Patent Application
20090080169
This invention relates to a method of forming a Ball Grid Array (BGA) package having increased standoff height. More particularly, this invention relates to a method that uses solder balls comprising a core formed of polymeric material or the like to form solder bump interconnections that increase the standoff height.
It is known to form a ball grid array package comprising an electronic assembly mounted to a substrate by solder bump interconnections. For example, an assembly with an encapsulated integrated circuit die may be mounted onto a mother board by solder bump interconnections. The substrate and assembly are parallel and spaced apart by a gap, with the solder bump interconnections disposed within the gap. The solder bump interconnections electrically and physically attach the substrate to the assembly via bond pads on the substrate and assembly. It has been proposed to form the interconnections using solder balls that include a polymeric core with a solder layer. The balls are placed on the pads, the substrate and assembly are aligned with the balls in between and then heated to reflow the solder. The core serves as spacer to assure minimal distance between the substrate and assembly.
During operation, the solder bump interconnections may crack due to stresses induced by thermal expansion mismatch between the assembly and the substrate, severing the electrical connection between the electronic assembly and the substrate and causing failure of the package. The distance between the surface of the substrate and the surface of the electronic assembly is referred to as the standoff height. Increasing the standoff height reduces the stresses and creates a more reliable connection. Further, it is known to locate electrical components on the substrate below the electronic assembly. The components generate heat. Increasing the standoff height between the substrate and the assembly enhances the flow of air or other cooling fluid within the gap and improves cooling of the electrical components underlying the assembly.
Accordingly, it is desired to form a package comprising an electronic assembly attached to a substrate by solder bump interconnections, wherein the standoff height between the assembly and the substrate is increased, thereby reducing stress on the interconnections and enhancing coolant flow within the space between the assembly and the substrate.
In accordance with this invention, a method is provided for forming a Ball Grid Array package having increased standoff height. A substrate is provided which includes a plurality of first solder bond pads. A solder ball is disposed onto each first solder bond pad. The solder ball is formed of a core, made of a material that remains solid at solder reflow temperature, and is encapsulated within a reflowable solder layer. An electronic assembly is arranged overlying the substrate such that second bond pads on the assembly are in contact with solder balls, thereby forming an arrangement wherein the substrate and the assembly are in a parallel, spaced relationship with solder balls in between the first and second bond pads and is characterized by a first standoff height between the substrate and the assembly. The arrangement is heated to a temperature for a time effective to melt and reflow the solder. The molten solder wets the first and second bond pads and coalesces between the solid core and at least one of the bond pads to increase the standoff height. Upon cooling and solidification of the solder, the ball grid array package is characterized by a second standoff height greater than the first standoff height of the pre-reflow arrangement.
Further features and advantages of the invention will appear more clearly on a reading of the following detailed description of the preferred embodiment of the invention, which is given by way of non-limiting example only and with reference to the accompanying drawings.
This invention will be further described with reference to the accompanying drawings in which:
In accordance with a preferred embodiment of this invention, a method is provided for forming a ball grid array (BGA) package 10 in
Substrate 14 preferably includes a board 18 formed of a polymeric or ceramic material and having a surface 20. A metallic trace is disposed on the surface and includes first bond pads 22. A solder resist layer 23 covers surface 20 and is patterned to create an opening that exposes first bond pads 22. A preferred substrate may be an FR-4 board. While in this embodiment the package includes a single substrate, the substrate may in turn be connected to one or more additional substrates to form product comprising multiple BGA packages, commonly referred to as a package-on-package (PoP) product. In this embodiment, substrate 14 also includes an electrical component 24 mounted to surface 20 of board 18 and underlying electronic assembly 16. Component 24 is spaced apart from electronic assembly 16 to allow coolant flow for heat dissipation of heat generated by component 24 during operation. A preferred component is a capacitor.
Electronic assembly 16 comprises a carrier 34 composed of polymeric or ceramic material and having a surface 26 facing the substrate. A metallic trace is disposed on surface 26 and includes second bond pads 28, in registration with first bond pads 22. A solder resist layer 29 covers surface 26 and is patterned to create an opening that exposes second bond pads 28. Electronic assembly 16 comprises a microelectronic die 30 and a component 31, spaced apart by insulator 38, and mounted to carrier 34 opposite surface 26. Microelectronic die and component are electrically connected to carrier 34 using wire bonds 33, which are in turn connected to second bond pads 28 using metal vias 35 through carrier 34. The microelectronic die 30 and component 31 are encapsulated by a polymeric overmolding 37 to protect the die and wire bonds during use.
Referring now to
After the substrate 14, electronic assembly 16, and solder balls 40 are arranged as depicted in
Upon examination of the solder bump interconnections 12, it is found that the solder coalesces between core 42 and bond pad 22 and also between core 42 and bond pad 28, to increase the distance between the first bond pad 22 and the second bond pad 28, as shown in
While not wishing to be limited to any particular theory, it is believed that molten solder surrounding the core of the solder ball tends to be drawn toward the bond pads during reflow as a result of the solder wetting the metal of the bond pads. When the size of the bond pad limits the spread of the solder, the solder coalesces between the core and the bond pads, increasing the distance between the core and the bond pad and thus the standoff height of the package. In accordance with this invention, it is believed that sizing the bond pad less than the diameter of the core is effective to increase the standoff height of the package. Preferably, the width of the bond pad is less than about 75 percent of the diameter of the core of the composite solder ball. In the described embodiment, both bond pads are sized to limit the spread of the solder. This process results in a solder bump interconnection characterized by a columnar shape. Referring to
Thus, this invention provides a method for forming a ball grid array package with increased standoff distance. During operation, BGA packages may be subjected to thermal cycling caused by normal operation of electronic components mounted on the carrier and substrate. The heat from these components generates stress on the solder bump interconnections between the substrate and the carrier due to thermal expansion mismatch. The stress may lead to cracks in the solder bump interconnection, severing the electrical connection and causing failure of the BGA package. Increasing the standoff distance in BGA array reduces the stress on the solder bump interconnections, resulting in a more reliable package.
Some components in the BGA package may require active cooling while in operation. Active cooling of components in a BGA package may be achieved by passing air or other coolant over the component to enable heat transfer from the component to the coolant. Increasing the standoff distance in the BGA array allows for more air or other coolant to pass by the component, increasing the efficiency of the cooling process, and ultimately resulting in a more reliable package.
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.
