Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor
Reexamination Certificate
1999-09-15
2001-02-20
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Reexamination Certificate
active
06190941
ABSTRACT:
PRIORITY CLAIM
This application is based on and claims the priority under 35 U.S.C. §119 of German Patent Application 198 42 590.2, filed on Sep. 17, 1998, the entire disclosure of which is incorporated herein by reference.
1. Field of the Invention
The invention relates to a method of fabricating a circuit arrangement, for example including a surface mount device mounted on a printed circuit board, having thermal through-hole contacts or vias provided through the circuit board.
2. Background Information
Many electronics applications utilize electronic circuit arrangements including electronic components that generate a rather high power dissipation and therefore heat dissipation during operation. This is especially true in circuit arrangements including power components such as power modules for controlling and driving other assemblies or the like. The various components of the circuit arrangement are typically mounted on a suitable support body or substrate. For example, in the field of motor vehicle electronics, it is typical to use surface mountable power electronic components, which are surface mounted with their respective rear contact surfaces in contact with a printed circuit board as the supporting substrate.
In order to adequately remove the heat generated by the power dissipation of the components, and especially the power components, one known solution relates to improving the vertical heat transfer through the supporting substrate. This is achieved by providing thermal through-contacts, i.e. so-called thermal vias, extending through the supporting substrate from the top surface to the bottom surface thereof. To form such thermal vias, it is typical to form one or more through-holes through the substrate directly beneath the rear contact or mounting surfaces of the respective components and particularly the power components, and then metallize the through-holes, for example with a copper coating, continuously through the entire thickness of the substrate and covering the entire inner surfaces of the through-holes.
A second known solution to the above mentioned heat dissipation problem relates to the external heat removal. In this regard, the substrate may be mounted on a metallic cooling body or heat sink such as a copper plate, which in turn conveys the power dissipation heat to a cooling system. Such a cooling body may be separated from the supporting substrate by an electrical insulating layer such as an insulating film or foil.
After applying the components on the top surface, i.e. the mounting surface, of the substrate, the components are electrically contacted with contact surfaces or particularly contact pads and/or circuit paths or traces of a metallic circuit path structure provided on the top surface of the substrate. In order to achieve this, a soldering paste is printed onto the various contact surfaces and the top surface of the thermal vias and is then melted in a reflow soldering process in order to solder-connect the component to the contact surfaces.
During the reflow soldering process, droplets or emitted sprays of solder or even melted portions of the metallic conductor path structure can permeate through the thermal vias to the bottom or back surface of the substrate, whereby such permeation of the molten metal material is especially actively driven due to the capillary action caused by the small diameter via holes. Once the molten metal material reaches the bottom surface of the supporting substrate, it damages or completely breaks through the electrical insulating layer such as an insulating film applied on the bottom surface of the supporting substrate. This results in electrical short circuits to the metallic cooling body or to a metal housing or the like that supports or encloses the substrate as well as the circuit arrangement.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the invention to provide a process for fabricating a circuit arrangement which achieves advantageous characteristics with reference to the heat dissipation or removal, the reliability, the costs, and the simplicity of the fabrication process. More particularly, it is an object of the invention to provide a method for preventing the penetration of molten metal material during a reflow soldering process from the top surface to the bottom surface of a supporting substrate through via holes provided therein. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved in a method for fabricating a circuit arrangement according to the invention, comprising providing a through-hole and particularly a thermal through-contact via in a supporting substrate, carrying out a screen printing process including at least two printing passes or stages so as to close the through-hole from the bottom surface of the supporting substrate by means of the applied screen printed material, and then mounting and soldering a component onto the top surface of the substrate.
More particularly, the invention provides that all of the thermal through-contacts or thermal vias in the substrate are closed or sealed by carrying out a screen printing process on the bottom or back surface of the substrate opposite the top mounting surface thereof, before the soldering process is carried out. The screen printing process involves arranging a screen printing film around the thermal vias, and then pressing the screen printing material into the openings of the thermal vias from the bottom surface of the substrate in a screen printing process including at least two printing stages or passes. Preferably, the screen printed materials used in the screen printing process are highly viscous, thixotropic, pasty and preferably solvent-free materials. The screen printed material may especially be selected corresponding to the material of the substrate. Herein, the term “highly viscous material” refers to a material having a viscosity that is so high that it is just still screen printable by any known screen printing process.
After the thermal via openings have been closed or sealed with the screen printed material, a suitable testing is carried out, for example by means of an optical examination against a back light or by means of an automated vacuum test in order to determine whether the opening of the respective via hole has been completely closed and sealed. Specimens that fail the testing are rejected or again subjected to the screen printing process. Acceptable specimens are passed to a process of drying the substrate and curing and hardening the screen printed material.
When the subsequent soldering process, for example a reflow soldering process, is carried out for mounting the components of the circuit arrangement on the top surface of the substrate, the printed closure or sealing of the via holes prevents the molten solder material from permeating through the via holes from the top surface to the bottom surface of the substrate, and thereby protects the bottom surface of the substrate from soiling or damage that would otherwise be caused by the molten solder. Nonetheless, the heat transport from the top surface to the bottom surface of the substrate through the thermal vias is not significantly diminished by the screen printed material that seals the thermal via holes. This is especially true if the screen printed material is confined to the opening of the via hole and does not overlap onto the rim of the metallization layer around the opening of the hole, or if any such overlap has a limited thickness.
By using a solvent-free screen printed material, a volume reduction or shrinking of the screen printed material during the curing and hardening thereof can be avoided, so that the formation of bubbles and/or cracks in the screen printed material is prevented. Otherwise, such bubbles or cracks would negatively influence the reliability of the closure or seal of the thermal via holes provided by the screen printed material. The respective diameter of the t
Heinz Helmut
Schuch Bernhard
Daimler-Chrysler AG
Fasse W. F.
Fasse W. G.
Jones Josetta
Niebling John F.
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