Wiring substrate and stiffener therefor

Details Wiring substrate and stiffener therefor Wiring substrate and stiffener therefor

2000-05-30

2002-05-21

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including aperture

C428S457000, C428S458000, C428S469000, C428S901000, C361S807000, C361S809000

Reexamination Certificate

active

06391422

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wiring substrate which includes a wiring substrate body and a metallic plate serving as a stiffener or the like, as well as to the metallic plate itself. More particularly, the invention relates to a wiring substrate which includes a metallic plate formed from a rolled metallic sheet and serving as a stiffener or the like, as well as to the metallic plate.
2. Description of the Related Art
Conventionally, there is known a wiring substrate which includes a metallic stiffener bonded to a main surface of a wiring substrate body. Such a stiffener is used in instances wherein, due to low rigidity, the wiring substrate body easily deforms upon being subjected to an external force, or wherein the wiring substrate body easily deforms due to thermal expansion or other causes. The stiffener suppresses deformation of the wiring substrate and thus maintains the flatness of the wiring substrate, thereby increasing reliability of the wiring substrate.
An example of such a prior art wiring substrate is shown in
FIGS. 9A and 9B
, wherein
FIG. 9A
is an enlarged sectional view of a wiring substrate
220
, and
FIG. 9B
shows a stiffener
201
for use in the wiring substrate
220
. As illustrated, the wiring substrate
220
includes the stiffener
201
, a wiring substrate body
211
, and an IC chip
221
. The stiffener
201
is a metallic plate of a predetermined rectangular shape formed from a rolled metallic sheet, and has a through-hole
202
formed at a central portion thereof so as to accommodate the IC chip
221
.
The boundary profile or perimeter
211
C of the wiring substrate body
211
has a rectangular shape substantially identical with that of the stiffener
201
. The wiring substrate body
211
includes connection pads
212
, which are formed on a first main surface
211
A and which correspond to terminals
222
of the IC chip
221
to be mounted thereon, and electrode pads
215
, which are formed on a second main surface
211
B such that electrical continuity is established between the connection pads
222
and the corresponding electrode pads
215
. The stiffener
201
is bonded to the first main surface
211
A of the wiring substrate body
211
using an adhesive layer
217
such that the profile
211
C of the wiring substrate body
211
is in substantial alignment with the profile
201
C of the stiffener
201
. The IC chip
221
is mounted on the first main surface
211
A in such a manner as to be accommodated within the through-hole
202
formed in the stiffener
201
. The terminals
222
of the IC chip
221
are soldered to the corresponding connection pads
212
.
In some cases, a space defined by the IC chip
221
and the first main surface
211
A of the wiring substrate body
211
is filled with underfill resin so as to improve reliability of connection between the IC chip
221
and the connection pads
212
. In some cases, a heat radiation plate with or without fins is attached to the stiffener
201
and the IC chip
221
in order to effectively radiate heat generated by the IC chip
221
.
However, when the stiffener
201
is bonded to the wiring substrate body
211
in the course of manufacture of the wiring substrate
220
, the wiring substrate
220
may suffer deformation, such as warpage or waviness. This conceivably results from a difference in coefficient of thermal expansion between the stiffener
201
and the wiring substrate body
211
or from a small variation in coefficient of thermal expansion within the wiring substrate body
211
.
In the case where the IC chip
221
is mounted on the wiring substrate body
211
after the stiffener
201
is bonded to the wiring substrate body
211
, such a deformation may cause failure in one or more of the connections between the terminals
222
of the IC chip
221
and the corresponding connection pads
212
of the wiring substrate body
211
, thereby resulting in an impairment in the yield of acceptable devices produced by the corresponding manufacturing process. In the case where the stiffener
201
is bonded to the wiring substrate body
211
after the IC chip
221
is mounted on the wiring substrate boy
211
, a strong stress force is produced which acts on connections between the terminals
222
of the IC chip
211
and the connection pads
212
of the wiring substrate body
211
, potentially resulting in cracking in one or more of these connections and a resultant electrical defect. When the wiring substrate
220
is mounted on another printed wiring board, the connection provided between the printed wiring board and the electrode pads
215
of the wiring substrate
220
may be unacceptable, again resulting in an impairment or reduction in yield.
The warpage or waviness of the wiring substrate
220
varies with temperature. Accordingly, when the process of mounting the IC chip
221
is repeatedly started and stopped, i.e., when the wiring substrate
220
is repeatedly subjected to cooling and heating cycles, stress forces acting on connections between the terminals
222
of the IC chip
221
and the corresponding connection pads
212
of the wiring substrate body
211
can cause progressive metal fatigue over time in the connection(s), resulting in cracking in the connection(s) and a resultant electrical defect.
SUMMARY OF THE INVENTION
One aspect of the present invention concerns the discovery or inventive appreciation by the present inventors that deformation of a wiring substrate such as that shown at
220
does not arise isotropically, but rather tends to arise intensively in a given direction. Specifically considering the prior art device described above, when the stiffener
201
is bonded to the wiring substrate body
211
such that the short sides of the wiring substrate body
211
of rectangular shape are oriented in substantially the same direction as the rolling direction of the stiffener
201
, the amount of deformation of the wiring substrate
220
is minimized. In contrast, when the stiffener
201
is bonded to the wiring substrate body
211
such that the diagonal direction of the wiring substrate body
211
is parallel with the rolling direction of the stiffener
201
, the amount of deformation of the wiring substrate
220
is maximized. Accordingly, by studying the relationship between the rolling direction of the stiffener
201
and the shape of the wiring substrate body
211
and that of the stiffener
201
, the amount of deformation of the wiring substrate
220
can be controlled.
The present invention is based on the findings of the inventors discussed above and an object of the invention is to provide a wiring substrate which is equipped with a metallic plate serving as, for example, a stiffener, and which is constructed so as to reduce or suppress deformation which arises in the course of manufacture, and deformation due to temperature variations during operation. Another object of the present invention is to provide the metallic plate serving as, for example, a stiffener.
To achieve the foregoing objects, the present invention provides a wiring substrate comprising a wiring substrate body having a main surface; and a metallic plate formed from a rolled metallic sheet and having a rolling direction, the metallic plate having a boundary profile or outline, as viewed in plan, which is substantially identical to, or smaller than, that of the wiring substrate body, and the metallic plate being bonded to the wiring substrate body within the main surface. When two parallel lines are in contact with and enclose therebetween the boundary profile or outline of the metallic plate (hereinafter such parallel lines will be described as simply “enclosing” the profile) in such a manner as to maximize the perpendicular distance between these lines, a first direction of the profile of the metallic plate extending perpendicular to the two parallel lines intersects the rolling direction of the metallic plate at an angle. Preferably, the metallic plate serves as a stiffener for reinforcing the wiring substrate body. However, th

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