Method of making package containing semiconductor devices and desiccant
1. A method of making a package, comprising: preparing a tray that retains a desiccant; providing one or more semiconductor devices on the tray; heating the tray; and sealing the tray in a moisture-proof container.
2. The method according to claim 1, wherein the tray and the desiccant retained therein are recovered after use.
3. A method of reusing a desiccant, comprising: recovering a tray in which a desiccant is retained; providing one or more semiconductor devices on the tray; heating the tray; and sealing the tray in a moisture-proof container.
4. A tray for containing semiconductor devices, comprising: a plurality of pockets for accommodating one or more semiconductor devices having individual packages; and a retainer that retains a desiccant, wherein the tray sustains enough heat to substantially remove any moisture absorbed by the individual packages.
5. The tray according to claim 4, wherein the retainer includes a groove in which the desiccant is placed, and a cover to retain the desiccant in the groove.
 1. Field of Invention
 The present invention relates to a method of making a package containing semiconductor devices and desiccant.
 2. Description of Related Art
 Semiconductor devices, which are individually packaged, are subjected to electrical testing and lead inspection. The devices are then heat-treated at, for example, 125.degree. C. for 16 to 24 hours to remove moisture that may be absorbed by the individual package before being shipped, for instance, to customers. Such heat-treatment is necessary at least when the individual package is a plastic package, because plastic material is susceptible to absorb moisture from the air. Removing the moisture allows to prevent separation and/or cracking of the individual package which may result from the rapid expansion of water inside the individual package during mounting of the semiconductor devices onto print-circuit boards.
 Although moisture can be removed from the individual package by heat treatment before, for instance, shipping, moisture may be re-absorbed by the individual package during the course of transportation of the semiconductor devices.
 To overcome this problem, magazines, trays, or tapes containing the individually packaged semiconductor devices are sealed in a moisture-proof container (e.g., a bag made of a film having a low moisture permeability). In addition, desiccant is put into the container together with the semiconductor devices thereby to make a moisture-proof package containing semiconductor devices and desiccant. The desiccant is included in the package so that, if the moisture-proof package is stored for a long period of time, or if the hermetic seal of the moisture-proof bag is defective, the elements present inside the moisture-proof package are kept moisture-free. Such a moisture-proof package containing semiconductor devices and desiccant is described in U.S. Pat. No. 4,971,196, which is incorporated by reference in its entirety.
 According to this conventional method, the desiccant, which is assumed to be effective in keeping the elements inside the moisture-proof package moisture-free, is prepared separately from the semiconductor devices. When the desiccant is left in a humid environment for a long period of time prior to being incorporated in the package with, for instance, semiconductor devices, however, it loses some of its moisture absorbing properties. As a result, it becomes difficult to ensure that the elements inside, i.e., the semiconductor devices, are kept moisture-free.
 In other words, additional care must be taken in order to keep the desiccant in a dry environment before it is put into the moisture-proof package, in order to ensure a moisture-free environment inside the package.
 Moreover, in this conventional method, the desiccant is discarded after a single use, thereby increasing the burden for proper waste control because large quantities of desiccant are discarded.
SUMMARY OF THE INVENTION
 An object of this invention is to provide a method of making a package, for instance, containing semiconductor devices, that ensures a moisture-free environment inside the package, without any necessity for additional care in the preparation of the desiccant. Another object of this invention is to reduce the burden of waste control due to frequent discard of desiccant.
 To achieve such objects, an exemplary embodiment according to this invention provides a method of making a package, which includes preparing a tray that retains a desiccant, placing semiconductor devices on the tray, heating the tray, and sealing the tray in a moisture-proof package.
 An exemplary embodiment according to this invention also provides a method of reusing a desiccant, which includes recovering a tray in which a desiccant is retained, providing semiconductor devices on the tray, heating the tray, and sealing the tray in a moisture-proof container.
 Another exemplary embodiment according to this invention provides a tray for containing semiconductor devices and which includes a plurality of pockets for accommodating one or more semiconductor devices having individual packages, and a retainer that retains a desiccant wherein the tray sustains enough heat to substantially remove any moisture absorbed by the individual packages.
BRIEF DESCRIPTION OF THE DRAWINGS
 Various exemplary embodiments of the methods of this invention will be described in detail, with reference to the following figures, wherein:
 FIG. 1 is an illustration of an exemplary tray used in a method of making a package containing semiconductor devices and desiccant according to an exemplary embodiment of this invention;
 FIG. 2 is a cross-sectional view of an exemplary tray according to an exemplary embodiment of the present invention; and
 FIG. 3 is a perspective view showing a hermetically sealed moisture-proof bag containing a plurality of trays.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the methods according to this invention.
 This invention was first described in Japanese Patent Application No. 2002-304458, which is incorporated herein by reference in its entirety.
 A method of making a package containing semiconductor devices and desiccant according to an exemplary embodiment of this invention will be described with reference to the drawings.
 FIGS. 1 and 2 show a tray, that may be used to contain semiconductor devices and desiccant according to an exemplary embodiment of this invention. FIG. 1 is a plan view of the tray and FIG. 2 is a magnified cross-sectional view taken along one end of the tray.
 As shown in FIG. 1 and according to an exemplary embodiment of the present invention, a rectangular tray 1 has long sides 1a, short sides 1b, and desiccant retainers 2 for retaining desiccant. The desiccant retainers 2 are disposed at both ends of the long side 1a. The center portion of the tray 1 between the desiccant retainers 2 has pockets 3 for accommodating, or for insertion of, semiconductor devices. An opening 3a, smaller than the typical size of the individual package of the semiconductor device, is formed in the bottom of each pocket 3.
 The tray 1 is made of heat-resistant conductive material, such as the material described in laid-open Japanese patent application No. Hei 9-123239, which is incorporated herein by reference in its entirety. This laid-open patent describes a heat resistant tray formed by injection molding of heat-resistant resin such as PPE (poly-phenylene-ether) with a conductive filler such as carbon fibers or carbon black.
 Each desiccant retainer 2 has a groove 6 (not shown in FIG. 1) extending along the short side 1b of the tray 1. The groove is covered by a cover 5 having openings 5a arranged in a grid.
 Referring now to the cross-sectional view described in FIG. 2, the desiccant retainer 2 has a groove 6, in which an amount of silica gel 9 is placed. Protrusions 8 project from the two sidewalls of the groove 6. The cover 5 having the openings 5a is brought into engagement with the protrusions 8. The opening 5a is smaller than the particles of silica gel 9, therefore the silica gel particles 9 are securely retained in the groove 6 once the cover 5 is brought into engagement with the protrusions 8.
 In other exemplary embodiments of this invention, the placement and structure of the retainer may be modified. For example, the cover 5 may be made to fit snugly on the opening of the groove 6. Furthermore, a heat resistant adhesive may be used as the retainer to retain the silica gel particles 9 in the groove 6.
 Before the tray 1 is used to ship semiconductor devices, the desiccant retainers are filled with an appropriate amount of desiccant that, in an exemplary embodiment of this invention, may be silica gel particles. Alternatively, the tray 1 may be purchased from a tray manufacturer with desiccant already present in the retainer. In another exemplary embodiment of this invention, if the tray 1 is recovered from a customer and still contains desiccant 9 in the retainer 2, it is not necessary to re-fill the desiccant retainer 2 with desiccant particles 9.
 Semiconductor devices that are encapsulated in individual plastic packages are subjected to electrical testing and lead inspection, and then are placed in the pockets 3 of the tray 1, which carry the desiccant which, in an exemplary embodiment of this invention, is silica gel. In an exemplary embodiment of this invention, the tray 1 carrying the semiconductor devices and the silica gel 9 is heat treated at, for example, 125.degree. C. for approximately six hours. Upon completion of the heat treatment, the trays designed to accommodate semiconductor devices and desiccant are stacked, bound, and sealed in a moisture-proof bag.
 FIG. 3 is a perspective view showing a package that contains a plurality of trays that carry semiconductor devices and desiccant (not shown).
 In an exemplary embodiment of this invention, the air inside the moisture-proof bag 10 containing the trays is removed and the moisture-proof bag 10 is hermetically sealed. Consequently, a moisture-proof package 10 containing semiconductor devices and desiccant is made.
 The heat treatment before the sealing of the moisture-proof package 10 contributes to substantially removing any moisture absorbed by the individual package. That is, the moisture absorbed in the plastic individual package is sufficiently removed to an extent that prevents separation and/or cracking of the individual package during mounting onto a print-circuit board. Furthermore, in various exemplary embodiments of this invention, because the desiccant 2 is integral with the tray 1 and is heat-treated together with the semiconductor devices in the tray 1, the desiccant 9 sealed in the moisture-proof package 10 together with the semiconductor devices possesses high moisture absorption capability. Therefore, the environment inside the moisture-proof bag is kept moisture-free even when the moisture-proof package is stored for a long period of time in a humid environment, or the hermetic seal of the bag is defective.
 In other words, the moisture-free environment inside the moisture-proof package is ensured without any additional care being necessary to keep the desiccant 9 in a dry environment prior to the use of the desiccant 9.
 In various embodiments of this invention, the moisture-proof package 10 is packed in a cardboard box or the like before being shipped to a customer.
 The trays holding the semiconductor devices that are sent to customers are collected after the semiconductor devices arc removed from the trays. Because the trays are collected with silica gel particles still retained in the desiccant retainers, they can be reused. That is, the same method of making the moisture-proof package including placing semiconductor devices on the recovered tray, heat treating the semiconductor devices and the desiccant retained by the tray 1, and sealing the heat treated semiconductor devices together with the heat treated desiccant 9 in a moisture-proof container 10 can be repeated.
 The recovered tray may be used to ship the same semiconductor devices to the same or different customers, but also may be used to ship different semiconductor devices to the same or different customers.
 Because the desiccant 9 and the tray 1 can be used repeatedly, the amount of chemical waste stemming from the desiccant particles 9 being discarded after each use can be reduced. That is, even if the desiccant 9 loses its moisture-absorbing ability after repeated heat treatments and uses, and should be discarded after a certain number of times of reuse, the total amount of waste can be significantly decreased compared to a conventional use. Therefore, the burden of waste control is significantly reduced. The costs incurred from purchasing the desiccant will also be significantly reduced because the amount of desiccant 9 to purchase will be greatly reduced.
 While this invention has been described in conjunction with the exemplary embodiments outlined above, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent to those having at least ordinary skill in the art. Accordingly, the exemplary embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention. Therefore, the claims as filed and as they may be amended are intended to embrace all known or later-developed alternatives, modifications variations, improvements, and/or substantial equivalents.
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