Metal fusion bonding – Process – With pretreating other than heating or cooling of work part...
Reexamination Certificate
2002-12-24
2004-11-09
Edmondson, L. (Department: 1725)
Metal fusion bonding
Process
With pretreating other than heating or cooling of work part...
C228S209000, C228S262100, C228S262610, C174S256000, C174S260000
Reexamination Certificate
active
06814276
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims a priority under 35 U.S.C. §119 to Japanese Patent Application No. 2001-400696 filed on Dec. 28, 2001, entitled “PROCESS FOR SOLDERING AND CONNECTING STRUCTURE”. The contents of that application are incorporated herein by the reference thereto in their entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for soldering a member to another member. More specifically, the present invention relates to a process for soldering a land formed on a substrate to an electrode (e.g. a lead) of an electronic component in the production of an electronic circuit board. Furthermore, the present invention also relates to a connecting structure, more specifically an electronic circuit board, produced by such process for soldering.
2. Description of Related Art
In the production of an electronic circuit board to be used for an electronic device and so on, a reflow soldering process is known as a process for mounting an electronic component on a substrate, more specifically, for physically connecting a lead taken out from the electronic component, to a land formed on the substrate.
In a general reflow soldering process, a so-called cream solder is applied by screen printing onto a land that is a part of a patterned wiring formed on a substrate. The cream solder is usually a mixture of soldering powder of a solder material, and a flux including rosin, an activator and a solvent. After that, an electronic component is located on a predetermined portion of the substrate so that a lead taken out from the electronic component adheres to the cream solder applied on the land. Thus obtained substrate, on which the electronic component is located through the cream solder, is heated at a temperature of at least the melting point of the used solder material so as to activate the flux and melt solder material of the soldering powder in the cream solder, and to evaporate (or volatilize) off other components such as the flux in the cream solder. Thereafter, the resultant product is cooled (or subjected to radiational cooling) such that the molten solder material is solidified. The solidified solder material forms a connecting part between the lead of the electronic component and the land of the substrate to electrically and physically connect them each other. Although other component(s) such as the flux other than the solder material may be present in the connecting part, such other component is excluded by phase separation form the solder material during the heating. Therefore such component is not present inside the connecting part and only remains in a small amount on the surface of the connecting part. Thereby, an electronic circuit board is obtained, wherein the electronic component is mounted on the substrate by the connecting part (or the soldering part) which is substantially composed of the solder material.
As the solder material, an Sn—Pb based material, in particular an Sn—Pb based material having an eutectic composition (hereinafter simply referred to as an Sn—Pb eutectic material also) is generally used. The eutectic composition of the Sn—Pb based material is an Sn-37Pb composition (i.e. a composition consisting of 37% by weight of Pb and the rest (63% by weight) of Sn). It is known that the Sn—Pb based material has a melting point of 183° C. in the case of this eutectic composition.
Recently, a manner of disposing an electronic device including the electronic circuit board described above becomes an issue, and this causes a concern about influence on the global environment or human bodies due to lead (Pb) contained in the conventional solder material. For that reason, there is a movement for using a solder material containing no lead, namely a lead-free solder material, in place of the conventionally used Sn—Pb based material, and efforts has been made to put the lead-free solder material into practical use.
At present, materials having various compositions have been suggested as the lead-free solder material. One of them is an Sn—Zn based material. As a result of recent studies, the eutectic composition of the Sn—Zn based material is roughly an Sn-9Zn composition (i.e. a composition consisting of 9% by weight of Zn and the rest (91% by weight) of Sn). It has turned out that the Sn—Zn based material has a melting point of 199° C. in the case of this eutectic composition.
Considering the thermal resistance of the electronic component to be mounted on the substrate as well as the adaptability for the existing soldering process, the lead-free solder material desirably has a melting point that is sufficiently low so as not to damage the electronic component, and is close to the melting point of the conventional Sn—Pb based material. The melting point of the Sn—Zn based material as above is lower than the melting point of other lead-free solder material such as an Sn—Ag based material, and is relatively close to the melting point of the Sn—Pb based material. Accordingly, the Sn—Zn based material can be regarded as a strong candidate for a substitute for the Sn—Pb based material.
By using the Sn—Zn based material in place of the Sn—Pb based material, there is an advantage in that soldering of an electronic component to a circuit board can be conducted while avoiding thermal damage to the electronic component. However, from a result of a test for continuous use of the electronic circuit board obtained thereby under a high temperature condition, the inventors found that a connecting part between the land of the substrate and the lead of the electronic component degraded and a sufficiently high thermal fatigue strength could not be obtained.
It is considered that such degradation (or deterioration) results from following mechanism. Zinc (Zn) contained in the Sn—Zn based material is contacted with copper (Cu) used for a material of the land and the lead. Then, an intermetallic compound of Cu—Zn is formed at (or in the vicinity of) interfaces between the connecting part and the land and between the connecting part and the lead.
More detail explanation hereinafter will be made with reference to
FIG. 2
, as to an electronic circuit board
80
which is produced by soldering an electronic component to a substrate while using a cream solder containing soldering powder of the Sn—Zn based material in place of the Sn—Pb material in the conventional reflow soldering process as described above.
In this electronic circuit board
80
, a lead
69
taken out from an electronic component
67
is electrically and mechanically connected to a land
63
formed on a substrate
61
through a connecting part
65
. The land
63
is generally made of Cu and is formed integrally with a patterned wiring. The lead
69
is generally composed of a base material
69
a
of Cu and a plating
69
b
of an Sn—Pb eutectic material which coats the base material
69
a
. The connecting part
65
results from the cream solder by a heat treatment, and it is substantially made of a solder material derived from the soldering powder as mentioned above.
At present, the movement away form lead (Pb) has also been advanced as to a plating material of a lead of an electronic component. In the current situation during a transitional period, however, there is a case where the Sn—Pb based material is still used.
During the heat treatment, Cu consisting in the land
63
diffuses into the solder material to be combined with Zn since the solder material is directly contacted with the land
63
. As a result, an intermetallic compound
73
of Cu—Zn is formed at (or in the vicinity of) an interface of the contact between the connecting part
65
and the land
63
.
Further, during the heat treatment, the solder material of the soldering powder melts, and the plating
69
b
of the Sn—Pb eutectic material also melt since the melting point of the Sn—Pb eutectic material is generally lower than the temperature of the heat treatment. A part of the plating
69
b
which part is contacted with the molten solder material, melts and diffuses into the
Edmondson L.
Wenderoth , Lind & Ponack, L.L.P.
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