Chip mounting device

A chip mounting device which is hereinafter also referred to as an "interconnection preform placement device", includes a retaining member having a predefined pattern of holes in which are positioned preforms of joint-forming material such as solder. Each preform is of a predefined configuration and has a height or length greater than is cross-sectional dimension. The preform retains its general configuration after the interconnection or soldering process to form a resilient joint which is more capable of withstanding stress, strain and fatigue. A method of forming resilient interconnections comprises placing the interconnection preform placement device between parallel patterns of electrically conductive elements, such as the conductive pads on an electronic component and a circuit board, and effecting the bonding of the conductive elements with the preforms.

BACKGROUND OF THE INVENTION 
This invention relates to electrical interconnections, and to methods and 
apparatuses for forming such interconnections. More particularly, the 
invention is directed to a method and apparatus for attaching electronic 
components, especially chips or chip carrier packages, to supporting 
substrates, such as circuit boards. 
The microelectronics industry is steadily moving toward the use of large 
chips and chip carrier packages (CCP) which have connection pads on the 
faces and/or edges. These are generally used where there are limitations 
with the use of dual inline packages (DIP). The number of connections on 
the most popular packages can range from 64 to 156. Chip carrier packages 
can be produced with leads attached (leaded) or they can be leadless. 
Leaded CCPs can be soldered directly onto printed circuit boards (PCB) or 
printed wire boards (PWB). Leadless CCPs can be soldered onto ceramic 
boards or installed into connectors. However, with glass/epoxy printed 
circuit boards or printed wire boards, leadless CCPs are usually mounted 
into connectors which are in turn mounted on the PCBs because of the 
effect of the differential thermal expansion coefficients of the materials 
involved when subjected to temperature fluctuations. These connectors are 
complex to manufacture and costly to use. 
As the CCP technology improves and their reliability increases, more 
emphasis is directed to soldering these packages directly onto the PCBs to 
make more use of the board space, and dispensing with the connectors even 
though the use of connectors permits replacement of faulty CCPs. The cost 
of conventional connectors relative to the cost of the CCPs can be 
disproportionately high. This is a strong incentive to use CCPs without 
connectors. 
However, the direct soldering of the CCPs on PCBs without the use of 
connectors is associated with a number of problems. (1) The variation of 
the surface flatness and non-parallel contours between the CCP and the 
boards produces varying solder joint heights. (2) The solder will have a 
tendency to wick out of the joint area into crevices or castellations in 
the CCP, thus "starving" the joint area. (3) Gold alloying with the solder 
will produce embrittlement of the "starved" joint. (4) Differential 
thermal expansion between the CCP and the board will fracture a thin 
solder joint due to the high shear strains in the joint. (5) Bridging 
between joints may occur if excess solder is present or if the distance 
between the joints is small. (6) Solder location tolerances are small and 
getting smaller yet with increasing packaging density, with a typical 
connection pad having a width of 0.012 inches and a 0.022-inch 
center-to-center spacing. (7) Flux removal from the space between the CCP 
and the PCB and subsequent inspection thereof. (8) The solder pastes used 
to attach the CCP will produce loose solder balls which cause electrical 
problems. 
Problems (1) through (4) and problems (7) and (8) can be substantially 
overcome by introducing tall columns of solder or similar joint-forming 
material to form each joint. This provides a "solder-rich" condition which 
minimizes the gold alloying effect, provides excess solder to accommodate 
surface irregularities and non-flatness, the tall solder columns will have 
lower shear, bending and/or combined stress and strain proportional to 
their greatest height, and no solder balls will be created. Problems (5) 
and (6) require the precise control of solder quantity and location. 
Different solutions have been proposed for the foregoing problems. The 
proper positioning of a predetermined quantity of solder may be achieved 
with the use of solder preforms spaced on a carrier or template in the 
locations corresponding to the points where the solder joint are to be 
formed. Examples of this technique may be found in U.S. Pat. Nos. 
3,320,658, issued to Bolda et al; 3,396,894, issued to Ellis; 3,472,365, 
issued to Tiedema; 3,719,981, issued to Steitz; 3,744,129, issued to 
Dewey; 4,209,893, issued to Dyce et al.; and 4,216,350, issued to Reid. 
Dyce et al. and Reid relate more specifically to the use of ring-type 
solder preforms for the solder connection of pin-type joints. The solder 
preforms of Dewey are hollow cylinders. Tiedema relates to a flexible 
carrier ribbon having spaced apertures which receive solder discs to 
provide a convenient means to handle and transport solder discs. 
The Ellis patent discloses a device for simultaneously applying a plurality 
of solder or other bodies of heat-fusible material, in which the solder 
bodies are disposed in heat-recoverable cups formed from or positioned on 
a sheet of material. The cups are spaced to correspond to the location of 
solder application, and, when heat is applied to the device, the solder 
melts and the cups recover to a flat configuration, and the recovering cup 
material forces the solder material out therefrom and into contact with 
the elements to be soldered. 
The Bolda et al. patent provides a thermoplastic carrier sheet onto which a 
plurality of conductive elements, such as solder preform spheres, are 
positioned. The individual conductive elements are heated to a temperature 
which is sufficient to soften the carrier material, but insufficient to 
deform the conductive elements. During the heating process, the conductive 
element nestles in the softened thermoplastic material and, upon removal 
of the heat, the carrier material resolidifies and rigidly supports the 
conductive elements. When the solder ball carrier assembly is used, heat 
is applied to melt the solder ball and soften the thermoplastic carrier 
material, permitting the material to form an insulation between those 
portions of the electrical conductors not electrically interconnected by 
the solder elements. 
Another approach has been proposed by Bell Labortories which is developing 
techniques employing vacuum equipment to pick up and place small solder 
balls on the underside of the CCPs, and retaining them by using a solid 
phase bonding method. Additional information may be found in the paper by 
R. H. Minetti entitled "Solid Phase Solder Bonding for Use in the Assembly 
of Microelectronic Circuits." 
Steitz provides a method of joining solder balls to solder bumps spaced on 
a semiconductor flip chip by forming an array of solder balls on the tacky 
side of a pressure-sensitive tape, with the balls being spaced like the 
solder bumps. The array of solder balls is placed in contact with the 
solder bumps, and both are then heated to reflow the solder and cooled to 
fix the contacts after which the tape is removed. 
In U.S. Pat. No. 3,614,832, Chance et al. form connections between the 
contacts on a solid-state device and conductive lands on a substrate by 
placing a decal over the device, the decal having a plurality of 
conductive strips attached adhesively to a backing sheet. Application of 
heat and pressure effects bonding of the strips, after which the adhesive 
is dissolved and the decal removed. 
Although the foregoing techniques provide for the correct placement of a 
predetermined quantity of solder or other suitable joint-forming material, 
and with the proper dimensioning of the carrier or template, sufficiently 
small quantities of solder can be positioned on close spacing between 
centers, these proposals do not address the problem of high shear strains 
in the solder joints. 
As noted above, among the factors considered in forming acceptable 
electrical connections between the CCP and the PCB is that the connections 
must be able to withstand stresses developed due to the effect of 
temperature fluctuations and the differences in thermal expansion 
coefficients between the material of the CCP and the substrate or circuit 
board on which it is mounted. Thus, a CCP may be made of a ceramic 
material and the circuit board may be made of an epoxy-glass composition, 
and when subjected to elevated temperatures these elements will expand at 
different rates, inducing stresses in the connections. 
Even if the materials used in the CCP and the circuit board have thermal 
expansion coefficients which are close in value to minimize the 
differential expansion effects, heating/cooling cycles which result when 
power is applied across the CCP induce a temperature differential between 
the CCP and the PCB to produce stresses in the joints. 
It is well known, and as summarized below, that if the solder joint is 
formed into a "long" column configuration in which the height of the 
column is much greater than the diameter or transverse dimension of the 
joint, less stress is induced in the joint and consequently the joint has 
greater reliability and longer life. 
A common deficiency of the solder connections described in the foregoing 
references is that the height of the joint is smaller than its width or 
diameter, and the height is significantly smaller than the 
center-to-center distance between consecutive joints. This is true since 
the joints are formed from free solder preforms in a variety of shapes. 
None of these preforms can sustain a column-type shape after melting and 
during the soldering process due to the low strength and viscosity and 
high surface tension of molten solder. These joints contrast with the 
present invention in which the height of the resultant joints is greater 
than the transverse dimension of the solder joints. 
In the patent to Krall, U.S. Pat. No. 3,921,285, a method is described for 
joining microminiature components to a carrying structure in which the 
height of the electrical connections may be adjusted during original 
joining of the component to the carrying structure or in a two-step solder 
reflow process. The method involves the elongation of the solder joints 
between the component and its carrier, and is accomplished by the use of a 
vaporizable material which is either liquid at room temperature or becomes 
liquid before the solder melts. A bridge is positioned over the component 
and the vaporizable material is placed between the bridge and the 
component surface opposite the surface on which the connections are 
formed. Heating to achieve soldering causes the material to vaporize, and 
the combined action of vaporization and surface tension pulls the 
component closer to the bridge, which in turn elongates the solder joint. 
Upon cooling the joint remains fixed in its elongated shape. 
While Krall provides for elongated solder joints, the device is 
structurally complex and difficult to use. Specifically, an additional 
lifting structure is required to operate while the solder is in a molten 
state. If the motion of the lifting device is too great, or if the solder 
quantities are not uniform, then some solder joints may be ruptured. 
Additionally, the lifting structure is an obstacle to cleaning and 
inspection. Finally, this method will function only when the ratio of 
solder column height to diameter is relatively small. 
OBJECTS OF THE INVENTION 
It is a primary object of the present invention to provide an apparatus and 
a method for the precise placement of a predetermined quantity of material 
for the formation of a solder-type connection between 
electrically-conductive elements. 
Another object of the invention is to provide an apparatus and a method of 
the foregoing type for the placement of a connection-forming material 
having a predefined configuration. 
Another object of the invention is to provide an apparatus and a method of 
the foregoing type for the placement of a connection-forming material 
preform having a height dimension greater than its transverse dimension. 
Yet another object of the invention is to provide an apparatus and a method 
of the foregoing type for the formation of solder-type connections which 
are resilient and better able to withstand fatigue or repetitive thermal 
cycling. 
Still another object of the invention is to provide an apparatus and a 
method of the foregoing type for the simultaneous formation of a plurality 
of resilient solder-type connections between a plurality of 
parallel-disposed electrically-conductive elements. 
A further object of the invention is to provide an apparatus and a method 
to attach an electronic component to a circuit board. 
Yet a further object of the invention is to accurately align the CCP with 
the PWB. 
A specific object of the present invention is to provide an apparatus and a 
method to attach an electronic component to a circuit board with resilient 
solder joints having a height/length dimension greater than the transverse 
dimension which are better able to withstand thermal cycling/fatigue 
stresses and to accommodate dimensional irregularities in the component, 
the circuit board and the conductive elements thereon. 
SUMMARY OF THE INVENTION 
The foregoing and other objects are achieved by the interconnection preform 
placement device of the present invention which includes a retaining 
member having a predefined pattern of holes in which are positioned 
preforms of joint-forming material, such as solder. Each preform is of a 
predefined configuration and has a height greater than its cross-sectional 
dimension. The preform retains its configuration after the interconnection 
or soldering process to form a resilient joint which is more capable of 
withstanding stress, strain and fatigue. 
The method comprises placing the interconnection preform placement device 
between parallel patterns of electrically-conductive elements, such as the 
conductive pads on an electronic component and those on a circuit board, 
and effecting the bonding and interconnection of the conductive elements 
with the preforms. 
The preferred preform configuration is a slender column, but other shapes 
are encompassed within the invention, and the preforms may be reinforced 
with suitable material disposed therein. The preform carrier may be left 
in the assembled mounting, or it may be designed to be removed afterwards. 
The preform carrier may be left in the assembled mounting to perform a 
structural, thermal and/or electrical function.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the description herein, the solder joining of a chip carrier package, 
which is referred to hereafter as a "chip device" or a "chip carrier", and 
a circuit board is used as an illustrative application of the present 
invention. The joining of other electronic components is also 
comprehended, and the invention may be employed wherever and whenever a 
solder connection of increased reliability and longer life is desired. 
Referring to the drawings, and more particularly to FIG. 1, a joint or 
connection made in accordance with techniques known in the prior art is 
shown formed between a chip device 10 and a circuit board 12. The chip 
device 10 has a plurality of electrical contacts which may be disposed 
along its edge. Only one contact area is shown for the sake of simplicity. 
The circuit board 12 has a plurality of complementary contact areas on its 
surface (only one shown). The circuit board 12 may be a printed circuit 
board (PCB) or a printed wire board (PWB), and may be referred to 
hereinafter simply as a "board" or "circuit board." In a known fashion, 
the chip device will be disposed above the surface of the circuit board 
such that the contact areas are vertically aligned and mechanically and/or 
electrically joined, such as with a solder connection. In the illustration 
of FIG. 1, the opposed, facing surface of the chip device 10 and the 
circuit board 12 is provided with a layer 14 of conductive material, such 
as copper, and these layers are interconnected with a suitable solder 
composition 16, which may be of known tin-lead alloy. 
As noted above, a solder connection such as shown in FIG. 1 is subjected to 
stresses induced by thermal and mechanical forces. These stresses are the 
result of strains produced by mechanical deformation, differences in 
temperature between the chip device and the printed circuit board and/or 
differences in the thermal coefficients of expansion between the chip 
device and the PCB. For example, thermal stresses may result from power 
cycling even when the materials of the chip device and circuit board do 
not exhibit any differences in the coefficient of thermal expansion, but 
because of the power applied to the chip device, a temperature 
differential exists between the chip device and the circuit board. 
It can be seen that as the connection is subjected to repeated heating and 
cooling cycles, it will ultimately fail due to fatigue, and the connection 
will fail earlier if the stresses induced therein are high. Therefore, to 
prolong the life of the joint and to increase its reliability, the 
resultant stresses therein should be reduced. In order to determine how 
the stresses can be reduced, the factors governing the creation of the 
stresses in the joint should be considered. 
When the temperature of a CCP mounted on a PWB is raised by .DELTA.T, the 
length of the CCP increases by the amount .delta..sub.c 
EQU .delta..sub.c =L.sub.c .multidot..alpha..sub.c .multidot..DELTA.T (1) 
and the length of the PWB increases by an amount 
EQU .delta..sub.B =L.sub.B .multidot..alpha..sub.B .multidot..DELTA.T (2) 
where 
L.sub.C =Original length of the CCP 
L.sub.B =Original length of the PWB 
.alpha..sub.c =Thermal coefficient of expansion of the CCP material 
.alpha..sub.B =Thermal coefficient of expansion of the PWB material. 
The difference between .delta..sub.B and .delta..sub.C is the net change in 
length between the CCP and the PWB due to the .DELTA.T and is defined as 
.delta.. 
##EQU1## 
So 
EQU .delta.=L.multidot..DELTA.T (.alpha..sub.B -.alpha..sub.C) (4) 
since L.sub.B =L.sub.C. 
This change in length between the CCP and the PWB is usually accompanied by 
an internal force F which causes an elastic/plastic deformation in the 
three main elements of the assembly, i.e., the CCP, the PWB and the 
interconnections. It is known to those skilled in the art that if all 
three of these elements are very stiff then the force F will, of 
necessity, be high and potentially destructive. It is similarly known that 
if the stiffness of the interconnections can be reduced (i.e., made more 
flexible), then there will be a concommittant reduction in the peak force 
F for any given value of .delta.. 
The stiffness of the various joint element designs may be approximated as 
follows: 
By definition stiffness is K=F/f 
where 
F=Applied force 
f=Deflection 
K=Constant of stiffness 
If H is small, as in the prior art jointing methods illustrated in FIG. 1, 
then the joint stiffness is: 
EQU K=GA/H 
where 
G=Shear modulus of elasticity 
A=Joint area 
H=Height of the joint 
From this expression it is evident that to reduce the stiffness K of the 
prior art joint, it is necessary to decrease the area of connection A or 
the shear modulus of elasticity G or to increase the height H. 
The shear stress .sigma. in the joint may be expressed; 
EQU .sigma.=F/A (5) 
so if the area of the joint A were decreased to reduce the stiffness, this 
would simultaneously increase the stress proportionally, as shown by 
equation (5). Increasing the area of the joint is not generally beneficial 
and, of course, this strategy would take additional space, thus defeating 
the primary goal of this type of package, i.e., closer lead spacing. 
Increase in the joint height, however, has the benefit of lower joint 
stiffness, without an increase in stress. 
A more detailed consideration of joints with a larger H dimension, as shown 
in FIG. 2, will now be undertaken. 
If the height H between the opposed surfaces of the conductive paths, such 
as 10 and 12 in FIG. 1, is relatively large, then the resultant critical 
stresses are due to bending deformation of the joint. This can be 
represented as shown in FIG. 2 wherein the joint between the contact 10 
and the conductive land 12 is represented by a column 18 of solder. The 
stresses in the joint can be evaluated by considering the column as a 
cantilevered beam, fixed at one end, such as to the conductive land 12, 
with the force F, again assumed to be constant, acting transversely upon 
the free end of the column. The deflection f at the tip of the beam and 
the bending stress .sigma..sub.b are given by the well-known equations: 
EQU f=FH.sup.3 /(3EI) (2) 
EQU .sigma..sub.b =FH/Z (3) 
For a round beam, or a round column, the relationship between the section 
modulus Z and the moment of inertia I is: 
EQU Z=I/c=2I/d (4) 
where 
EQU I=.pi.d.sup.4 /64 (5) 
where d is the diameter. The section modulus Z then is: 
EQU Z=.pi.d.sup.3 /32 (6) 
Substitution of equation (5) into equation (2) results in the deflection 
equation (7) below, and substitution of equation (6) into equation (3) 
yields the stress equation (8) below: 
EQU f=(64FH.sup.3)/(3.pi.Ed.sup.4) (7) 
EQU .sigma.T.sub.b =(32FH)/.pi.d.sup.3 (8) 
The force F in FIG. 3A is produced as the result of dimensional expansion 
or contraction of the joint due to the thermal cycling, which is 
represented by the deflection f in equation (7) above. Then, to get the 
relationship between the deflection and the stress, equations (7) and (8) 
can be combined as follows: 
EQU F=(3.pi.EdE.sup.4 f)/64 H.sup.3 (9) 
EQU .sigma..sub.b =3 Edf/2 H.sup.2 (10) 
From the relationship of the stress .sigma..sub.b given in equation (10) 
above, it can be seen that in order to reduce this stress, the value of 
the numerator should be reduced, principally by using a smaller value for 
d, i.e., decreasing the diameter of the cylindrical column; or by reducing 
E, i.e., using a more flexible joint material; or by increasing the value 
of the denominator, such as by increasing the height H of the column. 
Since the height H is squared, an increase in this parameter from one 
value to twice that value reduces the stresses in the column to one fourth 
of the initial level. This indicates that the height H need not be 
drastically increased in order to get an appreciable reduction in the 
stresses in the column. 
Additionally, by using a somewhat higher column of smaller diameter, not 
only are the resultant stresses therein reduced, but this permits higher 
density of joints between the chip devices and the circuit board, a result 
which is very desirable for increased packing density. 
A more rigorous and more exact representation of the deflection of the 
column shown in FIG. 2 is shown in FIG. 3B, in which one end of the column 
is fixed and the other end is guided. In this situation, the following 
relationships exist for the deflections f, and the stresses .sigma..sub.g 
at each of the two ends and at the center of the beam: 
EQU f=FH.sup.3 /(12EI) (11) 
EQU .sigma..sub.g =FH/2Z at ends (12) 
EQU .sigma..sub.g =0 at center (13) 
Rearranging equations (11), (12) and (13) in the same manner as done 
previously, the following relationships are obtained for the force F 
acting on the joint and the stress .sigma..sub.g : 
EQU F=(12EIf)/H.sup.3 (14) 
EQU .sigma..sub.g =(6Edf)/H.sup.2 =4 (.sigma..sub.b) (15) 
From equation (15), it can be seen that the stress is four tims larger than 
if the connection is considered as a simple cantilevered beam (equation 
10). However, the diameter of the beam d and the modulus of elasticity E 
are still in the numerator, and the height H is squared and appears in the 
denominator, which is identical to the stress given by the equation (10). 
Thus, the stress relationship derived from the more rigorous model of the 
joint still supports the above conclusions. 
Furthermore, from equation (13) it can be seen that at the center of the 
column there are no bending stresses. Consequently, a column-type joint, 
with varying diameters, such that the diameter at the cenfter of the 
column is smaller than the diameter at both ends (i.e., like an 
hourglass), would be more uniformly stressed and would provide greater 
flexibility. 
A more detailed finite element analysis of similar joint structures was 
presented by E. A. Wilson, Honeywell, Phoenix, Ariz., and E. P. Anderson, 
Honeywell, Bloomington, Minn., in their paper entitled "An Analytical 
Investigation into Geometric Influence on Integrated Circuit Bump Strain," 
which was presented at the 33rd Electronic Components Conference, May 
16-18, 1983, In Orlando, Fla. (Proceedings page 320-327), incorporated by 
reference herein. This analysis confirms the advantages of columnar 
structures. 
It is understood that by reducing the peak stresses as described above in a 
repetitive loading situation, the number of cycles required to produce 
failure is greatly increased. 
From the foregoing evaluations, it can be seen that the life of the solder 
joints can be increased substantially by a relatively small increase in 
solder joint height. Likewise, a reduction in solder joint diameter will 
extend the life of the solder joints. A concomittant advantage in using 
joints of increased height and/or reduced diameter is the opportunity for 
increased packing density. 
The foregoing advantages of reduced stresses and extended life in the 
solder joints and the opportunity for increased packing density are 
incorporated in the present invention which provides a device for the 
precise positioning of solder preform of slender cylindrical shapes at 
predefined locations. In the embodiment of the invention illustrated in 
FIG. 4, the interconnection preform placement device 20 includes a 
retaining member 22 of electrically nonconductive material having a 
configuration and size substantially the same as the chip carrier for 
which the preform placement device is used in mounting the chip carrier on 
a suitable substrate, such as a circuit board. The holder 22 has a 
cenftral cut-out 24 to form a perimeter or bordering portion in which are 
spaced at predetermined locations a plurality of holes 26 which receive 
preforms 28 of solder in the shape of slender cylindrical columns, such as 
shown in FIG. 5. The retaining member may also hereinafter be referred to 
as a "holder." 
The location and positioning of the holes 26 on the retaining member 22 are 
determined by the spacing of the conductive contacts disposed on the edge 
of the chip carrier to be mounted. Generally, the height of each solder 
preform 28 will be somewhat taller than the thickness of the retaining 
member 22 such that the upper and lower edge portions of each solder 
preform 28 will extend above the corresponding surfaces of the retaining 
member, and in use these exposed surfaces will make physical contact with 
the corresponding conductive pads on the chip carrier and on the circuit 
board. 
While not specifically shown in FIG. 4, the preform placement device is 
provided with appropriate means for properly locating and orienting the 
retaining member 22 with respect to the chip carrier and the circuit board 
so that the conductive pads on the chip carrier, the conductive pads on 
the circuit board and the column solder preforms are properly aligned. 
Such locating means are known, and may include a chamfered corner which 
mates with a similarly-configured surface on the chip carrier. One or more 
of the corners may have an indexing notch. Additionally, two pine may be 
provided on the bottom surface of the retaining member 22, or two holes to 
accept two pins which register with positioning holes provided on the 
circuit board. A combination of such positioning techniques may be 
incorporated into the holder 22. 
It is possible to vary the composition of the solder material such that the 
solder preform has a consistency which will give or compress under the 
weight of the chip carrier to be mounted. This compressibility of the 
solder preform is important because the chip carrier and the circuit board 
are typically not truly flat so that the lengths of the solder columns 
must change during the soldering process to accommodate these 
irregularities. 
The retaining member 22 may be made from any suitable material, and 
preferably is of an electrically-nonconductive material. The material may 
be a single sheet or layer of desired thickness, or may be a laminate of a 
plurality of thin sheets or layers of suitable material or may be a 
plurality of elements which form a sheet-like structure. Such material 
would include, but is not limited to, glass matt and high-temperature 
polymeric materials such as Ultem.TM.*. The material of the retaining 
member 22 should be sufficiently rigid and temperature-resistant to 
maintain the solder preform's position during the soldering process. 
FNT *Ultem is a trademark of General Electric Company. 
Preferably there are two categories of retaining members, those that are 
removable and those that become a permanent part of the interconnection. 
The removable type could be dissolvable, frangible, segmented, or 
deformable without harming the preforms. The permanent or nonremovable 
type can be passive or can perform electrical, mechanical and/or thermal 
functions. The removable type can therefore be destroyed by dissolving or 
breaking apart. The glass matt embodiment can be physically pulled away in 
whole or piece by piece. The Ultem.TM. embodiment can be dissolved by 
well-known chemical means without harming the preform. 
It is within the scope of the invention to make a segmented retaining 
member that can be stripped away after interconnection. The retaining 
member 22 in FIG. 4 can be provided with lines of weakness or cuts as 
shown at 29 in phantom line. In this way, the retaining member 22 may be 
removed in one or more pieces and directions. The retaining member may 
also originally be made from individual elements as will be discussed 
later. 
Thus it can be seen that the removable retaining member may also be 
conductive such as an aluminum foil in order to maintain the position of 
the preforms during interconnection. It is important that such a retaining 
member not bond to the preforms during the process. 
Finally, the retaining member can be made from a thin sheet of solder 
material which could melt and flow into the solder-like preforms during 
the soldering process. 
The retaining member which remains in place should be generally 
nonconductive and relatively flexible so that it does not interfere with 
the motion of the preform columns. Additionally, it may provide an 
impedence-matched interconnection if it is a combination of conductive and 
dielectric materials so that it provides a transmission line or microstrip 
effect, or a coaxial type shield around the preforms. 
In another embodiment the retaining member could be made from a 
nonconductive material which, however, becomes conductive when a specific 
voltage threshold is exceeded, this threshold being just above the normal 
operating voltage of the circuit. This type of retaining member would 
provide protection to the chip device by shorting out any potentially 
damaging transient overvoltages which may result from electrostatic 
discharge or other electrical faults. 
In some applications, it may be desirable to make the retaining member from 
a material which exhibits heat-recovery. This type of retaining member 
could be reinforced with glass fiber or the like to control its 
coefficient of thermal expansion but locally the holes which are provided 
for retaining the solder-like columns could be fabricated so as to 
contract in diameter during soldering and thereby extrude the solder 
column up to meet the CCP and PWB. 
Although the retaining member has been shown to position only 
interconnection preforms in FIGS. 4 and 7-10, it is understood that other 
elements could also be positioned by the retaining member and attached to 
the chip device and board. A heat-sink device could be carried in the 
central opening of the retaining member of FIG. 4 and bonded to the CCP 
and PWB during soldering. Similarly, a vibration damper, structural 
reinforcement, or a Peltier-type cooler could be positioned and attached 
to the CCP and the PWB. Also, an electrical ground plane could be 
positioned near the interconnection preforms to beneficially modify their 
electronic impedance characteristics similar to a microstrip board trace. 
FIG. 4 shows a square peripheral array of interconnections. However, other 
patterns or arrays could also be accomplished by this invention. Any nxm 
matrix of interconnections on a regular rectilinear format could be 
provided. Non-rectilinear or partially filled arrays could also be 
provided. 
If the interconnections are arranged sufficiently close together, a 
random-type interconnection system could be produced. In this type of 
system, since the interconnection density is much higher than the density 
of the pads on the CCP and PWB, then statistically there will always be at 
least one interconnecting preform located between each pad set to be 
interconnected. The arrangement of the interconnections is completely 
random in this type of holder system. This type of retaining member may 
ideally be made from a plurality of elements bonded together, each having 
at least one aperture therethrough. The elements such as elements 31, 
shown in phantom in FIG. 4, may be of uniform or random cross-section in 
order that they may be efficiently bundled and permanently or temporarily 
joined together such as by fusion or proper adhesive material to form a 
sheet-like structure. In such a bundled configuration the retaining member 
elements may be formed by continuously extruding the retaining member 
material over the preform, cutting the extruded composite into discrete 
pieces, bundling the axially-aligned pieces and preferably joining the 
retaining member material and then slicing the assembly at right angles to 
the axial alignment to make a preform placement device. 
Also shown in the configuration shown in FIG. 4, the retaining member 22 
can be ideally stamped from a sheet of material, or otherwise cut from 
sheet material. Quite possibly the retaining member fabricating process 
would provide for the simultaneous stamping of the holder shape and the 
central cut-out 24 and formation of the properly-spaced holes 26, and 
insertion of the solder preforms into the holes. Of course, it is possible 
that the production of the retaining member 22 can be achieved in a 
sequence of steps in which the retaining member with its cut-out and 
orientation surfaces is stamped from a sheet material, and subsequent 
techniques are used to form the holes into which the solder preforms are 
inserted. Other manufacturing methods such as molding or insert molding 
are withining the scope of the invention. 
The retaining member 22 may be fabricated from a sheet of a laminate of 
heat-recoverable material in which the retaining member is stamped from 
the material sheet, the holes formed therein, and the solder preforms 
position within the holes. Subsequently, the retaining member is heated to 
a temperature sufficient to cause the material to recover such that the 
diameters of the holes 26 are reduced, but at a temperature below the 
melting point of the solder preforms. The use of a heat-recoverable 
material for the retaining 22 affords the advantages that the holes can be 
formed to a predefined shape, such as an hourglass, with varying 
diameters, such that the diameter of the middle section is smaller than 
the diameter at the ends. These holes would be expanded to a uniform 
diameter larger than the diameter of the preforms so that the preforms can 
be more readily positioned in the holes, after which the retaining member 
22 is heat-recovered, causing each hole to constrict to a size 
substantially identical to the diameter of the preform, thus securely 
gripping the preform within the retaining member. Furthermore, the use of 
a heat-recoverable material affords the advantage that in use, when heat 
is supplied to effect the solder joint, such as by a solder reflow 
process, the firm gripping of the solder preforms by the heat-recovered 
retaining member material will impart the original hourglass shape of the 
holes to the preform, thus improving the flexibility of the column-type 
joints, and in turn reducing the stresses and improving the reliability of 
the joints. 
It is also possible to further control the ultimate configuration of the 
solder joint by the use of a heat-recoverable material in the retaining 
member 22. This can be achieved by partially recovering the material, 
causing the holes to securely grip the preforms. Then, during the 
soldering process, sufficient heat is applied to cause the material of the 
retaining member to further recover, thereby further decreasing the 
diameter of the holes to apply a constrictive force onto the molten or 
softened preform. This will have a tendency to produce column joints of 
height greater than the thickness of the retaining member 22. 
While the retaining member 22 has been shown in FIG. 4 as a perimeter 
configuration, it is possible that the retaining member can be a layer of 
material without any cut-outs therein, and in which the necessary number 
of solder preforms are properly located throughout the entire surface of 
the retaining member as necessitated by the number, location and 
configuration of the conductive pads to be joined. 
The solder preforms 28 may be made of any suitable material, such as a 
tin-lead alloy of appropriate composition or a conductive elastomer. The 
preforms can be produced by any suitable technique, such as by the 
continuous extrusion of the solder material through appropriate-sized dies 
and cutting the extrudate to the proper lengths. The preforms may be 
molded. To improve the mechanical strength of the preforms and help 
maintain the columnar configuration during the soldering process, 
particles of metallic or non-metallic material may be embedded in the 
solder preforms, such as shown in FIG. 6. For example, discrete pieces 30 
of metallic material, such as pieces of copper, may be embedded in the 
solder preforms by mixing the particles in the composition prior to 
extrusion, and then extruding the mixture in a known fashion. As shown in 
FIG. 6, the discrete particles may be aligned along the longitudinal axis 
of the extrudate. This alignment can be achieved with appropriate 
techniques, such as the application of a magnetic force during the 
extrusion process, or simply by the shearing force applied by the extruder 
on the extruding material. 
The particles mixed into the solder should have a melting point above the 
melting point of the solder, and good metallurgical, mechanical and 
electrical properties. 
In addition to copper, discussed above, fillers could include nickel, iron, 
and metal-coated high-temperature polymer or glass films with a high 
aspect ratio. These materials could be discrete particles or continuous 
lengths with a single strand or many strands in each preform. These 
materials may also be a continuous strand of conductive material such as 
wire with filled or unfilled solder material located at the ends thereof 
for interconnection purposes as will be appreciated and discussed with 
respect to FIGS. 11-13. Solder could completely coat the strands or 
fibers, or could be deposited only at the ends of inherently conductive 
fibers. Additionally, the interconnection bonding agent or solder could be 
added in a separate operation. Thus, continuous conductors or fiber 
bundles could be retained by the retaining member and then attachment 
would be accomplished by immersing the assembly in molten solder which 
will wick and wet the components and make the electrical and mechanical 
interconnection. 
Additionally, the particles may be oriented in any other desirable 
alignment, and the relative content of the particles in the solder 
preforms, as well as the size of the particles relative to the height of 
each preform, can be tailored to the requirements of the joints to be 
formed. Furthermore, the surfaces of the solder preforms may be coated 
with a suitable flux, or the flux may be coated only on the end portions 
of the preform so that during the soldering process the flux will coat the 
respective contact areas on the conductive pads to ensure proper flux of 
the solder. The flux may also be incorporated within the preform. 
The use of the preform placement device 20 in mounting a chip carrier 32 to 
a suitable substrate, such as a circuit board 34, is illustrated in FIGS. 
7 and 8. The retaining member 22, with the solder preform 28 secured 
therein, is positioned between the lower surface of the chip carrier 32 
and the opposing upper surface of the circuit board 34, and properly 
located so that the end portions of each solder preform 28 make contact 
with the conductive contacts on the chip carrier and the conducting land 
on the circuit board 34. Means may be provided for orienting the chip 
carrier 32 relative to the circuit board 34, such as by providing 
orientation holes 36 in the chip carrier which are vertically aligned with 
corresponding holes 38 in the circuit board and holes 39 in chip carrier 
32 and by inserting pins through the aligned holes. after the soldering 
process, the ends of the column solder preforms 28 are securely bonded to 
their respective conductive contacts and lands the chip carrier 32 and the 
circuit board 34, as shown in FIG. 8. 
FIG. 4 illustrates a flat stamped retaining member but, of course, the 
holder could be formed by stamping, folding or molding into a cup-shaped 
structure into which the CCP could be accurately placed. Detent features 
could be provided as pressure-sensitive or hot-melt adhesives could be 
provided within the retaining member so that the chip carrier could be 
preassembled to the CCP before application to the PWB. This subassembly 
could then be located to the PWB using fixtures, pick and place equipment, 
alignment features such as holes or pins, or the like. Adhesives or pins 
could be provided in the center surfaces of this retaining member as 
discussed earlier with respect to FIG. 4 to maintain position during the 
soldering operation. 
FIG. 4 illustrates a single retaining member but, of course, these 
compounds could conveniently be supplied connected together like a 
bandboiler for convenient assembly packaging and application. 
During the soldering process, it is understood that suitable means will be 
utilized to maintain good contacts between the conductive elements on the 
chip carrier 32 and the circuit board 34 until the solder joint has 
solidified. Techniques for maintaining this contact are known. Another 
technique for providing this retaining force is shown in FIGS. 9 and 10 
and can be incorporated into the retaining member for the solder preforms. 
As shown in FIG. 9, an illustrative number of solder preforms 28 are 
disposed in holes provided in the retaining member 40 which is made from a 
layer of heat-recoverable material, and each surface is formed with a 
depression or recess 42. Formation of the recess 42 causes the opposite 
surface of the retaining member to be raised in a correspondingly-shaped 
protrusion or bump 44. The elevated planar surface of the protrusion 44 is 
coated wih a suitable adhesive 46. The recesses 42 and the protrusions 44 
can be conveniently formed by a stamping operation in which 
appropriately-shaped dies are pressed onto the opposed surfaces of the 
holder 40, causing recesses 42 to be formed in one surface and forcing the 
material out in the form of protrusion 44 on the other surface, as shown 
more clearly in the cross-section of FIG. 10. 
Once applied between the chip carrier 32 and the circuit board 34, the end 
portions of each solder preforms 28 make contact with the conductive 
elements 10 and 12 on the chip carrier and circuit board, and the adhesive 
46 on the surfaces of the protrusions 44 make contact, respectively, with 
the opposed surfaces of the chip carrier and the circuit board, thus 
holding the chip carrier to the circuit board. During the soldering 
process, the application of heat causes the heat-recoverable material of 
the retaining member 40 to recover in a known fashion, causing the 
recesses 42 and the protrusions 44 to revert to the flat configuration of 
the retaining member, thus pulling the chip carrier 32 toward the circuit 
board 34 and causing the solder to wet the contact elements on each 
respective device. 
The shape at the recesses and protrusions shown in FIGS. 9 and 10 are 
illustrative only; other configurations may be equally suitable. The 
trapezoidal configuration of the recesses 42 and protrusions 44 shown in 
the drawings are particularly advantageous in that they provide a 
relatively large flat surface onto which the adhesive 46 may be applied, 
and the form of the recesses and protrusions can be easily made in the 
retaining member 40. In use, the large-area adhesive layers provide a 
strong gripping force to the respective surfaces of the chip carrier 32 
and the circuit board 34, and the contractive force produced by the 
recovering material of the retaining member 40 exerts sufficient force to 
pull the chip carrier down toward the circuit board 34. The shapes also 
aid in venting between chip and board. Obviously, adhesive can be put as 
well on flat retaining members, without any protrusions, simply to secure 
the device on the board and the CCP on the device (with or without 
heat-shrinking) or for vibration damping, etc., as discussed earlier. 
Although the solder preforms considered thus far have been of slender 
cylindrical configurations, other shapes are equally suitable, depending 
upon the requirements of the mounting. Preforms with sqaure, hexagonal or 
other shapes of cross-sections can be used. 
Furthermore, some examples of other configurations are shown in FIGS. 
11A-C, FIG. 12 and FIG. 13. The S-shape of the preforms shown in FIGS. 
11A-C provides joints of great flexibility which permit relatively large 
displacements between the chip carrier 32 and the circuit board 34 without 
inducing undue stresses in the fixed portions of the joint. With the 
reverse S-shape preform 48 shown in FIG. 11A, a substantially large 
contact area is afforded at the upper portions of the preform making 
contact with the conductive pads 10 and 12 on the chip carrier 32 and the 
circuit board 34. The preform 48 is supported by two parallel-disposed 
holder layers 50 and 52. 
The S-shaped preform 54 of FIG. 11B affords the same advantages as the 
preform 48 of FIG. 11A and, additionally, provides two probe areas P and 
P.sup.1 which may be used to test for electrical continuity of the 
connection. This configuration is particularly advantageous when used in 
making connections along the periphery of the chip carrier when the 
contacts are spaced along the edges of the chip carrier inasmuch as the 
probe area are readily accessible. As with the preform 48, the preform 54 
is supported by parallel-disposed retaining member layers 50 and 52. In 
fabricating the preforms 48 and 54 and positioning them into the retaining 
members 50 and 52, the preforms are initially straight elements which are 
inserted into the corresponding, vertically aligned holes in the retaining 
member layers 50 and 52 and the ends are bent into the configuration shown 
in FIGS. 11A and 11B. 
The S-shaped preform 56 as shown in FIG. 11C is provided in a single 
retaining member layer 58 having a thickness substantially greater than 
the individual retaining member layers 50 and 52 shown in FIGS. 11A and 
11B. Otherwise, the preform 56 affords the same advantages as the preforms 
48 and 54. 
The preform 60 shown in FIG. 12 is of C-shape configuration which is 
suitably attached along the edge of the retaining member 58. Due to its 
unique configuration the C-shape preforms 60 is most advantageously used 
when disposed along the peripheral edges of the retaining member 58. The 
preform 60 provides a very substantial joint surface which may be used to 
test for electrical continuity between the chip carrier 32 and the circuit 
board 34. 
Although the preforms have been generally described as being made of solder 
filled with discrete particles or continuous strands, it is within the 
scope of the invention to make the preforms from a continuous length of 
conductive material such as wire with filled or unfilled solder material 
located at the ends thereof for interconnection purposes. 
For maximum flexibility and resilience between the chip carrier 32 and the 
circuit board 34, the coiled spring configuration 62 shown in FIG. 13 is 
ideal. The preform 62 could be readily formed by extruding the solder 
composition as a continuous extrudate, forming it onto a coiled 
configuration of suitable spring material of desired diameter and length, 
and appropriately supporting each spring preform within the retaining 
member 58. The degree of resilience afforded by the preform 62 can be 
controlled in substantially the same fashion that the parameters 
regulating the performance of conventional springs are controlled, such as 
controlling the diametric sizes of each turn of the spring, the length of 
the spring and the diameter of the extrudate from which the spring is 
made. 
Since each solder preform is individually placed in a given location, the 
preform configuration can be tailored to meet the specific requirements of 
a particular joint. Thus, for example, the diameter and/or height of the 
cylindrical preform at certain locations can be different from the 
preforms at other locations to meet the specific needs of the joint being 
formed. Conceivably, each joint may be unique and may incorporate a 
different solder preform. Additionally, it is possible to combine 
different preform configurations, such as combining the cylindrical 
preforms with any of the preforms shown in FIGS. 11-13 to meet the 
specific needs for resilience and stress reduction in one or more 
particular solder joints. 
The interconnection preform placement device of the present invention 
provides a unique and convenient technique for accurately positioning a 
plurality of preforms between a chip carrier and a circuit board to which 
the carrier is mounted. The use of cylindrical column preforms of solder 
results in solder joints of low bending stiffness and hence low shear 
stresses, which contribute to high fatigue resistance in the joints. The 
use of the column-shaped perform in the present invention ensures that the 
desirable column configurations will be retained during the solderng 
process and that the formed solder joints will be of column shape having 
low shear stresses distributed therethrough. 
In addition to the embodiment of FIGS. 9 and 10 described above, the 
interconnection perform placement device may have a layer of 
pressure-sensitive adhesive on its surfaces, with or without a "release" 
paper or cover. This adhesive will maintain the device on the circuit 
board and the chip package on the device during handling prior to the 
soldering process. The adhesive is applied in such a way as not to 
interface with the soldering or solder reflow process. 
The retaining member may be made of a suitable high-temperature material 
capable of sustaining the heat applied during the soldering process and be 
of an electrically insulating material to be left in place after soldering 
to provide an electrical insulator and an environmental seal. 
Alternatively, the retaining member may be made of material which is 
heat-soluble, chemically soluble, or disintegrable such that after 
soldering the retaining member can be dissolved or disintegrated and 
removed from the mounting to provide clearance for flux removal, for 
instance, or for other procedures to complete the installation. 
The concepts embodied in the present invention may be adapted for use in 
attaching a chip to a chip carrier or a chip directly to a circuit board, 
or to attach leaded CCPs or hybrid thick-film-type chip carriers to 
circuit boards. Multiple preform placement devices or larger-scale 
placement devices can accommodate the simultaneous bonding of numerous 
chip carrier packages. Further, the interconnection preform placement 
device may be placed between two circuit boards to interconnect vertically 
the conductive pads of boards. 
While preferred embodiments of the invention have been illustrated and 
described, it will be appreciated that variations therefrom may be made 
without departing from the scope of the invention as defined in the 
appended claims.