Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With contact or lead
Reexamination Certificate
2001-01-10
2002-03-19
Wilson, Allan R. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With contact or lead
Reexamination Certificate
active
06359332
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printed-wiring substrate having lead pins. More particularly, the invention relates to a printed-wiring substrate which is formed primarily from an insulating material, such as resin or ceramic, and has a number of pin-bonding portions (electrodes) formed on a main surface thereof. Lead pins (input/output terminals) are brazed to the corresponding pin-bonding portions; for example, to a printed-wiring substrate of a PGA (pin grid array) type (IC package), on which an electronic component, such as a semiconductor integrated circuit device (IC), is mounted in a sealed condition.
2. Description of the Related Art
A PGA type printed-wiring substrate (hereinafter, also referred to as a substrate) has a number of bonding portions (electrodes), each of which assumes the form of a pad, formed on one main surface for bonding to an IC, as well as a number of lead pins (hereinafter, also referred to as pins) provided on the other main surface for insertion into a socket provided on a motherboard. Each lead pin assumes the form of, for example, a nail and is brazed to a substrate such that a flange (a portion of the pin having a diameter greater than that of a shaft portion of the pin) located at an end portion of the pin abuts a pin-bonding portion of the substrate.
The above-mentioned pin-bonding structure is disclosed in Japanese Utility Model Application Laid-Open (kokai) No. 60-106375. Specifically, a convex portion is formed on the bonding surface of the flange located at an end portion (head portion) of the nail-shaped pin. The bonding surface of the flange faces a pin-bonding portion of the substrate. The pin is brazed to the substrate such that the convex portion abuts the pin-bonding portion. In the case of brazing a pin having a flange whose end portion serving as a bonding surface is flat, the amount of brazing filler metal present between the flange and a pin-bonding portion is small, resulting in small bonded area. By contrast, a convex portion allows for a larger amount therebetween. Since the large amount of brazing filler metal effects a stress-absorbing action, the bonding strength of the pin is increased.
A printed-wiring substrate formed from resin employs solder, which has a relatively low melting point as compared with brazing filler metal, for bonding a pin. As a result, the bonding strength of the pin tends to become insufficient. According to conventional practice, in order to cope with the problem involved in a printed-wiring substrate formed from resin, a lead pin serving as an input/output terminal has a flange formed at an intermediate position of a shaft portion thereof, and a hole is formed at a pin-bonding portion of the substrate. The lead pin is soldered to the substrate such that an end of the shaft portion thereof is inserted into the hole. However, preparation of the hole decreases space for wiring within the substrate, thereby decreasing the degree of freedom in design. Meanwhile, even in the case of a printed-wiring substrate formed from resin, it is conceivable that the bonding strength of even a nail-shaped pin can be increased considerably by using solder in a sufficient volume and by employing the bonding structure disclosed in the above-mentioned publication. In this case, soldering practice shown in
FIG. 6
is considered appropriate. Specifically, a pin
121
is brazed to a pin-bonding portion
111
of a substrate
101
such that a brazing filler metal
131
covers the entirety of a flange
123
as in the case of internal chill (i.e., enveloped casting).
3. Problems Solved by the Invention
However, the above-mentioned practice raises the following problem. When the brazing filler metal
131
is applied so as to cover the entirety of the flange
123
, the molten brazing filler metal extends (creeps) by wetting from the root of a shaft portion
122
of the pin
121
toward a tip end of the pin
121
(downward in FIG.
6
). This means that the brazing filler metal
131
adheres to at least the root of the shaft portion
122
. This impairs appearance and hinders insertion of the pin
121
into a socket of a motherboard after assembly into a semiconductor device, thereby impairing reliability of electrical connection.
As shown in
FIG. 7
, by employing a bonding structure for the pin
121
which reduces the amount of the brazing filler metal
131
to be consumed for bonding, extension by wetting of the brazing filler metal
131
to the shaft portion
122
of the pin
121
can be prevented. However, a narrow portion K, in the form of a meniscus, of the brazing filler metal
131
is formed. As a result, when an external force is applied to the pin
121
, stress concentration tends to occur, thereby impairing the bonding strength of the pin
121
and the reliability of electrical connection. That is, when pin bonding is performed so as not to allow extension by wetting of a brazing filler metal of low melting point to the shaft portion as in the case of brazing with respect to a printed-wiring substrate formed from resin, a failure to obtain a desired bonding strength is highly likely; therefore, strict inspection of pin-bonding strength or quality control must be conducted.
SUMMARY OF THE INVENTION
In view of the above-mentioned problem involved in a printed-wiring substrate having lead pins, such as a PGA type printed-wiring substrate, an object of the present invention is to provide a printed-wiring substrate having lead pins brazed to corresponding pin-bonding portions of a substrate in which solder does not adhere to a shaft portion of the lead pin or impair pin-bonding strength, to thereby provide a highly reliable electrical connection.
The above object of the invention is achieved by providing a printed-wiring substrate comprising a substrate having pin-bonding portions formed on a main surface thereof and lead pins, the lead pins each having a flange and a shaft portion and being brazed to corresponding pin-bonding portions via the corresponding flanges, the flange having a convex portion formed on a bonding surface thereof in opposition to the pin-bonding portion, the printed-wiring substrate characterized in that:
a brazing filler metal brazing the lead pin to the pin-bonding portion extends by wetting toward the tip end of the lead pin beyond the outermost edge of a surface of the flange opposite the bonding surface such that the end of an extension of the brazing filler metal formed by wetting is located between the outermost edge and the shaft portion.
In the printed-wiring substrate having lead pins of the present invention, the brazing filler metal which extends by wetting does not reach the shaft portion and thereby does not hinder insertion of the lead pins into a socket. The brazing filler metal extends by wetting toward the tip end of the lead pin beyond the outermost edge of the surface of the flange opposite the bonding surface of the same. Such extension of applied solder means that the amount of applied solder is appropriate so as not to cause insufficient bonding strength.
When a molten brazing filler metal fails to extend beyond the outermost edge of the opposite surface, a narrow portion, in the form of a meniscus, of the brazing filler metal is formed as shown in
FIG. 7
, or the contour of a section of the brazing filler metal as sectioned by a plane including the axis of the shaft portion of the lead pin assumes the form of a concave arc (hereinafter, the term “narrow portion” includes the form of a concave arc). As a result, when an external force is applied to the lead pin, a brazed (bonded) portion tends to break due to stress concentration. By contrast, when the brazing filler metal extends beyond the outermost edge of the opposite surface as in the case of the present invention, the applied brazing filler metal is free of a narrow portion, because of sufficiency in the amount of applied brazing filler metal, thereby avoiding a substantial impairment in brazing strength. Thus, the present invention can provide a printed-wiring su
NGK Spark Plug Co. Ltd.
Sughrue & Mion, PLLC
Wilson Allan R.
LandOfFree
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