BRIEF DESCRIPTION OF THE DRAWINGS
Additional characteristics and advantages of the invention ensue from the description below of the preferred embodiments and from the accompanying drawings, in which:
FIGS. 1A to 1D are side views of a semiconductor device according to a first embodiment of the present invention;
FIG. 1E is a top view of a semiconductor device according to the first embodiment of the present invention;
FIG. 2A is a side view of a semiconductor device according to a second embodiment of the present invention;
FIG. 2B is a top view of a semiconductor device according to the second embodiment of the present invention;
FIGS. 3A to 3D are side views of a semiconductor device according to a third embodiment of the present invention;
FIGS. 4A to 4D are side views of a semiconductor device according to a fourth embodiment of the present invention; and
FIG. 4E is a top view of a semiconductor device according to the fourth embodiment of the present invention.
DETAILED DESCRIPTION
FIGS. 1A to 1E show a semiconductor device having a chip or die 11 provided with solder bumps 12 for attaching the die 11 to a leadframe 13. The leadframe 13 has a central heat sink portion 14, located so as to be centered in the die 11 when the die 11 is attached to the leadframe 13, and pins 15, located at peripheral portions of the leadframe 13. The pins 15 bond with the die 11 so as to electrically connect the die 11 to the leadframe 13. In this embodiment the central heat sink portion 14 is electrically separated from the pins 15. Recesses 16 are provided in the leadframe 13 in the pins 15 at positions where the solder bumps 12 on the die 11 will be bonded to the leadframe 13. The recesses 16 are formed by selectively thinning down the leadframe 13 from the bonding side, typically by patterned etching. The leadframe 13 is thinned by an amount equal to the expected height of the bumps 12 after soldering of the bumps 12 has taken place and the die 11 has been bonded to the leadframe 13.
FIG. 1A shows the device before the die 11 is bonded to the leadframe 13. In FIG. 1 B, the die 11 is brought into contact with the leadframe 13 so as to begin the process of attaching the die 11 to the leadframe 13.
In FIG. 1C the solder bumps 12 are melted. As the bumps 12 melt and collapse downwards, a central section of the die 11 is pressed against the central heat sink portion 14 of the leadframe 13. The heat sink 14 supports the die 11, thus preventing the solder bumps 12 from collapsing too far. It can be seen that the recesses 16 are configured to be the same height as the solder bumps 12 after bump reflow (melting of the solder or solder paste) has taken place. Thus, when the die 11 is attached to the leadframe 13, the bonding surface of the die 11 is flush with the uppermost surface of the leadframe 13 and there is at least partial contact between the bonding surfaces of the die 11 and the leadframe 13. This allows heat to be conducted away from the die 11 through the leadframe 13.
The device may be given added mechanical strength by encapsulating the die 11 and leadframe 13, as shown in FIG. 1 D. The device may be encapsulated in whole or in part by a mold compound 17, for example epoxy resin based or other appropriate plastic material. However, the contact areas of the device at pins 15 are not encapsulated so that they are free to allow electrical connection to the device.
FIGS. 2A and 2B show a semiconductor device having a die 11 attached to a leadframe 13. The leadframe 13 comprises pins 15 provided with recesses 16. The die 11 is provided with solder bumps 12 configured to fit into corresponding recesses 16 when the die 11 is bonded to the leadframe.
In this embodiment, some of the pins 15 extend from a peripheral portion of the leadframe 13 into a central portion so that the pins themselves are used as the heat sink. The recesses 16 are formed in the leadframe 13 by top etching of the leadframe 13. As with the first embodiment, the recesses 16 are configured to be the same height as the solder bumps 12 after melting of the bumps 12 or solder paste has taken place and the die 11 is bonded to the leadframe 13, so that the recesses 16 can receive and accommodate the bumps 12.
FIGS. 3A-3D show an alternative embodiment where the bumps 12 are not soldered to the leadframe but are instead bonded to the leadframe thermo sonically or by thermal compression. In this case the recesses 16 may not completely accommodate the bumps 12, causing an air gap to be left between the die 11 and the leadframe 13. Therefore a thermal conductor 18 is placed between the die 11 and the leadframe 13 to promote thermal conduction between the die 11 and the leadframe 13. The thermal conductor 18 may be a thermal grease, a liquid adhesive or a film adhesive, for example.
In FIG. 3A the thermal conductor 18 is applied to the central heat sink portion 14 of the leadframe. The die 11, being provided with bumps 12, is then brought into position over the leadframe 13, as shown in FIG. 3B, such that the die 11 itself is centered over the central heat sink portion 14 of the leadframe 13 and the bumps 12 are positioned over the recesses 16 provided in the pins 15. In FIG. 3C the bumps 12 are attached to the pins 15 in the recesses 16 by thermal compression or thermo sonic bonding. During that process the die 11 is brought into contact with the conductor 18. A thermal connection between the die 11 and the leadframe 13 is thus established via the thermal conductor 18 that is sufficient to allow heat to be conducted away from the die 11.
The device of this embodiment may also be encapsulated by a mold compound 17 to provide additional mechanical strength. Alternatively, the mold compound 17 may be made from a thermally conducting material and be configured to fill the gap between the die 11 and the central heat sink portion 14 so as to establish a thermal connection between the die 11 and the leadframe 13. In this case the mold compound 17 would replace the thermal conductor 18 as a means of conducting heat away from the die 11 to the leadframe 13.
A further embodiment of the semiconductor device is shown in FIGS. 4A-4E. The die 11 is provided with bumps 12 for attachment to the leadframe 13. However, in this embodiment, the leadframe 13 itself does not act as a heat sink—all thermal conductivity takes place through the bumps 12.
FIG. 4A shows the die 11 positioned over the leadframe 13. Both the central section of the leadframe 13 and the pins 15 in the periphery of the leadframe 13 have been top etched to form recesses 16. The recesses 16 are configured to accommodate bumps 12. In FIG. 4B the die 11 is moved towards the leadframe 13 so that the bumps 12 are brought into contact with the leadframe 13 and in FIG. 4C the bumps 12 are bonded to the leadframe 13 so that they are accommodated in the recesses 16. A thermal and electrical connection is thus established between the die 11 and the leadframe 13. As with the previous embodiments, the recesses 16 are etched so as to have the same height as the bumps 12 after the die 11 is attached to the leadframe 13. In this way, the bonding surface of the die 11 is flush with the uppermost surface of the leadframe 13. However, the central portion of the leadframe 13 does not have to be completely etched. It could also be selectively etched so as to provide for dies having different bump heights in combination with heat sink sections.
The device is then encapsulated by a mold compound 17, except at the contact points, as shown in FIG. 4D.
The invention has been described hereinabove with reference to specific embodiments. However the invention is not limited to these embodiments and no doubt alternatives will occur to the skilled person which fall within the scope of the claims.
For example, the embodiments described where the bumps 12 are soldered to the leadframe 13 could also be realized for the case where the bumps 12 are bonded to the leadframe 13 by an alternative means (for example by thermal compression), and vice versa.
Patent Application
20070273010
