Stock material or miscellaneous articles – All metal or with adjacent metals – Having metal particles
Reexamination Certificate
2001-09-28
2002-11-26
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having metal particles
C428S617000, C428S618000, C428S675000, C428S929000, C257S779000, C257S781000
Reexamination Certificate
active
06485843
ABSTRACT:
BRIEF DESCRIPTION OF THE INVENTION
This invention.relates to an improved method for mounting surface-mount technology (SMT) devices such as solder ball-grid array (BGA) or solder column grid array (CGA) devices onto a printed circuit board.
BACKGROUND OF THE INVENTION
Among the variety of SMT attach methods available in the electronic packaging industry, BGA and CGA devices are often used for attaching devices with high I/O requirements. In these SMT device attach systems, an array of solder balls or solder columns form both the electrical and structural interconnections between the SMT device and the printed circuit (PC) board.
In a BGA attach system, the PC board is provided with one or more BGA device bonding sites and each site is provided with an array of bonding pads that matches the array of solder ball interconnections on the BGA device. To attach the BGA device onto the PC board, low-temperature solder paste, whose reflow temperature is lower than that of the BGA solder balls, is deposited onto each of the bonding pads on the PC board. Solder paste made from eutectic Pb—Sn solder is typically used for this application. The BGA device is then aligned and placed on a device bonding site so that the array of BGA solder balls is aligned with the array of bonding pads on the PC board. The whole assembly is then heated to a temperature sufficiently high to melt the low temperature solder paste and form metallurgical bonds between the BGA solder balls and the bonding pads. Because the solder paste has a lower melting point than the BGA solder balls, only the solder paste melts during the BGA bonding process step.
In addition to the bonding pads, a PC board typically has surface-wiring structures that provide electrical connections between various points on the surface of the PC board. Many of the bonding pads are connected to other bonding pads within the same surface-mount device bonding site or to various other points on the surface of the PC board through these surface-wiring structures. For example,
FIG. 1
illustrates a conventional BGA device assembly system. In
FIG. 1
, a prior art PC board
20
is shown with a BGA device bonding site
25
. BGA device bonding site
25
has an array of bonding pads
23
configured to match the array of solder balls
24
on BGA device
26
. And as discussed above, some of the bonding pads are shown as being connected to surface-wiring structures
22
a
and
22
b
. Two types of surface-wiring structures
22
a
and
22
b
are shown for illustrative purposes. Surface-wiring structure
22
a
connects those bonding pads to other parts of the PC board and surface-wiring structure
22
b
connects two bonding pads.
Bonding pads
23
and surface-wiring structures
22
a
and
22
b
are typically formed from the PC board's surface metallurgy generally comprising a layer of copper (Cu) base conductor layer deposited on the PC board's surface. The Cu base conductor layer is deposited in patterns forming bonding pads
23
and surface-wiring structures
22
a
and
22
b
. Then very thin layers of nickel (Ni) and gold (Au) are deposited over the Cu at bonding pads
23
. The remaining Cu base conductor layer portions without any additional metal layers form surface-wiring structures
22
a
and
22
b
. The metal layers are typically deposited using processes such as electrolytic plating or electro-less plating that are well-known in the electronics industry.
The layer of Ni plated on top of the Cu layer at bonding pads
23
functions as an adhesion promoting layer between the Cu and Au layers and also as a diffusion barrier between the Cu and Au layers. The Au layer provides corrosion protection.
The BGA device bonding site is then coated with a layer of solder mask
28
. Solder mask
28
is typically a layer of organic material, such as polyimide and covers the whole BGA device bonding site but has apertures exposing the Au surface of the bonding pads
23
to allow the BGA solder balls to bond to them. Because solder does not wet to the solder mask material, the solder mask functions as a dam around each bonding pad and prevents the molten solder from bleeding out along the surface-wiring structures during the BGA bonding process. And because of its electrically insulating property, solder mask
28
also functions as a passivation layer protecting the surface-wiring structures
22
a
and
22
b.
FIGS. 2 and 3
illustrate the details of a conventional BGA assembly structure using a solder mask.
FIG. 2
illustrates a cross-sectional view of BGA bonding site
25
from
FIG. 1
before the BGA device is bonded to the PC board. The Cu/Ni/Au multi-layer structure of bonding pads
23
is shown by the reference numbers
34
,
32
,
30
respectively. As discussed above, surface-wiring structure
22
consists of the Cu layer only and does not have Ni or Au plated over it.
The BGA bonding site is coated with a solder mask layer
28
that has apertures
37
. Apertures
37
expose Au layer
30
of the bonding pads for bonding to BGA solder balls. To bond a BGA device to BGA bonding site
25
, low-temperature solder paste
29
is first deposited onto the bonding pads using typical solder paste deposition techniques used in the industry. Some examples are an off-contact printing process, such as screen printing, or an on-contact printing process, such as stenciling.
After the solder paste has been deposited, a BGA device is aligned and placed onto the bonding site so that BGA solder balls
24
align with corresponding bonding pads. The whole assembly is then heated to an elevated temperature sufficiently high to melt the low temperature solder paste. Upon cooling, the low temperature solder forms metallurgical bonds with BGA solder balls
24
and the bonding pads. PC board
20
is also shown with a multi-layer internal structure with internal wiring metallurgy
27
such as is found in many PC boards.
FIG. 3
illustrates the cross-sectional view of PC board
20
after BGA device
26
has been bonded to BGA bonding site
25
. Solder balls
24
are bonded to bonding pads
23
and form the interconnections between BGA device
26
and PC board
20
.
A problem with this conventional BGA attach system is that because the solder masks are formed from materials with relatively high coefficients of thermal expansion (CTE) compared to the CTE of the PC board, the mismatch of CTEs between the solder mask and the PC board causes the PC board to warp after the assembly is exposed to elevated temperature during the BGA bonding process. For example, the CTE of a typical polyimide solder mask is about 500×10
−7
/° C. compared to the CTE of Fr-4, a typical PC board material, which is about 158×10
−7
/° C. Because the solder mask has higher CTE, it expands more than the PC board. But, because the BGA bonding process temperature is above the glass transition temperature (Tg) of the solder mask, there is no stress transmitted to the PC board during the high temperature stage of the bonding process. However, upon cooling down to the room temperature, below the Tg of the solder mask, the solder mask and the PC board shrink at different rates, causing the PC board to warp.
This poses a significant reliability concern. In many electronic devices, the component assemblies undergo many thermal cycles as the devices are turned on and off during their use. These thermal cycles strain the solder interconnections because the solder mask and the PC board expand and contract different amounts and eventually may result in metal fatigue failure of the solder interconnections.
An additional problem associated with the use of a conventional solder mask is that the interface between the solder mask and the Au plated surface-wiring is subject to degradation. The degradation may be in the form of de-lamination or peeling.
SUMMARY OF THE INVENTION
The invention provides a PC board with at least one SMT device bonding site for mounting SMT devices comprising: at least one surface for mounting one or more SMT devices; at least one base conductor layer plated on a portion of
Altera Corporation
Lam Cathy
Pennie & Edmonds LLP
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