A semiconductor package having mechanically and electrically bonded supportive elements

Abstract: A semiconductor package having an array of solder pads (110) and supportive elements (200) that are mechanically and electrically bonded to said solder pads wherein the supportive elements do not collapse during the bonding process.

Description: The present invention relates to semiconductor packaging. One example of an application where this invention may be practised is on grid array semiconductor packages. The following figures which illustrate the various embodiments of the background art and the present invention may incorporate the same or similar elements. Therefore, where the same or similar elements occur throughout the various figures, they will be designated in the same manner. A Ball Grid Array (BGA) semiconductor package has mechanical and electrical external connections that are made via balls of solid solder that attach to an array of solder pads both upon the BGA package and upon the medium, hereinafter referred to as the substrate or receiving substrate, to which the BGA package is to be electrically and mechanically attached. Figure 1 illustrates the cross section of part of a BGA package and receiving substrate, together with the cross section of a solder ball, as known in the art. The ball of solid solder 100, that is actually illustrated as a solid hemispherical solder ball, is attached to a solder pad 110, which in turn attached to the lower surface of a BGA package 120. Also shown in this figure is the receiving substrate 130 together with an attached solder pad 140. The U.S. patent 5,241,133, MULLEN III et al, entitled LEADLESS PAD ARRAY CHIP CARRIER illustrates an example of a typical BGA semiconductor package U.S. patents 5,468,995 and 5,446,635 and European Patent Application EP 0 724 289 A2 illustrate examples of grid array packages comprising solder halls and supportive elements. For plastic BGA packages, i.e. BGA packages wherein the integrated circuit is protected by an epoxy resin, the solder balls are typically implemented using a tin-lead (Sn-Pb) alloy having a respective 63-37 weight percentage or a tin-lead-silver (Sn-Pb-Ag) alloy having a respective 62-36-2 weight percentage. The method of attaching solder balls to both the BGA package and the receiving substrate basically involves a thermal step which causes the solder ball to melt, known in the art as reflow, such attachment being known to those skilled in the art. For ceramic BGA packages, which are heavier than an equivalent plastic BGA package, the solder balls are typically implemented using a tin-lead (Sn-Pb) alloy having a respective 10-90 weight percentage. The attachment of these types of solder balls is assisted by the addition of a solder cream, that typically comprises a flux and a Sn-Pb-Ag (62-36-2) alloy, that assists in the softening and diffusion of the solder ball. Solder has a melting range from approximately 179øC (solidus) to 380øC (liquidus), depending upon the composition of the solder. The soldering of a BGA package, complete with attached solder balls, to a substrate is carried out by the reflow method. The solder balls ensure electrical contact between the BGA package and the substrate and also compensates for defects in the flatness of the surfaces to be electrically and mechanically bonded. One disadvantage of these solder balls is that, mechanically they are not very strong, which is due to their composition. As a result these solder balls quickly foul the machinery employed in their manipulation and placement. Another disadvantage with solder balls relates to the area and geometry of the pads both on the package and the substrate to which the package is to be bonded. These pads set the tolerances relating to the shape, and thus the height, and spacing of the solder balls that are to mechanically and electrically bond the package and the substrate. Other disadvantages associated with solder balls is that they are: difficult to clean; difficult to desolder; and difficult to inspect. Accordingly, an object of the present invention is to overcome a disadvantage by reducing the amount and frequency of fouling of the machinery used in manipulation and placement. Another object of the present invention is to overcome the other aforementioned disadvantages associated with the solder balls of BGA packages. Another object of the present invention is to increase the density of the electrical and mechanical attachments between a semiconductor package and a receiving substrate. Other objects of the present invention are to provide cheaper and lighter elements for making the electrical and mechanical attachment between a semiconductor packages and a receiving substrate. In order to achieve these objects, the present invention proposes a semiconductor package having an array of solder pads and supportive elements according to claim 1. According to other examples, the supportive elements are spheres, hemispheres, spheroids, elliptical, cylindrical, square, rectangular, and/or annular. According to other examples , the supportive elements are conductive, for example, metallic or alloys of copper, brass, aluminum, steel or any combination thereof and each has an operative mechanical and electrical bond coating. According to another example , the mechanical and electrical bond is achieved by solder. According to other examples , the supportive elements are non-conductive, for example, ceramic, glass or a macromolecular polymerised material or any combination thereof and each has an operative mechanical and electrical bond coating. According to another example, mechanical and electrical bonding is achieved by an adhesive that contains a metallic powder. According to another example , mechanical and electrical bonding is achieved by an adhesive that contains silver and/or copper powder in epoxy based glue. According to another example , the mechanical and/or electrical bond coating, for example the adhesive and/or solder, covers part or the totality of the surface of said supportive elements. According to another example , a semiconductor package according to the present invention operatively incorporates an integrated circuit, the semiconductor package being incorporated or used in a system or apparatus. The system or apparatus is, or is incorporated within, or is used in conjunction with, a computer, a domestic or consumer appliance, a vehicle, a telephone or a telephone network. These and other objects, as well as other advantages and features, of the present invention will become apparent in light of the following detailed description and accompanying drawings among which: Figure 1 illustrates an already depicted cross section of part of a BGA package, receiving substrate, and solder ball, as known in the art; Figure 2 illustrates a hemispherical ring as a supportive element according to the present invention; and Figure 3 illustrates examples of two two-dimensional forms of supportive elements according to the present invention. Rather than using a solder ball and suffering the disadvantages already described, this invention proposes the use of supportive elements that are intended 'to replace the solder balls. These supportive elements, which are substantially rigid, are firstly bonded to a semiconductor package, for example a grid array type semiconductor package. The package, together with its attached supportive elements, can then be bonded to a receiving substrate, for example a printed circuit board (p.c.b.), at a later stage. These supportive elements can be of any desired operative form and can be made from a number of different materials. As way of an example regarding the form of the supportive element, the following lists some forms that can be employed in place of traditional solder balls according to the present invention, these example forms include: a sphere; a hemisphere; a spheroid; an ellipse; a cylinder; a square; a rectangle; a rhombus; and a polygon or any combination thereof. Furthermore, these forms can be annular, i.e. hollow from one end to another, or they can be partly hollowed. The forms can be chosen such that they best meet the specific mechanical criteria and various other necessary criteria. As way of an example regarding the material of the supportive element, the following lists some materials that can be employed according to the present invention, these example materials include: elemental metals and metal alloys, such as copper, nickel, aluminum, steel, tin etc. ; non-metallic materials such as ceramic, glass, polymerised materials such as Teflon, or epoxy resins, coated with a conductive material that can be an adhesive; a combination of metallic and non-metallic materials such as copper powder in a epoxy polymerised matrix and metallised non-conductive bulk material. The materials can be chosen such that they best meet the specific electrical, mechanical and thermal criteria and various other criteria such as weight and cost. The choice of a suitable operative material can be readily deduced by those skilled in the art. Regarding bonding a supportive element to a grid array package, again this can be readily deduced by those skilled in the art. However, by way of some examples according to the present invention, consider the following. If the supportive element is metallic, then it is possible, for example, to operatively cover the supportive element with a material which is easily soldered such as tin and tin alloys, gold and palladium. The supportive element can be covered, for example, with a 10 micrometer layer of brazing cream or tin-rich coating. If the supportive element is non-metallic, then it is necessary to metallise its surface or use a conductive adhesive such as metallic powder in a macromolecular matrix, as for example, copper powder in epoxy glue. In the case where the supportive elements are fabricated from a combination of metallic and non-metallic materials, the choice of bonding material must be made in accordance with the materials and composition of those materials that make up the supportive elements. The choice of such an operative bonding material can be readily deduced by those skilled in the art. The use of a ring or a hemispherical ring as a supportive element in place of a solder ball is illustrated in Figure 2. The hemispherical ring 200 has, in this particular example, two flat sides A and B. The top of side A is attached to a bond or solder pad 110 of the grid array package 120. The bottom of side B is intended to be attached to the bond or solder pad of a receiving substrate (not illustrated). An advantage of using a hemispherical ring, as illustrated in figure 1, as opposed to a spherical ring, is that there is greater stability during bonding, a greater surface area on which to bond to and thus a greater mechanical strength. According to the present invention, an advantage of using hollow forms of supportive elements, such as the hemispherical ring 200 illustrated in Figure 2, is that it makes the automation, i.e. the manipulation and attachment/detachment, of the supportive elements easier than is currently achieved using solder ball technology. With such supportive elements it is now possible to deal with individual supportive elements. Another advantage, according to the present invention, of using hollow forms of supportive elements, such as that illustrated in Figure 2, is that these supportive elements can be aligned in an identical manner so as to improve the quality and speed of inspection i.e. in the case of a metallic ring 10, the solder would flow in and around the sides and edges of sides A and B. Figure 3 illustrates some two two-dimensional examples of forms for supportive elements according to the present invention. These forms include a square, a sphere, a rectangle, a rhombus and a hexagon. It should be understood that the present disclosure is to be considered as an exemplification of the principles of the invention.

Claims: A semiconductor package having an array of solder pads (110) and supportive elements (200) that are mechanically and electrically bonded to said solder pads, characterized in that each supportive element has the shape of a hollow cylinder bounded on its lateral surface to a solder pad. A semiconductor package according to claim 1, characterized in that said supportive elements have the shape of a hollow body with a cross section selected among circular, elliptical, square, rectangular. A semiconductor package according to claim 1, characterized in that said supportive elements are conductive and each has an operative mechanical and electrical bond coating. A semiconductor package according to claim 3, characterized in that said supportive elements are metallic or alloys of copper, brass, aluminum, steel or any combination thereof. A semiconductor package according to claim 3, characterized in that the mechanical and electrical bond is achieved by solder. A semiconductor package according to claim 1, characterized in that said supportive elements are non-conductive and covered with a conductive material. A semiconductor package according to claim 6, characterized in that said supportive elements are ceramic, glass or a macromolecular polymerized material or any combination thereof. A semiconductor package according to claim 3, characterized in that mechanical and electrical bonding is achieved by an adhesive that contains a metallic powder. A semiconductor package according to claim 8, characterized in that mechanical and electrical bonding is achieved by an adhesive that contains silver and/or copper powder in epoxy based glue.

Inventors: PETIT LUC, PETIT, LUC
Applicants: ST MICROELECTRONICS SA, STMICROELECTRONICS S.A.
Application year: 1996
Main classification: H01L23
IPC codes: H05K3, H01L23