Method for forming connections of a semiconductor device on base

Method for welding fine wires to connecting terminations born by the base of a semiconductor device, using a tool for welding and cutting the wire in the form of a tube (9) which acts by its end face (11) containing the wire (12) to be welded, of which the outer strand (13) is to be welded to the termination (6), an incipient rupture (16) being formed by the inner angle (17) of the tube, and a limited flattening (H) being obtained by contact (40) between part of the end face of the tube and the termination, the value (H) being determined by the choice of an angle (P) created between the termination (6) and the end face (11) of the tube.

This invention relates to the welding of electrical connecting wires to 
metallic terminals. 
In a certain number of electrical devices or apparatus, the electrodes of 
the active part of these devices have to be connected to conductive 
terminals which are supported by an insulating support or base and which, 
by virtue of their shape, their dimensions and their robustness, provide 
for easy connection to external circuits. Each terminal thus has an outer 
part for establishing this connection and an inner part designed to be 
connected to one of the electrodes of the device fixed to the base. This 
connection is frequently established by means of a flexible conductive 
wire, the inner part of the terminal having a flat part or "termination," 
to which the wire is applied and normally welded by one of its ends. 
However, the use of a connecting means such as this, which is easy in the 
general case of electrical circuits of normal dimensions, becomes 
increasingly more difficult when the dimensions of the elements to be 
connected decrease, in particular in cases where the devices to be 
connected are on the scale of microelectronic circuits which, in addition, 
are often formed on a single block of semiconductor material. 
In that case, the elements to be connected are generally assembled on an 
insulating support or base supporting the block of semiconductor material, 
the base and the block of semiconductor material being respectively 
provided around their circumferences with termination type terminals and 
contact electrodes connected to the various parts of the circuit existing 
on the block. 
The conductive wire is continuously delivered to the end of a tubular guide 
into which it is introduced and from which it issues at one useful end. At 
the outlet end of the tube, the strand of wire thus exposed is alternately 
applied at a predetermined temperature and pressure to the terminations 
and to the contact electrodes where it is fixed by welding and, after each 
application, is cut to enable the following connecting operation to be 
carried out. For cutting the wire, use is made of the partial flattening 
which the wire undergoes in the welding zone which considerably reduces 
its mechanical resistance. Accordingly, a moderate tractive force applied 
to the strand of wire present in the guide tube is sufficient to break the 
wire at the actual level of the welding point. 
However, as will be explained in detail hereinafter with reference to the 
accompanying drawings, a process such as this requires that the force with 
which the wire is applied to the termination, which determines both the 
satisfactory completion of the weld and the partial flattening of the 
wire, be regulated with a high degree of precision. Accordingly, it is 
necessary to use a high-precision device for controlling the movements of 
the guide tube to ensure that, at the outlet end of the tube, the wire 
undergoes predetermined flattening in accordance with the requirements 
mentioned above. Now, the connecting wire used has a very small diameter, 
typically of the order of 20 to 50.mu., whilst the wire guide tube, of 
which the exit face is used for welding, typically has an external 
diameter of 200.mu.. For this reason, it is often referred to as a 
"capillary tube". The distance to be established between the exit face of 
the tube and the surface of the termination is therefore very small, 
typically of the order of half the diameter of the wire, i.e. 
approximately 10 to 30.mu.. 
In practice, the wire itself forms the sole spacing wedge defining the 
distance to be established when it reaches its final thickness after 
flattening. Under these conditions, the definition of the corresponding 
distance is very imprecise because, for a constant application force, it 
depends upon the diameter of the wire and upon the nature of its 
constituent material. In contrast to conventional welding processes 
developed on this principle, the process according to the present 
invention does not have any of these disadvantages. 
In the method according to the invention, the final welding and breaking 
distance between the exit face of the wire and the termination on which 
the welding operation is carried out, is defined independently of any role 
of the wire to be welded. This distance is even defined in the absence of 
the wire. 
More precisely, the present invention relates to a method for forming 
connections between a semiconductor device and a base, each of these 
connections being established by a wire of which one end is connected to 
the device whilst its other end is to be welded to a termination forming 
part of a contact element integral with the base, this wire being 
previously introduced into a tool in the form of an open tube or 
"capillary" from which it emerges at one end to form, on the one hand, an 
inner strand accommodated in the capillary and, on the other hand, an 
outer strand of which the direction is defined by the connection between 
said end and the semiconductor device, said method comprising the 
following steps: 
guiding the wire by displacement of the end face on the welding zone of the 
surface of the termination, 
welding the wire in this zone by clamping it under heat between this zone 
and a portion of the end face, 
partial notching by limited flattening of the inner strand of the wire in 
the vicinity of the weld, 
cutting the wire by the application of a tractive force to the inner 
strand, this process being characterised in that flattening is limited by 
the fact that another portion of said end face and the surface of the 
termination come into contact during the notching step.

The circuit 1, formed on a semiconductor block 2, is fixed to a base 4 made 
of an insulating material and comprises a plurality of output electrodes, 
such as 3. 
The base itself supports a plurality of terminals, such as 5, each of which 
comprises an inner part 6 or "termination" and an outer part 7 or 
connecting tag. 
A conductive wire, of which the diameter and constituent material are 
selected to provide for high flexibility, establishes the electrical 
connection between each electrode 3 and the corresponding terminal 6. The 
circuit itself is thus protected from the mechanical stresses applied to 
the connections from outside, a cover (not shown) subsequently being fixed 
to the base to provide protection against environmental influences. 
FIGS. 2(a), (b) and (c), respectively show three steps involved in the 
connection of a semiconductor circuit to the terminal of a base. The same 
elements are denoted by the same reference numerals as in FIG. 1. The 
known method uses a tube 9, having an inner passage 10 and an end face 11 
of which the role is fundamental. In view of its very small dimensions, 
typically an internal diameter of 70 micrometers, this tube will be 
referred to hereinafter as a "capillary tube." 
The connecting wire is continuously introduced into the passage, where it 
forms the inner feed strand 12, and issues at the end face from which it 
extends outwards and forms the outer connecting strand 13. 
This strand having been welded to the electrode 3 beforehand, the process 
comprises a first welding step in which the capillary tube 9 is displaced 
by a mechanism (not shown) in the direction of the arrow 14, its end face 
parallel to the surface of the termination, thus approaching the 
termination to form a clamp for the wire 13 of the parallel jaw type. 
This clamping operation takes place under heat and pressure under known 
conditions in such a way that, taking into account the type of materials 
used for the wire and the termination, the wire is welded to the 
termination over a length A. According to typical values, a temperature of 
350.degree. C. and a force of 80 gr are satisfactory for a wire 25.mu. in 
diameter and a termination 300.mu. in length. 
On completion of welding, the wire is left with a flattened portion 22 of 
constant thickness H over its entire welded length, as shown in FIG. 2(b). 
However, a second result is obtained by continuing the movement of the 
capillary tube in the direction of the arrow 14. For a suitable value of 
the movement, the constituent metal of the wire is flattened beyond its 
ductility limits and an incipient rupture or break is advantageously 
produced by the end face of the capillary tube in a region 16 adjacent the 
inner strand 12. 
In a second step, the capillary tube 9 is removed in the direction of the 
arrow 18 from the surface of the termination, exposing a given length of 
the inner strand 12. 
When this length reaches a value considered as adequate h, a tractive force 
is applied in the same direction 21 to the wire 12 and, due to the 
presence of the incipient rupture 16, the wire is broken at 20, as shown 
in FIG. 2(c). 
The length h of wire now becomes an outer strand for another connecting 
operation and the process may be repeated with another sequence of steps 
as just described. 
On an industrial scale, it is essential for this third step, namely the 
exposure outside the capillary tube of a given length of the inner strand, 
to be carried out first, followed by breaking of the wire at the level of 
the incipient rupture produced on completion of the first step because, if 
the wire is broken before exposure or if it is exposed without breaking, 
the outer strand will ultimately disappear, with the result that another 
length of wire will have to be taken from the delivery spool and 
introduced into the passage of the capillary. This so-called "loss of 
wire" gives rise to considerable losses of time and, hence, money in 
production. 
Now, the satisfactory completion of the third step depends upon the 
mechanical resistance of the wire after flattening which in turn depends 
upon the residual distance left between, on the one hand, the end face of 
the capillary tube and, on the other hand, the surface of the termination 
during clamping of the wire to form the incipient rupture. In order to 
regulate this distance, the known process described above relies solely on 
the presence of the wire and, more precisely, upon the extent to which it 
is flattened during the welding step. 
Now, there is no practical correlation between these two factors and, in 
the machines in which the method is carried out, the need to obtain a weld 
of high quality leads to applied pressures and, hence, to flattening of 
the wire, often giving rise to premature breaking of the wire before 
formation of another outer strand, the troublesome consequences of this 
premature breaking having been explained above. 
FIG. 3 is a section through a capillary tube equipped with means for 
establishing an optimal clamping distance which is sufficient ot determine 
an incipient rupture in the breakage zone, but insufficient ot cause 
premature breakage before the complete formation of the outer strand, the 
importance of this requirement having been explained above. 
This means consists of a block B which forms a stop joined to the capillary 
tube by a means, such as the weld S, and projecting by a distance H in 
relation to the end face thereof. 
During the second step, this stop is applied to the surface of the 
termination, leaving between the surface of the termination and the end 
face of the capillary tube parallel thereto a clamping distance H which 
satisfies the requirements explained above. This distance and, hence, the 
distance by which the stop B has to project can only be experimentally 
determined because numerous factors are involved, such as the diameter of 
the wire, the constituent material of the wire, its physicochemical state, 
such as its strain hardening, its temperature, the mechanical resistance 
of the weld produced in the preceding step and the actual surface area of 
the end face. 
However, a solution such as this, which may be used for a scale of 
dimensions of the order normally encountered in industry, becomes 
impracticable on a microelectronic scale where the distance H is typically 
of the order of 10.mu. micrometers. 
FIG. 4, in the form of an explanatory drawing, shows one method of carrying 
out the process according to the invention. 
In this method and in other methods which will be described hereinafter, 
the second step described above comprises the use of simple means for 
applying the capillary tube to the surface of the termination, these means 
being perfectly adapted to the microelectronic scale and having no effect 
upon the nature or succession of the preceding steps. 
The means illustrated in FIG. 4 establishes the required contact by 
establishing an angle different from zero between the clamping faces 
respectively formed by the end of the capillary tube and the termination 
to be welded. On completion of the welding step, the region of the end 
face of the capillary which is directed towards the apex of the angle 
comes into contact at 40 with the surface of the termination, thus 
performing the same function as the stop B illustrated in FIG. 3, but 
without having any of its disadvantages and difficulties of production. 
FIG. 5 shows a first way for carrying out the method according to the 
invention. 
The base of the circuit to be welded (partly shown) carries a termination 5 
to be welded in known manner. By contrast, the welding capillary 9 is 
inclined at an angle P relative to the straight line D perpendicular to 
the plane of the welding surface of the termination. This method 
necessitates a modification to the mechanism used to displace the 
capillary tube to enable it to be inclined. 
FIGS. 6 and 7 show another two methods for carrying out the invention. 
In these methods, the welding equipment such as already illustrated in FIG. 
1 remains the same. By contrast, it is the termination themselves which 
bear the modifications due a to the invention. 
In FIG. 6, the termination 6 has, at one of its ends to be welded, a 
portion 60 with an inclined plane 61 to which the wire 13 may be welded, 
as shown in FIG. 4, by application of the welding surface 11 of the 
capillary tube 9. 
In FIG. 7, the termination 6 is characterised by a particularly simple 
arrangement, i.e. the inclined plane 61 required for carrying out the 
method according to the invention is formed by partly bending the end of 
the termination. 
FIG. 8 shows another advantageous way for carrying out the method according 
to the invention. 
The method illustrated in the preceding Figures necessitates modifications, 
some of which are complicated, either to the mechanical welding apparatus 
by providing it with additional mechanism for obtaining particular angles 
different from 90) between the plane of the base and the axis of the 
capillary tube, or to the actual terminations of the bases by providing 
them with geometric characteristics or structures differing from those of 
the terminations normally encountered in practice. 
In this other method, the welding apparatus is responsible for moving the 
capillary tube perpendicularly of the surface of the termination to be 
welded under the conditions already described and known, the termination 
to be welded being of a known type which has not been subjected to any 
particular bending operation. By contrast, the end 11 of the capillary 
tube has undergone a machining operation to provide its end face with an 
angle P relative to the plane perpendicular to the direction of movement 
of the capillary. The stop responsible for establishing the optimal 
clamping distance H is thus formed in a particularly simple and 
advantageous manner by the region of the end face adjacent the apex B of 
the angle P. It is pointed out that, during the plurality of welding 
operations to be carried out on terminations for a microelectronic circuit 
support, the part B will always have to be diametrically opposite the 
direction defined by the strand of wire already connected to the 
corresponding electrode of the circuit. This orientation means that the 
welding apparatus has to be provided with means for rotating the capillary 
about its axis in dependence upon this direction. 
Another advantage is afforded by the embodiment illustrated in FIG. 8 by 
virtue of the fact that the optimal angle of the end face of the capillary 
tube relative to the plane perpendicular to its axis depends upon the 
diameter of the wire and upon the nature of its constituent material. Now, 
the embodiment of the capillary tube shown in FIG. 8 enables a wide 
variety of wires differing in their characteristics to be welded simply by 
changing the corresponding capillary within a range comprising exit face 
angles graduated in value. 
In cases where the terminations are formed by the embodiment illustrated in 
FIG. 6 or 7, the combination between a particular angle value of the 
termination and another particular value of the capillary enables the 
possibilities of welding wires having very different characteristics to be 
even further extended. 
The embodiments of the welding process according to the invention described 
above and illustrated in the accompanying drawings have only been given by 
way of example. Any other embodiment comprising the fundamental 
characteristic of the invention, namely bringing part of the end face of 
the capillary tube into contact with the surface of the termination to be 
welded during the welding step, said contact determining a given interval 
between this surface and another part of the end face of the capillary 
tube, falls within the scope of the invention.