Consider your materials carefully for microprocessors and ASIC Design

Posted on March 19, 2010, 11:57 am

Microprocessor and ASIC designers must focus on the thermal and mechanical protection of the IC die while considering the system cost and reliability. Covers and radiators are common solutions for mechanical protection. To ensure reliability, designers seek to minimize the junction temperature and die often considered the high thermal conductivity to be the most important attribute of cover material. However, the thermal performance and reliability hinges on other factors: match or mismatch of coefficient of thermal expansion (CTE) between the cover and assembly hardware, stiff cover Flatness, weight, dimensional tolerances, and package design. THERMAL MANAGEMENTThermal provide technical management of adequate heat dissipation without adding mechanical stress to the AC differences of thermal expansion between the integrated circuit, cover, support, interface materials, and other materials in packaging. The most common materials cover for microprocessors and ASICs are copper (Cu), aluminum (Al), aluminum and silicon carbide (ALSIC). With a thermal conductivity value of about 400 W / mK at room temperature, copper has the highest thermal conductivity of materials available. The thermal conductivity of ALSIC forged aluminum are 190 and 200 W / mK, respectively. Designers must also consider issues associated with thermal cycling CTE values of the matrix and cover as many other combinations. CTE is usually not a problem with a die size of less than 5 mm and heat flux less than 10 W/cm2. As die size and increase the flow of heat, the differences between the lid CTE die, the flatness of the cover and weight have a significant effect on thermal performance, and selecting a cover material with a CTE compatible with the matrix becomes important. Compatible with CTE values of the cover will reduce die Assembly bending and distortion during thermal cycling. Comparing the average values of CTE of the lid and die materials common to 150 ° C, ALSIC most closely matches gallium-based IC materials. A solder connection between the lid and die yields maximum heat dissipation in the flip-chip applications. The CEC slightly higher ALSIC puts the chain in a slight compression during assembly and thermal cycling. However, rising raw CTE May catastrophic spread of tensile forces on the IC with temperature. In any event, the results of more ALSIC CTE is to minimize distortions package during assembly and thermal cycling. With twice the ETC ALSIC, copper system incurs greater flexion, but it has a higher thermal conductivity. Aluminum, with 23 ppm / ° C CTE, is unsuitable for high power applications due to the large CTE mismatch. DENSITYAnother MATERIAL consideration is the material density of the cover. Density (weight) is not a thermal property, but may affect protection of dying during assembly and service. Consider the weight per solder ball of the IC. During assembly, the heavy weight can distort the cover of solder balls during welding (creep of materials). It can also cause short circuits between the bullets. High speed automated assembly, cover weight poses a significant influence on the package stress during acceleration or deceleration of the Assembly of changes. Lid weight also affects the resistance to shock and vibration and the stress due to the orientation package for the service. These situations favor materials with lighter weight. Weight becomes more important for large groups with lids more than 40 mm2. ASSEMBLIESAs systems become larger, the combination of cover materials, shape, rigidity, flatness and dimensional tolerances is also important that the CEC and the values of thermal conductivity. Rigidity and dimensional tolerances affect Adjust the lid of the industry. The depth of the cavity of the lid is important to minimize the gap between the die and the lid. This depth, somewhat dependent on the flexibility lid must be big enough to protect the industry. For more rigid materials, shallow depth is acceptable because the stiffness will not distort the eyelid during assembly, the heatsink attachment, and / or service. Lid-growing stiffness material with a thick cover, but this may not be acceptable due to weight constraints. With a cover less steep, designers may need to impose tighter dimensions on the depth of the cavity to maintain an acceptable thickness bond line. However, tighter tolerances increase the cost of manufacturing the lid. MANUFACTURABILITYManufacturing processes and costs are other considerations in the choice of cover materials. Each material has a preferred manufacturing process for the lowest cost, but designers must take into account the full costs of the system, including the rate of quality. A process of manufacturing low cost, stamping lids in the material sheet is the conventional method of manufacturing copper and aluminum covers, limiting them mainly to form 2D and 3D environment functionality. An aluminum lid embossed targeting low power applications only due to CTE aluminum and low thermal conductivity. When dying is small and low power, covers stamped copper may provide a cost effective solution. ALSIC uses a casting process a little more expensive but offer greater geometric capabilities. In addition to its compatibility with the materials ETC IC ALSIC also allows large caps because of lightness and rigidity.