Method and system for sealing the edge of a PBGA package

A system and method for providing plastic ball grid array ("PBGA") packages is disclosed. In one aspect, the method and system provide a plurality of PBGA packages. Each of the plurality of PBGA packages including a semiconductor die and a portion of a substrate. The semiconductor die is electrically coupled to the portion of the substrate. The portion of the substrate has an edge. In this aspect, the method and system include forming the plurality of PBGA packages on the substrate and separating the portion of the substrate for each of the plurality of PBGA packages. The portion of the substrate for one of the plurality of PBGA packages is separated from the portion of the substrate for another of the plurality of PBGA packages by a gap. In a preferred embodiment, the gap is created by punching the substrate. The method and system further include filling the gap with a moisture sealant and cutting the moisture sealant to separate the plurality of PBGA packages. The edge of the portion of the substrate for each of the plurality of PBGA packages is substantially covered by the moisture sealant. In another aspect, the method and system include providing a semiconductor die, a substrate having an edge, and a moisture sealant. The semiconductor die is electrically coupled to the substrate. The substrate has an edge. The moisture sealant substantially covers the edge of the substrate.

FIELD OF THE INVENTION
 The present invention relates to plastic ball grid array packages and more
 particularly to a method and system for sealing the edges of the substrate
 of the packages, thereby providing improved reliability and performance of
 the package.
 BACKGROUND OF THE INVENTION
 Conventional plastic ball grid array ("PBGA") packages are used in a
 variety of semiconductor applications. Both micro-BGA and PBGA packages
 are currently available. Conventional micro-BGA packages are chip-scale
 packages. As their name suggests, conventional micro-BGA packages are
 significantly smaller than PBGA packages.
 Conventional micro-BGA packages are formed on a tape substrate.
 Semiconductor dies are attached to the tape. Electrical connection is made
 between the dies and metal traces within the tape. The gaps between
 semiconductor dies are then filled with an adhesive encapsulant. The
 adhesive encapsulant aids in sealing and protecting the edges of the die.
 The tape is then cut between the dies, forming micro-BGA packages.
 Conventional PBGA packages are significantly larger than conventional
 micro-BGA packages. Because of the difference in size, conventional PBGA
 packages are typically manufactured using very different processes and
 materials than micro-BGA packages. The conventional PBGA package includes
 a semiconductor die attached to a substrate. However, the substrate is
 similar to a printed circuit board. Typically, the substrate used is a BT
 (Bismaliemide Triazine) substrate. The substrate not only provides a
 relatively stiff surface to which the semiconductor die can be attached
 but also electrically couples the die to the solder balls. Consequently,
 the substrate includes conductive traces with insulating layers
 interspersed between the conductive traces.
 Contacts on the semiconductor die are typically electrically coupled to the
 metallic traces in the substrate through wires bonded to a contact on the
 surface of the substrate. The semiconductor die is also typically covered
 in a molding compound, such as an epoxy. The molding compound aids in
 protecting the die from the environment and contributes to the robustness
 of the package. Solder balls on a side of the substrate opposite to the
 die can electrically connect the PBGA package to another circuit.
 Typically, conventional PBGA packages are formed by attaching a number of
 dies to a long strip of substrate that is designed to accommodate the
 dies. The dies are then electrically coupled and molded to the substrate.
 After the PBGA packages are substantially formed, the substrate is cut to
 separate the conventional PBGA packages. The conventional PBGA packages
 may then be used in other circuits.
 Although conventional PBGA packages are useful for many applications, the
 PBGA package may be prone to failure. For example, moisture may enter the
 substrate, and adversely affect the reliability of the PBGA package.
 Accordingly, what is needed is a system and method for improving the
 reliability of the substrate in a PBGA package. The present invention
 addresses such a need.
 SUMMARY OF THE INVENTION
 The present invention comprises a system and method for providing plastic
 ball grid array ("PBGA") packages. In one aspect, the method and system
 provide a plurality of PBGA packages. Each of the plurality of PBGA
 packages includes a semiconductor die and a portion of a substrate. The
 semiconductor die is electrically coupled to the portion of the substrate.
 The portion of the substrate has an edge. In this aspect, the method and
 system comprise forming the plurality of PBGA packages on the substrate
 and separating the portion of the substrate for each of the plurality of
 PBGA packages. The portion of the substrate for one of the plurality of
 PBGA packages is separated from the portion of the substrate for another
 of the plurality of PBGA packages by a gap. This gap may be created by a
 punching or cutting process. The method and system further comprise
 filling the gap with a moisture sealant and sawing along the center of the
 gap (now filled with the moisture sealant) to separate the plurality of
 PBGA packages. The edge of the portion of the substrate for each of the
 plurality of PBGA packages is substantially covered by the moisture
 sealant. In another aspect, the method and system comprise providing a
 semiconductor die, a substrate having an edge, and a moisture sealant. The
 semiconductor die is electrically coupled to the substrate. The substrate
 has an edge. The moisture sealant substantially covers the edge of the
 substrate.
 According to the system and method disclosed herein, the present invention
 seals the edge of the substrate of the PBGA packages, thereby increasing
 reliability of the PBGA package.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention relates to an improvement in semiconductor packages.
 The following description is presented to enable one of ordinary skill in
 the art to make and use the invention and is provided in the context of a
 patent application and its requirements. Various modifications to the
 preferred embodiment will be readily apparent to those skilled in the art
 and the generic principles herein may be applied to other embodiments.
 Thus, the present invention is not intended to be limited to the
 embodiment shown but is to be accorded the widest scope consistent with
 the principles and features described herein.
 FIG. 1A is a diagram of a conventional plastic ball grid array ("PBGA")
 package 10. The conventional PBGA package 10 includes a semiconductor die
 14, a substrate 20, wire bonds 18, a molding compound 16, and a plurality
 of solder balls 12. The semiconductor die 14 is electrically coupled to
 the substrate 20 through the wires 18. The molding compound 16 protects
 the die 14 from the environment. Although not shown, micro-BGA packages
 are also currently available. Conventional micro-BGA packages are
 significantly smaller than PBGA packages. For example, conventional PBGA
 packages are typically on the order of centimeters on each side.
 Conventional micro-BGA packages, however, are only slightly larger than
 the die itself.
 Because of the difference in size, conventional PBGA packages are typically
 manufactured using very different processes and materials than micro-BGA
 packages. Conventional micro-BGA packages are formed on a tape substrate.
 Electrical connection is made between the dies and metal traces within the
 tape. The gaps between semiconductor dies are then filled with an adhesive
 encapsulant. The adhesive encapsulant aids in sealing and protecting the
 edges of the die. The tape is then cut between the dies, forming micro-BGA
 packages.
 In contrast, the substrate 20 of the conventional PBGA package 10 is
 similar to a printed circuit board. FIG. 1B depicts a cross sectional view
 of an edge of a portion of the substrate 20. The substrate 20 includes
 metal traces 21, 23, and 25. The substrate 20 also includes insulating
 layers 22, 24, 26, and 28. In a preferred embodiment, the insulating
 layers 22, 24, 26, and 28 are actually composed of polymer fibers in an
 epoxy. Also in a preferred embodiment, the metal traces 21, 23, and 25 are
 copper layers. Moreover, although not shown, there may be electrical
 connection made between one or more of the metal traces 21, 23, and 25 and
 another metal trace 21, 23, or 25, one or more of the solder balls 12, and
 the semiconductor die 14. The solder balls 12 are shown on the lower
 surface of the substrate 20. The metal traces 21, 23, and 25 provide
 electrical connection between the semiconductor die 14 and a portion of
 the solder balls 12.
 FIG. 2 depicts a conventional method 50 for fabricating conventional PBGA
 packages 10. Several individual PBGA packages are formed on a single large
 substrate strip via step 52. Thus, several semiconductor dies 14 are
 affixed to the substrate 20 and electrically coupled to the substrate 20.
 The substrate 20 is then cut, via step 54. As a result, several individual
 PBGA packages 10 are fabricated.
 Step 54 is depicted in FIG. 3. As depicted in FIG. 3, two cuts 30 and 32
 have been made in the substrate strip 40. Thus, two conventional PBGA
 packages 10 have been separated from the substrate strip 40. Thus, the
 substrate 20 for each PBGA package 10 is a portion of the substrate strip
 40 shown in FIG. 3.
 Although the method 50 is capable of providing conventional PBGA packages
 10, one of ordinary skill in the art will realize that conventional PBGA
 packages 10 can be unreliable. It has been determined that when the
 conventional PBGA package 10 is subjected to certain stress tests, the
 conventional PBGA package 10 fails. Substrates absorb moisture, which can
 lead to leakage failures, corrosion, delamination, and other problems.
 Referring back to FIG. 1B, it is hypothesized that failure of electrical
 connection through the substrate 20 could be due to corrosion and/or
 delamination of some of the metal traces 21, 23, or 25 within the
 substrate 20. It is also hypothesized that the moisture causing the
 corrosion and/or delamination of some of the metal traces 21, 23, or 25
 enters through the edge of the substrate 20.
 The substrate 20 includes metal traces 21, 23, and 25 as well as insulating
 layers 22, 24, 26, and 28. The layers are exposed at the edge of the
 substrate 20 after the cut is made in step 54 of the method 50. Moisture
 may penetrate the edge of the substrate 20 for each PBGA package 10.
 Because the substrate 20 includes metal traces 21, 23, and 25 and
 insulating layers 22, 24, 26, and 28 the penetration of moisture is
 enhanced. This is because moisture penetrates at a higher rate along a
 joint between two materials. As a result, moisture may penetrate into the
 substrate 20 and corrupt the performance of the conventional PBGA package
 10.
 The present invention provides for a method and system for providing
 plastic ball grid array ("PBGA") packages. In one aspect, the method and
 system provide a plurality of PBGA packages. Each of the plurality of PBGA
 packages includes a semiconductor die and a portion of a substrate. The
 semiconductor die is electrically coupled to the portion of the substrate.
 The portion of the substrate has an edge. In this aspect, the method and
 system comprise forming the plurality of PBGA packages on the substrate
 and cutting or punching the substrate to separate the portion of the
 substrate for each of the plurality of PBGA packages. This cutting or
 punching process creates gaps. The portion of the substrate for one of the
 plurality of PBGA packages is separated from the portion of the substrate
 for another of the plurality of PBGA packages by the gap. The method and
 system further comprise filling the gap with a moisture sealant and sawing
 along the center of the gap (now filled with moisture sealant) to separate
 the plurality of PBGA packages. The edge of the portion of the substrate
 for each of the plurality of PBGA packages is substantially covered by the
 moisture sealant. In another aspect, the method and system comprise
 providing a semiconductor die, a substrate having an edge, and a moisture
 sealant. The semiconductor die is electrically coupled to the substrate.
 The substrate has an edge. The moisture sealant substantially covers the
 edge of the substrate.
 The present invention will be described in terms of a PBGA package using a
 particular substrate and a particular moisture sealant. However, one of
 ordinary skill in the art will readily recognize that this method and
 system will operate effectively for other types of substrates and other
 sealants.
 To more particularly illustrate the method and system in accordance with
 the present invention, refer to FIG. 4A, which depicts one embodiment of a
 method 100 for providing a PBGA package in accordance with the present
 invention. Individual PBGA packages are substantially formed on a
 substrate via step 102. Preferably, the substrate used is similar to a
 printed circuit board. In one preferred embodiment, the substrate is BT
 (Bismaliemide Triazine). FIG. 4B depicts one embodiment of the step 102.
 In one embodiment, step 102 includes placing a plurality of semiconductor
 dies on the substrate and attaching the dies to the substrate, via step
 110. Electrical connection is made between the semiconductor dies and the
 portion of the substrate to which each semiconductor die is attached, via
 step 112. In one embodiment, step 112 is performed by wirebonding contacts
 on each semiconductor die to contacts on the portion of the substrate on
 which the semiconductor die is placed. The semiconductor dies are then
 molded to the substrate, via step 114. Thus, the semiconductor dies are
 fixed to the substrate.
 Referring back to FIG. 4A, once the PBGA packages have been substantially
 formed in step 102, the individual PBGA packages are substantially
 separated, via step 104. In a preferred embodiment, the substrate is
 punched in step 104. Punching the substrate in step 104 separates the
 individual PBGA packages except for small portions in two corners of each
 package. However, in an alternate embodiment, the substrate may be cut in
 step 104. Each PBGA package includes a portion of the substrate. After
 step 104 is performed, the edge of the portion of the substrate for one
 PBGA package is separated from the portion of the substrate for another
 PBGA packages by a gap. A substrate which has been punched is depicted in
 FIG. 5. The outer edge of each PBGA package 200 is the edge of the portion
 of the substrate for that PBGA package 200. The PBGA packages 200 are
 separated by gaps 201, 202, and 203 and connected at two corners. However,
 in an alternate embodiment, each PBGA package 200 is separate after step
 104 is performed. Therefore, the PBGA packages 200 of the alternate
 embodiment are not connected at the corners.
 Referring back to FIG. 4A, the gaps 201, 202, and 203 are then filled with
 an adhesive moisture sealant, via step 106. In a preferred embodiment, the
 moisture sealant is a liquid. Thus, step 106 may also include curing the
 adhesive moisture sealant so that the adhesive moisture sealant
 solidifies. In a preferred embodiment, the adhesive moisture sealant is
 baked in order to accelerate the curing process. Once the gaps are filled
 with the adhesive, then the PBGA packages are cut again, via step 108.
 However, the cut in step 108 is performed so that some adhesive remains at
 the edges of the PBGA packages. Thus, in a preferred embodiment, the cut
 in step 108 is performed substantially at the center of the gap to ensure
 that some of the adhesive moisture sealant remains at the edges of the
 PBGA packages. Referring to FIG. 5, the cut performed in step 108 is
 preferably performed along dashed lines 250, 252, and 254.
 FIG. 6 depicts one embodiment of a PBGA package 200 in accordance with the
 present invention. The PBGA package 200 includes a semiconductor die 214
 on a portion of the substrate 210. The portion of the substrate 210 was
 separated from the remainder of the substrate (not shown) in step 104. On
 an opposing surface, the portion of the substrate 210 is connected with a
 plurality of solder balls 212. The portion of the substrate 210 includes
 metal and insulating layers (not shown). The semiconductor die 214 is
 electrically coupled to the portion of the substrate 210 by wires 218. The
 semiconductor die 214 and wires 218 are surrounded by molding compound
 216.
 Because steps 106 and 108 have been performed, the PBGA package 200 also
 includes adhesive 220 at the edges of the portion of the substrate 210.
 The adhesive 220 seals the edges of the portion of the substrate 210.
 Consequently, the penetration of moisture into the substrate is greatly
 reduced. Corrosion and delamination of metal traces within the portion of
 the substrate 210 is also reduced. Reliability of the PBGA package 200 is,
 therefore, enhanced.
 A method and system has been disclosed for providing a PBGA package having
 reduced penetration of moisture into the substrate. Although the present
 invention has been described in accordance with the embodiments shown, one
 of ordinary skill in the art will readily recognize that there could be
 variations to the embodiments and those variations would be within the
 spirit and scope of the present invention. Accordingly, many modifications
 may be made by one of ordinary skill in the art without departing from the
 spirit and scope of the appended claims.