Electricity: conductors and insulators – Boxes and housings – Hermetic sealed envelope type
Reexamination Certificate
2001-04-19
2004-03-09
Ngo, Hung V. (Department: 2831)
Electricity: conductors and insulators
Boxes and housings
Hermetic sealed envelope type
C257S704000
Reexamination Certificate
active
06703560
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a grid array module, and more particularly to a land grid array (LGA) module and a method for forming the same.
2. Description of the Related Art
Traditionally, electronic components, or modules, have been connected to circuit cards (or printed wiring boards (PWB)), by solder, either by pins inserted into plated through holes, or by direct solder attached to the PWB surface. The attachment of an electronic module to a PWB is generally referred to as a “second packaging level”.
With the increasing complexity of PWBs and their components, reworking modules (e.g., removing modules defective or otherwise from a “card” and replacing the module) has become increasingly necessary, such that module reworkability is now an extremely important design criterion.
Reworking of soldered modules on location is difficult and expensive. Typically, a special apparatus is required for heating a selected module to a temperature beyond the melting point of the solder joints, without disturbing adjacent components.
In response to the demands of component rework, the Land Grid Array (LGA) concept has been developed. In LGA technology, contacts on the module are mechanically held against mating pads on the card, generally augmented by an array of springs functioning as an interposer. The spring array provides the necessary contact force to each module and PWB contact, while providing mechanical compliance to absorb vertical tolerances. The minimum allowable contact force is determined by the properties of the contact force to each module and the anticipated environmental exposure. Typical values are substantially within a range of about 50 grams-force to about 150 grams-force (0.49-1.47 Newtons per contact).
The LGA thus replaces a soldered interconnect array with an array of mechanical pressure contacts, which may be readily separated for module rework.
The LGA has the additional advantage (in addition to easier component rework) that thermal mismatch strains between module and PWB may be absorbed by contact sliding, or in sideways deformation of the interposer contact springs. However, the amelioration of thermal stress is replaced by the introduction of high mechanical forces, which must remain on the module and PWB throughout the product lifetime, and which can potentially compromise the structural integrity of this module.
Thus, land grid array socketing can subject a substrate or the like to a very high level of loading force (e.g., due to a heat sink pushing on a cooling cap affixed to the substrate through a “seal band”) from the array of spring contacts. The loading is typically balanced by mechanical socket forces, which are distributed along the underside of the substrate.
Hence, the substrate undergoes essentially peripheral loading on the top and a distributed loading on the bottom. This imbalance can produce a substantial upward camber and mechanical bending stresses which remain as long as the module remains attached.
As a result, such unbalanced loading can break the substrate catastrophically (especially if made of a weaker material such as glass-ceramic or the like), can reduce the force on central contacts, thus compromising or destroying electrical performance, can squeeze out thermal paste used to thermally connect the chip to the cap, to a degree that it cannot recover during module rework, can squash chip-to-substrate interconnections, especially if not protected with an underfill. Further, the die may be fractured or the seal band damaged or broken.
The conventional methods and designs have attempted to solve the above problems by providing a thicker substrate (e.g., making the substrate have a thickness within a range of about 2 mm to about 8 mm depending on the application involved) and/or a more rigid substrate (e.g., by using stiffer materials, reinforcement members, or the like), but at the expense of performance, space and complexity of manufacturing.
FIG. 1
illustrates an example of a conventional multi-chip LGA module
1
. The module
1
includes a substrate
2
, which mounts one or more chips or discrete electronic components
3
, and a cap
4
. The cap
4
serves to mechanically protect the chip, and to provide a heat transfer path from the back of the chip
3
to the external cooling environment. To enhance heat transfer, a highly thermally conductive material
5
, such as a paste containing ceramic, metal and/or metal oxide particles or the like, is typically placed between the back of the chip
3
and the cap
4
. The cap
4
is attached to the substrate
2
along a peripheral band, or “picture frame”-like structure, by a thin layer of adhesive
6
(e.g., a so-called “seal band”). Preferably, the adhesive layer
6
forming the seal band has a thickness substantially within a range of about 10 &mgr;m to about 100 &mgr;m.
The substrate
2
is attached on the top to the chips or discrete devices
3
by an array of solder joints
7
, which may be encapsulated with an underfill material
9
such as silica-filled epoxy or the like. Alternatively, the chips
3
may be back bonded and wire bonded to the substrate
2
(not shown). The bottom of the substrate
2
contains an array of metallized pads
8
which serve to subsequently interconnect the module to the printed wiring board
10
. Thus, the module is formed.
The module
1
is clamped to the board
10
by a plate
11
(which may double as a heat sink, and which may have fins
15
, to enhance heat transfer) or the like, which is attached to posts
12
, which protrude from the board
10
. The plate
11
may be attached to the posts
12
by screws or another suitable fastening mechanism, and which may be augmented by springs (not shown).
Alternatively, as shown in
FIG. 2
, the cover plate
11
(e.g., heat sink) may be shaped so as to attach directly to the board
10
, thereby making the posts
12
unnecessary in this design. A soft medium (not shown), such as metal-filled grease, may be placed between the plate
11
and cap
4
to enhance heat transfer.
The substrate pads
8
are connected to mating pads
18
, positioned on the surface of the PWB
10
, generally through a spring carrier, or interposer,
19
containing LGA contacts
20
, which is usually clamped to the board
10
together with the module
1
. It is noted that while just two mating pads
18
are shown in
FIG. 1
, such pads
18
are provided along the entire underside of the substrate
2
.
Referring now to
FIG. 3
, potentially damaging stress, shown by arrows
20
and
21
, is imparted to the substrate
2
when the module
1
is attached, due to actuation forces (e.g., from the cover plate
11
(not shown in
FIG. 3
) and heat sink fins
15
(not shown) or the like) reaching the substrate
2
, through the seal band
6
.
Since the reaction force on the substrate
2
is not collinear with the seal band
6
, but because the reaction force arises from the pads
8
, the reaction force is distributed on the bottom surface of the substrate
2
. This imbalance leads to upward flexure of the substrate
2
, as shown by arrows
21
.
Substrate flexure can cause a number of fatal problems, either immediately on actuation, or, even worse, over time. These include failure of the substrate
2
itself (e.g., either catastrophic fracture, internal line tearing, or surface via-via cracks), fracture of the chips
3
, excessive squeeze-out of the heat transfer medium
5
, delamination of the chip/substrate underfill
9
, or rupture of the seal band
6
. Clearly, as substrate size increases, or as its thickness decreases, the substrate's tendency to flex increases.
Minimizing substrate flexure under actuation loading thus becomes a critical factor in the performance and reliability of any LGA.
As mentioned above, several obvious steps have been attempted to remedy the above problems such as by providing for a thicker substrate, a rigid fill material between chip
3
and cap
4
, and a very stiff seal band material. However, using any of these generally compromises some othe
Coico Patrick Anthony
Covell James H.
Fasano Benjamin V.
Goldman Lewis S.
Hering Ronald L.
Blecker Ira D.
Ngo Hung V.
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