Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2000-07-21
2001-08-07
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S613000, C438S614000, C438S615000
Reexamination Certificate
active
06271109
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related in general to the field of semiconductor devices and processes and more specifically to structures, materials and fabrication of substrates to be used in surface mount assembly of semiconductor devices.
DESCRIPTION OF THE RELATED ART
In order to successfully attach a leaded semiconductor surface mount device onto a board by soldering, all the device leads have to touch the board surface simultaneously, or at least be within a certain small distance from that surface; they have to exhibit “coplanarity”. The coplanarity is a function of the lead pitch. As an example of industry practice, for a lead pitch of 0.65 mm, the acceptable coplanarity is 0.1 mm, representing a tolerance window of ±0.05 mm. For a lead pitch of 0.3 mm, the acceptable coplanarity is only 0.05 mm (for comparison, the diameter of a human hair falls into the 0.1 to 0.3 mm range).
With the advent of the Ball Grid Array (BGA) package for semiconductor devices, the coplanarity of leaded devices is no longer an issue, since the leads are replaced by solder “balls” for surface mount assembly. However, in plastic BGA's, the overall packages are usually somewhat flexible, since they are composed of flexible materials. There is typically a significant difference in the coefficients of thermal expansion between the silicon chip, the plastic substrate used for chip mounting, and the encapsulation material (commonly a plastic molding compound). Consequently, in processes at elevated temperatures, the package may slightly warp and then represent, as a whole package, coplanarity problems in subsequent assembly processes. Due to the warping and coplanarity problem, only a limited number of solder balls attached to the warped package surface will contact the board in assembly, while a substantial number of solder balls will not contact the board surface and will not be able to form solder joints in solder reflow attach processes.
Typically, assembly and packaging processes at elevated temperatures include:
Transfer molding at 175° C. in less than 1 minute.
Polymerization of the molded device at 175° C. for up to six hours in “cure” ovens.
Reflow of attached solder balls or bumps. Typically, solder bumps are reflowed in chain type furnaces at temperatures dependent on the melting of the solder mixture (typically between about 150 and 250° C.).
After these temperature treatments, plastic BGA packages may exhibit warping to the extent that uniform solder ball attachment onto substrates is difficult, if not outright impossible. The resultant coplanarity problems are particularly pronounced for BGA packages using plastic films or other thin plastic materials as supporting parts. As a consequence, serious yield losses and reliability problems have been encountered in board attach processes of plastic BGA's.
The proposal to remedy the coplanarity problem by using an array of solder balls having different diameters dependent on the location on the package, is completely impractical; in addition, the degree of warping varies with device type, size and materials of packages, thermal process history, and so on. As an example, in BGA's having a convex warping, this proposal would require smaller diameter solder balls for the center portion of the package and larger diameter balls for the peripheral portions—a proposition which mass manufacturing would have great difficulties in handling.
An urgent need has therefore arisen for a low-cost and reliable approach, involving both package and board structures and the assembly fabrication method, to provide uniform board attachment of warped plastic Ball Grid Array packages, with the goal of assembling the plastic BGA device substantially parallel to the substrate. The structure of the substrate and the assembly method should be flexible enough to be applied for different semiconductor product families and a wide spectrum of design and assembly variations, and should achieve improvements towards the goals of enhanced process yields and device reliability. Preferably, these innovations should be accomplished using the installed equipment base so that no investment in new manufacturing machines is needed.
SUMMARY OF THE INVENTION
According to the present invention for a semiconductor integrated circuit (IC) assembly, a Ball Grid Array (BGA) package with the solder balls arrayed on a warped surface can be attached substantially parallel onto a flat substrate when the substrate has contact areas featuring at least one distributed characteristic to cause the solder balls to become thinner during reflow. One such distributed characteristic may be the size of the contact areas. Another such feature may be the metallic thickness of the areas. Most frequently, the warped surface has an outward concave contour, but the invention applies also to convex BGA surface contours.
The present invention is related to high density ICs packaged as plastic BGA's, especially those having high numbers of inputs/outputs, and also to low end, low cost devices. These ICs can be found in many semiconductor device families such as standard linear and logic products, digital signal processors, microprocessors, digital and analog devices, high frequency and high power devices, and both large and small area chip categories.
It is an aspect of the present invention to provide a substrate with solder contact areas having at least one characteristic which exploits the wetting of solder on metallic surfaces, the dissolving strength of liquid solder, and the self-aligning feature of liquid solder surfaces based on surface tension.
Another aspect of the present invention is to design these characteristics so that certain categories of substrates match certain BGA package types having their known statistical degree of warping.
Another aspect of the invention is to reach these goals without cost of equipment changes and new capital investment and using the installed fabrication equipment base.
Another aspect of the invention is to teach guidelines for designing the substrate contact areas in order to match the corrective characteristic to the statistical coplanarity distribution of the BGA-to-be-assembled.
These aspects have been achieved by the teachings of the invention concerning the structure, geometries and material selection of the substrates, and the assembly methods suitable for mass production. Various modifications have been successfully employed.
In the first embodiment of the invention, the sizes of the metallic substrate areas are modified to accommodate a BGA package with concave warping of its solder ball surface. The resultant lowering of the solder joint heights and resolved coplanarity problem are illustrated.
In the second embodiment of the invention, the metallic thickness and solder-dissolving characteristic of the substrate contact areas are modified to accommodate a BGA package with concave warping of its solder ball surface. The resultant lowering of the solder joint heights and resolved coplanarity problem are illustrated.
The technical advances represented by the invention, as well as the aspects thereof will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.
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Arabe Ferdinand B.
Arguelles Ronaldo M.
Weygan Teddy D.
Gurley Lynne A.
Honeycutt Gary C.
Navarro Arthur I.
Niebling John F.
Telecky Fred
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