Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1997-05-15
2001-01-09
Young, Lee (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S844000, C029S842000, C228S179100
Reexamination Certificate
active
06170155
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a system of components to be hybridized adapted to a hybridization technique by melting of solder balls of the type referred to as “flip-chip”. The invention also relates to a process for preparing these components and a hybridization process using these components.
The invention will be applied particularly in the electronic and optical fields for interconnection of components made of different materials.
For example, the invention is particularly useful for interconnection of silicon components with AsGa or InSb components.
For the purposes of this invention, a component may be an electronic component such as an electronic chip, an electronic or opto-electronic circuit support, or a mechanical component such as a cover or a physical magnitudes sensor.
In particular, the invention may be used for the manufacture of infrared detectors, the manufacture of lasers with a vertical cavity, or to transfer a matrix of AsGa read photodiodes onto an Si read circuit.
2. State of Prior Art
A distinction is made between two main techniques for hybridization of components by solder balls.
The first technique is called the “hybridization by melting technique” (or “flip-chip”), and uses balls of a meltable material, for example such as a tin and lead alloy (SnPb) or a tin and indium alloy (SnIn). This technique is shown in the drawing on
FIGS. 1 and 2
in the appendix.
On these figures, references
10
and
12
refer to first and second components to be interconnected, respectively. The first component
10
comprises the first ball reception pads
14
a
, made of a material wettable by the solder balls material. Pads
14
a
are surrounded by an area
16
a
of material not wettable by the material in the balls.
Similarly, the second component
12
has second pads
14
b
also made of a material wettable by the balls material and surrounded by an area
16
b
of unwettable material.
The first and second pads
14
a
,
14
b
are associated to form pairs of pads, in locations complementary to the first and second components respectively. Thus the first and second pads in each pair of pads are located approximately opposite each other when the components to be hybridized,
10
,
12
are placed in contact with each other as shown on FIG.
1
.
It is considered that
FIG. 1
shows the closed structure consisting of the two components at ambient temperature denoted Ta. Let L be the distance that separates the two pads on which the balls fit on each of the component.
Pads
14
a
of one of the components, in the event component
10
, are equipped with balls
18
made of a meltable material. These balls
18
are designed to create a mechanical and/or electrical link between the pads in each pair of pads.
In order to do the hybridization, component
12
is moved onto component
10
in order to bring the pads
14
b
into contact with the corresponding balls
18
. The entire structure is then heated up to or greater than the melting temperature of the balls so that balls
18
are soldered onto pads
14
b.
All balls are thus soldered simultaneously onto their corresponding pads
14
b
on component
12
. The structure shown on
FIG. 2
is obtained after cooling. On this figure, each pad
14
a
of component
10
is mechanically and electrically connected to a corresponding pad
14
b
on component
12
, by a solder ball
18
.
The precision of the mutual positioning of components when component
10
is transferred to component
12
is not very critical. When the ball material melts, components
10
and
12
automatically align with each other under the effect of the surface tension of the material making up the molten balls.
In the hybridization procedure described above, all solders between the balls and reception pads are made at the same time. The hybridization by fusion process is particularly suitable for hybridization of a number of components, such as chips, on a reception component forming substrate. A high hybridization efficiency can be obtained for these structures.
FIGS. 1 and 2
illustrate the use of the hybridization by melting process for small components, and for components which have very similar coefficients of expansion.
When components to be hybridized have different coefficients of expansion, and particularly when there is a non-negligible distance between the different pads, the pads on the two components are no longer aligned when the structure temperature is increased to or above the temperature at which the ball material melts.
This situation is illustrated on
FIG. 3
which shows components at the hybridization temperature Th. It is considered that the material in component
12
has a coefficient of expansion &agr;
2
exceeding the coefficient of expansion &agr;
1
of the component material
10
. When the structure temperature is increased to the hybridization temperature Th, the pads
14
a
on component
10
, and consequently balls
18
, are separated by a distance L′ such that:
L′=L(1+&agr;
1
−&Dgr;T)
where L is the distance between the same pads
14
a
at ambient temperature Ta (FIG.
1
), and &Dgr;T is defined by &Dgr;T=Th−Ta.
In the same way, the pads
14
b
on component
12
are separated by a distance L′ at the hybridization temperature Th such that:
L″=L(1+&agr;
2
−&Dgr;T)
In the special case shown on
FIG. 3
in which each component only has two ball reception pads, the misalignment &Dgr;L between each pad
14
b
and each corresponding ball
18
is then such that:
&Dgr;L=L/2.&Dgr;&agr;.&Dgr;T,
where &Dgr;&agr;=&agr;
2
−&agr;
1
.
In some cases the misalignment between ball reception pads on substrates to be hybridized can be sufficient to compromise the hybridization operation.
For example, when substrate
10
is made of silicon (&agr;
1
(Si)=2.10
−6
), and substrate
12
is made of gallium arsenate (&agr;
2
(AsGa)=8.10
−6
), and when the distance between the balls
18
at an ambient temperature of 20° C. is 2 cm, and when the hybridization temperature of the 60/40 tin-lead alloy balls
18
is of the order of 220° C., the lateral offset between each pad
14
b
and each corresponding ball
18
is &Dgr;L=12 &mgr;m.
For example, this order of magnitude is reached for components in the form of modules with 1000 aligned connection pads at a pitch of 20 &mgr;m.
In this case, if the diameter of the wettable surface of connection pads is for example 12 &mgr;m, only balls for which offset &Dgr;L is less than or equal to 6 &mgr;m can be connected using the hybridization by melting process. Only these balls will be in contact with the reception pads on the second component.
In order to overcome these difficulties, there is a second technique for hybridization by solder balls. This second technique is referred to as the hybridization by pressure technique.
This technique starts with a structure similar to the structure shown on FIG.
1
. Component
12
is transferred to component
10
by making the pads
14
b
coincide very precisely with the balls
18
. Then, balls
18
are pressed firmly into contact with pads
14
b
in order to form a connection by thermo-compression, by exerting appropriate forces on components
10
and
12
.
This operation may take place at a temperature below the ball material melting temperature, and in particular at a temperature close to ambient temperature.
Thus, by limiting temperature excursions &Dgr;T, the problem of a misalignment between pads
14
b
and balls
18
, or pads
14
a
, does not arise, even when the components are made of materials with different coefficients of expansion.
However, the hybridization by pressure technique does have a number of disadvantages compared with the hybridization by melting technique.
For example, for hybridization by pressure, components to be hybridized must be aligned with very high precision. The self-alignment phenomenon described above does not occur at temperatures below the melting point of the ball material.
Furthermo
Chantre Chantal
Marion Francois
Commissariat A l'Energie Atomique
Pearne & Gordon LLP
Smith Sean
Young Lee
LandOfFree
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