Ultra mold for encapsulating very thin packages

Abstract

A method of encapsulating a workpiece, particularly a microelectronic device, to achieve a very thin encapsulating layer and reduce the finished device size. The method includes positioning the workpiece in the mold cavity of a mold capable of reducing its volume while the mold compound is in a liquid state from a first volume, where mold compound may be easily added without creating voids, to a second smaller volume which defines the finished workpiece size. The second volume is below the size which would permit the void-free encapsulation of the workpiece in a conventional thermosetting plastic transfer molding machine. The mold may be opened in two stages to prevent damage to thin molded microelectronic devices by opening the perimeter of the mold first while the molded device is still being supported by large molding surfaces. The invention also includes the mold used in the method.

Description

[0001] 1. Field of the Invention[0002] The present invention relates to the encapsulation of thin workpieces, particularly electronic and microelectronic devices, to achieve a very thin, but void-free seal around the workpiece.[0003] 2. Description of Related Art[0004] Microelectronic devices must be protected against moisture as well as assembly process and other environmental contaminants. This is commonly done by encapsulating the device in a mold compound, such as a thermosetting plastic, applied by a transfer molding process.[0005] In a typical transfer molding machine used in the microelectronics industry, a thin electronic workpiece mounted on a lead frame is clamped between two halves of a split mold. The mold defines a mold cavity around the device with sufficient clearance to allow mold compound to be injected and flow around the device to encapsulate it. During the molding process mold compound is injected into an inlet and air inside the mold escapes from a vent.[0006] T...

AI Technical Summary

Benefits of technology

[0016] Yet another object of the present invention to provide a mold for encapsulating an electronic workpiece that can vary in volume from a first volume where mold compound can be injected easily and completely surround the device with a relatively thick coating to a second reduced volume in which only a thin encapsulating coating remains.

[0019] In the preferred method of the invention, the mold has a first volume with a size sufficiently large to permit laminar flow of the mold compound around substantially all sides of the electronic workpiece during the step of adding mold compound to the mold cavity. The second volume has a size less than the size necessary to permit such laminar flow. This allows the mold to produce very thin coatings of a thickness less than would otherwise be possible by conventional transfer molding techniques.

[0020] In the most highly preferred method of the invention, the mold cavity is adapted to receive a substantially planar electronic workpiece and includes a pair of opposed mold surfaces on opposite sides of the device. The volume of the mold cavity is reduced to the second volume by reducing the distance between the pair of opposed mold surfaces of the mold cavity.

[0021] The volume is reduced in one aspect of the method of this invention by providing a tapered clamp cavity defined between two opposed clamp plates having inclined ramp surfaces. The mold is held between the opposed clamp plates and is moved deeper into the tapered clamp cavity to reduce the distance between opposed mold surfaces of the mold cavity.

[0022] The mold compound used in connection with this method is typically a thermosetting plastic. In accordance with the method, the mold compound includes a filler. The filler is typically silica, but other fillers can be used to enhance thermal, electrical or mechanical properties of the mold compound.

Problems solved by technology

However, as the distance between the inner mold surfaces and the electronic workpiece is decreased, it becomes more difficult to obtain a high quality void-free encapsulant around the entire device.

If the mold compound reaches the vent before the mold is completely filled, an air bubble is trapped in the mold, creating a void.

However, as the distance between the upper and lower mold surfaces and the device is reduced, so as to make the encapsulating coating thinner, it becomes more difficult for the mold compound to penetrate these regions.

The result is a void in the encapsulation material as an air bubble is pinched off in the center of the device.

The exact minimum distance limit is, of course, a function of the specific mold compound used, the fillers it contains and process parameters, such as temperature, but, in general, reducing the distance from the inner mold surfaces to the device to less than some minimum distance results in unacceptable manufacturing losses due to the formation of voids.

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