CORROSION RESISTANT PINS FOR METAL PACKAGED MICROCIRCUITS

Abstract: A pin arrangement for use in metal packaged microcircuits. The pin (20) includes an annular boss (21), sometimes referred to as an upset. The upset (21) is formed on the pin (20) at the point where the pin (20) is to be embedded in the glass hermetic seal (12). The upset (21) provides a horizontal plane against the surface of the glass (12), giving substantially greater mechanical strength to the pin (20) in the glass (12). Due to this greater surface area, minute cracking and chipping of the glass (12) at the point of contact is minimized. Further, due to the upset (21), the flow of glass (12) up the shaft of the pin (20) during the hermetic sealing process is retarded, thereby avoiding the critical area and all the problems inherent therewith. Additionally, not only does the upset (21) provide greater mechanical strength to the glass (12), the upset (21) also provides a focal point for pin flexion. When flexion occurs, the pin (20) will tend to bend on the outside of the hermetic seal (12) at the juncture of the pin shaft and the upset (21), rather than at the interior juncture of the pin shaft and the upset. Accordingly, the upset (21) acts as a strain relief point.

Description: CORROSION RESISTANT PINS FOR METAL PACKAGED MICROCIRCUITS BACKGROUND OF THE INVENTION Technical Field The present invention relates to pins used in metal packaged microcircuits. More specifically, the present invention relates to those pins having a boss thereon to inhibit the formation of a meniscus at the exterior portion of the pin otherwise caused by hermetic sealing. Background Information Typically, metal packaged microcircuits of the plug-in type have a body and round pins whose base material is made of KOVAR, a material which is a combination of nickel, iron and cobalt and is therefore readily corrodible. In order to prevent corrosion, metal packaged microcircuits are typically plated with various types of corrosion resistant finishes. Because the metal package user performs a number of operations on the metal package received from the vendor, the types of finishes that can be used to prevent corrosion must be compatible with the processing which the user performs on the packages. Typical corrosion resistant finishes used on the metal package and pins include gold, nickel, a combination of gold and nickel, silver and copper. For the most part, though, gold is the corrosion resistant finish that is used. Gold not only aids the electrical connection process of the microcircuit to the pins, but also provides a very high degree of corrosion resistance. Turning now to Figure 1, a typical configuration of a pin mounted in the eyelet of a metal packaged microcircuit is shown. Pin 10 is shown located in the eyelet 11 of the metal package, held in place by a hermetic seal formed from glass 12. Typically, metal packages are made as matched seals. The matched seals obtain their hermeticity by the wetting of molten glass to the oxides on the surface of the pin. If the pin is plated with a corrosion resistant finish, the wetting action is inhibited. Because gold does not oxidize, hermetically reliable seals cannot be obtained with gold plated surfaces. Accordingly, the metallic area of the pin where the glass seal is to be created must be free of corrosion protection, as must the interior of the eyelet where the glass-to-metal seals are made. Therefore, the pins are typically hermetically sealed to the metal package prior to any plating. As the glass seals are being made and the glass is molten, the glass has a high affinity to the metallic oxides on the pin surface. The molten glass therefore runs along the metallic surface by cohesive action. As the molten glass moves along the surface of the pin, it becomes increasingly thin and finally decreases to zero thickness. Thus, meniscus 13 is formed. Additionally, at the point where the glass decreases to zero thickness, critical point 14 is formed. After the pins have been assembled in their packages, the pins and metal packages are typically plated with corrosion resistant finishes. Major problems of corrosion exist due to the critical area formed by the glass meniscus. The glass, which becomes increasingly thin up to the critical point, also becomes increasingly weak. The geometry of the metal packaged microcircuit is such that eyelet 11 is a relatively thick and strong piece of material relative to the glass seal. The pin is usually thin and relatively flexible, typically extending for a considerable distance, relative to its cross-sectional area, external to the package. Because the pins are long, they are easily flexed. This flexure is transmitted to the glass seal, and the glass, especially at and around the critical area, is easily chipped and cracked. Not only do small pieces of glass fall away from the surface of the pin, but TIT T fine fissures also appear in the surface of the glass. This not only destroys the hermeticity of the seal, but it also leaves extremely minute areas of base metal of the pin exposed. Accordingly, pin-prick size areas of base metal are adjacent to the length of the lead which is gold plated. When two metallic areas of dissimilar metals are juxtaposed, a galvanic cell may result. A galvanic cell results in the destruction of one of the metallic surfaces, typically the material having the lowest nobility, at a rate proportional to the ratio of the more noble material to the less noble material. When gold, the most typical plating for pins, and KOVAR, the most typical based material for pins, are the two metal surfaces in question, a galvanic cell does indeed result. The galvanic cell results in the extremely rapid destruction of the unplated area because the ratio between plated and unplated area is rather high. The rapid destruction either causes the leads to fall off, rendering the microcircuit useless, or it causes the integrity of the lead to be so poor as to make the reliability of the package highly suspect. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to hermetically seal a pin in a metal packaged microcircuit with molten glass, yet avoiding the formation of a meniscus, on the exterior of the package, thereby avoiding the problems inherent in the prior art. It is further an object of the present invention to provide a method of plating the pins during assembly of the metal packaged microcircuit so as to avoid possible galvanic deterioration effects. The present invention solves the problems inherent in the prior art by including on the pin an annular boss, sometimes referred to as an upset. The upset is formed on the pin at the point where the pin is to be embedded in the glass hermetic seal. The upset provides a horizontal plane against the surface of the glass, giving substantially greater mechanical strength to the pin in the glass. Due to this greater surface area, minute cracking and chipping of the glass at the SUBSTI point of contact is minimized. Further, due to the upset, the flow of glass up the shaft of the pin during the hermetic sealing process is retarded, thereby avoiding the critical area and all the problems inherent therewith. Additionally, not only does the upset provide greater mechanical strength to the glass, the upset also provides a focal point for pin flexion. When flexion occurs, the pin will tend to bend on the outside of the hermetic seal at the juncture of the pin shaft and the upset, rather than at the interior juncture of the pin shaft and the upset. Accordingly, the upset acts as a strain relief point. In addition to the prior configuration, the pin is plated during assembly of the metal packaged microcircuit so as to avoid possible galvanic deterioration effects. After the upset is formed but prior to assembly in the hermetic seal, the portion of the pin which is to reside in the hermetic seal is masked and a preplating, preferably nickel, is applied. The*-preplated pin is then hermetically sealed in the metal packaged microcircuit and the exposed pin portion and metal package are subsequently post-plated, preferably with gold. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the prior art configuration of a pin hermetically sealed by glass in a metal package. Figure 2 illustrates the preferred embodiment of the pin of the present invention embedded in a hermetic seal formed by glass. DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to Figure 2, the preferred embodiment of the pin of the present invention is shown. Pin 20 is embedded in eyelet 11 of a hermetic seal formed by glass 12. As the hermetic seal is being formed, pin 20 is inserted through the molten glass to upset 21, stopping at exterior surface 22 of upset 21. Although meniscus 13 forms on the interior side of the pin (the area which will be attached to the microcircuit), no meniscus is formed at the exterior surface of the hermetic seal. Due to the absence of the meniscus at the exterior of the hermetic seal, and to the substantial, relative to the cross-sectional area of the pin, horizontal plane formed by upset 21, greater mechanical strength is given to the pin. Additionally, the horizontal plane created by the upset produces a stronger seal due to the large diameter of the upset relative to the cross-sectional area of the pin. Not only does the upset provide greater mechanical strength to the glass, but it also provides a point for pin flexion. When flexion occurs, the pin will tend to bend on the outside of the hermetic seal at the juncture of the pin shaft and the upset, rather than at the interior juncture of the pin shaft and the upset. Accordingly, upset 21 becomes a strain relief point for the pin. A myriad of manufacturing techniques to provide upset 21 in pin 20 will be obvious to those skilled in the art. For example, pin 20 could be fitted with a washer whose inside diameter is substantially equal to the outside diameter of the pin. In the preferred embodiment, pin 20 is formed from straight cut wire of KOVAR having uniform diameter. The upset is created by running the wire through a four slide. We have found that locating the upset in the hermetic seal such that its exterior portion is exposed provides the greatest mechanical strength to the pin and hermetic seal assembly while simultaneously avoiding the formation of the meniscus. After the upset has been formed in the pin, upset 21 and exterior portion 23 of pin 20 are preferably preplated with a corrosion resistant material, most preferably nickel. The remainder of pin 20, including the area at which the hermetic seal is to be formed, is not preplated so as not to negatively affect the hermetic glass seal. Many ways are available for insuring that the preplating applied to the pin attaches only to the upset and the external length of the pin. For example, the pin could be placed in a rack and dipped to the specified depth in a plating bath. In the preferred embodiment, the internal portion of the pin is masked, most preferably with heat shrink tubing, and the preplating is attached. After preplating, the tubing is removed. SUBSTITUTE SHEET The preplated pin is then inserted into the molten glass to form the hermetic seal. Glass 12 is melted, by a procedure well known to those skilled in the art, and the preplated pin is inserted into the molten glass to upset 21. As will be appreciated by those skilled in the art, upset 21 shields critical area 14 (also defined as that point where the preplating finish on the pin and the unplated portion of the pin meet). After the hermetic seal is formed, the exposed portion of the pin and the exterior portion of the metal package is post-plated. In the preferred embodiment, the post-plating material is gold. Because critical area 14 is buried in the surface of the glass, even if the glass portion exposed to the exterior of the metal package is chipped, the unplated portion of the pin starting at critical area 14 is not exposed. Because of this, metal packaged microcircuits having pins according to the present invention can withstand damage to glass 12 so long as the damage does not expose any of the unplated surface of the pin, or otherwise destroy the integrity of the hermetic seal. We have found that metal packaged microcircuits using pins of the present invention offer outstanding corrosion resistance test results using conventional salt spray testing procedures. Although illustrative embodiments of the present invention have been described in detail with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments. Various changes or modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.

Claims: CLAIMS What we claim as our invention is: 1. In a metal packaged microcircuit having an eyelet of predetermined cross-sectional area and of predetermined height to accommodate a pin and a compound for forming a hermetic seal between said pin and the eyelet, said pin comprising: a shaft having a first portion to be external to the hermetic seal and a second portion to be in contact with the hermetic seal; and a boss juxtaposed between the first portion and the second portion of said shaft to inhibit the formation of a meniscus on the second portion of said shaft during the formation of the hermetic seal. 2. In the metal packaged microcircuit of claim l, said boss having a cross-sectional area less than the cross-sectional area of the eyelet. 3. In the metal packaged microcircuit of claim 1 wherein said boss is annular. * . 4. In the metal packaged microcircuit of claim 3, said boss having a cross-sectional area less than the cross-sectional area of the eyelet. 5. In a metal packaged microcircuit having an eyelet to accommodate a pin and a compound for forming a hermetic seal between the pin and the eyelet, the pin including a shaft having a first portion to be external to the hermetic seal, a second portion to be in contact with the hermetic seal, and a boss juxtaposed between the first and the second portions of the shaft, the method of plating the pin comprising the steps and order of: preplating only the first portion and the boss of the pin with a first material; hermetically sealing the second portion of the pin in the eyelet of the microcircuit; and SUBSTITUTE SHEET post-plating the first portion and the exposed portion of the boss of the pin with a second material. 6. In the metal packaged microcircuit of claim 5, said method of preplating comprising the steps of: masking the second portion of the pin to inhibit plating by the first material; plating the non-masked surfaces of the pin with the first material; and removing the mask from the second portion. 7. In the metal packaged microcircuit of claim 5, said method of hermetically sealing also including the step of hermetically sealing a portion of the boss in the eyelet of the microcircuit. 8. In the metal packaged microcircuit of claim 5, the first material comprising nickel. 9. In the metal packaged microcircuit of claim 8, the second material comprising gold.

Inventors: BAIRD PHILLIPS, SCHERER JEREMY D, BAIRD, PHILLIPS, SCHERER, JEREMY, D.
Applicants: AEGIS INC, AEGIS, INC.
Application year: 1987
Main classification: H01L23
IPC codes: H01L23, H01R13