www.public4you.fora.pl

osiks5y2utan

ensure stability, designers seek to minimize die junction temperature and constantly think high thermal conductivity to be the maximum important attribute of lid material. Yet thermal rendition and stability hinges on other factors: mate or mismatch of coefficient of thermal expansion (CTE) between the lid and assembly materials, lid stiffness/flatness, weight, dimensional tolerances, and package chart.
THERMAL MANAGEMENT
Thermal management techniques provide adequate thermal dissipation without joining mechanical stress to the IC from thermal expansion differences between the IC, lid, substrate, interface materials, and other materials in the package.
The most mutual lid materials for microprocessors and ASICs are bronze (Cu), aluminum (Al) [link widoczny dla zalogowanych], and aluminum silicon carbide (AlSiC). With a thermal conductivity value approximately 400 W/mK by chamber temperature, copper has the maximum thermal conductivity of accessible materials. The thermal conductivity of AlSiC and wrought aluminum are 190 and 200 W/ mK, respectively.
Designers must also consider thermal cycling issues associated with the CTE values of the die and lid as well other combinations. CTE generally isn’t one publish with a die size less than 5 mm and heat flux fewer than 10 W/cm2. As die size and heat flux boost, CTE inconsistencies between lid, die, lid flatness, and weight have a significant efficacy on thermal performance [link widoczny dla zalogowanych], and choosing a lid material with a CTE compatible with the die becomes important.
Compatible lid material CTE values will dwindle die assembly flexing and distortions during thermal cycling. Comparing average CTE values of lid and common die materials at 150°C, AlSiC most closely matches gallium-based IC materials. A solder articulation between lid and die yields most thermal dissipation in flip-chip applications.
The slightly higher CTE of AlSiC puts the die in slight compression during assembly and thermal cycling. However, higher CTE materials may impart catastrophic tensile forces on the IC with rising temperature. In any event, the closer CTE match of AlSiC will minimize package distortions during assembly and thermal cycling.
With twice the CTE of AlSiC, copper incurs greater system flexing, though it does have a higher thermal conductivity. Aluminum, with a 23-ppm/°C CTE, is unsuitable for high-power large applications due to the CTE mismatch.
MATERIAL DENSITY
Another attention is lid material density. Density (weight) namely not a thermal attribute, merely tin influence die protection during assembly and service. Consider the weight per weld ball of the IC. During assembly, tall lid weight tin deform solder balls during soldering (material crawl). It also can potentially occasion shorts among the balls.
In high-speed automated assembly, lid heaviness poses significant inspire aboard archive stress during acceleration/deceleration assembly shifts. Lid heaviness likewise affects shock and vibration resistance and stress state deserving to package orientation during service. These situations like substances with lighter weight. Weight becomes more important because larger assemblies with lids larger than 40 mm2.
LARGE ASSEMBLIES
As systems transform larger, the fusion of lid material, shape, stiffness, flatness, and dimensional tolerances becomes for important as CTE and thermal conductivity amounts. Stiffness and dimensional tolerances influence the lid’s eligible to the die.
The hole depth of the lid is important in minimizing the gap between the die and lid. This depth, somewhat dependent above lid flexibility, have to be great enough to protect the die. For stiffer material, a shallower depth is receivable [link widoczny dla zalogowanych], as stiffness will ensure not distortions of the lid during assembly, heatsink attachment, and/or service.
Lid-material stiffness increases with lid thickness, but this may not be acceptable due to weight limitations. With a less rigid lid, designers may need to tax tighter dimensions on cavity depth to maintain