Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Flip chip
Reexamination Certificate
1998-06-30
2001-10-02
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Flip chip
C257S738000, C257S779000, C257S780000, C228S180220, C361S760000, C361S768000, C361S779000
Reexamination Certificate
active
06297559
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to techniques for connecting an electronic device to external circuitry, and more particularly to ball grid array interconnections between microelectronic packages and printed circuit boards.
BACKGROUND OF INVENTION
Rapid advances in microelectronic devices are continuously demanding a finer pitch connection between electronic packages and a printed circuit board (about a few hundred micrometer pitch or less). To meet this demand as well as the demand for low cost electronic packages, surface mount technology (SMT) has expanded its share over the conventional plated-through-hole (PTH) technology for the last twenty years. At present, more than two thirds of integrated circuits (IC) including both memory and logic devices are assembled by SMT. SMT packages commonly found in a PCB are leaded chip carriers such as small outline integrated circuits (SOIC), plastic leaded chip carrier (PLCC), quad flat pack (QFP), thin small outline package (TSOP), or tape carrier package (TCP). These leaded chip carriers, mostly plastic packages, depend on a perimeter connection between an IC package and a PCB. The perimeter connection scheme of SMT packages has reached its limitation in terms of connection pitch and I/O capability.
To relieve the limitations of perimeter connections and thereby to increase the packaging density, area array connection schemes have recently become popular. Some of the area array packages developed for SMT include ball grid array (BGA) package, solder column grid array (SCGA), direct chip attach (DCA) to PCB by flip chip connection, tape ball grid array (TBGA), or chip scale packages (CSP). Among them, BGA is the most popular one, where solder balls connect a module carrying an IC to a PCB. This technology is an extension of the controlled collapse chip connection (C4) scheme originally developed for solder bump connection of multiple chips to a ceramic substrate.
The IC on the module can be connected to the module in several ways as taught by Mulles et al., U.S. Pat. No. 5,241,133; Massingill, U.S. Pat. No. 5,420,460; and Marrs et al., U.S. Pat. No. 5,355,283 among others. Ceramic or organic module substrates can be employed depending on the performance, weight and other requirements. The common feature, however, is that the connection between the IC carrier and the next level PCB is accomplished by an array of solder balls which are attached to the module by a solder alloy with a lower melting temperature.
BGA packages have several advantages over the conventional leaded chip carriers: small and low profile package, large, standard pitch for a same I/O count, high assembly yield due to self-alignment, rugged package (no lead deformation), better electrical/thermal performance, and compatible with SMT assembly process. A few drawbacks of BGA packages are noted such as a difficulty of visual inspection of solder joints, cost issues of BGA modules, control of solder ball connection process, lack of field reliability data, and others.
There are several options depending on the choice of module materials, such as plastic BGA, ceramic BGA, and tape BGA. Ceramic BGA is more costly than plastic BGA, but it has a better proven reliability over the plastic BGA. However, one major weakness of ceramic BGA is a large mismatch of thermal coefficient of expansion (TCE) between a ceramic module and a polymeric PCB. This limits the maximum size of ceramic BGA module to be mounted on a PCB, which is about 32 mm
2
with state-of-art technology. For a ball pitch of 50 mil, this BGA module can have about 625 I/O connections. Plastic BGA is better in terms of TCE mismatch because of a better materials compatibility between the module and the PCB substrate materials. Since most of plastic BGA's have a perimeter connection to a chip by wire bonding, the overall packaging density is much lower than that of a ceramic BGA which has an area array connection to a chip by flip chip or C4 technology.
FIG. 1
schematically illustrates the cross section of a ceramic BGA module
13
connected to an organic substrate
15
by use of a two dimensional array of solder balls
14
. On the BGA module
13
, an IC chip
11
is already attached to it by another array of smaller solder bumps
12
.
Solder balls used are typically 90% Pb-10% Sn in composition for ceramic BGA, 63% Sn-37% Pb eutectic solder for plastic BGA. As schematically shown in
FIG. 2
, the solder ball
24
is connected by reflowing Sn—Pb eutectic solder paste
23
as commonly used in SMT soldering. During the reflow of ceramic BGA, only the Sn—Pb eutectic solder paste
23
is melted, not the solder ball
24
of a high melting temperature. During the reflow process, several reactions occur simultaneously at the soldering interfaces; dissolution of terminal metalluriges such as Au or Cu layer into the molten Sn—Pb eutec tic solder, formation of Sn-containing intermetallic phases, interdiffusion of Sn and Pb across the liquid-solid interface, void formation in the solidifying Sn—Pb eutectic solder paste materials, and others. These reactions would affect the joint integrity or degrade the long-term reliability. The large mismatch in thermal expansion coefficients between a ceramic module
21
and an organic printed circuit board
26
, often causes thermal fatigue failure along the interface
22
,
25
close to the ceramic module during a thermal cycling test. In order to improve the joint integrity and reliability, several solutions have been proposed; replacing Pb-rich solder balls with more flexible materials or placing a diffusion barrier layer on solder balls to prevent interdiffusion, or using a different joining material other than solders such as electrically conductive adhesives like Ag-filled epoxy.
SUMMARY OF INVENTION
In accordance with the present invention, a strong and compliant interconnection of BGA to ceramic or plastic substrates is made possible. Moreover, the present invention makes it possible to provide a stable BGA joint structure which does not cause an excessive interdiffusion between solder balls and adjoining solder paste. Also, according to the present invention, a BGA structure which gives a longer fatigue life can be obtained. Furthermore, it is possible according to the present invention to provide a joining material and method which can handle a larger module size than that of the standard EGA module.
More particularly, an aspect of the present invention is an interconnection scheme of ball grid array packages to ceramic and organic substrates by an application of an electrically conductive material formed from a plurality of conducting particles. Each conducting particle has an electrically conductive coating which is fused to an electrically conductive coating on an adjacent particle to form a network of fused conducting particles.
Another aspect of the present invention is to replace lead (Pb)-containing solder paste material by Pb-free conducting material for the assembly of BGA packages.
A still further aspect of the present invention is a method of forming an electrically conductive joint between a solder ball and a contact pad by forming a paste of particles having an electrically conductive coating and a polymeric material wherein the paste is disposed to be adhesively and electrically joined. Heat and pressure are provided to fuse the electrically conductive particles to themselves, and to metallurgically bond them to the contact pads.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described only the preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as
Call Anson J.
DeLaurentis Stephen Anthony
Farooq Shaji
Kang Sung Kwon
Purushothaman Sampath
Chambliss Alonzo
Chaudhuri Olik
Connolly Bove Lodge & Hutz
International Business Machines - Corporation
Morris Daniel P.
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