朋友們
羅恩博士在接下來的幾篇文章中，我想和 Indium Corporation 的金屬熱介面材料 (TIM) 產品經理 Jon Major 談談金屬 TIM。Jon，你能告訴我們一些關於你自己的事嗎；你的技術背景、你如何與 Indium Corporation 結緣、你如何對 TIMs 感興趣等等？
Jon: I’ve always been passionate about product development, engineering, materials, and manufacturing. I was fortunate enough to start my career in Silicon Valley, working with the brightest engineers on the planet; I had the opportunity to work on several groundbreaking products, such as the first iPad Air, the first cloud-based smartphone called the “Sidekick”, the first internet connected radio, and several other mobile devices, as well as an IoT platform for connected vehicles.
Jon: Thermal management was always considered at the design level, especially when dealing with consumer products. At Indium Corporation, I have the opportunity to dive deep, not only with the materials themselves, but how they perform with various surfaces, pressures, under varying warpage conditions, and how long they will survive under different use conditions. Our principal thermal engineer recently developed an in-house thermal test vehicle that provides the representative environment for examining performances of thermal interface materials. It’s rather fascinating! I was happy to be a part of a project that enables us to give our customers valuable data on how metal-based TIMs perform under varying conditions.
羅恩博士 Jon，您能簡單解釋一下為什麼需要金屬 TIM，以及它們是如何工作的嗎？
Jon: As integrated circuit (IC) technology has advanced, the amount of heat generated by a high-performance IC is staggering, sometimes exceeding 1,000 watts when the IC is only slightly bigger than an inch (2.5 cm) on one side. The IC typically needs to operate at less than 100°C or its life will be too short. Without TIMs to conduct the heat away from the IC and to the heat-sink, this goal would be impossible.
(圖 1 顯示一個 IC 的示意圖，其中有兩個 TIM 將熱傳導至散熱片）。
圖 1.TIM1 將 IC 的熱傳導至 IC 封裝蓋。TIM2 將 IC 封裝蓋上的熱傳導至散熱片。
Jon: In the past, polymeric (traditional) TIMs, gels, and other non-metal TIMs were used. In some applications, they are still used today. The most common was thermal grease, which has been used for many decades. Thermal grease has a carrier that is almost like Vaseline®. The carrier is loaded with conductive particles. The thermal grease is then applied where the metal TIMs are in Figure 1. Thermal grease has two shortcomings. One is that its thermal conductivity is not sufficient to meet higher-heat fluxes generated by high performance computing (HPC), AI, accelerated process unit (APU), and graphics processing unit (GPU) trends. The other is that the on/off cycles of electronics can cause “pump-out.” Pump-out occurs when the thermal grease is pumped-out from the space that it occupies to conduct heat away from the IC. With the thermal grease pumped out, it can no longer perform its function.
Jon: This is where metallic based TIMs come in. They can provide the lowest thermal resistance and be customized for package-specific needs. They also do not typically experience pump-out.
Jon:隨著 HPC 的進步，我們看到客戶因為晶粒變薄和翹曲、熱交談 (來自鄰近元件的熱量) 以及其他各種設計挑戰，而面臨額外的挑戰。由於金屬基 TIM 可解決上述許多挑戰，並提供高功率密度應用所需的效能與可靠性，因此市場對金屬基 TIM 的需求持續成長。
Jon：雖 然 TIM 的主要目的是幫助將熱量從熱表面傳遞到冷表面，但在某些應用中還需要考慮其他屬性（例如易於組裝、可靠性、可持續性）。金屬基 TIM 可分為焊接型 (回流焊)、壓縮型 (非回流焊)、液態型 (液態金屬 TIM) 或相變 TIM。相變 TIM 的設計目的是在達到一定溫度時改變相位。我們將在未來的文章中涵蓋所有這些金屬 TIM。
Jon：金屬 TIM 的優點是具有最高的 TIM 材料體積熱導率，但必須認識到，僅僅體積熱導率並非選擇 TIM 的唯一標準。热接触电阻或界面电阻通常主导 TIM 的整体热阻。因此，高表面润湿性以最小化热接触电阻是 TIM 性能的关键标准。
羅恩博士據我瞭解，TIM1 通常是焊接 TIM。您能解釋一下它們是如何工作的嗎？
Jon: TIM1 is commonly referred to as the interface between the backside of a die and the underside of an integrated heat spreader (IHS) and component cap. A soldered TIM (sTIM) at this interface is the “Cadillac” of TIMs. Once reflowed, sTIMs form intermetallic bonds that provide low interfacial resistance. Coupled with the fact that metal-based TIMs have high bulk thermal conductivity, the sTIM provides very low overall thermal resistance. sTIMs also mechanically fasten the die and IHS together given there is an intermetallic compound (IMC) formed at the interface. Often, we are asked if the rigidity of the solder joint could cause problems during power cycling. With the proper alloy and process, the sTIM can provide the ductility necessary during the life of the package, so rigidity issues are not a concern.
喬恩 There are many process considerations when selecting a sTIM. Indium Corporation has the experience and guidelines to help customers realize the benefits of sTIMs. One of the challenges in assembling TIM1s is voiding during reflow (see Figure 2). Voiding becomes worse after multiple reflows.
圖 2.TIM1 置於晶片 (或裸片) 與 IHS 之間。
Ron 醫生： 我知道在減少排尿方面有一些突破，您能解釋一下嗎？
Jon： 過去，sTIM 主要用於 LGA 或 PGA 型封裝。這些封裝需要回流一次以回流 sTIM。由於 sTIM 所提供的優點，人們正努力尋找最佳的 sTIM 材料和製程，讓封裝能在多次 BGA 回流週期（通常峰值溫度為 240-250°C）中經得起考驗。隨後的每次回流，傳統的 sTIM 材料都會出現空隙增生，導致熱性能變差。
(與純铟相比，InAg 合金在減少後續回流的空隙增長方面有顯著改善。見圖 3)。
圖 3.與 In TIM1s 相比，InAg TIM1s 能顯著減少排尿。
Jon: However, there are trade-offs to adding Ag to the solder joint. More Ag also means lower bulk thermal conductivity and a more rigid solder joint leading to reduced mechanical reliability. There is significant research underway to understand how different compositions of InAg wet to various surfaces and how they perform during reliability testing. High surface wetting, to minimize thermal contact resistance, is a critical TIM performance criterion. In addition, poor wetting can result in higher voiding, also leading to poor thermal performance. With the proper alloys selection, flux and process considerations sTIM can be adopted in Flip Chips BGA(FCBGA)style packages that will undergo multiple reflow cycles (see Figure 4).
圖 4.InAg sTIM 非常適合 FCBGA。
朋友們
敬請期待我們下一篇關於 mTIMS1.5 的文章！
乾杯
羅恩博士
