Electricity: electrical systems and devices – Safety and protection of systems and devices – Impedance insertion
Reexamination Certificate
2000-12-21
2003-02-25
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Impedance insertion
C361S100000, C361S115000
Reexamination Certificate
active
06525916
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electronic device with double-wire bonding, a manufacturing method thereof, and a method for checking the intactness of the bonding wires of this electronic device.
BACKGROUND OF THE INVENTION
As is known, in electronic power devices, the connection between one or more of the contact pads of the die in which the device is provided and the respective pins of the lead frame to which the die is secured (typically by a bonding layer), is frequently obtained by double-wire bonding formed by two bonding wires that are typically made of gold and are arranged in parallel with one another.
The double-wire bonding is particularly useful in cases in which it is important to guarantee bonding having low resistivity during the conduction phase or a high current conduction capacity or both. In fact, for example, a single 2 mil gold wire cannot conduct a current greater than 2 A for an indefinite time, whereas if a second wire is added in parallel to the first, the current conduction capacity is in fact doubled.
In addition, in some applications, in order to reduce further the resistivity of the bonding during the conduction phase, and to increase further the current conduction capacity, use is also made of two pins of the lead frame, which have the same function, and are thus connected to one another outside the electronic device, and each have double-wire bonding to the respective pads.
In double-wire bonding, it is therefore necessary to ensure in a final test phase, that there is intactness and satisfactory bonding of all the bonding wires, by an automatic checking process, since, with the present technology, the bonding process has a defectiveness of approximately 50-100 ppm, and the intactness of the bonding wires is not guaranteed by the assembly operation, and the lack of one or more bonding wires could make the electronic power device unreliable.
The conventional measuring methods, such as pin to pin continuity measurements, require execution of operations that are somewhat complex, and particularly demanding in terms of testing time, in order to distinguish faulty bonding, in which a single bonding wire is present, from satisfactory bonding, in which both bonding wires are present, since the resistance of the gold wires is negligible compared with the overall resistance of the circuit being measured.
For example, a 2 mil gold wire 3 mm long has a resistance of 33 m&OHgr;, and on the assumption that a measurement is carried with a current of 1 A, along the bonding wire there is consequently a voltage drop of 33 mV, which is approximately 100 times smaller than the voltage drop (approximately 3 V) on the diode (and on its internal resistor) which is typically arranged in series with the bonding wire.
Therefore, taking into account the dispersion of the values of the voltage drop on the diode, the voltage drop caused by the presence of a single bonding wire cannot be distinguished from that caused by the presence of two bonding wires.
A known solution used in order to overcome this problem consists of doubling the pad to which one end of the bonding wires is connected and the bonding wire, thus providing two separate pads which are connected to the same pin by different bonding wires.
This solution is shown schematically in
FIGS. 1 and 2
. In particular,
FIG. 1
shows schematically an electronic device
1
comprising an output transistor, in the example an MOS transistor, one terminal of which is connected to a pad
3
, which in turn is connected to a pin
4
via double-wire bonding indicated as
5
a
and
5
b.
FIG. 2
shows schematically the same electronic device, in this case indicated as
8
, as modified on the basis of the above-described known solution. In particular, in
FIG. 2
, the output transistor
2
is doubled into the output transistors
2
a
and
2
b
, the output terminals of which
6
a
,
6
b
are each connected to a corresponding pad
3
a
,
3
b
, into which the pad
3
has been doubled. The output transistors
2
a
and
2
b
have control terminals
7
a
,
7
b
, which are separated so that their operation can be controlled independently; the pin
4
is connected by the bonding wire
5
a
to the pad
3
a
, and by the bonding wire
5
b
to the pad
3
b.
By means of this solution, by switching on separately the output transistor
2
a
or
2
b
, it is possible to check the correct bonding of the respective bonding wire
5
a
or
5
b.
Although this solution makes it possible to check separately each bonding wire easily, by switching on separately each electronic component which is connected to the latter, it has the disadvantage that it requires an increase in the surface area occupied by the electronic device, due to the doubling of the electronic components that are connected at the output, and of the corresponding pads, as well as to the need to isolate electrically these doubled electronic components, and to the presence of additional electronic components for execution of the checking functions, which is highly undesirable in view of the continual trend towards ever-increasing miniaturization of integrated devices.
This disadvantage is all the more serious in cases in which, instead of having a single output electronic component to be doubled, there is a bridge configuration, in which all four of the electronic components of the bridge must be doubled.
This solution has the further disadvantage in that it involves a complication in the driving stage (not shown) of the electronic power device, which must be able to switch on separately each electronic component doubled, and that it requires introduction of a further command in order to activate the checking function and to select the final stage to be activated.
A solution proposed in order to eliminate the above-described disadvantages is described in European Patent EP-0622733 in the name of the applicant.
This solution does not require doubling of pads, and is based on the fact that, when a somewhat high current is passed through double-wire bonding, this gives rise to substantial dissipation of power by the bonding wires, and thus to heating of the latter, such that, since the resistance of gold wires is strongly dependent on the temperature, by monitoring the development over a period of time of the voltage drop caused by the current, it is possible to distinguish the double-wire bonding from that which is defective, in which only one of the bonding wires has been bonded correctly.
In fact, when the same current is supplied to the bonding wires, if there is a single bonding wire in tact, all of the current flows in this single bonding wire, thus generating dissipation of power which is four times greater than in the case of correct bonding, in which the current is distributed substantially equally between the two bonding wires.
As a result, in the case of incorrect bonding, there is greater heating of the bonding wire, a greater increase in its resistance, and thus a higher voltage drop, and therefore, when the development over a period of time of the voltage drop is compared with the corresponding development over a period of time for the same bonding in a part which is known to be intact, it is possible to detect any absence of a bonding wire.
However, practical implementation of this solution has the disadvantage that it requires firstly generation and control of relatively high currents, and secondly, use of highly sensitive measuring devices, since, in view of the resistance values of the gold wires concerned, the devices must be able to distinguish variations of voltage that are always very small, of approximately a few mV.
A solution proposed in order to eliminate the disadvantages inherent in the known art is described in patent application EP-74930, in the name of the applicant.
This solution involves doubling not only of the pad, but also of a section of the line which connects the pair of doubled pads electrically to a terminal of the electrical component, thus obtaining a pair of current paths arranged in parallel with one another, between the d
Capici Salvatore
Marino Filippo
Jackson Stephen W.
Jorgenson Lisa K.
Seed IP Law Group
STMicroelectronics S.r.l.
Tarleton E. Russell
LandOfFree
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