朋友們
A short time ago, I posted “Metal TIMs 101: Chapter 1” with Jon Major. Now it is time for Chapter 2!
羅恩博士Jon, TIM1.5 是什麼？
Jon: TIM1.5 commonly refers to the interface between a bare die (a die without a lid) and a heat-sink or cold plate. Unlike TIM1, there is no heat spreader, and therefore the die is in direct contact with the cooling solution. See Figure 1. We refer to TIM1.5 as a system level TIM because the CPU/GPU/ASIC is already soldered onto a PCB, so the TIM is applied during system assembly (sometimes referred to as FATP (Final Assembly, Test and Pack)). Most TIM1.5 packages are considered lidless BGA style packages. TIM2 is also a system level TIM in many products.
圖 1.TIM1、TIM2 和 TIM1.5 的應用
羅恩博士TIM1.5 何時使用？
Jon: Each interface in a thermal stack up adds thermal resistance. By eliminating the lid in a design, the total thermal resistance between the die and cooling solution is reduced in theory. However, the thermal interface material used in a TIM1.5 application plays a very important role in the performance of the overall thermal stack-up. Often, packages used in High Performance Computing (HPC) applications like GPUs, CPUs, or sometimes ASICs are designed without a lid to minimize thermal resistance, so a TIM1.5 material is then needed. Figure 2 shows a TIM1.5 covering four integrated circuit (IC)dies ready for the attachment of the heat-sink.
圖 2.覆蓋四個 IC 晶粒的 TIM1.5 正等待散熱片的應用。
羅恩博士TIM1.5 有哪些挑戰？
Jon: The primary purpose of a TIM is to fill air gaps between a hot surface, such as the semiconductor die, and a cold surface, such as a heat-sink. Although metal-based TIMs have very high bulk thermal conductivity compared to polymeric based TIMs, thermal conductivity is not the sole criteria for selecting a TIM; the resistances at the interfaces of the TIM with the IC and the heat-sink are also important. So, for metal TIMs, it is important to have a metal solution with low thermal interface resistance. In addition, it is important to understand that a TIM that works well for one package style may not be ideal for others. Understanding how the TIM will be applied during system assembly is also critical. We see compressible metal TIMs, soldered TIMs, as well as liquid metal TIMs used in TIM1.5 applications. The challenges we help customers solve in TIM1.5 applications are primary focused around thermal resistance, surface wetting, surface flatness, surface roughness, clamping force, reflow temperature (if soldered), surface metallizations, void inspection methods, long-term reliability, as well as process implementation and optimization. Some customers prefer to use a solder TIM for TIM1.5, others compressible TIMs and many are interested in liquid metal TIMs because of its thin bondline thickness (BLT), excellent wetting, and low interfacial resistance.
羅恩博士請告訴我們 TIM1.5 中使用的可壓縮金屬 TIM。
Jon: Flat metallic foils have been used as a compressible TIM for several decades. Typically, they are indium metal or copper shims used in the telecom, military, or RF spaces, among others. In 2008, Indium Corporation patented a patterned metallic foil called “Heat-Spring®“, which is now a family of various alloys and pattern types designed to address a variety of high-performance applications, including TIM1.5, TIM2, burn-in, large area baseplates to cold plates for high power applications like IGBTs (insulated gate bipolar transistor) and SiC packages, as well as immersion cooling applications. The patterned metal helps to improve the thermal interface conductivity by eliminating air pockets between the TIM and heat-sink and the TIM and IC. Good thermal interface conductivity is a critical performance criterion for a compressible metal TIM.
Heat-Spring®是一種軟金屬合金熱介面材料 (SMA-TIM)。它是一種非回流可壓縮 TIM，非常容易應用，而且因為它是圖案化的。與平面金屬箔相比，它能增加模具和散熱表面的順應性。壓力集中在圖案中的小接觸區域，造成塑性變形並改善接觸，長時間使用可降低熱阻。當壓縮在兩個表面之間時，Heat-Spring® 可提供超低的熱阻，即使這些表面是扭曲、粗糙或不共面的。當用作 TIM1.5 時，Heat-Spring® 可提供金屬基 TIM 解決方案，而無需複雜且昂貴的系統層級回流製程。
We recently developed an innovative new pattern called “HSx” that uses a new fabrication technique, offering increased compliancy for bowed or higher warpage dies (>150 μmof warpage) that also provides ultra-low thermal resistance with less pressure (30 psi) compared to our current patterns (HSD and HSHP (high profile)). See Figure 3.
圖 3.不同的Heat-Spring®模式。
Heat-Spring® compliancy improves with time and thermal cycling, referred to as “burn-in”. HSx has an especially short burn-in period and performance improves with thermal cycling. See Figure 4.
Figure 4. Power cycling of HSx, 0-600 Watts for 35,000 cycles. Tj reduces over time and stabilizes as seen above.
Customers continue to see warpage as a major challenge, given the CTE mismatch between the substrate and silicon chip. Thus, we developed the new pattern with warpage in mind. Because there is no backside metallization needed, HSx is an ideal TIM1.5 option for application where at least 30 psi of clamping force is available.
羅恩博士 為什麼不使用熱潤滑脂？這似乎可以很好地處理翹曲，而且已經存在很久了。
Jon: For some applications, thermal grease is a perfectly acceptable TIM, so long as the power is low. For such low-power density applications, grease often will perform just fine, but there is always a risk of “pump-out,” which happens when thermal cycling and CTE mismatches actually pump the thermal grease out of the TIM interface, often leading to thermal failure. Solid metal TIMs, however, are not subject to the pump-out failure mode and are ideally suited for high-power density components.
Ron 博士從圖 5 來看，銅的體積熱阻較低。為什麼不用銅呢？
圖 5.代表材料的熱阻 vs bondline 厚度 (BLT)
Jon: Although copper has an impressive bulk thermal conductivity of almost 400 W/mK, it is too hard and non-compliant to connect directly to a die as a TIM. It is not possible to apply enough pressure to plastically deform copper to fill in air gaps without crushing the die. The challenge of copper TIMs can be seen in Figure 6. Note the high-gap thermal resistance of copper as a function of pressure compared to indium. Figure 5 also shows the very high thermal resistance of thermal grease.
圖 6.代表金屬的熱間隙電阻與壓力的關係。
Ron 博士：Heat-Spring® 如何在浸入式冷卻中發揮作用？
Jon: Heat-Spring® is a proven TIM in both direct-to chip (DTC) and immersion cooling applications. It’s rather common to see Heat-Spring® used as a TIM in immersion systems. Unlike thermal greases or popular polymer phase change materials (PCMs), Heat-Spring®s will neither dissolve nor contaminate standard organic or fluorinated dielectric immersion fluids. It’s easy to install either as a TIM2 or TIM1.5 – simple place the Heat-Spring® and clamp the heat-sink or cold plate to the assembly. We also have new novel ways of applying the Heat-Spring®, which further reduces contact resistance. We are looking forward to introducing these new methods to industry in the near future.
羅恩博士 熱彈簧可以回收嗎？
Jon: 是的！铟基Heat-Spring®是 100% 可回收的，對可持續發展計劃有極大的益處。與其將材料送往垃圾掩埋場，我們會買回使用過的铟和許多其他金屬，這些材料可以再次使用。
羅恩博士 Jon, 謝謝。讓我們在不久的將來討論一些其他 TIM 解決方案，以解決熱管理方面的挑戰
Jon:當然！
