Mọi người ơi,
A short time ago, I posted “Metal TIMs 101: Chapter 1” with Jon Major. Now it is time for Chapter 2!
Tiến sĩ Ron : Jon, TIM1.5 là gì?
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.
Hình 1. Ứng dụng của TIM1, TIM2 và TIM1.5
Tiến sĩ Ron: Khi nào thì sử dụng TIM1.5?
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.
Hình 2. Một TIM1.5 bao phủ bốn khối IC đang chờ lắp bộ tản nhiệt.
Tiến sĩ Ron: Những thách thức của TIM1.5 là gì?
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.
Tiến sĩ Ron: Hãy cho chúng tôi biết về TIM kim loại nén được sử dụng trong TIM1.5.
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 ® là vật liệu giao diện nhiệt hợp kim kim loại mềm (SMA-TIM). Đây là TIM nén không chảy lại, rất dễ áp dụng và vì nó có hoa văn. Nó cung cấp khả năng tuân thủ cao hơn với bề mặt khuôn và tản nhiệt so với lá kim loại phẳng. Áp suất tập trung vào các vùng tiếp xúc nhỏ trong hoa văn, gây ra biến dạng dẻo và cải thiện tiếp xúc, giảm điện trở nhiệt theo thời gian. Khi được nén giữa hai bề mặt, Heat-Spring ® cung cấp điện trở nhiệt cực thấp ngay cả khi các bề mặt đó bị cong vênh, gồ ghề hoặc không đồng phẳng. Khi được sử dụng làm TIM1.5, Heat-Spring ® cung cấp giải pháp TIM gốc kim loại mà không cần các quy trình chảy lại cấp hệ thống phức tạp và tốn kém.
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.
Hình 3. Các mẫu Heat-Spring ® khác nhau.
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.
Tiến sĩ Ron: Tại sao không sử dụng mỡ tản nhiệt? Có vẻ như nó xử lý cong vênh tốt và đã có từ lâu rồi.
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.
Tiến sĩ Ron : Từ Hình 5, đồng có điện trở nhiệt khối thấp hơn. Tại sao không sử dụng đồng?
Hình 5. Độ bền nhiệt của vật liệu đại diện so với độ dày đường liên kết (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.
Hình 6. Độ bền khe hở nhiệt của các kim loại tiêu biểu theo áp suất.
Tiến sĩ Ron : Heat-Spring ® hoạt động như thế nào khi làm mát ngâm?
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.
Tiến sĩ Ron: Lò xo nhiệt có thể tái chế được không?
Jon : Vâng! Heat- Spring® gốc Indium có thể tái chế 100% và có lợi ích to lớn cho các chương trình phát triển bền vững. Thay vì đưa vật liệu đến bãi chôn lấp, chúng tôi sẽ mua lại indium đã qua sử dụng và nhiều kim loại khác, có thể sử dụng lại.
Tiến sĩ Ron: Jon, cảm ơn. Chúng ta hãy thảo luận một số giải pháp TIM khác cho các thách thức quản lý nhiệt trong tương lai gần
Jon: Chắc chắn rồi!
