Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Ball or nail head type contact – lead – or bond
Reexamination Certificate
2002-09-04
2004-02-03
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Ball or nail head type contact, lead, or bond
C257S780000, C438S612000
Reexamination Certificate
active
06686665
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to integrated circuit device packaging, and more particularly, to apparatuses and methods for improved bonding of solder balls to package structures.
2. Description of the Related Art
Typically, integrated circuit devices include multi-level structures contained within a substrate material. The multi-level structures can include passive components connected by metallization lines. In the field of radio frequency (RF) and wireless applications, use of Low Temperature Co-Fired Ceramic (LTCC) substrates are becoming more popular for defining multi-level structures having passive components connected by metallization lines. LTCC substrates are capable of embedding passive components while providing superior performance at high frequencies. In this manner, LTCC substrates are generally attached to a printed circuit board (PCB), a number of semiconductor devices, or a number of discrete components, or a combination thereof, to define a larger electronic device. The LTCC substrates are commonly attached to the PCB using a ball grid array (BGA) attachment technique. Attachment of the semiconductor devices to the LTCC substrate can be accomplished by flip chip or wire bonding.
FIG. 1A
shows an illustration of a BGA-to-PCB attachment configuration
100
, in accordance with the prior art. In the BGA-to-PCB attachment configuration
100
, an LTCC substrate
101
is attached to a PCB
102
using a number of electrically conductive balls
111
. Each of the balls
111
are disposed between a LTCC ball attachment pad
105
and a PCB ball attachment pad
106
. Solder
108
is used to mechanically and electrically attach each of the balls
111
to both the LTCC ball attachment pad
105
and the PCB ball attachment pad
106
.
FIG. 1B
shows an illustration of a flip chip attachment configuration
120
, in accordance with the prior art. A semiconductor device
121
is attached to the LTCC substrate
101
using a number of electrically conductive balls
127
. Each of the balls
127
are disposed between an LTCC via (not shown) or an LTCC ball attachment pad
125
and a semiconductor under bump metallurgy (UBM) pad
123
. The ball
127
can be pre-deposited onto the semiconductor UBM pad
123
, the LTCC via, or the LTCC ball attachment pad
125
. A solder reflow process is used to form joints between the semiconductor device
121
, the LTCC substrate
101
, and the balls
127
. A fluxing agent is often used to aid joint formation during the solder reflow process.
In certain applications where device cost is secondary (e.g., military applications), gold (Au) conducting material is used within the LTCC substrate to fabricate the chip. However, in commercial applications where competition is a motivating factor for reducing cost, it is generally more desirable to use less expensive silver (Ag) conducting material within the LTCC substrate. Unfortunately, use of Ag conducting material within the LTCC substrate introduces material compatibility and component interface issues when using the BGA attachment technique. Specifically, use of Ag conducting material has traditionally required the use of a palladium (Pd)/Ag material mixture as the LTCC ball attachment pad
105
. The Pd/Ag LTCC ball attachment pad
105
adhesion characteristics are adversely affected by reaction with solder materials during a typical device fabrication process. Consequently, the Pd/Ag LTCC ball attachment pad
105
is prone to delaminate from the LTCC substrate
101
resulting in BGA attachment failure during either fabrication or subsequent use of the device. Such BGA attachment failure causes product reliability to be unacceptably poor.
FIG. 1C
shows an illustration of the BGA-to-PCB attachment configuration
100
with respect to the LTCC substrate
101
, in accordance with the prior art. The LTCC ball attachment pad
105
in disposed above a via
103
in the LTCC substrate
101
. The via
103
is a Ag or Ag/Pd conductor configured to electrically connect the Ag metallization lines (not shown) within the LTCC substrate
101
with the ball
111
. In the prior art, the LTCC ball attachment pad
105
is composed of 20% Pd and 80% Ag. It has been traditionally assumed that the Pd enhances the resistance of the LTCC ball attachment pad
105
to leaching by the solder
108
, wherein the solder
108
is composed of either a 96.5% tin (Sn) and 3.5% Ag mixture or a 63% Sn and 37% lead (Pb) mixture. It has been further assumed that the Pd inhibits Ag migration when exposed to a voltage bias. Also, the prior art suggests using a lower Pd content Pd/Ag mixture as a transition layer between the Ag via
103
and the Pd/Ag solder pad. The difficulty with using Pd/Ag for the LTCC ball attachment pad
105
as suggested by the prior art becomes apparent during fabrication when successive reflow operations are performed.
FIG. 1D
shows an illustration of the BGA-to-PCB attachment configuration
100
with respect to the LTCC substrate
101
after a reflow operation, in accordance with the prior art. During the reflow operation, the Sn in the solder
108
diffuses toward the LTCC substrate
101
. Correspondingly, the Ag/Pd material of the LTCC ball attachment pad
105
is displaced toward the ball
111
. As a result of the Sn diffusion, a Sn diffusion layer forms within the LTCC ball attachment pad
105
and extends to the surface adjacent to the LTCC substrate
101
. After successive reflow operations, the Sn diffusion layer can be composed of more than 50% Sn. Such a high percentage of Sn indicates a significant consumption of the LTCC ball attachment pad
105
through leaching by the solder
108
. Thus, the presence of Pd in the LTCC ball attachment pad
105
does not provide enhanced resistance to leaching by the solder
108
, as suggested by the prior art.
FIG. 1E
shows an illustration of the Sn diffusion and resulting LTCC ball attachment pad
105
delamination
112
caused by the reflow operation, in accordance with the prior art. As previously discussed, the Sn contained within the solder
108
diffuses into the LTCC ball attachment pad
105
causing a displacement of the Pd/Ag toward the ball
111
. The Sn diffusion layer formed within the LTCC ball attachment pad
105
at the LTCC substrate
101
interface weakens the adhesion between the LTCC ball attachment pad
105
and LTCC substrate
101
. The weakened adhesion in combination with the mechanical and thermal stresses induced by the reflow operation causes delamination
112
of the LTCC ball attachment pad
105
from the LTCC substrate
101
. Once delamination
112
occurs, the via
103
alone is required to withstand the mechanical and thermal stresses resulting from continued fabrication and subsequent use of the device. Generally, the via
103
is not strong enough to withstand these stresses. Thus, via
103
failure (i.e., cracking) causes the electrical conductivity from the via
103
through the ball
111
to be interrupted.
FIG. 1F-1
shows a scanning electron microscope (SEM) image of the BGA-to-PCB attachment configuration
100
following a typical reflow operation sequence, in accordance with the prior art. The LTCC substrate
101
is shown to be mechanically and electrically connected to the PCB
102
by a number of balls
111
.
FIG. 1F-2
shows a SEM image of the ball
111
configured between the LTCC substrate
101
and the PCB
102
following a typical reflow operation sequence, in accordance with the prior art. The delamination
112
is visible on each side of the via
103
between the LTCC substrate
101
and the PCB
102
.
FIG. 1F-3
shows a SEM image of the LTCC ball attachment pad
105
interface with the LTCC substrate
101
following a typical reflow operation sequence, in accordance with the prior art. The delamination
112
is clearly visible on each side of the via
103
. Also, via failure
113
is visible at a location proximate to the LTCC ball attachment pad
105
interface with the LTCC substrate
101
.
A prior art solution to the solder leaching, Sn diffusion,
Gao Guilian
Lewis David John
Murphy Stephen Thomas
Clark Sheila V.
Martine & Penilla LLP
Zeevo, Inc.
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