|
Manufacturing Probe Needles With Vision
by Dave Senders and Steve Neely, Point Technologies, and John Lewis, Cognex
Combining motion and machine vision expands inspection equipment
capabilities.
Motion control and machine vision are used throughout the semiconductor manufacturing process, from monitoring the diameter of
ingots as they are being formed from a crystal seed to aligning a die lead frame
prior to wire bonding. In nearly every step of the process, motion and vision
can be found working together to align, inspect, measure, and identify wafers
and die so the various pieces of equipment can do their tasks.
Point Technologies, a supplier of precision electrochemical pointing and
micromachining services and products for small-diameter wire and tubing to the
semiconductor, medical, and biotech communities, recently has applied motion and
vision to a new area in the semiconductor industry: probe needle inspection
(Figure 1). This new approach replaces a slow, labor-intensive, and error-prone
method that required extensive operator training.
|
Figure 1. View of Probe Needle Inspection Subsystem |
Eyeing the Needle
Semiconductor manufacturers rely on probe needles to test integrated circuits
(ICs) while still part of a complete wafer to ensure that only the functional
ones are packaged for final use. A probe needle is a straight, small-diameter
metal wire with one end that tapers down into a sharp point. It establishes an
electrical connection between tester and IC by contacting the metal bond pads on
the wafer.
Generally, hundreds or thousands of probe needles are assembled into an array on
a probe card tailored to interface between the specific type of IC being tested
and the wafer prober. During testing, precise needle geometry is essential to
ensure test-data reliability and consistency for several reasons.
First, when the wafer being tested contacts the probe card, the needles flex,
causing the tips to slide across the metal pads. Consequently, probe-tip
diameter is a critical dimension that determines the area of the pad that is
scrubbed.
Next, the needle’s diameter and taper shape determine how much it will flex and
the amount of force with which it touches down on the wafer. This is called the
balance contact force, a critical contact pressure specification set by
manufacturers that affects probe-card life and how well the probe tip will break
through a thin layer of aluminum oxide on the metal bond pads.
Finally, the probe tips must be accurately bent before assembling the probes on
the probe card; this requires precise and careful work. Most probe-card
manufacturers use reference diameter to determine where to bend the probe
(Figure 2). If probe geometry is inconsistent, the reference diameter, bend
angle, tip diameter, and tip length will be inconsistent. Any of these problems
can cause probe misalignment and result in poor test-data consistency.
|
Figure 2. Probe Tip Criteria
(For larger image click here) |
In the past, Point Technologies relied on a combination of manual video
inspection systems and optical comparators to provide probe-card manufacturers
with needles that met stringent requirements. However, as IC production volumes
rose and demand for probe needles increased, this process was far too
labor-intensive. Because needle inspection accounted for a significant
proportion of the total manufacturing time, it was a prime candidate for
automation.
Setting Sights on Automation
In late 2003, Point Technologies initiated an investigation of options for
improving the needle inspection process. Commercially available equipment that
could do the job was much too cumbersome, slow, and expensive to offer a viable
solution, so the team decided to design a system to better meet its needs.
The primary design goals were to increase measurement throughput, accuracy, and
repeatability while reducing inspection time. The system also would
automatically document all measurements onto a shared server, minimize operator
training, and maximize operator comfort. Finally, it would automatically plot
the measurement data on a graph for comparison to customer specifications and
provide statistical analysis for process improvements.
The engineering team set out to integrate motion control and machine vision.
Upfront work was done to evaluate the feasibility of using a machine vision
system to improve the measurement efficiency and still get measurements
repeatable within 0.00002 inches.
The team started with an In-Sight® Vision Sensor from Cognex because it is easy
to learn and comes with user software. The sensor consists of a DSP-based vision
processing unit, a high-speed digital camera, onboard light control, and
built-in discrete I/O. It also provides a standard VGA output for real-time
display, built-in Ethernet communications, and an onboard serial port.
Without attending any formal training classes, it took approximately 30 hours to
set up the inspection routine. In-Sight was a good validation tool; however, it
wasn’t the right product for the final application.
Instead, the design team chose a Cognex VisionPro® PC-Based Vision System
because it offered higher performance and provided more seamless integration
with the motion-control system. Moving to the VisionPro platform took a lot more
time to learn and required a couple of weeks of help from a Cognex Certified
Vision Integrator (CVI). The CVI was instrumental in developing the vision code
for the original system, a decision that also proved to be cost-effective since
the code was used for subsequent systems.
The PC-based vision system uses a Cognex MVS-8100D Series Frame Grabber inside a
basic Dell PC and a high-resolution (1,280 × 1,024) Cognex CDC-200 Digital
Camera. One of the most challenging parts of any machine vision application is
selecting the image formation system to produce a good picture. After some
lighting and optics experimentation, the team implemented a high-magnification
telecentric lens to eliminate optical distortions and a 2 × 2 inch diffuse LED
backlight to provide suitable images with the necessary resolution.
Vision Meets Motion
The motion-control part of the system is essentially a tabletop, Cartesian robot
with stepper motors to drive three axes of motion. One axis moves the loading
table into and out of the inspection area. The other two axes control vertical
and horizontal camera movement. The motion system combines assorted components
such as a three-axis motion controller, precision linear slides, leadscrews, and
stepper motors.
Logic and control are managed with Visual Basic, which allows stepper motors to
move the camera and position the needles. Vision-guided motion applications such
as this require a very fast vision system that will synchronize image capture,
analysis, and measurement with the motion required to find and focus the needle
and move from needle to needle. The team integrated the vision, motion, operator
interface, network communications, and database using MicroSoft Visual Basic 6.0
on the PC into the PointScan Automated Needle Inspection System (Figure 3).
|
Figure 3. PointScan Automated Needle Inspection System |
In all, the team went from concept to working system in about six months. Since
late 2004, five systems have been built and are being used in production.
Although there were some challenges, the design team’s previous experience
building multi-axis automation systems made the mechanical design pretty
straightforward. For example, due to the very small size of the probe needles
and the high magnification of the vision system, special care was taken to
eliminate sources of vibration in the machine design by using vibration dampers.
Another challenge arose when considering camera resolution, optics, and working
distance to achieve the desired field of view (FOV). The entire length of a part
being measured didn’t fit into the FOV, so the part had to be moved a fixed
distance under the camera to complete the taper length measurement. This
complicated the application because it required combining the information from
more than one image.
Integrating vision and motion also requires that engineers carefully calibrate
the vision system with the motion system. If there’s optical distortion from the
lens or perspective changes in the camera mounting angle, this involves more
than determining a scaling factor that relates pixels to a measured dimension.
The vision system tool suite includes special algorithms to correct for these
errors.
Lights, Camera, Inspection
Prior to inspection, 200 blank straightened wires are loaded into a transferable
fixture or block and then tapered in a proprietary Electro-Chemical Pointing
(ECPTM) process. The needles are inspected in process and after completion. The
specifications for every job are stored in a shared database, so after setting
the fixture down on the loading table, the operator simply starts the
inspection.
The loading table automatically indexes the probe needles into the inspection
area. The camera then moves over the top of the needles from the side. When its
FOV intersects the first needle, the table backs up until the point is located.
Tight coupling of the vision with the motion allows the two systems to
communicate and then center the point in the FOV.
To optimize measurement accuracy, repeatability, and reproducibility, PointScan
includes an automatic focus routine. During this step, the vision system
communicates with the motion system as it moves the camera up and down to focus
on the needle. Once focused, the vision system acquires and analyzes the image
and makes the appropriate measurements with the required accuracy. The data is
automatically recorded, and the camera jogs to the next needle to be measured.
As measurements are recorded, the data is plotted automatically on the screen so
the operator can see if they are within specification (Figure 4). At the end of
the inspection, the system automatically unloads the needles, and the pins are
accepted or rejected.
|
Figure 4. Display of Inspection Data
(For larger image click here) |
If the parts are rejected, the operator must rework or make new parts. The
inspection data can be used to adjust the process. Additionally, inspection data
is archived on parts that have successfully passed the inspection process. At
the end of the job, the system generates an inspection report to accompany each
shipment that includes all of the data for the measured parts and compares it
with customer specifications.
Intelligent Inspection
Machine vision has enabled the design team to create an inspection system where
a computer does all of the inspection and most of the data analysis. Tests
indicate more than a ten-fold reduction in inspection time.
In the past, it took a person about 20 minutes to measure 12 needles and record
the data on paper. PointScan measures 12 needles in less than 2 minutes, records
the data, and automatically graphs the data for the operator.
Machine vision has proven invaluable for accurately gauging very small parts and
providing qualitative results that can be used to track parts and productivity
and help troubleshoot inefficiencies in the process. As a result, Point
Technologies sees machine vision and motion control as key for the future of
manufacturing. Integrating vision with motion is not as difficult as some would
have you believe. Although it does take time, money, and expertise, if done
right, even the most challenging applications can pay off.
About the Authors
Dave Senders is a manufacturing design engineer at Point Technologies
specializing in machine and product designs for the semiconductor, medical, and
biotech industries. Mr. Senders holds a degree in mechanical engineering from
the University of Colorado. Point Technologies, 6859 N. Foothill Hwy., Boulder,
CO 80302, 800-557-7059, e-mail: dave@pointtech.com
Steve Neely works in Berlin, Germany, as the director of engineering in Europe
for Point Technologies. Since 2001, Mr. Neely has held positions in
production/operation management and was an engineering manager focusing on
product development and process development. He received a mechanical
engineering degree from the University of Colorado. e-mail:
steve@pointtech.com
John Lewis is a writer and public relations specialist at Cognex. Formerly a
technical editor for an engineering magazine, he has nine years experience
writing about factory automation, machine vision, and other technology topics.
Cognex, 1 Vision Dr., Natick, MA 01760, 508-650-3000, e-mail:
john.lewis@cognex.com
FOR MORE INFORMATION
on Cognex machine vision systems
www.rsleads.com/505ee-179 |