WO1991009699A1

WO1991009699A1 - Welding of solder frame to ceramic lid in semi-conductor packaging

Info

Publication number: WO1991009699A1
Application number: PCT/US1990/007690
Authority: WO
Inventors: Jerry C. Laplante
Original assignee: Williams Advanced Materials Inc.
Priority date: 1989-12-29
Filing date: 1990-12-28
Publication date: 1991-07-11

Abstract

Metallic solder frames (12) are tack welded to a metallized portion (11) of a ceramic lid (10) using an electrode (13) which is heated in contact with the frame and pressed theragainst. Tack welds can be made without melting any metallic component or by melting only solder material.

Description

WELDING OF SOLDER FRAME TO CERAMIC LID IN SEMI-CONDUCTOR PACKAGING Background of the Invention Preassembled metal lids having applied solder pre¬ forms for use in sealing electronics packages, particu¬ larly for high reliability devices, have been known for some time. Such assemblies consist, generally, of a solder preform in the shape of a "frame" applied peripherally about a metal "lid".

Lids are usually made of a low expansion alloy such as ASTM F15 (known under various trade names such as Kovar, Rodar, Therlo, etc.) or 58Fe-42Ni (alloy 42), to match the thermal expansion of a ceramic package con- taining at least one electronic device. The ceramic most often employed is aluminum oxide (A1₂0₃) . The lid is also usually clad and/or electroplated with layers of addi¬ tional metals, particularly nickel and gold, to improve the corrosion resistance of the lid and to allow easy wetting of the solder used to attach it to the ceramic package.

The frame may be made of one or more of a variety of elements or alloys that can function as solders. Gener¬ ally, the material used is a relatively low melting solder, with a flow temperature below 400°C. One of the most common solders consists of a gold-tin alloy at or near a eutectic composition of 80Au-20Sn.

The frame is usually assembled to a metal lid by a spot welding technique, producing weld "spots". Two common methods of spot welding are used. One method is known as "series" welding, wherein welding electrodes are brought into contact with both frame and lid in such a fashion that a pulse of electrical current can be made to flow through the frame and the lid, causing heating at- the high resistance interface between the two components and causing localized melting and subsequent soldering or "welding" of the frame to the lid. In the second method. known as "parallel" welding, current enters and exits the assembly on the same side. Electrodes are here brought down against the frame at distanced points. A current pulse is made to flow through the entire assembly between electrodes. Conduction is through both frame and lid. Increased resistance at the contact points again causes localized heating and consequent welding of the compon¬ ents.

There are certain disadvantages to the use of metal frame-lid assemblies. One such disadvantage is their susceptibility to corrosion, particularly when subjected to salt spray testing. Another disadvantage is the difficulty, under some circumstances, of "marking" a lid for purposes of identifying the package it seals. It is difficult to achieve good adherence between the marking ink and a gold plated surface, due to the lack of re¬ activity of gold. Another disadvantage is the cost of gold plated over the entire surface of the metal lid.

A further disadvantage is loss of hermeticity after extended thermal cycling. Even though the metal lid is chosen to match A1₂0₃ as closely as possible in thermal expansion, the metal lid, because of its higher thermal conductivity and lower specific heat, heats and cools more rapidly during testing and during on and off operation of the contained device. Therefore, thermal stresses arise on each cycle.

A ceramic lid of the same material (A1₂0₃) matches the thermal conductivity and specific heat of the package and therefore heats and cools at the same rate. Extensive thermal shock testing has demonstrated appreciably in¬ creased seal life using ceramic lids.

One approach to bypassing these disadvantages has been to produce a ceramic lid rather than a metal lid. Ceramic lids are well known, and have been used for some time to seal ceramic packages. However, the common tech¬ nique for sealing them to their corresponding package has been to use glass as a "solder". This is quite effective, but the sealing temperatures necessary are well above those needed for the metal solders, as for instance the above mentioned 80Au-20Sn. This causes adverse effects to the circuitry and/or electronic devices sealed in many packages.

It is possible to seal a ceramic lid to a package using metal solder provided the ceramic is first coated in the appropriate areas with a layer of metal or metal plus oxide. A lid made of ceramic, e.g., aluminum oxide, is covered in a desired pattern such as a peripheral frame with a metal layer by a process known as "metallizing". This process can be accomplished by a variety of means well known to the ceramics manufacturing industry.

Using such a ceramic lid, it is desirable to produce frame-lid assemblies in which the solder frame is again pre-attached to the metallization and the lid. This has been done by producing a "paste" of solder, flux, and various organic binders that is inked or painted onto the lid and heated to melt the solder and drive off the non- metal constituents of the paste. This then produces a frame-lid assembly with the solder already wet to the lid. Though usable in some applications, this type of assembly has a major disadvantage. The pre-wet solder tends to stay preferentially wet to the lid and does not as readily wet the package to which it is being attached. Also, the pre-wetting operation causes a certain amount of diffusion of the metallization into the solder. This results in the formation of intermetallic compounds that often tend to be deleterious to the strength of the solder joint. This phenomenon occurs in all solder operations, but it is aggravated by time. Reflowed solder joints of necessity are above the melting point of the solder for much longer periods than are joints made in a single melting oper¬ ation. It would be more preferable to utilize a ceramic frame-lid assembly made in the same fashion as the con¬ ventional metal assembly, wherein the solder frame is locally spot welded to the metallized surface. However, conventional tack welding procedures are not applicable to ceramic lids. Series welding is most obviously not poss¬ ible because the ceramic lid is an insulator and will not conduct current.

The parallel process would appear to be feasible but does not work for more subtle reasons. In order for welds to be formed using the two-electrode arrangement, it must be possible for a substantial portion of the welding current to be carried by both components to be welded.

Otherwise there can be no flow through the high resistance point between the two. It is the nature of most metal¬ lized layers that they be quite thin relative to either the typical metal lid or to the solder preform. Also, it is usual that the materials used in the metallization tend to have a higher intrinsic electrical resistivity than the solder alloys. Therefore, current flow in the parallel welding mode is almost exclusively through the solder frame, resulting in no weld formation. Increasing current will eventually cause so much resistive heating in the solder that it melts the frame, thus defeating the basic purpose of the assembly.

Attempts were made to extend the area of the metal¬ lization to allow one electrode to be brought down on the metallization and one on the solder frame, thus forcing the welding current to flow through the contact area between the frame and metallization layer. This was defeated by the high resistance of the metallization layer. By the time sufficient current had been used to cause a spot weld, the metallizing layer was heated to the point at which it melted part or all of the frame lying in loose contact with it. U.S. Patent No. 4,746,583 mentions tack welding a metallic solder frame to a ceramic lid, but no details are given. Summary of the Invention

The invention is directed to a process for tack weld¬ ing a metallic solder frame to a ceramic lid for an elec- tronic packaging device wherein the portion of the lid surface to receive solder is metallized and the solder frame is tack welded thereto using an electrode which is heated in contact with the frame and is pressed against the frame to hold it firmly in contact with the metallized area until a tack weld is produced. Description of the Drawings In the drawing:

Figure 1 depicts a ceramic lid having metallizing in place;

Figure 2 depicts a ceramic lid having metallizing and a metallic solder frame in place;

Figure 3 depicts the lid of Figure 2 with tack welding in progress; Figure 4 depicts a form of tack welding electrode for use in accordance with the invention; and

Figure 5 depicts the type of weld produced using the eletrode of Figure 4. Detailed Description of the Invention The invention will now be described in conjunction with the drawing in which Figure 1 depicts a ceramic lid 10 having a metallized area 11 in place and Figure 2 depicts a ceramic lid 10 having metallized area 11 and a metallic solder frame 12 registered therewith. The assembly of Figure 2 is ready to be tack welded so that the solder frame 12 will be held precisely in position when the lid is assembled with a flat package holding an electronic device such as a semi-conductor and the assem¬ bly is soldered together by melting the solder derived from frame 12.

In Figure 3 the technique usually known as ther o- compression bonding is illustrated. In this technique, the material or materials to be joined are heated under pressure to a temperature below either or any of their- melting points, but high enough to cause relatively rapid diffusion of one material into or unto the other after or while bringing the joinable surfaces into intimate con- tact. This process has been used to bond pure gold sur¬ faces, or gold to other metals, but its ability to bond solder alloys such as gold-tin or lead-tin is surprising. In Figure 3, an electrical resistance heated elec- trode 13 with a tip 14 of a suitable configuration is shown. The tip temperature is measured by a thermocouple connected to the suitable control mechanism which regu¬ lates the output of power supply 15 feeding electrical current to the resistance heating member. The tip 14 is brought down against the frame 12, pressure 16 is applied, and power is fed to the tip to bring it to a temperature sufficient to effect a solid state bond. The current is then shut off to allow the tip to cool, after which it is raised and moved to the next weld position. For conven- ience in maintaining a rapid cycle, the tip may be kept at an elevated temperature between welding operations. A completed tack weld is indicated at 17.

As an example, bonds have been made between 80 Au, 20 Sn solder preforms and Au plated tungsten metallization, using a tip 14 of .020 inch dia. , held at 240°C before and after bonding, raised to 370°C after applying a load of about 600 grams, for a time of about 0.5 second. The tip pressure was about 2-3 kg/mm2. It will be noted that the tip temperature exceeds the 280° melting point of the solder. Under the chosen conditions, however, the solder does not melt, as it does not have time to reach the tip temperature. It is obvious that other combinations of tip temperature, pressure, and time may be used to accomplish the same result. The use of this procedure is not limited to forming a bond below the melting point of the solder. A very ade¬ quate bond may be formed by partial or complete melting of the solder in the region of the heated tip. For reasons of appearance only, a solid state bond is preferred. Another method in accordance with the invention involves the use of a pulse of electric current to effect localized melting and consequent soldering of the solder to the metallization. This method avoids the difficulties involved in controlling current flow through the metal¬ lized layer by bypassing this layer altogether.

Figure 4 schematically represents the electrode assembly 18 used for this procedure. Electrode 18 has two contact faces 19 and 20, separated by an insulating spacer 21. Spacer 21 can be a solid insulating material or an air gap. The size of the spacer is governed to control the distance between electrodes 19 and 20 such that cur- rent flow is confined to a small area when the overall electrode assembly is brought down against the material to be welded. Again, this assembly is used in the same fash¬ ion as welding tool 13 in Figure 3. This is represented in Figure 5 which is a magnified view of the weld region. After pressure is applied to electrode 18, a pulse of cur¬ rent is sent from the power supply 15 causing resistive heating of the solder frame 12 sufficient to melt it loc¬ ally 22 and cause it to wet and adhere to the metalliza¬ tion 11. The degree of heating and the consequent size and shape of the weld puddle 22 are controlled in conven¬ tional manner. The two electrode contact faces can be mounted in a common head or can be mounted independently. Metallization can be accomplished in accordance with the invention using known means to produce a tungsten, molybdenum-tungsten, molybdenum-manganese layer, or other metallizations known to the art, with a thickness about 5 to about 20 microns. In some applications it is possible to use thick film metallizations that are more in the range of 5-30 microns thick, and are composed of precious metal (e.g., Pd-Ag, Pt-Ag, etc.) or copper or copper alloys. A preferred solder alloy is the 80Au-20Sn alloy, although other solders having melting points not exceeding 400°C can be employed, e.g., 88Au-12Ge; 10Au-90Sn; 95Sn- 3.5Ag-1.5Sb; 99Sn-lSb; 95Sn-5Ag.

Claims

WHAT IS CLAIMED IS;

1. In the process for soldering a ceramic lid to a ceramic electronic package wherein the portion of the lid to be soldered is metallized and a frame of a metallic solder is welded to the metallized area, the improvement in tack welding said solder frame to said metallized area which comprises using an electrode heated in contact with said frame and pressed thereagainst in contact with said frame.

2. The process in accordance with claim 1 wherein said electrode is heated to a temperature above the melt¬ ing point of said solder but said tack weld is produced by heat and pressure without melting said solder.

3. The process in accordance with claim 1 wherein said electrode pressed against said solder frame is div¬ ided to present spaced apart conductive tips such that a current pulse passed between said conductive tips through said solder frame melts a portion of said solder and forms a tack weld to the underlying metallized layer.

4. The process in accordance with claim 3 wherein said conductive tips are separated by an atmosphere gap.

5. The process in accordance with claim 3 wherein said conductive tips are separated by a solid insulator.

6. The process in accordance with claim 3 wherein said conductive tips are mounted in a common head.

7. The process in accordance with claim 3 wherein said conductive tips are mounted independently.