Testing head having cantilever probes

Details Testing head having cantilever probes Testing head having cantilever probes

2001-08-03

2004-01-06

Karlsen, Ernest (Department: 2829)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

C324S754090

Reexamination Certificate

active

06674298

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a testing head having cantilever probes, and more particularly to a testing head for use on semiconductor integrated devices.
2. Description of the Related Art
As is well known, a testing head is basically a device suitable to electrically interconnect a plurality of contact pads of a microstructure and the corresponding channels of a testing machine that is to perform the tests.
Integrated circuits are factory tested in order to spot and reject any circuits which are already defective during the production phase. The testing heads are normally employed to electrically test the integrated circuits “on wafer”, prior to cutting and mounting them in a chip package.
As schematically shown in
FIGS. 1 and 2
, a testing head
1
having cantilever probes usually comprises a backing ring
2
, made of aluminum or ceramics, to which a resin holder
3
is attached, and that is suitable to hold a plurality of movable contact elements or probes
4
, being normally wires made of special alloys having good electrical and mechanical properties, the probes being mounted to jut out of the resin holder
3
at plural points
5
and at a suitable angle from a plane &bgr;. Such emerging probes are commonly known as “cantilever probes”.
In particular, each probe
4
has an end portion or contact tip
6
, which is bent with an angle &ggr; from the rest of the probe so that a plurality of contact pads
7
of a device to be tested is contacted. The bent contact tips
6
are commonly referred to as the “crooks”.
The good connection of the probes
4
of the testing head
1
to the contact pads
7
of a device to be tested is ensured by the testing head
1
exerting a pressure on the device, whereby the probes
4
are vertically flexed in the opposite direction from the device movement towards the testing head
1
.
As schematically shown in
FIG. 3
for a single probe
4
, as the device to be tested vertically moves against the contact tip
6
, the probe
4
flexes, and its elbow point X, situated at the transition from the contact tip
6
to a probe section
8
emerging from the resin holder
3
, describes a circular arc.
Thus, the jutting probe section
8
forms a working arm of the probe
4
adapted to flex vertically, and is commonly referred to as the “free length” of the probe.
The crooked shape of the probes
4
is designed to allow the contact tips
6
of the probes
4
to skid, upon coming in touch with the contact pads
7
of the device to be tested and during the pad overtravel beyond a pre-set point of contact, across the contact pads
7
along a direction dictated by the arrangement geometry.
It should be noted that the force exerted on the contact pads
7
by each probe
4
depends on many factors, among which are especially the type of material forming the probe
4
, the probe shape, the angle &agr; made by the probe contact tip
6
, the length of the probe jutting section or free length
8
, and the amount of overtravel of the pads to be measured. These factors also determine the extent of the contact tips
6
skidding on the contact pads
7
, this being commonly known as the “scrub”.
It should be noted that, with a dense distribution of the contact pads
7
, the probes
4
must be arranged in plural rows, and the lengths L
1
, . . . Ln of the crooked ends vary accordingly, as schematically shown in FIG.
4
.
Also known is to use backing rings
2
, generally made of aluminum or ceramics, having different shapes depending on the set of contact pads
7
to be tested, so that the free lengths of the probes
4
, and hence the forces exerted by the latter to the contact pads
7
, can be equalized in the interest of even wear and performance of the testing head
1
.
In particular, when the probes
4
are arranged in a plurality of rows or levels, as schematically shown in
FIGS. 5A
,
5
B and
5
C, the emerging points
5
of the probes
4
from the resin
3
, when viewed frontally, make either a diagonal (FIG.
5
A), or straight (FIG.
5
B), or combination pattern (
FIG. 5C
) that is dependent on constructional requirements.
The portions of the probes outside the backing ring
2
are usually soldered on a PC board
9
, as shown in
FIG. 1
, to establish an electrical connection between the testing head
1
having cantilever probes and the testing machine.
It is therefore necessary that the outer portion of any probe
4
can be recognized unfailingly in the probe bunch, so that it can be soldered on the PC board
9
in the correct manner.
In addition, the probes
4
extend with their sections outside the backing ring
2
parallel to one another, as shown in
FIG. 6A
(side A), and the probes
4
for soldering on the PC board
9
are not easily singled out. It is also known the use of probes
4
with a radial spreading in their portion outside the backing ring
2
, as schematically shown in
FIG. 6A
(side B).
The probes
4
can be arranged in a plurality of rows or layers such that they have a diagonal or a straight configuration, in either the case of parallel or radial probes, as shown in FIG.
6
A.
FIG. 6B
shows, by way of example, an arrangement of the probes
4
in three rows with a radial diagonal configuration, and
FIG. 6C
shows an arrangement of the probes
4
in three rows with a radial straight configuration.
It is, moreover, a known fact that certain electronic devices, e.g. memories, have contact pads disposed along two sides only. Accordingly, a number of such devices can be tested in parallel if they are set in a single row.
A row of devices can be tested by suitably calibrating the inside dimensions of the backing ring
2
.
When several rows of devices are to be tested in parallel—usually two rows of eight devices or four rows of eight devices—multi-bridge backing rings, schematically shown in
FIGS. 7A and 7B
, are used.
In particular, a multi-bridge backing ring
2
b
includes a plurality of bridges
2
c
having a width dimension P inside the ring
2
b
perimeter, which bridges are suitable to carry probes for several devices to be tested in parallel. There are various techniques that can be used in order to obtain the desired pressure uniformity on the probes
4
against the corresponding contact pads
7
.
A first known technique uses a multi-bridge backing ring
2
b
having plural bridges
2
c
inside its perimeter to define plural device rows FILA
1
, FILA
2
, . . . as schematically shown in
FIGS. 7A and 7B
.
The shape and dimensions of the multi-bridge backing ring
2
b
and the inner bridges
2
c
are selected such that the jutting sections or free lengths FL
1
, FL
2
, FL
3
, FL
4
, . . . of all the probes will be equalized. In this way, the probes
4
are all caused to abut on the contact pads
7
with the same force.
A limitation comes to this prior technique from that the minimum width Lmin of a device to be tested cannot be less than the sum of the minimum length FLmin of the jutting section or minimum free length FL
1
, FL
2
, . . . of the probes
4
and the minimum theoretical width Pmin of each inner bridge
2
c
, i.e.:
Lmin≧FLmin+Pmin,  (1)
as schematically shown in FIG.
7
A.
A second prior technique uses probes of different types bound to the same backing ring
2
, as schematically shown in
FIGS. 8A and 8B
.
In particular, probes
4
of a larger diameter are used for the innermost contact pads
7
within the backing ring perimeter, to have equal forces exerted on the contact pads
7
even though the jutting sections or free lengths FL
1
, FL
2
, . . . may be different.
Using this technique, however, the dimensions and free lengths of the probes
4
are difficult to calibrate for an even pressure on all the contact pads
7
. In addition, the probes
4
which are to reach devices located farther inwards than the probe emergence points on the backing ring
2
will be those having the largest dimensions, as having the greatest jutting sections or free lengths
8
, thus enforcing reduced density for the contact pads
7
on the devices.
Also, neither of the abo

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