METHOD AND APPARATUS FOR MOLDING PLASTIC PACKAGES

Abstract: The present invention has a top mold, cavity plate and bottom mold for encapsulating a variety of BGA packages using a transfer molding process. The top mold features a network of runner for communicating with at least one spruce. The cavity plate has on the side facing the bottom mold at least one cavity for forming a mold over a substrate sandwiched there between when clamped. On the side of the cavity plate facing the top mold is at least one conical channel for receiving resin from the runner. The tip of the conical channel terminates in a submarine gate disposed in the centre of the cavity for introducing resin therein while minimising wire sweep. The network of runner and combination of conical channels encapsulates BGA packages in a variety of matrix reliably and economically.

Description: METHOD AND APPARATUS FOR MOLDING PLASTIC PACKAGES FIELD OF THE INVENTION The present invention relates to a method and apparatus for molding plastic packages for semiconductors. In particular, the present invention pertains to a method and apparatus for molding ball grid array (BGA) packages reliably and economically. BACKGROUND OF THE INVENTION BGA packages promise to overcome the limitations of conventional packages such as quad flat packs (QFP) in cost, power dissipation, speed and input/output (I/O) pin counts. Crucial to deliver these promises is the electronic industry's ability to mold BGA packages reliably and economically with existing surface mount technologies and equipment. In general, BGA packages are smaller and thinner in physical dimension than the conventional packages of equivalent speed and I/O pin counts. Not surprisingly, non-issues such as wire sweep in conventional packaging technology become significant when BGA packages are adopted for higher speed and I/O pin count applications. Furthermore, the trend towards building tightly interconnected systems such as multichip modules ( CM) continues. CM refers to a package having more than one chip in it, combining several to more than a hundred high- performance silicon ICs with a custom-designed substrate structure. BGA packages works naturally with MCM as the electronic industry have experience assembling standard single-chip module on printed circuit board (PCB) with high yield and low cost. Nevertheless, the prevailing practice of using glob top encapsulation for MCMs detract from the yield and cost advantage of surface mount technologies. OBJECT OF THE INVENTION It is an object of the present invention to encapsulate BGA packages reliably. It is another object of the present invention to encapsulate MCM BGA packages reliably and economically. SUMMARY OF THE INVENTION The present invention has a top mold, cavity plate and bottom mold for encapsulating a variety of BGA packages using a transfer molding process. The top mold features a network of runner for communicating with at least one spruce. The cavity plate has on the side facing the bottom mold at least one cavity for forming a mold over a substrate sandwiched there between when clamped. On the side of the cavity plate facing the top mold is at least one conical channel for receiving resin from the runner. The tip of the conical channel terminates in a submarine gate disposed in the centre of the cavity for introducing resin therein while minimising wire sweep. The network of runner and combination of conical channels encapsulates BGA packages in a variety of matrix reliably and economically. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a cross section view of a cavity down type ball grid array package where the molded or encapsulation resin is shown in dotted lines. FIG. 1B is another cross section view of a cavity up type ball grid array package where the overmold resin is also shown in dotted lines. FIG. 2 is a prior art gate plate molding method for encapsulating plastic packages. FIG. 3 is another prior art removable cavity plate molding method for encapsulating plastic packages. FIG. 4 is yet another prior art globbing method for filling molded resin for plastic packages. FIG. 5 illustrates the warpage of a matrix BGA substrate resulting from the application of globbing method of molding; subsequent trimming of the substrate into the proper die size is made virtually impossible. FIG. 6 is a top, elevational view of the present invention where a BGA matrix substrate may be molded using unique runner and conical channel system to achieve superior results. FIG.7 is a cross section, elevational view of the present invention according to section A-A in FIG. 6. DESCRIPTION OF THE EMBODIMENT OF THE INVENTION A method and apparatus for encapsulating BGA packages is described. In the following description, numerous specific details are set forth such as spruce, runner and mold plate, etc. in order to provide a thorough understanding of the present invention. It will be obvious to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known parts such as those involved with the resin heating and transfer as well as clamping operation of a transfer mold are not shown in order not to obscure the present invention. FIGS 1A and 1B are simplified cross sectional views of the two most common structures of the BGA packages. In FIG. 1A, the BGA package is shown in a cavity down configuration. Typical of this range of BGA package is one taught by U.S. Pat. No. 5,583,378. BGA package 1 comprises an integrated chip (IC) 2 attached to a heat sink 4. Not shown in FIG 1A is the details in which the chip 2 is bonded to the substrate 6. It should be understood by one skilled in the art that chip is either wire bonded or TAB leaded to the substrate 6. An array of solder balls 8 is attached to the underside of the BGA package 1. The area between the dotted line and the chip 2 represents a volume 10 to be encapsulated with resin and is the area of interest of the present invention. The prevailing industry practice employs a dam and fill encapsulation process to protect the chip. First, a rectangular dam is optionally dispensed around the die using a high-viscosity encapsulant. Then, a rotary pump dispenses a spiral fill inside the dam using a low- viscosity encapsulant. See, for example CAM/ALOT liquid dispensing systems Referring to FIG. 1B, BGA package 11 is illustrated in a cavity up configuration. The package 11 incorporates a IC 12 attached to the upper side of a substrate 16, while an array of solder balls 18 is attached to the under side of the substrate 16. Like the BGA package 1 in FIG. 1A, the details of how the IC is bonded to the substrate is omitted. A volume 20 represents the encapsulation resin. Package 11 exemplifies a broad class of plastic BGA structures which was pioneered by Motorola and Citizen; they called it Over Molded Plastic Pad Array Carrier. FIGS. 2 - 4 show graphically prior art molding methods that have been used to encapsulate plastic BGA structures. In FIG. 2, a gate plate molding method for encapsulating plastic packages is illustrated. The package 20 is molded laterally with a gate 22 connected to a cavity 21 (not shown). This approach is described in detail in U.S. Pat. No. 4,442,056 issued to Dusan Slepcevic. In FIG. 3, a removable cavity plate 30 incorporating at least a cavity 21 and also a lateral gate 32 is taught in U.S. Pat. No. 4,332,537 also issued to Dusan Slepcevic for encapsulating plastic packages. Finally, FIG. 4 highlights the most popular method in encapsulating BGA packages: glob top method. For instance, see FIG 6 of U.S. Pat. No. 5,355,283 issued to Marrs et al. Glob top encapsulation fills the BGA die by dispensing epoxy in layers in order to prevent trapping air bubbles inside the resin. The prior art methods of encapsulating plastic packages have a number of limitations which are not easily addressed with existing surface mount technologies and equipment. First, the lateral introduction of encapsulant into the cavity often cause wire sweep problem in high I/O pin counts BGA packages. Second, in context of MCM where testing procedures lack standards, electrical problem caused by wire sweep may exacerbate the reliability of such advanced BGA packages. Third, the glob top approach is time consuming and susceptible to quality control problem. The result of encapsulating a BGA matrix substrate by using a glob top method is illustrated in FIG. 5 where the warpage 42 of the substrate is drastic due to mismatch in the coefficient of thermal expansion of the encapsulant and the substrate. It renders the subsequent trimming of the substrate into individual BGA die difficult or near impossible. FIGS 6 and 7 are top and side elevational view of the present invention for encapsulating plastic packages. The present invention combines applicant/assignee's know-how in transfer molding for gang-pot mold as well as the removable cavity plate molding technique. It should be understood by one skilled in the art that transfer molding distinct from injection molding, especially with respect to the exclusive of thermosetting resins or encapsulants in transfer molding for encapsulating semiconductor devices. In contrast, injection molding equipment suitable for working with thermoplastic materials. Referring to FIGS 6 and 7, the present invention 50 features a top mold 53, a removable cavity plate 55, and a bottom mold 57 for clamping a substrate 66 incorporating a matrix of plastic packages including BGA structures. It should be understood by one skilled in the art that a top mold base 51 and a bottom mold base 59 are usually required to work with the present invention. The mold bases 51 and 59 serve to retain the heat generated by the pot for heating the resin pellets or strips (not shown in the figures). Again in FIG. 6, the top mold 53 of the present invention comprises at under side at least one spruce 54 or cull for receiving and redirecting the heated resin which is compressed by a plunger 52. With respect to the substrate 66 which incorporates a matrix of BGA packages 60 thereon, a multiple plunger or gang-pot mold such as the present invention 50 is recommended. In the preferred embodiment of the present invention, the spruce 54 are coupled to a network of runners 56 over the centre portions of the BGA packages. The runners 56 facilities the flow of resin into the respective cavities below the top mold. As the runners 56 are interconnected, the flow of resin in the runners would be balanced. In FIG. 7, a cross section view of the removable cavity plate 55 is shown interposed between the top mold 53 and the bottom mold 57. On the surface fronting and flushed with the top mold 53 is at least one conical channel 58 for receiving the flow of resin from the runners. The narrow end of the conical channel 58 terminates preferably in a submarine gate which communicates with at least one cavity 60 for the BGA package. Referring again to FIG. 7, clamped between the cavity plate 55 and the bottom mold 57 is the substrate 66. The positioning of the conical channel with respect to the cavity is one of the inventive features of the present invention: the introduction of resin from the centre portion of the cavity rather than laterally ensures the flow of resin within the cavity is even. The flow of resin In fact is in the same direction as the fan out of either the wire bonding or TAB leads; as such, the chances of wire sweep occurring is highly unlikely. Furthermore, the attendant problems associated with the increasing viscosity of the flow of resin within the cavity are also minimised. Overall, the present invention offers the electronic industry all advantages of transfer molding technology without the disadvantages of the conventional cavity molding or plate molding approaches. The process of encapsulating the BGA substrate is as follows: the plunger 52 in FIG. 7 moves upward in the pot 62 and forces heated resin into the spruce 54 and via the runners 56 and conical channels 58 into the cavities 60. When the plunger 52 detects a back pressure from the network of runners, it indicates that the resin has filled the cavities completely. The clamping operation of the present invention terminates when the appropriate curing time for the encapsulant has elapsed. The configuration of the removable cavity plate 55 facilitates the smooth breaking of the runner from the cavities, thus reducing the cycle time in encapsulating the BGA substrates. Although FIGS. 6 and 7 show the encapsulation of a matrix BGA substrate, the present invention may easily be adopted to encapsulate a custom- tailored MCM BGA substrate where the placement of the BGA structure may be random. Thus, the present invention has the flexibility to catered a variety of substrates. While the present invention has been described particularly with reference to FIGS. 1 to 7 with emphasis on a method and apparatus for encapsulating plastic packages with emphasis on BGA packages, it should be understood that the figures are for illustration only and should not be taken a limitation on the invention. In addition, it is clear that the method and apparatus of the present Invention has utility in many applications where packaging of semiconductor devices is required. It is contemplated that many changes and modifications may be made by one of ordinary skill in the art without departing from the spirit and the scope of the invention as described. For instance, the present invention may be applied in a straight forward manner to other substrates such as ceramic, TAB and etc.

Claims: CLAIMS 1. A system for encapsulation plastic packages comprising: a top mold including at least one spruce and at least one runner system for transferring resin from at least one plunger and pot assembly; a bottom mold for supporting a substrate upon which at least one plastic package is to be made; a cavity plate interposed between said told mold and said bottom mold for receiving resin from said runner system, said cavity plate having on its surface facing said top mold at least one conical channel, said channel communicating with at least one cavity for encapsulating said substrate, whereby said system encapsulating said substrate reliably while minimising wire sweep problem within said plastic package. 2. The encapsulating system in claim 1 wherein said cavity plate further terminating at its tip with a submarine gate.

Inventors: CHEW HWEE SENG JIMMY, TAN PENG HUI, CHEW, HWEE, SENG, JIMMY, TAN, PENG, HUI
Applicants: ADVANCED SYSTEMS AUTOMATION LI, ADVANCED SYSTEMS AUTOMATION LIMITED
Application year: 1998
Main classification: B29C45
IPC codes: B29C45, H01L21