Computing power is surging across high-performance markets like AI, data centers, HPC, and automotive power electronics. As devices push the boundaries of performance, managing heat becomes a critical challenge. Thermal interface materials (TIMs) are central to this battle, ensuring reliability, efficiency, and longevity. While polymeric TIMs are nearing their performance ceiling, solder TIMs, also called sTIMs, have become the preferred solution for superior heat dissipation.
TIM1 and TIM1.5 are two fundamental interfaces in advanced electronic packaging. TIM1 is used at the chip level, directly between the die and lid or heat spreader during device assembly. TIM1.5, while also aiding chip-level heat removal, is applied later at the board level, between the packaged device and the cooler or cold plate.
To unlock the next generation of device performance, it’s essential to understand their unique roles and the specific engineering challenges they present. Here, we’ll explore their key differences, technological drivers, and best practices for incorporating these materials in advanced assemblies.
Understanding TIM1 vs. TIM1.5 Applications
The most common metallic option for both TIM1 and TIM1.5 applications in high-power-density assemblies is a pure indium solder TIM. This is due to its high thermal conductivity and strong mechanical adhesion to large, warping surfaces. For both applications, a solder TIM requires metallization on the back side of the silicon die and on the heat spreader or heat-sink.
TIM1
TIM1 is the thermal interface material placed between the semiconductor die and the lid or heat spreader during assembly on the chip. The TIM2 is then placed on top of the lid, along with a heat-sink. Some notable qualities of solder TIM implementation are:
- Large die area, sometimes 50–100mm in length and width.
- The existence of a lid adhesive curing profile while the solder TIM is within the stack-up.
- A precisely managed reflow process, which depends on the type of lidded package (PGA, LGA, BGA, etc.).
- It is the most common application for solder TIMs in the HPC and AI space.
TIM1.5
TIM1.5 is used between the finished package and the heat-sink or cold plate during board-level assembly. It also supports chip-level heat dissipation but is applied later in the manufacturing process, bridging the package to the system-level cooling solution. Considerations for TIM1.5 implementation include:
- Theoretically better thermal performance due to fewer interfaces for heat to pass through.
- The final solder reflow assembly process. Low reflow temperatures do not impact any existing SMT on the main board.
- The necessity of robust process controls to conform to die warpage, prevent bondline tilt, and mitigate voiding.
At its core, TIM1 is designed for manufacturing precision and chip-level reliability, while TIM1.5 is engineered for scalability and robust assembly on a larger scale.
Technology Drivers in TIM1 Applications
TIM1 plays a vital role in chip-level heat removal. Achieving the lowest possible thermal resistance while maintaining long-term reliability is critical. Three main engineering drivers shape TIM1 applications:
- Reducing Thermal Resistance (Rth): Lower Rth means less temperature rise for a given power load, keeping junction temperatures down and performance up.
- Thinner, Uniform Bondlines: Thin bondlines not only cut thermal resistance but also enhance uniformity, helping reduce voids and avoid mechanical issues like TIM pump-out.
- Handling Die Height Variation: As packages integrate multiple dies of varying thicknesses, TIM1 must reliably bridge these differences to ensure solid thermal contact across all dies.
Another critical factor is void minimization. Even small percentages of voids—air pockets that insulate heat and concentrate mechanical stress—can cause performance loss and reliability failures. Equally important is controlling issues like hot tearing (microcracks during solidification) and dewetting (failure to wet all surfaces), which can disrupt the crucial thermal pathway between the chip and lid.
Technology Drivers in TIM1.5 Applications
The complexity of TIM1.5 centers on large-scale assembly, mechanical robustness, and long-term field reliability.
Key drivers include:
- Coplanarity Control: Large assemblies are prone to height differences between the lid, dies, and heat-sink. These variations cause inconsistent bondline thickness, leading to hotspots and reliability problems.
- Process Robustness: The sheer size of TIM1.5 bond areas magnifies minor defects. Robust, consistent processes are needed to produce void-free, dependable bonds across uneven surfaces. As with TIM1 applications, lower Rth means less temperature rise for any given power load.
- Reworkability: TIM1.5 is often used in high-value, expensive modules. Repairs and adjustments are more feasible at this level, reducing scrap and overall costs.
- Longevity: TIM1.5 assemblies are expected to last 10–15 years. Failures are costly, often resulting in downtime and revenue loss, so reliability and resilience under stress are paramount.
The TIM1 and TIM1.5 Assembly Landscape
While their goals differ, both interfaces demand careful process management.
The TIM1 assembly process starts with wafer preparation and back-side metallization application. After die singulation and placement, a solder TIM is placed on the die surface. An adhesive is dispensed around the substrate’s perimeter. The lid is precisely aligned, and the assembly is heated under strict controls to create a uniform, continuous bond. Heating cycles depend on the specific chip design, adhesive choice, and solder TIM alloy. Slow, even cooling prevents stress build-up and voids. Inspection, often with X-ray or acoustic analysis, ensures the absence of defects, as even minor voids can significantly impair chip-level heat removal.
In the TIM1.5 process, scale and robustness are paramount. Substrate and die surfaces are prepped before a solder TIM is placed to cover all dies or the full lid. Specialized stand-offs help maintain coplanarity during alignment with the heat-sink. The TIM1.5 process is the final reflow assembly stage, so temperature control is critical, not just for the integrity of the solder TIM joint, but also for all SMT components already on the main board. For larger area soldering, novel reflow technologies such as vacuum and pressure systems can help encourage void removal. X-ray inspection can check for void formation, but scanning acoustic microscopy (SAM) can be incompatible depending on the fin architecture of the soldered heat-sink. Usually, the full board assembly is very costly by this stage. Luckily, TIM1.5 is reworkable due to the lack of a lid sealant in the stack-up and the high ductility of the solder TIM alloy.
Conclusion
As device power and performance demands grow, understanding the key process differences between TIM1 and TIM1.5 is more important than ever. TIM1 provides precision and ultra-low thermal resistance at the chip level, while TIM1.5 enables reliable, scalable solutions for large areas and multi-die modules. By mastering advanced solder TIM strategies for both, engineers can better manage heat, boost performance, and support the longevity of tomorrow’s technologies.
Ready to explore more? Watch our InSIDER series webinar, “1kW and Beyond: Indium solder TIMs (sTIMs) for Bare Die Assembly,” for the latest insights, challenges, and solutions in high-power packaging.
