Metal fusion bonding – Process – Metal to nonmetal with separate metallic filler
Patent
1994-08-09
1998-11-03
Ramsey, Kenneth J.
Metal fusion bonding
Process
Metal to nonmetal with separate metallic filler
B23K 120
Patent
active
058296654
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a fluxless soldering process adapted to be used where minute mating portions are required to be soldered together with a high degree of gastightness without using a flux as encountered in a pressure sensor unit.
BACKGROUND ART
As an example, a pressure sensor unit has a joint portion to be soldered, and this joint portion measures only about 0.5 mm. In addition, a high degree of gastightness such as a leakage of less than 1.times.10.sup.-11 atm.multidot.cc/s is required. In the past, therefore, in order to secure sufficient solder-wettability, a flux has been applied to the soldered joint surface.
However, when a flux is applied and heated, gases are given off from the flux, creating voids in the joint portion. These remaining voids are a main cause of insufficient gastightness in the soldered joint portion. Accordingly, where the joint portion is very small, it is difficult to improve the gastightness in the joint portion. Furthermore, it is necessary to wash away flux residues after soldering. This increases the number of the soldering steps. Also, the use of a detergent made from a Freon is not desirable for protection of environment.
Accordingly, an improved fluxless soldering method has been proposed in Japanese Patent Laid-Open No. 218950/1991. In particular, two members to be bonded together are spaced from each other vertically. The balance between the solder-wettabilities of the bonded members is improved by making use of both a load and the weight of the solder itself which acts to push the solder and cause it to flow. However, where a glass substrate used in a pressure sensor having a sensor chip on which a semiconductor circuit of a large scale integration is integrally formed is bonded to a metal stem by this method, the surface of the sensor chip extends in such a direction that the chip makes contact with a jig during assembly, thus scratching the surface of the sensor chip. This deteriorates the electrical characteristics.
In view of the foregoing, the present invention has been made. It is an object of the invention to provide a fluxless soldering process capable of bonding a pressure sensor unit having a sensor chip incorporating a semiconductor circuit of a large scale integration, for example, to a metal stem without scratching the surface of the sensor chip so as to improve the balance between the solder-wettabilities of both joint surfaces of portions to be soldered.
DISCLOSURE OF THE INVENTION
The above object is achieved by forming a nickel layer for soldering on the surface of a first member to be soldered and an oxidation-preventing layer for preventing the nickel layer from oxidizing. The first member undergoes a thermal hysteresis before the soldering. The thickness of this oxidation-preventing layer is so determined that movement of nickel atoms of the nickel layer toward the top surface of the first member is restricted in relation to the thermal hysteresis. A solder foil is melted by heating it in a reducing atmosphere. In this way, this first member is soldered to a second member.
In this soldering method, even if the first member undergoes the thermal hysteresis, the oxidation-preventing layer overlying the nickel layer is thick enough to effectively prevent nickel atoms of the nickel layer from being diffused up to the top surface of the first member. In consequence, substantially no deterioration in the solder-wettability of the joint surface of the first member takes place.
Accordingly, the solder-wettability of the first member is improved without increasing the load applied to the joint portion during soldering. Also, the wetting of the opposite surface by the solder is promoted. This maintains the balance between the solder-wettabilities of the joint surfaces of the two members which are placed vertically. In this manner, the joint surfaces of the two members to be bonded together are well wetted by the solder. In addition, a soldering operation providing a high degree of gastightness can be realized without usin
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Okada Hiroyuki
Yoneyama Takao
Nippondenso Co. Ltd.
Ramsey Kenneth J.
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