Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2000-12-28
2004-02-24
Gibson, Randy (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S807000, C361S810000, C361S779000, C257S737000, C257S738000, C257S779000, C174S253000, C174S260000
Reexamination Certificate
active
06697261
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printed circuit board unit including stacked solder or conductive bumps between a printed wiring substrate and a mounted component, and particularly, to an interposer or relay substrate interposed between the stacked conductive bumps.
2. Description of the Prior Art
A printed circuit board unit is well known to include a mounted component such as a ball grid array (BGA) device, for example. When the electric circuit generates heat in the printed circuit board unit, a heat or thermal stress is in general induced in the printed wiring substrate, made of a glass epoxy or polyimide resin, and the ceramic substrate of the BGA device. Since the printed wiring substrate made of resin and the ceramic substrate have different thermal expansion coefficients, the substrates usually suffer from a relative movement or shift along the surface of the printed wiring substrate due to the thermal expansion.
Such a relative movement between the printed wiring substrate made of resin and the ceramic substrate induces a shearing stress in solder bumps disposed between the printed wiring substrate and the ceramic substrate. The shearing stress may repeatedly be generated in the solder bumps in response to switching between on and off statuses of the electric circuit in the printed circuit board unit. The solder bumps may sometimes suffer from cracks when the shearing strain finally overcomes the strength of the solder bumps. The thus repeated and intermittent generation of the shearing stress is supposed to deteriorate the durability of the solder bumps.
It is conventionally known that a higher or taller solder bump leads to reduction in the shearing stress induced in the solder bump. A higher solder bump contributes to a broader distribution of the shearing stress, so that a smaller shearing stress can be defined within a horizontal cross-section of the solder bump. However, an increase in the height of a spherical solder bump or solder ball inevitably induces an increase in the width of the solder ball. A higher or taller solder ball in this manner is supposed to suffer from a smaller distribution density of the solder balls.
In view of the above-described disadvantage, it is proposed that solder bumps are vertically stacked on the printed wiring substrate below the mounted component. The stacked solder bumps are supposed to realize an increase in the height without increasing the width. In this proposal, the solder bumps and relay substrates are alternately stacked on one another on the printed wiring substrate. The relay substrate serves to connect the adjacent stacked solder bumps.
As disclosed in Japanese Patent Application Laid-open No. 09-214088, for example, it is proposed that the thermal expansion coefficient of the relay substrate is set at an intermediate level between the thermal expansion coefficients of the printed wiring substrate and the mounted component. In this case, a shearing stress is equally distributed over the stacked solder bumps. Japanese Patent Application Laid-open No. 62-18049 still proposes to interpose a relay substrate in the form of a film having a smaller Young's modulus between the stacked solder bumps. The relay substrate is supposed to absorb a larger shearing stress.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide an interposer or a relay substrate for a multileveled printed circuit board unit capable of more efficiently absorbing a repetitive shearing stress induced in conductive bumps stacked between a printed wiring substrate and a mounted component, and a printed circuit board unit employing the same interposer.
It is another object of the present invention to provide an interposer or relay substrate capable of introducing additional functions in a multileveled printed circuit board unit.
According to a first aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit in which conductive bumps are stacked between a printed wiring substrate and a mounted component, comprising an interposer substrate made of a porous material interposed between stacked conductive bumps.
In the multileveled printed circuit board unit employing the interposer, when any difference in the expansion is caused between the printed wiring substrate and the mounted component, one side of the interposer substrate receives a relatively smaller displacement force while the other side of the interposer substrate receives a relatively larger displacement force. A shearing stress is induced in the interposer substrate. Deformation of the porous material serves to absorb the shearing stress in the interposer substrate. The conductive bumps bonded on one side of the interposer substrate as well as the conductive bumps bonded on the other side of the interposer substrate may be relieved from a shearing stress. Accordingly, the durability of the conductive bumps can be improved. The conductive bumps are allowed to keep a stronger bonding in a longer duration.
The porous material is preferably designed to define pores penetrating through the interposer substrate. The pore may be utilized to provide a via establishing an electric connection between the conductive bumps on the opposite sides. It is not necessary to bore the interposer substrate solely for the via. Production process of the interposer substrate can be facilitated.
The pore is designed as a slit located between adjacent ones of the conductive bumps arranged on an identical level. Even when one side of the interposer substrate receives a relatively smaller displacement force while the other side of the interposer substrate receives a relatively larger displacement force in the aforementioned manner, a shearing stress can be absorbed by deformation of the slit. In this case, the size of the slit at a central area of the interposer substrate is preferably set smaller than that of the slit located at a area remoter from the central area, since the planar displacement or shift gets larger at a location remoter from the center of the interposer substrate upon a thermal expansion of the printed wiring substrate and the mounted component.
According to a second aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit in which conductive bumps are stacked between a printed wiring substrate and a mounted component, comprising multilayered films made of different materials interposed between stacked conductive bumps.
It is possible to provide different characteristics at the opposite sides of the interposer substrate, respectively. For example, when the film contacting the mounted component may be made of a material having a thermal expansion coefficient identical to that of the mounted component, no shearing stress is induced in the conductive bumps between the mounted component and the film. On the other hand, when the film contacting the printed wiring substrate may be made of a material having a thermal expansion coefficient identical that of the printed wiring substrate, no shearing stress is likewise induced in the conductive bumps between the film and the printed wiring substrate. The material of the film contacting the mounted component may be selected from inorganic materials forming the substrate of the mounted component. The material of the film contacting the printed wiring substrate is selected from organic materials forming the printed wiring substrate. A relative sliding movement between the films may be accomplished by an adhesive layer interposed between the films. The adhesive layer may be made of an adhesive for coupling the films with each other.
According to a third aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit, comprising: a first conductive bump on a first level; and a second conductive bump on a second level stacked on the first conductive bump between a printed wiring substrate and a mounted component, said second co
Fujitsu Limited
Gibson Randy
Westerman Hattori Daniels & Adrian LLP
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