Structure to accommodate increase in volume expansion during solder reflow

#1. Field of the Invention

#The present invention relates to methods and structures for attaching a semiconductor chip or chip carrier to a substrate and, more particularly, to methods and structures for attaching a semiconductor chip or chip carrier to a substrate using solder ball technology.

#2. Background and Related Art

#These foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings, wherein like reference members represent like parts of the invention.

#FIG. 1 shows a cross-section of a typical arrangement wherein a semiconductor chip is positioned for electrical connection to a substrate through an array of solder balls.

#FIG. 2A shows an enlarged section of the arrangement shown in FIG. 1 with one form of structure used to release pressure on reflow of solder balls.

#With reference to FIG. 1, there is shown a conventional arrangement of semiconductor chip and substrate. Substrate 3 may be a PCB type of substrate or a ceramic substrate, for example. Substrate 3 may also be a single chip module or a multi chip module (MCM) which is, in turn, attached to a substrate, such as a PCB. Chip 1 is shown positioned on substrate 3 with C4 solder balls or bumps 5, for example, positioned therebetween. Solder balls 5 may, in fact, not be ball shape but may be shaped like bumps or be, very generally, globular in shape. FIG. 1 shows the balls 5 somewhat elongated in shape but slightly truncated at their ends by conductive pads 7 and 9. Thus, the terms “solder balls” or “solder bumps” should not be taken to be limiting in shape but taken to be more as a mass of solder. In this regard, it is clear that connection is not necessarily limited to a C4-type or a flip chip solder connection but may, for example, be a BGA solder interconnect. Typically, solder balls 5 are first attached to conductive pads 7 on substrate 3. Pads 7 may, for example, be copper pads. Chip 1 is then aligned so that its copper pads 9, or other bump limiting metallurgy (BLM) structures, align with solder balls 5.

#As further shown in FIG. 1, a layer of insulating material 11 surrounds and encapsulates solder balls 5. Typically, the chip and substrate pads are aligned to solder balls 5 and then the arrangement heated to reflow the solder to make the connection. After connection is made, an underfill is then dispensed between chip and substrate to provide encapsulation of the solder connections and support therefor.

#Whatever technique is used to make connections and encapsulate same, it is clear that when encapsulated there is little room for expansion of the solder balls or connections on subsequent single or multiple reflow. Subsequent reflow may occur, for example, when there is subsequent attachment to a PCB, where substrate 3 is a single or MCM, or subsequent attachment to a card. It can also occur during preconditioning. This problem is particularly severe for low melt single alloy solders. Typically, the volume expansion associated with high tin content solders in going to the liquid phase is 3 to 6%. However, the problem may exist for any of a variety of solder alloys that exhibit high volume expansion (e.g. >3%) on melting and that will encounter additional reflow (melt) temperatures during assembly or preconditioning of the package.