Vision System for the Physical Measurements of Semiconductor Devices

Improved PCB designs are leading to more efficient packaging and denser component placement. At the same time, pick-and-place (P&P) equipment can employ blind insertion in which a device may be removed from its carrier (tape or tube) and placed on a PCB without vision inspection and coordinate correction. This component density, combined with blind insertion, demands that devices maintain tighter tolerances for board placement.

Critical device dimensions, such as lead position, are forcing the development of inspection systems that perform the proper physical measurements of the package body and component leads. To develop an inspection with these criteria, Motorola and Acuity joined forces on a project that would correctly measure those devices blindly inserted into a PCB by a P&P system.

Need for True-Position Measurements

The goal of the program was to perform outgoing measurements that would guarantee that the devices fit properly into circuit boards. The specification AMSE Y14.5M-1994 Dimensioning and Tolerancing is used to (defines the) dimension (of) the devices. In particular, basic dimensions for lead-tip pitch are defined and a positional tolerance is specified. The basic dimension defines a step distance and does not have a tolerance.

Positional tolerances are defined in the design phase for both the PCB and the device lead tips. For the lead-tip positions (features), these tolerances account for variances in lead-tip width, are determined for the maximum lead-tip width, and are referenced from package data.

Tolerancing results in 3-D regions where the features must lie and are referred to as “true positions.” PCB assembly P&P systems require that lead positions be referenced to the proper data to ensure that the board tolerances correspond to the device tolerances.

Figure 1 shows the true-position regions that confine the lead-tip edges. Qualifying parts in this manner assures that the part can be placed in the PCB without using integrated vision systems. That is, the PCB manufacturer’s system can grasp the device by the package data and be assured the leads will conform to the board tolerances.

Fault With Traditional Vision Systems

In the measurement of semiconductor devices, 2-D vision systems are most often used to inspect/measure lead positions in the field of view of the camera. Image- processing systems traditionally backlight the leads and use the light-to-dark-to-light transitions to determine lead edges.

Many systems report measurements on these images, such as lead-to-lead pitch, lead width and lead-to-lead gap. They also allow you to apply tolerances to these dimensions. Of these, the lead width is the only dimension that has a tolerance. Applying a tolerance to the other “false” dimensions can lead to disastrous results.

Figure 2 illustrates scenarios of defect devices that show the difference between a pitch tolerance and the positional rectangle for devices in which the leads fan out, fan in or are swept in one direction. The scenarios in Figure 2 may be acceptable with respect to pitch or gap tolerance, but all the devices would fail the true-position criteria.

Innovative New Approach

Motorola and Acuity engineers developed methods on the Intelligent Visual Sensor (IVS) platform. These methods determined the positions of the leads and evaluated the positions with respect to the true-position tolerances.

A new multicamera system measures, calculates and dispositions devices using the proper dimensions and tolerances. Two camera views are required to properly measure the device. One view is from the top (looking down) for measuring the left to right (X-axis) shift and the length (Y-axis) for the leads. The other view looks end-on at the lead tips to determine the lead Z position.

Per the Y14.5M drawings, the X, Y and Z tolerances are independent; the datum for each may also be independent. This defines a 3-D rectangular area in which the lead tip must reside. Provided that the lead widths are within tolerance, the lead-tip edges must not extend beyond the rectangle.

Diffuse coaxial lighting produces uniform illumination of the leads and package body. Strobing of the light assures that any system motion or vibration does not interfere with the measurement. A Telecentric lens is used on the lead-tip view. Telecentric lenses eliminate parallax so that the distortion between the lead tips and the package reference points (data) is minimized. Calibration of the image with traceable standards at the proper focal distance is paramount.

The measurement procedure is:

The coordinate system (also referred to as the datum or frame of reference) is determined by locating and measuring the device features per the Y14.5M drawing. Calculations are performed to determine the reference points.

The lead edges and tips are located and measured to ensure that the lead widths are within specified tolerances and that true-position dispositioning can take place.

The centers of each lead are determined and the deviation from the basic pitch is calculated.

Finally, a decision is made if actual position of the lead tip lies within the tolerance limits defined by this equation:

The IVS screen shows the datum locator tools (Figure 3). The device is a 23-lead single in-line package. The X-Y datum for this device is the midpoint between the two tooling holes. A robotic end effector grips the part by these holes and inserts the part into the PCB.

The lead tip view is used in the same manner to determine if lead Z is within the true-position region.

Summary

Measuring and dispositioning devices with true-position methodologies assures you that the product is readily manufacturable. An indicator that a vision system does not employ true-position dispositioning is the use of pitch tolerance or gap tolerance. Using this tolerance indicates that the vision supplier is not performing the correct measurement/dispositioning methodology.

True-position inspection tolerances can be larger than those allowed by human inspectors. Many manual inspection criteria are comparison based and will have limits determined by the lead geometries. That is, the leads may be allowed to shift by some fraction of the lead width. Using true-position dispositioning may reduce false reject rates in production. Customers currently employing blind P&P systems with this devices tested are reporting single-digit PPM insertion errors.

About the Authors

Wyatt Huddleston joined Motorola in 1988 and today is a Principle Engineer at the company’s Semiconductor Products Sector. Prior to Motorola, he was employed by General Electric. Mr. Huddleston received his bachelor’s degree in chemical engineering from Georgia Tech. Motorola, 2100 E. Elliot Rd., Tempe, AZ 85284, phone (602) 413-6072, email [email protected].

Jeff Snyder is Manager of Applications Engineering at Acuity Imaging. He joined Acuity in 1987 after completing a B.S. degree in electrical engineering at the University of Cincinnati. During his tenure at Acuity, Mr. Snyder has held several positions in the Applications Engineering Department. Acuity Imaging, 9 Townsend West, Nashua, NH 03063, (603) 577-5890, email [email protected].

July 1996