BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:
FIG. 1 is a schematic sectional view illustrating a TSOP (thin small outline package) type two-layered stack package using a conventional lead frame;
FIG. 2 is a photograph of a sectional view of a lead showing an example of the damage a plating layer can sustain after upper and lower leads are connected by solder dipping in the conventional stack package illustrated in FIG. 1;
FIGS. 3A and 3B are views illustrating a lead with the plating layer lost and damaged that is poorly connected to a connection pad of the semiconductor module by soldering according to the conventional art;
FIG. 3C is a photograph of a sectional view showing an example of the damage in the leads illustrated in FIGS. 3A and 3B;
FIG. 4 is a photograph of a sectional view showing a lead frame type stack package according to an embodiment of the present invention;
FIG. 5 is a photograph of a sectional view showing the lead after upper and lower leads are connected by solder balls in the stack package shown in FIG. 4;
FIG. 6 illustrates a soldering process using laser according to an embodiment of the present invention;
FIGS. 7A and 7B illustrate a lead having a plating layer formed thereon and connected to a connection pad of a semiconductor module by soldering according to an embodiment of the present invention; and
FIG. 7C is a photograph showing the lead illustrated in FIGS. 7A and 7B.
DETAILED DESCRIPTION
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout the specification.
FIG. 4 is a photograph of a sectional view showing a lead frame type stack package according to an embodiment of the present invention. The stack package of FIG. 4 is a two-layered package composed of a lower package and an upper package. Each package is a thin small outline package (TSOP) type package. Leads 33 and 43 are extended from bodies 31 and 41. The lead 33 of the upper package 30 is connected to the lead 43 of the lower package 40 by a solder ball 53. The solder ball 53 may be formed using laser soldering, and has a diameter similar to that of each of the leads 33 and 43. The bodies of the upper package 30 and the lower package 40 may be bonded by an adhesives 51 disposed therebetween. Since the lead 33 of the upper package 30 and the lead 43 of the lower package 40 are connected by a solder ball 53, the loss of a lead plating layer, which may be generated by the connection using solder dipping, does not occur.
FIG. 5 is a photograph of a sectional view showing a lead 43 of a lower package 40 after the lead is connected by a solder ball in the stack package shown in FIG. 4. As shown in FIG. 5, a plating layer 43b completely covers a metal alloy portion 43a. The metal alloy portion 43a may be composed of alloy having a good electric conductivity and comprising copper (Cu) or gold (Au) as main elements. The plating layer 43b may be composed of metal such as SnAgCu, in which tin (Sn) is a main element.
A stack package is mounted on the semiconductor module by the lead 43 of the lower package being connected to a connection pad of a semiconductor module with solder. In the current embodiment, since the plating layer 43b, which functions to help the wetting of the solder, is not lost, the solder is well wetted to the lead 43 of the lower package 40 so that the solder connection is successfully made.
In the current embodiment, the TSOP type stack package has been explained, but the present invention can be also employed to other types of packages using a lead frame, for example, a quad flat package (QFP). Further, the current embodiment has explained a two-layered stack package, but the present invention can be also employed to a three-layered or more stack package. In addition, a unit package of the stack package may comprise a single semiconductor chip, or may comprise a plurality of semiconductor chips.
A method of fabricating the lead frame type stack package as shown in FIG. 5 will be explained as follows. Lead frame type packages such as TSOP are stacked up and down. At this time, the leads of the upper package and the leads of the lower package are arranged to respectively contact each other. The bodies of the upper package and the lower package may further be bonded by adhesives. Then, the leads of the upper package and the leads of the lower package are connected using laser soldering. Three or more lead frame type packages may be stacked using the laser soldering. That is, another package may be stacked on the upper package of the stack package with the leads of the upper package being connected to the leads of the other package by laser soldering. Additional packages may be further stacked on the other package in a similar manner.
FIG. 6 illustrates a laser soldering process according to an embodiment of the present invention. Referring to FIG. 6, a solder ball 80 comes out from a nozzle of a laser soldering head 91 and is disposed at a connection point between the upper and lower leads 33 and 43. A laser beam is irradiated to the solder ball 80 supplied from the side 93 of the laser soldering head 91 to the nozzle of the laser soldering head 91 to melt the solder ball 80. The melted solder ball 80 is pulled out of the nozzle by exhaust gas such as an inert gas. The laser beam may use a Nd:YAG laser, for example. While the solder ball 80, which comes out to the connection of the leads and is melted, is cured, the leads 33 and 43 of the upper and lower packages 31 and 41 can be connected.
At this time, the laser soldering head 91 may be placed at any direction, for example, vertically or tiltedly. And also, the stack package may be placed at any direction.
According to the present invention, when the lead bonding method using the solder ball 80 is used, the problem of losing the plating layer is solved since the plating layer of the lead does not diffuse into a soldering pot.
FIGS. 7A and 7B illustrate a lead having a plating layer formed thereon and connected to a connection pad of a semiconductor module by soldering according to an embodiment of the present invention. FIG. 7C is a photograph showing the lead illustrated in FIGS. 7A and 7B. Referring to FIGS. 7A and 7B, a lead 43 of a stack package is placed on a connection pad 73 of a semiconductor module, which is coated with a soldering powder 55 including a metal powder, such as SnAgCu. When heat is applied to reflow the soldering powder 55, the soldering powder 55 is spread along the surface of a plating layer 43b of the lead 43 so as to form a fillet 55′. When the plating layer 43b is well formed on the surface of the lead 43, the fillet 55′ connecting the lead 43 to the connection pad 73 is uniformly formed along the plating layer so that a stack package is firmly connected to the connection pad of the semiconductor module. Hence, the stack packages are firmly connected in the module, thereby increasing the reliability of the products that contain these stacked packages.
The lead frame type stack package according to embodiments of the present invention is formed by connecting the leads of the upper package and the leads of the lower package using laser soldering. Since the leads of the upper and lower packages are connected using solder balls rather than being immersed into a soldering pot, loss of the plating layer of the lead caused due to solder dipping can be prevented. Thus, the leads of the lower package can be well connected to the connection pad of the semiconductor module without soldering defects.
While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.
Patent Application
20080079128
