U.S. patent application number 11/163094 was filed with the patent office on 2006-06-01 for method for attaching an integrated circuit package to a circuit board.
Invention is credited to Shao-Tsu Kung, Chen-Hua Liu.
Application Number | 20060115975 11/163094 |
Document ID | / |
Family ID | 29998850 |
Filed Date | 2006-06-01 |
United States Patent
Application |
20060115975 |
Kind Code |
A1 |
Kung; Shao-Tsu ; et
al. |
June 1, 2006 |
METHOD FOR ATTACHING AN INTEGRATED CIRCUIT PACKAGE TO A CIRCUIT
BOARD
Abstract
A method for attaching an IC package to a circuit board, the IC
package having a plurality of electrical contacts in an arrangement
having a perimeter, first positions the IC package adjacent to the
circuit board. Then, electrically connects the IC package to the
circuit board through the plurality of electrical contacts. The
method finally, disposes at least one anchor mechanically attaching
the IC package to the circuit board, the anchor disposed at a
location outside of the perimeter of the plurality of electrical
contacts. The type, quantity, and exact geometry of the anchors
depend on the specific design parameters of the IC package and
circuit board.
Inventors: |
Kung; Shao-Tsu; (Taipei
City, TW) ; Liu; Chen-Hua; (Taipei City, TW) |
Correspondence
Address: |
NORTH AMERICA INTELLECTUAL PROPERTY CORPORATION
P.O. BOX 506
MERRIFIELD
VA
22116
US
|
Family ID: |
29998850 |
Appl. No.: |
11/163094 |
Filed: |
October 5, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10064462 |
Jul 17, 2002 |
6978539 |
|
|
11163094 |
Oct 5, 2005 |
|
|
|
Current U.S.
Class: |
438/613 ;
257/E23.084 |
Current CPC
Class: |
H05K 2201/10598
20130101; H05K 1/0218 20130101; H05K 2201/09781 20130101; H05K
2201/10704 20130101; H05K 2201/10734 20130101; H05K 3/303 20130101;
Y10T 29/49179 20150115; Y10T 29/49144 20150115; Y10T 29/49169
20150115; Y02P 70/50 20151101; Y02P 70/613 20151101; H05K 3/3436
20130101; Y10T 29/4913 20150115 |
Class at
Publication: |
438/613 ;
257/E23.084 |
International
Class: |
H01L 21/44 20060101
H01L021/44 |
Claims
1. A method for attaching an integrated circuit (IC) package to a
circuit board, the IC package having a plurality of electrical
contacts, the plurality of electrical contacts being disposed on a
major surface of the IC package in an arrangement having a
perimeter, the method comprising: positioning the IC package
relative to the circuit board, such that the major surface of the
IC package is adjacent to a major surface of the circuit board;
electrically connecting the IC package to the circuit board through
the plurality of electrical contacts; and attaching at least one
metal strap directly to the IC package to mechanically attach the
IC package to the circuit board; wherein the metal strap does not
provide an electrical contact between the IC package and the
circuit board.
2. The method of claim 1 wherein the metal strap is disposed at a
location outside of the perimeter of the plurality of electrical
contacts.
3. The method of claim 2 wherein the major surface of the IC
package has a rectangular shape, and the arrangement of the
plurality of electrical contacts is a grid-like array.
4. The method of claim 3 wherein metal straps are disposed at four
corners of the major surface of the IC package.
5. The method of claim 3 wherein metal straps are disposed along
four edges of the major surface of the IC package.
6. The method of claim 3 wherein metal straps are disposed on edges
of the IC package.
7. The method of claim 1 wherein the electrical contacts are solder
balls.
8. The method of claim 1 wherein the electrical contacts are pins.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
10/064,462, filed Jul. 17, 2002.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an integrated circuit (IC)
package, and more specifically to a method for attaching an IC
package to a circuit board.
[0004] 2. Description of the Prior Art
[0005] An integrated circuit is typically housed inside of a
package made of ceramic or composite material. The package provides
mechanical, electrical, and thermal protection to the IC chip. The
package has contacts to which solder balls or pins are attached to
allow the IC chip to be electrically connected to a circuit board,
and at the same time to provide mechanical attachment of the
package to the circuit board. The solder balls or pins serve the
purpose of both electrical and mechanical connection.
[0006] FIG. 1 shows a bottom view of a prior art IC package 10
having an IC chip (item 16 of FIG. 2). A plurality of solder balls
12 is disposed on a bottom surface of the package 10 in a grid-like
array for attaching the package to a circuit board.
[0007] A prior art method for attaching the IC package 10 to a
circuit board 14 to manufacture an assembly 18 is illustrated in a
side view shown in FIG. 2. The IC package 10 holding the IC chip 16
is attached to the circuit board 14 by the plurality of solder
balls 12. During manufacture, the solder balls 12 are fused to both
the package 10 and the circuit board 14 to form an electrical and
mechanical connection.
[0008] The solder balls 12 can fail under stress from thermally
induced mechanical loading or direct mechanical loading of the
assembly 18. For instance, the package 10 and circuit board 14
typically have different coefficients of thermal expansion. As the
assembly 18 is heated or cooled the circuit board 14 and package 10
expand or contract at different rates, and consequently the solder
balls 12 can be subject to high stresses. Additionally, the circuit
board 14 can be flexed by externally applied forces, such as those
forces experienced during the manufacturing, assembly, and testing
processes. However, because the mechanical stiffness of the IC
package 10 is appreciably greater than that of the circuit board
14, the solder balls 12 can be subject to high stresses that can
also cause mechanical failure. While the principal stresses in the
solder balls 12 under these loading conditions may not be enough to
cause immediate failure, a cyclic mechanical or thermal load can
cause a fatigue failure.
[0009] FIG. 3 shows the assembly 18 bending about a single axis due
to a differential thermal contraction of the circuit board 14 and
the package 10. This condition can be caused, for example, by the
assembly 18 being cooled from a higher than ambient assembly
temperature resulting in the circuit board 14 contracting more than
the package 10. As the assembly 18 cooled, solder balls in two rows
of solder balls 12a and 12b were subject to excessive tensile
stress, and consequently cracked as shown. After a solder ball has
failed mechanically, the IC package 10 is no longer effectively
electrically connected to the circuit board 14. Furthermore, if the
loading on the circuit board 14 is not somehow released, the
package 10 may become completely mechanically separated from the
circuit board 14.
[0010] The prior art method of using solder balls to provide a
mechanical connection between an IC package and a circuit board
results in a weak connection that is prone to mechanical failure.
When the circuit board and package undergo a mechanical loading or
thermally induced mechanical loading, solder balls can fail
mechanically.
SUMMARY OF THE INVENTION
[0011] It is therefore a primary objective of the claimed invention
to provide a method for attaching an IC package to a circuit board,
so that the mechanical attachment strength is increased and the
problems of the prior art are solved.
[0012] According to the claimed invention, a method for attaching a
IC package to a circuit board, the IC package having a plurality of
electrical contacts in an arrangement having a perimeter, first
positions the IC package adjacent to the circuit board. Then,
electrically connects the IC package to the circuit board through
the plurality of electrical contacts. The method finally, disposes
at least one anchor mechanically attaching the IC package to the
circuit board, the anchor disposed at a location outside of the
perimeter of the plurality of electrical contacts.
[0013] The type, quantity, and exact geometry of the anchors depend
on the specific design parameters of the IC package and circuit
board.
[0014] It is an advantage of the claimed invention that the anchors
can prevent mechanical failure of the electrical connection of the
IC package to the circuit board, caused by mechanical and thermally
induced mechanical loading.
[0015] These and other objectives of the present invention will no
doubt become obvious to those of ordinary skill in the art after
reading the following detailed description of the preferred
embodiment that is illustrated in the various figures and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a bottom view of a prior art IC package.
[0017] FIG. 2 is a side view showing a prior art method for
attaching the IC package of FIG. 1 to a circuit board.
[0018] FIG. 3 is a side view of the assembly of FIG. 2 bending
about a single axis.
[0019] FIG. 4 is a perspective view of attaching an IC package to a
circuit board according to the present invention.
[0020] FIG. 5 is a bottom view of the IC package of FIG. 4.
[0021] FIG. 6 is a side view of the assembly shown in FIG. 4.
[0022] FIG. 7 is a side view of the assembly of FIG. 4 bending
about a single axis.
[0023] FIG. 8 is a side view of a second embodiment of the present
invention.
[0024] FIG. 9 is a side view of a third embodiment of the present
invention.
[0025] FIG. 10 is a bottom view of a fourth embodiment of the
present invention.
DETAILED DESCRIPTION
[0026] The present invention method will be described in a
preferred embodiment where four pins are used to strengthen the
mechanical attachment of an IC package to a circuit board. Three
alternative embodiments will also be described, wherein leads and
solder are used as means for strengthening the attachment. The
teachings of the present invention can apply to many modern IC
package attachment systems.
[0027] A perspective view of the preferred embodiment method of
attaching an IC package 20, accommodating an IC chip 26, to a
circuit board 22 to manufacture an assembly 24 is shown in FIG. 4.
The package 20 is first aligned with the circuit board 22. The
package 20 is then electrically connected to the circuit board 22
by solder balls (item 30 of FIG. 5). The package 20 is further
mechanically attached to the circuit board 22 by a group of four
anchor pins 28. As the assembly 24 is manufactured, the anchor pins
28 can be produced at the same time and by the same device, as the
electrically connecting solder balls. The pins 28 provide for a
strengthened mechanical attachment of the package 20 to the circuit
board 22.
[0028] FIG. 5 is a bottom view of the IC package 20 showing a
plurality of solder balls 30 disposed on a bottom surface of the
package 20 in a grid-like array for electrically connecting the
package to a circuit board 22. Each anchor pin 28 is located at a
corner of the package 20. In practical application, the anchor pins
28 are fit into holes formed in the package 20, but the anchor pins
28 could also easily be fused directly to the bottom of the package
20 to have the same anchoring effect. The anchor pins 28 are
arranged at the four corners of the package 20 outside of a
perimeter formed by the outmost solder balls 30, as this placement
ensures that the anchor pins 28 will support a significant portion
of any loading applied to the package 20 or the circuit board 22.
Alternatively, the anchor pins 28 could be intermingled with the
solder balls 30, but this arrangement is less desirable since the
solder balls 30 are typically clustered together quite closely. The
present invention is not limited by the arrangement of the solder
balls 30 shown in FIG. 5. Moreover, the solder balls 30 could be
pins or other means for providing electrical connections between
the package 20 and the circuit board 22.
[0029] The exact diameter, length, and material of the anchor pins
28 are determined based on the precise arrangement of and material
used for the solder balls 30, as well as on other design
parameters. The anchor pins 28, however, do not provide an
electrical connection between the IC package 20 and the circuit
board 22. For the preferred embodiment described herein, the anchor
pins 28 have a diameter about equal to an average diameter of the
plurality of solder balls 30, and are of a material having a
considerably higher allowable stress and stiffness than the
material of the solder balls 30.
[0030] A side view of the assembly 24 is shown in FIG. 6. The
anchor pins 28 are illustrated penetrating both the IC package 20
and the circuit board 22. In the preferred embodiment, each anchor
pin 28 has a beveled head at the package end and a non-removable
flattened fastener at the circuit board end. As mentioned
previously, the exact shape, size, and end connections of the pins
28 are determined at design time considering all of the necessary
factors. Essentially, the pins 28 are designed to anchor the
package 20 to the circuit board 22.
[0031] Also shown in FIG. 6 is the plurality of solder balls 30
providing the electrical connection of the IC package 20 to the
circuit board 22, the design of such connection being well known in
the art.
[0032] If the assembly 24 is subject to thermally induced
mechanical loading or direct mechanical loading, both of which may
be simultaneous and cyclic as previously described in detail for
the prior art, the assembly 24 may bend about a single axis as
shown in FIG. 7. Certainly, the assembly 24 may bend about multiple
axes but for the sake of clarity, only single axis bending will be
addressed in the description of the preferred embodiment herein.
Consideration of multiple axes of bending can be accomplished by
the well-known principle of superposition. Due to a difference in
coefficient of expansion, if the assembly 24 undergoes thermally
induced mechanical loading, or due to a difference in stiffness, if
the assembly 24 undergoes direct mechanical loading, the circuit
board 22 bends significantly more than the IC package 20.
Regardless of the cause of the bending, the solder balls 30 and
anchor pins 28 are subject to a tensile loading. The mechanical
properties of the anchor pins 28 allow them to support a
substantial portion of the loading that would otherwise be taken by
the solder balls 30. Note the region 32 of the circuit board 22
that is held by the anchor pins 28 to conform to the shape of the
package 20. As a result, none of the solder balls 30 experiences a
stress large enough to cause immediate or fatigue failure.
[0033] A second embodiment of the present invention method is
illustrated in FIG. 8, which shows a side view of the IC package 20
and the circuit board 22. The package 20 is anchored to the circuit
board 22 by a plurality of metal straps 32. The metal straps 32 are
disposed around the perimeter, on all four edges, of the IC package
20. The metal straps 32 are fused to four edges of the package 20
and the circuit board 22, and can be produced at the same time and
by the same device as the electrically connecting solder balls 30.
It is important to note that the metal straps 32 do not provide any
electrical connection between the package 20 and the circuit board
22. The exact quantity, dimensions, placement, and material of the
metal straps 32 are determined based on relevant design parameters,
such as expected manufacturing or operating temperature range and
external loading.
[0034] A third embodiment of the present invention method is
illustrated in FIG. 9, which shows a side view of the IC package 20
and the circuit board 22. The package 20 is anchored to the circuit
board 22 by a plurality of redundant solder balls 34. The solder
balls 34 are disposed at the four corners of the IC package 20.
These mechanically connecting solder balls 34 are fused to the
bottom of the package 20 and the circuit board 22, and can be
produced at the same time and by the same device as the
electrically connecting solder balls 30. As similar with the
previously described embodiments, the redundant solder balls 34 do
not provide any electrical connection between the package 20 and
the circuit board 22. The exact quantity, diameter, placement, and
material of the redundant solder balls 34 are determined based on
relevant design parameters, such as expected manufacturing or
operating temperature range and external loading.
[0035] A fourth embodiment of the present invention method is
illustrated in FIG. 10, which shows a bottom view of the IC package
20. For clarity, the circuit board 22 is not shown in FIG. 10. A
continuous strip of solder 36 is disposed along the perimeter of IC
package 20 to mechanically connect the package 20 to the circuit
board 22. The solder strip 36 can be produced at the same time and
by the same device as the electrically connecting solder balls 30.
No electrical connection between the package 20 and the circuit
board 22 is provided by the solder strip 36. The exact length,
diameter, location, and material of the strip of solder 36 are
determined based on relevant design parameters, such as expected
manufacturing or operating temperature range and external loading.
Alternatively, the solder strip 36 need not be continuous and may
have interruptions in its length.
[0036] In summary, the described method of attaching an IC package
to a circuit board offers a strengthened mechanical connection. In
contrast to the prior art, the present invention anchors support a
sufficient share of a thermally induced or direct mechanical load,
both of which may be simultaneous and cyclic, to prevent solder
balls from becoming overstressed and cracking. This enhanced
stiffness package attachment method will lead to less mechanical
failures of solder balls.
[0037] Those skilled in the art will readily observe that numerous
modifications and alterations of the device and method may be made
while retaining the teachings of the invention. Accordingly, the
above disclosure should be construed as limited only by the metes
and bounds of the appended claims.
* * * * *