U.S. patent application number 10/455863 was filed with the patent office on 2004-12-09 for method and system for adjusting a curvature of a load plate based on a target load.
Invention is credited to Cromwell, Stephen Daniel.
Application Number | 20040247925 10/455863 |
Document ID | / |
Family ID | 32682523 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040247925 |
Kind Code |
A1 |
Cromwell, Stephen Daniel |
December 9, 2004 |
Method and system for adjusting a curvature of a load plate based
on a target load
Abstract
A method of making a load plate for an attach hardware assembly
of a circuit assembly includes adjusting a curvature of the load
plate based on a target load to be applied by the attach
hardware.
Inventors: |
Cromwell, Stephen Daniel;
(Penryn, CA) |
Correspondence
Address: |
HEWLETT-PACKARD DEVELOPMENT COMPANY
Intellectual Property Administration
P.O. Box 272400
Fort Collins
CO
80527-2400
US
|
Family ID: |
32682523 |
Appl. No.: |
10/455863 |
Filed: |
June 6, 2003 |
Current U.S.
Class: |
428/548 ;
257/E23.08; 257/E23.084; 257/E23.101; 257/E23.114; 257/E23.142 |
Current CPC
Class: |
H05K 7/1061 20130101;
H01L 23/552 20130101; H01L 2924/0002 20130101; H01L 2924/0002
20130101; H01L 23/4006 20130101; Y10T 428/12028 20150115; H01L
2924/00 20130101 |
Class at
Publication: |
428/548 |
International
Class: |
B22F 007/00 |
Claims
What is claimed is:
1. A method of making a load plate for an attach hardware assembly
of a circuit assembly, said method comprising adjusting a curvature
of said load plate based on a target load to be applied by said
attach hardware.
2. The method of claim 1, wherein adjusting said curvature of said
load plate based on said target load, further comprises adjusting
said curvature based on a material thickness and material
composition of a material from which said load plate is being
made.
3. The method of claim 2, further comprising using stainless steel
as said material.
4. The method of claim 1, further comprising, for each new material
lot being used to make load plates, performing a test run of load
plate production.
5. The method of claim 4, further comprising: measuring a starting
height of at least one load plate of said test run as a measure of
curvature; and tracking changes to said starting height through
fabrication of said at least one load plate.
6. The method of claim 5, wherein said fabrication comprises
heat-treating the load plate.
7. The method of claim 5, further comprising adjusting an original
starting height for production of load plates based on said changes
tracked in said test run and said target load.
8. The method of claim 7, further comprising initiating a,full
production run of load plates to use up said material lot.
9. A method of making a circuit assembly, said method comprising:
making an electrical connection between a circuit to a circuit
board; determining a target value for a load to be applied with an
attach hardware assembly to secure the connection between said
circuit and circuit board; adjusting a curvature of a load plate of
said attach hardware assembly based on a target load to be applied
by said attach hardware.
10. The method of claim 9, wherein adjusting said curvature of said
load plate based on said target load, further comprises adjusting
said curvature based on a material thickness and material
composition of a material from which said load plate is being
made.
11. The method of claim 10, further comprising using stainless
steel as said material.
12. The method of claim 9, further comprising applying a load with
said attach hardware assembly to secure the connection between said
circuit and said circuit board.
13. The method of claim 9, wherein said making an electrical
connection comprises connecting an Application Specific Integrated
Circuit in a socket of a circuit board.
14. The method of claim 9, further comprising, for each new
material lot being used to make load plates, performing a test run
of load plate production.
15. The method of claim 14, further comprising: measuring a
starting height of at least one load plate of said test run as a
measure of curvature; and tracking changes to said starting height
through fabrication of said at least one load plate.
16. The method of claim 15, further comprising adjusting an
original starting height for production of load plates based on
said changes tracked in said test run and said target load.
17. The method of claim 16, further comprising initiating a full
production run of load plates to use up said material lot.
18. A system for making a load plate for an attach hardware
assembly of a circuit assembly, said system comprising: means for
sizing and shaping a load plate; and means for adjusting a
curvature of said load plate as formed by said means for sizing and
shaping said load plate, wherein said curvature is adjusted based
on a target load to be applied by said attach hardware.
19. The system of claim 18, wherein adjusting said curvature of
said load plate based on said target load, further comprises
adjusting said curvature based on a material thickness and material
composition of a material from which said load plate is being
made.
20. The system of claim 18, wherein said load plate is formed
comprising stainless steel.
21. The system of claim 18, further comprising: means for measuring
a starting height of at least one load plate of a test run; and
means for racking changes to said starting height through
fabrication of said at least one load plate.
22. The system of claim 21, wherein said fabrication comprises
heat-treating the load plate.
23. The system of claim 22, further comprising means for adjusting
an original starting height for production of load plates based on
said changes tracked in said test run and said target load.
Description
BACKGROUND
[0001] An Application-Specific Integrated Circuit (ASIC) is a
microchip designed for a special application. The ASIC is designed
to process information or complete tasks in a manner specific to
the intended application. For example, ASICs are used in such
diverse applications as auto emission control, environmental
monitoring, and personal digital assistants (PDAs). ASICs are
contrasted with general integrated circuits that can be used to
perform different tasks for different applications. Examples of
general integrated circuits include the microprocessor and the
random access memory chips in a typical personal computer.
[0002] An ASIC can be mass-produced for a special application or
can be custom manufactured for a particular customer application.
Custom production is typically performed using components from a
"building block" library of ASIC components. Each ASIC includes a
number of input/output (I/O) leads that allow the ASIC to be
connected to a larger circuit and receive the signals and data with
which the ASIC works. These I/O leads are typically arranged in an
array known as a Land Grid Array (LGA). The ASIC is usually
attached to a circuit board, such as a printed circuit board (PCB).
Leads or a socket on the circuit board make contact with the I/O
leads of the LGA and connect the ASIC to the larger circuit of
which it is a part.
[0003] The ever growing I/O count in today's large ASICs requires a
very high clamping load to secure the ASIC to the circuit board and
ensure continuous electrical contact between the ASIC and the
circuit on the PCB. Clamping loads in the range of 400 to 700
pounds are becoming common. As noted, a socket may be provided on
the PCB into which the ASIC is clamped.
[0004] The load necessary to secure the ASIC to the PCB is produced
by the hardware used to attach the ASIC to the circuit board. This
hardware is frequently referred to as the "attach hardware." The
attach hardware includes a bolster plate and a load plate. The load
plate is a rigid plate that is typically made of steel and is
sometimes referred to as a spring plate. The bolster plate is
similar.
[0005] The ASIC assembly is sandwiched between the load plate and
the bolster plate. Load studs connect the load plate and bolster
plate, and a load screw is tightened to push the load plate and
bolster plate apart causing the load plate to flex and generate the
desired clamping force to the ASIC and circuit board.
[0006] Because the clamping load required is so high, the force can
cause the PCB and/or the bolster plate to bow or deflect. This will
impede the operation and performance of the socket or other
connection between the ASIC and the PCB. Consequently, the bow of
the bolster plate must be minimized. In some applications, this
requires minimizing the load applied. On the other hand, the socket
or connection between the ASIC and PCB requires a certain minimum
load to be reliable. The result is that the load is constrained
from above and below. The load must be sufficient to provide a
reliable connection in the socket or between the ASIC and PCB, but
must not be strong enough to cause a significant bow in the bolster
plate or circuit board.
SUMMARY
[0007] A method of making a load plate for an attach hardware
assembly of a circuit assembly includes adjusting a curvature of
the load plate based on a target load to be applied by the attach
hardware.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The accompanying drawings illustrate various embodiments of
the present invention and are a part of the specification. The
illustrated embodiments are merely examples of the present
invention and do not limit the scope of the invention.
[0009] FIG. 1 is an exploded view of an ASIC assembly including an
attach hardware assembly designed to provide a desired load
according to an embodiment of the present invention.
[0010] FIG. 2 is an illustration of the ASIC assembly of FIG. 1
when assembled.
[0011] FIG. 3a illustrates a load plate at a starting, uncompressed
shape.
[0012] FIG. 3b illustrates a load plate at a deflected, working
shape.
[0013] FIG. 4 is a flowchart illustrating a method of controlling
the load applied by an attach hardware assembly according to
another embodiment of the present invention.
[0014] Throughout the drawings, identical reference numbers
designate similar, but not necessarily identical, elements.
DETAILED DESCRIPTION
[0015] The present specification describes a method and system for
controlling the curvature of the load plate in the attach hardware
of an ASIC assembly so as to produce the desired target load at the
target deflection when the attach hardware is in place. The
curvature and, consequently, the spring rate of the load plate are
chosen so that the deflection of the load plate when put in service
produces the target load.
[0016] FIG. 1 illustrates an ASIC assembly (100) including an
attach hardware assembly. As shown in FIG. 1, a typical ASIC
assembly includes an ASIC (106) that is electrically connected to a
circuit board (107), for example a printed circuit board.
Typically, the ASIC (106) is connected to the circuit board (107)
using a socket (105). An insulator (108) is disposed below the
circuit board (107) to insulate the circuit board (107) from the
attach hardware assembly which will be described below.
[0017] A heat sink (104) is typically included in the assembly to
dissipate heat generated by the ASIC (106). An ASIC (106) will
generate heat as it operates and, if this heat is not dissipated,
can cause damage to the ASIC (106) or other components of the
assembly (100). An Electro Magnetic Interference (EMI) gasket (
111) and EMI frame (112) are also positioned between the heat sink
(104) and the circuit board (107). A thermstrate (113) is
positioned between the heat sink (104) and the ASIC (106) to
enhance the thermal interface and improve thermal conduction from
the ASIC (106) to the heat sing (104).
[0018] As described above, the ASIC (106) and circuit board (107)
are held together with an assembly of attach hardware. Among other
things, the attach hardware includes a load plate (101) and a
bolster plate (109). Load studs (103) are also part of the attach
hardware and run between the load plate (101) and the bolster plate
(109).
[0019] As shown in FIG. 1, the load plate (101) has a curvature,
for example, the load plate (101) has a cylindrical curvature. This
curvature of the load plate (101) allows the load plate (101) to
act as a spring, with a resulting spring rate, to apply a load to
the ASIC assembly (100).
[0020] Another component of the attach hardware is the load screw
(102). The load screw (102) is designed to cause a deflection of
the load plate (101) that decreases the curvature of the load plate
(101) and causes the load plate (101) to apply the target load to
the assembly (100). The load screw (102) is screwed through a
threaded hole in the load plate (101). The load screw (102) is
driven through the load plate (101) until the end of the load screw
(102) contacts the heat sink (104). Continuing the drive the load
screw (102) against the heat sink (104) will cause the load screw
(102) to applying an upward force on,the load plate (101) that
deflects the load plate (101) reducing the curvature of the load
plate (101). The corners of the load plate (101) are held in place
relative to the assembly (100) by the load studs (103). Thus, the
center of the load plate (101) is deflected upward against the
curvature of the load plate (101). Consequently, the load plate
(101) applies a load to the assembly (100) that is determined by
the spring rate and deflection of the load plate (101).
[0021] The circuit board (107), the socket (105), the ASIC (106)
and the thermal interface material (113) are sandwiched between the
bolster plate (109) and the heat sink (104) under the load created
by deflection of the load plate (101). As shown in FIG. 1, the heat
sink (104), ASIC (106), socket (105), circuit board (107) and
insulator (108) are all sandwiched between the load plate (101) and
the bolster plate (109) to complete the ASIC assembly (100).
[0022] Assembly of the ASIC assembly (100) is performed as follows.
The bolster plate (109) and EMI Frame (112) are attached to the
circuit board (107) via the load studs (103). The socket (105) and
ASIC (106) are placed onto the circuit board (107). Then, the heat
sink (104) is lowered down onto the ASIC (106) over the load studs
(103). The load plate assembly (101, 102) is shuttled onto the load
studs (103) and the load screw (102) is driven through the load
plate (101) to contact the heat sink (104). The assembled unit is
illustrated in FIG. 2. However, in FIG. 2, the load screw (102) is
not fully inserted. The load screw (102) is tightened until the
head of the load screw (102) is brought into contact with the load
plate (101) or a washer and applies pressure to deflect the load
plate (101), which deflects the load plate to the target
deflection.
[0023] This specification disclosed a process by which spring rate
variation in the load plate (101) is significantly reduced by
geometric compensation. As will be appreciated, if the spring rate
of the load plate (101) varies during the manufacture of ASIC
assemblies, the load applied in any given assembly will also vary
accordingly. This may lead to inconsistent product reliability
and/or failure of specific assemblies that experience a load
outside the acceptable working range.
[0024] As described above, the load in the ASIC assembly comes from
compressing or deflecting a cylindrically curved load plate from a
starting height to a specific compressed height. FIG. 3a
illustrates a load plate (101) with a starting height or curvature
(S) which is the shape of the load plate (101) after production and
with no force exerted on the plate (101) by a load screw. FIG. 3b
illustrates a load plate (101) that is in service and deflected to
a working height (D), which is less than (S). The load plate (100)
will naturally resist this deflection and apply the desired load to
the assembly.
[0025] To deliver the high loads required and survive the resulting
stress, the load plate is often fabricated out of stainless steel
and then heat-treated and tempered to achieve very high yield
stresses. After forming and heat-treating, the load plate is
compressed to just beyond the target deflection to eliminate
first-cycle plastic yield. This results in a part that can be
reused with the consistent results. This is referred to as sizing
the part.
[0026] Ideally, each load plate produced through this process will
follow the same force deflection curve. However, in reality, the
load produced by an individual load plate in use is a function of
the change in height or the deflection of the plate from a free
state to an installed state multiplied by the spring rate of the
plate at that working height. The spring rate is a function of the
geometry of the load plate (e.g. the thickness of the plate or
plates used to form the load plate) and the material from which the
load plate is formed.
[0027] As noted above, it is desirable to minimize variation in the
load applied by the load plates in different ASIC assemblies to
promote consistency and product reliability. Since the installed or
deflected height of a load plate is constant, the variation in the
resulting applied load arises from the starting height (plate
curvature) and the spring rate variation. As noted, spring rate
depends on plate thickness and plate material. Thus, load variation
can result from variation in the thickness or material composition
of the load plate and its response to the application process
(e.g., heat-treating and tempering).
[0028] The load variation from both of these variables can be
eliminated by appropriately adjusting the starting height or
curvature of the load plate so that a target load is achieved at
the installed or deflected height of the load plate. Variation in
material composition, material thickness and response to the
fabrication process can be introduced with each new lot of load
plate material, e.g., stainless steel, that comes into the load
plate manufacturing process. Typically, the variation in material
composition and thickness within a single lot is negligible.
[0029] FIG. 4 illustrates an exemplary method that can be use under
the principles disclosed herein to eliminate these variables from
the applied load of the plate when the load plate is placed in
service. As shown in FIG. 4, the method includes adjusting the
starting height or curvature of the load plate to produce the
desired working load.
[0030] With each new lot of material (determination 130), a sample
run of the load plates is made and measured (step 131). The
starting height (S, FIG. 3a) or curvature of the plate and any
change in height as the plate goes through the heat treating and
sizing processes is tracked (step 132).
[0031] Then, measuring the deflection required to achieve the
target load a new starting height for this lot of material can be
determined (step 133). The tooling that shapes the curvature in the
fabrication process is designed to be adjustable and the starting
height is adjusted to match the material used and the desired
working load.
[0032] A full production run can then be initiation (134).
Consequently, the fabrication method is adjusted to minimize any
variation in the working characteristics of the load plates
produced from different material lots.
[0033] Previously, variation in the working load of load plates
produced in different production runs has been significant and has
decreased product consistency and reliability. The load tolerance
due just to the standard thickness variation of +/-0.002" is
+/-41.5 lbs at a target load of 330 lbs or about +/-12% of the
load. Further, the starting height of the finished plate varies by
as much as 0.025" as a new lot of material responds to the heat
treating and sizing. At a spring rate of around 1700 lbs/in, the
resulting load variation is +/-21 lbs.
[0034] Most of the combined +/-62 lbs tolerance in the final load
can be eliminated by adjusting the part height to match the
material properties as outlined in this disclosure. Consequently,
load plates produced according to the methods described herein will
promote product consistency and reliability.
[0035] The preceding description has been presented only to
illustrate and describe embodiments of invention. It is not
intended to be exhaustive or to limit the invention to any precise
form disclosed. Many modifications and variations are possible in
light of the above teaching. It is intended that the scope of the
invention be defined by the following claims.
* * * * *