Amigos,
Dr. Ron: En mis próximos artículos, me gustaría charlar con Jon Major, director de producto de materiales metálicos de interfaz térmica (TIM) de Indium Corporation, sobre los TIM metálicos. Jon, ¿puedes hablarnos un poco de ti, de tu formación técnica, de cómo entraste en contacto con Indium Corporation, de cómo te interesaste por los TIM, etc.?
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.
Dr. Ron: Jon, ¿puedes explicar brevemente por qué son necesarios los TIM metálicos y cómo funcionan?
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.
(La figura 1 muestra un esquema de un CI con dos TIM para conducir el calor al disipador).
Figura 1. TIM1 El TIM1 conduce el calor del CI a la tapa del paquete del CI. El TIM2 conduce el calor de la tapa del CI al disipador térmico.
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: Con el avance de la HPC, vemos que los clientes se enfrentan a retos adicionales debido al adelgazamiento y alabeo de las matrices, la interferencia térmica (calor de los componentes vecinos) y otros retos de diseño. La demanda de TIM metálicos sigue creciendo, ya que pueden resolver muchos de estos problemas y proporcionar el rendimiento y la fiabilidad necesarios en aplicaciones de alta densidad de potencia.
Jon: Aunque el objetivo principal de un TIM es ayudar a transferir calor de una superficie caliente a otra fría, hay otros atributos que deben tenerse en cuenta en determinadas aplicaciones (por ejemplo, facilidad de montaje, fiabilidad, sostenibilidad). Los TIM de base metálica pueden clasificarse como soldados (reflowed), compresibles (non-reflowed), de base líquida (liquid metal TIM), o como TIM de cambio de fase. Los TIM de cambio de fase están diseñados para cambiar de fase cuando se alcanza una determinada temperatura. En futuros artículos hablaremos de todos estos TIM metálicos.
Jon: Los TIM metálicos tienen la ventaja de poseer algunas de las conductividades térmicas más altas de los materiales TIM, pero es importante reconocer que la conductividad térmica no es el único criterio para la selección de un TIM. La resistencia térmica de contacto, o resistencia interfacial, suele dominar la resistencia térmica global de los TIM. Por lo tanto, una elevada humectación de la superficie, para minimizar la resistencia al contacto térmico, es un criterio crítico de rendimiento de los TIM.
Dr. Ron: Segúntengo entendido, el TIM1 suele ser un TIM de soldadura. Puede explicar cómo funcionan?
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.
Jon: 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.
Figura 2. TIM1 El TIM1 se coloca entre el chip (o troquel) y el IHS.
Dr. Ron: Tengo entendido que ha habido algunos avances en la reducción del vaciado, ¿puede explicarlo?
Jon: Históricamente, los sTIM se utilizaban principalmente en encapsulados tipo LGA o PGA. Estos paquetes se sometían a reflujo una vez para refluir el sTIM. Debido a las ventajas que ofrecen los sTIM, se están realizando esfuerzos para encontrar un material sTIM óptimo y un proceso para paquetes que puedan sobrevivir a múltiples ciclos de reflujo BGA, normalmente con una temperatura pico de 240-250°C. Con cada reflujo subsiguiente, los materiales sTIM tradicionales mostrarán un crecimiento de huecos que provocará un rendimiento térmico deficiente.
(Las aleaciones InAg muestran una mejora significativa en comparación con el indio puro para reducir el crecimiento de huecos en los reflujos posteriores. Véase la figura 3).
Figura 3. Los InAg TIM1 reducen significativamente el vaciado en comparación con los In TIM1.
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).
Figura 4. Los InAg sTIM se adaptan bien a los FCBGA.
Amigos,
Esté atento a nuestro próximo artículo sobre mTIMS1.5.
Salud,
Dr. Ron
