Method and apparatus for underfilling semiconductor devices

#This invention relates generally to liquid dispensing methods and apparatus used in semiconductor package manufacturing and, more particularly, to the underfilling of one or more semiconductor dies carried by a substrate.

#In the microelectronics industry, a die carrying an integrated circuit is commonly mounted on a package carrier, such as a substrate, a circuit board or a leadframe, that provides electrical connections from the die to the exterior of the package. In one such packaging arrangement called flip chip mounting, the die includes an area array of electrically-conductive contacts, known as bond pads, that are electrically connected to corresponding area array of electrically-conductive contacts on the package carrier, known as solder balls or bumps. Typically, the solder bumps are registered with the bond pads and a reflow process is applied to create electrical connections in the form of solder joints between the die and the package carrier. The process of flip chip mounting results in a space or gap between the die and the package carrier.

#The die and the package carrier are usually formed of different materials having mismatched coefficients of coefficient of thermal expansion. As a result, the die and the package carrier experience significantly different dimension changes when heated that creates significant thermally-induced stresses in the electrical connections between the die and the package carrier. If uncompensated, the disparity in thermal expansion can result in degradation in the performance of the die, damage to the solder joints, or package failure. As the size of the die increases, the effect of a mismatch in the coefficient of thermal expansion between the die and the substrate becomes more pronounced. In stacked die packages, the mismatch in coefficient of thermal expansion between the die laminate and the package may be even greater than in single die packages. The failure mechanism in stacked die packages may shift from solder joint damage to die damage.

#The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention.

#FIG. 1 is a schematic view showing an underfilling operation performed with capillary action and with the substrate heated to a uniform temperature in accordance with prior art practices;

#FIG. 2 is a schematic perspective view of a package assembly of a die, shown in phantom, and substrate during an underfilling operation;

#With reference to FIGS. 2 and 3, a semiconductor device package 30 consisting of a die 32 mounted on a package carrier or substrate 34 in a flip chip mounting arrangement is shown. As those skilled in the art will appreciate, substrate 34 may comprise an organic or ceramic substrate material such as a printed circuit board, a flip chip multi-chip module or a flip chip carrier. The die 32 is electrically and mechanically connected to the substrate 34 through an area array of solder bumps 36 on the underside of the die 32 that are registered or aligned with a corresponding area array of solder pads 38 on the substrate 34. Upon heating, the solder pads 38 on the substrate reflow and physically connect with the solder bumps 36 of die 32 to provide mechanical, thermal and electrical coupling therebetween in the form of solder joints. With this mounting arrangement, a gap 40 is formed between a contact side 41 of the die 32 and a top surface 42 of the substrate 34.

#The gap 40 is filled with an encapsulant material 44, such as a liquid epoxy, according to the principles of the present invention. Various different encapsulant materials are suitable for use in the invention, including but not limited to a line of encapsulants commercially available under the HYSOL® tradename from Loctite Corp (Rocky Hill, Conn.). As illustrated in FIG. 2, encapsulant material 44 is provided from an underfill dispenser 35 as an L-shaped bead 45 dispensed onto the surface of the substrate proximate to the gap 40 and on two contiguous sides of the die 32. Although the present invention is described for use with an L-shaped bead, the principles of the invention are applicable to any bead shape, including a single line of encapsulant material 44 disposed along one side edge of die 32, a U-shaped bead of encapsulant material 44 disposed along three side edges of die 32, or other dispensing patterns. The amount of encapsulant material 44 in bead 45 depends upon the desired fillet volume and the under-die volume determined by the size of die 32 and the height tolerances of the solder junctions created between bumps 36 and pads 38.

#The underfill dispenser 35 may take any form readily known in the art for dispensing liquid encapsulant or underfill material in a desired pattern relative to the die 32. One suitable underfill dispenser 35 is the DP-3000 pump commercially available from Nordson Asymtek (Carlsbad, Calif.).