection	3	4	5	7
TIMs ISSUE #7: PnP/Insertion process standard/ guidelines	PnP/insertion process standard	2	3	5	7

Conclusions

Addressing voiding to reduce thermal impedance/resistance in solder TIMs
Addressing voiding in other TIMs (non-metal TIMs) (e.g., thermal greases, gels, pads, TC adhesives) (Voiding is more of an issue with adhesives.)
Bleeding of TIMs (e.g., gel and pad type materials- silicones and urethanes) (TIM 2 and TIM 0 materials)
TIM pump-out/bleed-out/dry-out

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