Tracking sensor fixture and method for tracking reference locations on a moving semiconductor leadframe strip

A tracking sensor fixture for mounting an optic sensor and for positioning reference locations on semiconductor leadframe strips with the sight line of the sensor is provided. The tracking sensor fixture includes a movable support frame and a guide member mounted to the support frame. The guide member includes a roller bearing that is biased into contact with an edge of a moving leadframe strip, regardless of the vertical location of the leadframe strip, by a spring acting on the movable support frame.

FIELD OF THE INVENTION 
This invention relates to semiconductor manufacture and to the tracking of 
semiconductor packages formed on a moving leadframe strip. More 
particularly, the present invention relates to a novel method and 
apparatus for precisely positioning reference locations on a moving 
leadframe strip in a sight line of an optic sensor. 
BACKGROUND OF THE INVENTION 
In semiconductor manufacture, a single semiconductor die (or chip) is 
typically mounted within a sealed package. In general, the package 
protects the die from damage (e.g. breakage, physical abuse) and from 
contaminants in the surrounding environment. In addition, the package 
provides a substantial lead system for connecting the electrical devices 
formed on the die to a printed circuit board or other external circuitry. 
Typically, the initial component in the packaging process is a leadframe or 
leadframe strip. A leadframe strip is a metal frame which is several 
inches long. The leadframe strip supports each semiconductor die for 
packaging and provides the leads for the final semiconductor package. A 
typical leadframe strip is produced from metal sheet stock (e.g. a copper 
alloy) and is adapted to mount several (e.g. eight) semiconductor 
packages. A leadframe strip typically includes side rails on either side 
which are formed with a row of circular openings or holes. The side rails 
and the holes in the side rails, facilitate transport and indexing of the 
leadframe strip by automated packaging machinery. As an example, the 
leadframe strips may be removably clipped to carrier belts that move the 
leadframe strips through the various manufacturing operations. 
During a conventional packaging process, the dies are attached to mounting 
paddles formed on the leadframe strip. Also during the packaging process, 
the bond pads formed on each die are electrically connected to the lead 
fingers of the leadframe strip using fine bond wires. Following the 
application of a polymide protective layer to the faces of the dice, each 
die and a portion of the leadframe strip to which it is attached, is 
encapsulated in a plastic material to form the semiconductor. Following 
encapsulation, the semiconductor packages require a deflashing process for 
removing excess molding material from the package exterior. A trim and 
form operation is then used to separate the resultant interconnected 
packages and to bend the leads of each semiconductor package into the 
proper configuration. 
During the packaging process it is often necessary to track the location of 
the dies or of the semiconductor packages, on the moving leadframe strips. 
As an example, following encapsulation of a die, it may be necessary to 
track the location of the semiconductor packages on the leadframes in 
order to imprint a visual code on the exterior of each semiconductor 
package. Typical information coded on the package includes product the, 
product specifications and the date of manufacture. 
For imprinting such a visual code, a laser inscription process can be used. 
With laser inscription, a laser is pulsed in a predetermined pattern onto 
the semiconductor packages as the leadframe strip is moved by a carrier 
belt past the laser. The laser is typically fired by an optic sensing 
device that is triggered by the holes in the siderails, or other reference 
locations, on the leadframe strips. The optic sensing device includes a 
light source (i.e. sender) and a photocell (i.e. receiver) facing each 
other on opposite sides of the leadframe. As a reference location on the 
leadframe strips moves past the light source, the light beam from the 
light source passes through the reference location to the photocell to 
make (or break) a control circuit and fire the laser. 
A problem with triggering a laser in this manner is that the reference 
locations must be precisely aligned with the path of the light beam (i.e. 
optic sight line) from the light source. Any misalignment between the 
light source and the moving reference locations will block the path of the 
light beam causing the inscription laser to misfire or to not fire at all. 
This may cause the printed visual code to be illegible or missing on some 
of the semiconductor packages. 
It is sometimes difficult to align an optic sensor with reference locations 
on the leadframe strips, because the carrier belts for moving the 
leadframe strips are often very long (e.g. 200 feet) and move relatively 
fast (e.g. 30 ft/min). The carrier belts are thus prone to move up and 
down or oscillate in a wave like pattern with respect to a stationary 
sensing device. Since reference locations, such as the holes in the 
leadframe strips, are only about 5 mil in diameter, very little up and 
down movement of a carrier belt is required to completely misalign the 
light source and reference locations. 
The present invention is directed to a tracking sensor fixture that is 
adapted to precisely align reference locations (e.g. holes) of a moving 
leadframe strip with the optic sight line of an optic sensor. The tracking 
sensor fixture can thus be used in a semiconductor manufacturing 
operation, such as during laser inscription, to facilitate timing of the 
process. 
Accordingly, it is an object of the present invention to provide a novel 
and improved method and apparatus for mounting an optic sensor and for 
aligning reference locations on a moving semiconductor leadframe strip 
with the sensor. It is a further object of the present invention to 
provide a tracking sensing fixture for precisely aligning reference 
locations, such as reference holes, formed on a moving semiconductor 
leadframe strip, with the optic sight line of an optic sensor. It is yet 
another object of the present invention to provide a tracking sensor 
fixture for mounting optic sensors that is reliable, relatively 
inexpensive to construct and operate, and which is adaptable for use in 
large scale semiconductor manufacture. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a tracking sensor fixture, and 
method for tracking reference locations on a moving semiconductor 
leadframe strip are provided. In an illustrative embodiment of the 
invention, the tracking sensor fixture is adapted to mount an optic sensor 
and to align reference locations, such as holes, on a moving leadframe 
strip, with the optic sight line of the optic sensor. 
The tracking sensor fixture, generally stated, includes a slidably mounted 
support frame for mounting a sender and receiver component of the sensor 
and a guide member mounted to the support frame. The guide member is 
biased for contacting the leadframe strips and placing reference locations 
on the leadframe strips into alignment with the optic sight line of the 
optic sensor. 
During a semiconductor manufacturing operation, leadframe strips are 
carried by a carrier belt past the tracking sensor fixture. The support 
frame of the tracking sensor fixture is mounted with respect to the 
carrier belt such that a lower edge of each leadframe strip is contacted 
and guided by a v-shaped guide plate and roller bearing of the guide 
member as the leadframe strips moves past the tracking sensor fixture. The 
support frame of the tracking sensor fixture is mounted on a slide bearing 
and moves up and down with respect to the carrier belt to compensate for 
differences in the vertical location of the leadframe strips on the 
carrier belt. A spring biases the guide member of the tracking sensor 
fixture into contact with the leadframe strips. This precisely aligns the 
holes formed along the bottom edge of each moving leadframe strip with the 
optic sight line of the optic sensor. 
The tracking sensor fixture is adapted to track reference locations on a 
moving leadframe strip by a method that, generally stated, includes the 
steps of: 
providing a movable fixture having a guide member and an optic sensor 
thereon; 
positioning the movable fixture with respect to the moving leadframe strip 
so that an edge of the leadframe strip is contacted by the guide member 
regardless of its vertical location and reference locations on the 
leadframe strip are aligned with a sight line of the optic sensor; and 
using the reference locations to control a semiconductor manufacturing 
operation such as laser inscription. 
These and other objects, features and advantages of the invention will be 
apparent from the following more particular description of the preferred 
embodiments of the invention as illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A typical leadframe strip 10 is depicted in FIG. 1. The leadframe strip 10 
is adapted to mount eight semiconductor dice (not shown) for packaging. 
The leadframe strip 10 includes paddles 12 for mounting the individual 
semiconductor dice. In addition, the leadframe strip 10 includes parallel 
spaced strip rails 14, 16 formed with a pattern of holes 18 to facilitate 
handling by automated machinery. Some of the holes 18 on the leadframe 
strip 10 may also be utilized as reference locations in accordance with 
the present invention. 
From the point in the manufacturing process where the paddles 12 are coated 
with adhesive for die bonding, until the point where the individual 
semiconductor packages or chips are separated from the leadframe strip 
rails 14,16 during a trim and form process, a leadframe strip 10 is 
treated as a unit in the semiconductor manufacturing process. The 
leadframe strips 10 are transported through automated machinery which 
perform the different manufacturing steps (e.g. die bonding, wire bonding, 
encapsulation). 
One phase of a semiconductor manufacturing process is shown in FIG. 2. A 
deflash apparatus 20, is adapted to remove excess plastic molding material 
from the semiconductor packages 46 formed on the leadframe strip 10. In 
general, such a deflash apparatus 20, contains an electrolytic solution 
for contacting and removing the excess plastic packaging material. Prior 
to the deflash process however, a code is imprinted on the semiconductor 
packages 46 formed on the leadframe strip 10. A laser scribe apparatus 22 
is used to print the visual code. This is the laser inscription process 
that was previously described. 
A carrier belt 24 is adapted to continuously move the leadframe strips 10, 
with the semiconductor packages 46 formed thereon, through the laser 
scribe apparatus 22 and through the deflash apparatus 20. The leadframe 
strips 10 are loaded onto the carrier belt 24 at a load station 30 and 
unloaded at an unload station 32. The leadframe strips 10 are suspended 
from the carrier belt 24 on clips (not shown) which engage a strip rail 14 
or 16 (FIG. 1), of the leadframes 10. An upper edge 26 of each leadframe 
strip 10 is engaged by the carrier belt 24. A lower edge 28 of each 
leadframe strip 10 is not contacted by the carrier belt 24. The leadframe 
strips 10 thus hang from the carrier belt 24. As the leadframe strips 10 
are moved by the carrier belt 24, the holes 18 (FIG. 1) on the lower edge 
28 of the leadframe strips 10 function as the reference locations for 
triggering the laser scribe apparatus 22. 
The tracking sensor fixture of the invention is located upstream of the 
laser scribe apparatus 22 and is generally designated as 34. The tracking 
sensor fixture is adapted to mount the sender and receiver components of 
an optic sensor such as a fiber optic sensor that includes fiber optic 
components. Such fiber optic sensors are well known in the art. The optic 
sensor may also be an optic sensor that includes reflective elements. 
In addition to providing a mount for an optic sensor, the tracking sensor 
fixture 34 is also adapted to engage the lower edge 28 of the moving 
leadframe strips 10 and precisely align the holes 18 on the lower edges 28 
of the leadframe strips 10 with an optic sight line 36 (FIG. 4) of the 
optic sensor. As such, the holes 18 function as reference locations for 
triggering the laser scribe apparatus 22. 
Referring now to FIGS. 3 and 4, the tracking sensor fixture 34 is shown in 
detail. The tracking sensor fixture 34 includes a support frame 38 and a 
guide member 40. The leadframe strips 10 held on the carrier belt 24 (FIG. 
2) move as indicated by directional arrow 56 over the guide member 40 of 
the tracking sensor fixture 34. 
The optic sensor for the tracking sensor fixture 34 includes a sender 
component 42 and a receiver component 44. These components 42, 44 of the 
optic sensor are mounted to the support frame 38 for the tracking sensor 
fixture 34. An optic sight line 36 extends from the sender component 42 to 
the receiver component 44. As seen in FIG. 3 the support frame 38 is 
generally u-shaped and the optic sight line 36 passes through the path of 
the leadframe strip 10. A primary object of the tracking sensor fixture 34 
is to align the optic sight line 36 with the holes 18 in the leadframe 10 
strips to permit an accurate triggering of the laser scribe apparatus 22. 
To accommodate different vertical locations of the leadframe strips 10, the 
support frame 38 for the tracking sensor fixture 34 is slidably mounted 
with respect to the carrier belt 24 on a slide bearing 50. The slide 
bearing 50 may be mounted to a stationary support brace 48 attached to the 
frame (not shown) for the carrier belt 24 (FIG. 2). A spring member 52 is 
attached to the stationary support brace 48 and to a movable member of the 
slide bearing 50. With this arrangement the support frame 38 and guide 
member 40 of the tracking sensor fixture 34 may float or move up and down 
as indicated by arrow 54. As the leadframe strips 10 move past the 
tracking sensor fixture 34, the spring member 52 biases the guide member 
40 into contact with the moving leadframes strip 10. 
The guide member 40 of the tracking sensor fixture 34 includes a generally 
v-shaped guide plate 58 and a circular roller bearing 60. As shown in FIG. 
3, the movable support frame 38 for the tracking sensor fixture 38 is 
mounted at an angle with respect to the lower edge 28 of the moving 
leadframe strips 10. The v-shaped guide plate 58 of the guide member 40 is 
thus also situated at an angle with respect to the lower edge of the 
leadframe strip 10. As also shown in FIG. 3, the v-shaped guide plate 58 
has clipped upstream corners 64 and is positioned to not interfere with 
the sight line 36 of the optic sensor. 
The roller bearing 60 of the guide member 40 is freely rotatably mounted on 
a mounting block 62 (FIG. 3) attached to the v-shaped guide plate 58 and 
to the movable support frame 38. With this arrangement moving leadframe 
strips 10 are guided by the v-shaped guide plate 58 into contact with and 
over the surface of the roller bearing 60. The movable support frame 38 
for the tracking sensor fixture 34 is constructed and dimensioned such 
that with a leadframe strip 10 in contact with the roller bearing 60, the 
holes 18 (FIG. 3) in the leadframe strip 10 will precisely align with the 
optic sight line 36 (FIG. 4). Stated differently, the distance from the 
bottom edge 28 of the leadframe strip 10 to the center line of the holes 
18 in the leadframe strip 10 is equal to the distance from the outside 
diameter of the roller bearing 60 to the optic sight line 36. 
Since the v-shaped guide plate 58, the roller bearing 60 and the optic 
sight line 36 are all tied to the movable support frame 38, each of these 
elements also move with the movable support frame 38. Any up and down 
movement of the movable support frame 38 caused by a changing vertical 
location of the leadframe strips 10 will thus not affect the alignment of 
the leadframe holes 18 with the sight line 36. 
Operation 
In operation of the tracking sensor fixture 34, leadframe strips 10 are 
moved by the carrier belt 24 as indicated by directional arrow 56. As the 
leadframe strips 10 move past the tracking sensor fixture 34, they are 
guided by the v-shaped guide plate 58 onto the roller bearing 60. As 
previously explained, the movable support frame 38 of the tracking sensor 
fixture 34 floats on the slide bearing 50. At the same time, the spring 
member 52 biases the guide plate 58 and roller bearing 60 against the 
bottom edge 28 of the leadframe strip 10. Since the guide plate 58 and 
roller bearing 60 float with the slidably mounted support frame 38, the 
optic sight line 36 of the tracking sensor fixture 34 automatically self 
aligns with the holes 18 of the moving leadframe strips 10. Any vertical 
misalignment of the leadframe strips 10 caused by up and down movement of 
the carrier belt 24 is thus accommodated by the changing location of the 
tracking sensor fixture 34. 
The holes 18 in the leadframe strips 10 can thus be used as reference 
locations to trigger the laser inscription apparatus 22 (FIG. 2) and 
imprint a visual code on the semiconductor packages 28. Alternately, 
features other than the holes 18 on the leadframe strips 10 can be used as 
reference locations to trigger the laser inscription apparatus. In 
addition, processes other than laser inscription can be controlled by a 
tracking sensing fixture constructed in accordance with the invention. 
Thus, the tracking sensor fixture of the invention provides a simple yet 
effective apparatus for mounting an optic sensor and for aligning 
semiconductor leadframe strips with the sight line of the optic sensor. 
Although the invention has been described in terms of a preferred 
embodiment, as will be apparent to those skilled in the art that other 
applications of the inventive concepts disclosed herein are possible. It 
is intended therefore that the following claims include such alternate 
embodiments.