Patent Application
20050252681
This invention relates to a microelectronic assembly wherein a component is attached to a substrate by a solder layer, and more particularly, to such assembly wherein the solder layer is thinner at the central section of the component than at the perimeter.
In a typical microelectronic assembly, heat generated by a power die or other component may be dissipated to a substrate to maintain the component within a desired operating temperature range. For example, a power die, such as a field effect transistor, referred to as an FET, or an insulated gate bipolar transistor device, referred to as an IGBT, may be attached to a heat sink on a printed circuit board. In another type of assembly, the power die may be attached to a tab and incorporated into an overmolded package that is subsequently clamped to a heat sink. The substrate to which the die is attached is preferably formed of a metal having high thermal conductivity, typically copper, whereas the power die is formed of silicon and is soldered to the substrate. To form the assembly, the die and the substrate are arranged with a solder material therebetween, and the arrangement is heated and cooled to reflow the solder, thereby forming a solder joint bonding the component to the substrate.
During operation, the assembly experiences cyclic heating and cooling that causes the power die and the substrate to expand and contract, but at different rates characteristic of the materials. The rate of expansion is referred to as the coefficient of thermal expansion, referred to as CTE, and is measured a parts per million (ppm) per degree Centigrade (° C.). Because of the difference between the CTE of the silicon die and the CTE of the copper heat sink, stress occurs within the solder layer that causes fatigue of the solder metal and lead to cracking of the joint and failure of the assembly. Heretofore assemblies have been formed having solder layers having a uniform thickness. In general, increasing the thickness of the solder reduces the concentration of stress therein. However, solder tends to exhibit a relatively low thermal conductivity compared to copper or other heat sink metals, so that thicker solder reduces heat transfer form the die and is less effective for purposes of maintaining the die within a desired operating temperature range.
Therefore, a need exists for a microelectronic assembly having an improved solder joint for attaching a power die or other component, to a substrate, such as a copper heat sink or tab, that reduces thermally induced stress within the joint and prolongs the useful life of the assembly, while facilitating heat dissipation from the component into the substrate to maintain the component within a desired operating temperature range.
In accordance with this invention, a microelectronic assembly comprises a component, such as a power die, attached to a substrate by a solder layer. The component face includes a central region surrounded by a perimeter region. The solder layer is characterized by a first thickness underlying the central region, and a second thickness underlying the perimeter region that is greater than the first thickness. Thus, the solder layer is thinner in the central section, to promote heat transfer from the component to the substrate. Further, the solder layer is greater at the perimeter where stresses due to thermal expansion mismatch tend to accumulate, and so is better able to resist fatigue that might otherwise result in failure of the device.
This invention will be further described with reference to the accompanying drawings in which:
In accordance with a preferred embodiment of this invention, referring to
In this embodiment, component 12 features a major face 18 that is planar and parallel to board 16. Face 18 includes a central region 20 surrounded by a perimeter region 22. Substrate 14 also includes a central section 24 underlying component central region 20 and a perimeter section 26 outboard from central section 24 and underlying component perimeter 22. In this embodiment, central section 24 of substrate 14 has a height h1 perpendicular to board 16 that is greater than the height h2 of the perimeter. Substrate 14 may be suitably made by attaching or forming a metal layer having a uniform thickness to the board and machining the perimeter to reduce the thickness thereof. Alternately, the substrate may be formed by affixing a pedestal to a thin metal film, so that the film forms the perimeter and the spacer provides the added height for the central section.
Component 12 is attached to substrate 14 by a solder joint 30 having a variable thickness. Solder joint 30 is formed of a continuous layer of solder alloy, and includes a central portion 32 and a perimeter portion 34. Joint 30 may be formed of any suitable solder alloy adapted to form a metallurgical bond to the component and the substrate. A preferred solder alloy is composed of a near-eutectic tin-lead alloy comprising 60 percent tin and the balance lead. Central portion 32 is disposed between central region of component face 18 and central section 24 of substrate 14 and is characterized by a first thickness t1. Perimeter portion 34 is disposed between perimeter 22 of component face 18 and perimeter 26 of substrate 14 and is characterized by a second thickness t2. In accordance with this invention, central thickness t1 is less than the perimeter thickness t2.
During operation, heat generated by component 12 is dissipated to substrate 14 through joint 30. This is accompanied by a rise in the temperature of the component and, to a lesser extent, the substrate, and expansion of the component and heat sink. Because of the differences in CTE, for example, in assemblies comprising silicon and copper, a mismatch occurs between the expansion of the component relative to the substrate and produces shear stress within the solder layer. While not limited to any particular theory, it is believed that a solder joint having a variable thickness in accordance with this invention reduces stress within the solder layer without retarding heat transfer to the substrate. In general, it is desired to provide a sufficient thickness of solder within the central portion to assure a continuous metallurgical bond to prevent detachment of the component, while minimizing thermal resistance. It is believed that central portion 32 may be suitably formed having a thickness less than about 0.12 mm (5 mils), and preferably between about 0.05 and 0.10 mm (2 and 4 mils). At the perimeter, it is desired to provide a sufficient thickness of solder to distribute the thermally induced stresses to below tolerable limits for the solder material that might otherwise result in crack formation. The optimum thickness for the perimeter solder is dependent upon several factors including the width of the component and the relative CTE of the materials, as they relate to the heat generated by the component and the maximum expected operating temperature. In general, it is believed that a perimeter thickness greater than about 0.20 mm (8 mils), and preferably between about 0.20 and 0.25 mm (8 and 10 mils), is suitable to enhance joint strength and avoid fatigue crack formation in assemblies of silicon die and a copper substrate. The overall strength of the joint is also dependent upon the width of the perimeter portion. Thus, joint strength may be increased by increasing the width of the perimeter portion. It is generally desired to maximize the central portion to facilitate heat transfer. In general, the joint may be designed so that the width of the central portion is between about 60 and 90 percent of the component width, with the perimeter evenly divided thereabout and between about 5 and 20 percent of the component width.
A variable thickness solder joint in accordance with this invention was evaluated and compared to solder joints having uniform thickness using finite element analysis. The analysis was based upon a square silicon die mounted on a copper substrate, and having a total width of 7 mm and a central region that was 5 mm wide. The joints were formed of near-eutectic solder and included a central thickness of 3 mils (0.076 mm) and a perimeter thickness of 10 mils (0.25 mm). When compared to a uniform solder layer that is 10 mils (0.25 mm) thick, the variable solder joint exhibited comparable shear stress values and a 22 percent improvement in thermal resistance. When compared to a uniform solder joint that is 3 mils (0.076 mm) thick, the variable thickness solder joint exhibited a 50 percent reduction in shear stress, with only a 22 percent increase in thermal resistance. Thus, the variable thickness solder joint exhibited an optimum combination of both lowered shear stress and enhanced thermal conductivity.
Referring now to
Therefore, this invention provides an assembly that includes a component attached to a substrate by a robust solder joint. The solder joint resists fatigue due to thermally induced stresses that result from CTE mismatch between the component and the substrate. In addition, the thin section of the solder joint promotes heat transfer to the substrate to maintain the component within a desired operating temperature range. This may be readily accomplished using conventional processes to pattern the surface of either the component or substrate, and without requiring additional components or assembly steps. In the described embodiments, the component is attached to a substrate that comprises a relatively large mass of high thermal conductivity metal to serve as a heat sink and optimize thermal dissipation. Alternately, the component may be attached to tab such as found in an overmolded package, or other suitable metal carrier or support, where it is desired to enhance joint strength while reducing central solder thickness.
While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.
