The rate of change in the PCB manufacturing process is staggering, keeping automated inspection system makers on their toes to support the latest packaging innovations. Often they must devise customized solutions to help manufacturers solve their newest inspection challenge.
Some of the newer, more difficult-to-inspect packaging technologies include the ball grid array (BGA) and the quad flat pack (QFP). These high-density component-lead devices are invisible to the visual inspection process, making accurate examination nearly impossible.
The failure rate of BGAs, however, is considered low enough to warrant little or no inspection, suggested Dr. John Kasik, President of Control Automation. A report by a major manufacturer stated that an investigation of hundreds of thousands of BGA devices found a failure rate among joints of only 6 parts per million, he said.
For BGA pads where solder connections are not visible after reflow, measuring the volume of wet paste is the most effective method of predicting BGA reliability, said Jeff Jalkio, Vice President of Research for CyberOptics. A recent study of BGA pad printing performed by IBM determined that sampling is a valid alternative to 100% inspection for controlling BGA solder-paste quality. The investigation concluded that monitoring solder volume is also necessary because of the solder variations caused by environmental factors, paste-property differences, operator setup inexperience and improper print intervals.
The quality of other devices, such as fine-pitch QFP joints, is difficult to assess because the heel joint, the key to reliability, is hidden from the inspector’s line of sight, said Bruce Bolliger of the Four Pi product line at Hewlett-Packard. X-ray laminography displays a cross-sectional view of critical solder-joint features. The defects are identified by measurements and user-defined acceptance thresholds, and reported to a process control system.
Another technology is transmission X-ray, which normally cannot isolate distinct features that overlap vertically. To overcome this shortcoming, some companies, such as FeinFocus, incorporate a decoupling feature that creates an oblique angle so characteristics that overlap can be isolated and magnified for close inspection, said Wai-Yee Lim of FeinFocus.
Even with this feature, X-ray systems cannot detect opens with 100% accuracy. That is why opens testing is usually done during electrical test, she said.
Other inspection technologies, such as laser-based systems, have increased in use over the past few years, especially for scanning the coplanarity of many packaging technologies, including QFPs, BGAs, micro BGAs and flip chip arrays, said Elaine Winter-May of RVSI. Backlight measurement techniques cannot make these measurements because the leads or balls are spread throughout the package surface.
The common thread among successful entrants into the inspection field is the quality of the image, said Harold Wasserman, Director of Engineering for Control Automation. If the image provides sufficient contrast between the defect and the acceptable condition, and the good condition has a manageable number of variables, then the inspection can be reliably performed. However, arriving at a high-contrast image and a means for inspecting it is not a simple matter.
For example, the soldered surface of the metalized end of a device appears highly reflective under normal ambient light conditions when viewed from directly above. The component pad may also be highly reflective in the absence of solder and cause a problem for an automated system.
Possible automated inspection solutions include viewing the joint from the front at a 30° angle to the perpendicular and illuminating the joint from the sides, or inspecting the solder joint from the sides and lighting the joint from the front. In both cases, the reflections from the meniscus of the solder joint are completely absent from the unsoldered pad. The choice of method depends on the solder process, pad geometry and component density.
There is no single answer for all scenarios, said Mr. Wasserman. X-ray systems, for example, suffer from confusion due to internal traces and components mounted on the back side of the board. And laser units have problems caused by extreme reflectivity and retro-reflections of the laser beam.
Process Yield Improvements
An automated inspection system that identifies only a defective condition just provides attribute data, which is not effective for improving assembly process yield, said Mr. Bolliger. On the other hand, systems that use statistical process control (SPC) software to collect, store and analyze solder-joint measurement data in real time can generate control charts, histograms and Pareto charts to characterize the assembly process, he said.
For example, variations in the heel-fillet height of gull-wing solder joints across a single board indicate uneven heat distribution during the reflow process step, said Mr. Bolliger. To prevent solder-joint defects from occurring, the SPC software automatically triggers real-time alarms when specific solder-joint variables drift outside control limits.
Next, the historical analysis of measurement data is used to improve assembly yields by determining the cause of manufacturing defects and monitoring the success of process corrections. Additionally, the SPC charts of quantitative measurements provide information to certify the process capabilities and quality levels to customers, added Mr. Bolliger.
For lead scanners, process yield improvements are obtained with parametric characterization of IC packages and with an SPC software package, said Ms. Winter-May. The software organizes the data and presents it graphically, making it easier to characterize and monitor the production process. You can generate charts by selecting the desired parameters in a window without leaving the test environment. Other services transfer lead scanning data in real time to a networked PC, allowing data to be analyzed from a central location.
Applications
The newest packaging devices often inspire fresh solutions to inspection dilemmas. Sometimes, ideas flourish into success stories like these:
Laser Scanning Device Leads
RVSI manufactures 3-D machine vision systems for inspecting leads on a large variety of semiconductor devices typically packaged in trays. Inspection of a complete tray is done with stand-alone equipment by scanning a laser beam over the leads of all devices and using a reflected light signal to record the lead positions in 3-D.
Recently, a major semiconductor manufacturer asked RVSI to inspect the leads on devices still attached to their lead frames. The manufacturer needed an inspection module that operated in-line, was capable of accepting lead frames from the conveyor, and would return them to the line with high throughput, accuracy and repeatability.
RVSI accepted the challenge and designed a new in-line lead scanner. The system adapted laser scanning technology to this application, providing quality and yield improvement through real-time feedback about the performance of tools upstream in the semiconductor production line.
Automated Video Inspection of Microwave PCBs
A designer and manufacturer of precision-machined heavy ground-plane microwave PCBs needed to improve a tedious, labor-intensive and time-consuming inspection process for single- and double-sided boards. The company was using an optical comparator, a stereo microscope and a coordinate measuring machine to perform the inspection and quality assurance tasks. Additionally, the SPC procedures and customer reports were hand-written.
Operations Technology suggested a three-axis, CAD-programmable, video, noncontact, automatic coordinate measuring system. This inspection system was used to perform precise measuring of first article and production samples by incorporating CAD data to automatically connect the product design and customer tolerances to the inspection process. SPC data, spreadsheet reports and video printouts were generated in real time from the automated program measuring routine.
The manufacturer realized fewer mistakes through immediate process control. A quality assurance system, programmed from CAD data, substantiated the results of the process corrections.
X-Ray of Densely Packed PCBAs
Electrical test and visual inspection are often used to evaluate solder-joint quality. However, the densely packaged printed circuit board assemblies (PCBAs) used in cellular phones, notebook computers and telecom transmission systems make it especially challenging to inspect these boards.
One inspection method that has been used successfully to inspect the PCBAs is cross-sectional X-ray imaging. It provides an effective alternative to electrical process test and visual inspection because it isolates and measures critical solder-joint features with high measurement repeatability.
Hewlett-Packard’s line of Four Pi systems used cross-sectional X-ray technology to help a manufacturer of cellular-phone PCBAs inspect a high volume of double-sided boards. These products had more than 2,000 joints per board, with one-third of them hidden by RF shields.
Previous inspection techniques left the manufacturer with a large inventory of PCBAs that failed the functional test and with defects that troubleshooters could not isolate. By using the Four Pi 5DX X-ray laminography system, the manufacturer achieved a test coverage of 99.3% and a defect-call repeatability of 99.5%. The total test time, including load/unload, alignment and surface map, was less than 50 s.
Initially, the manufacturer used the X-ray system to isolate defective solder joints and missing or misaligned components on the PCBAs that could not be diagnosed. Used in this manner, the manufacturer paid for the inspection system with money saved on scrap costs.
Machine Vision Checks DIPs
A Fortune 500 semiconductor manufacturer wanted to improve the inspection process of dual in-line (DIP) chips. It needed to weed out chips with smears, smudges and broken, mislocated or incorrectly oriented surface-printed character codes.
The inspection method was performed by technicians who examined the chips as they passed under a fixed magnifying glass. This process was slow, arduous and prone to errors. Also, potential flaws caused inspectors to stop the production line to compare the questionable device to a shadow graph, a template which reveals any discrepancies.
The manufacturer selected the I-Pak Machine Vision System from Acuity, which captured flaws such as absence/presence, unreadable or poorly printed characters. The system was programmed via the user interface to recognize characters and mark parameters to ensure they were within specification. The software also allowed the user to program limits of acceptability for criteria such as contrast, font style and shape.
The system helped the manufacturer increase its production line speed from 95 units/h to 1,200 units/h. The company also saved $10,000 annually in production costs.
Automated Optical Inspection of PCBs
A major manufacturer of hybrid-mix boards for intelligent power systems used in PCs, storage devices and printers wanted to automate the inspection process of these complex boards to achieve the desired quality and reliability levels. The operation was characterized as a low-mix, high-volume activity using top- and bottom-mounted surface-mount components and plated through-hole technology.
The manufacturer requested assistance from Control Automation to automate the inspection of the hybrid mix of pin through-hole and pin-through-edge boards. It was a complicated task because most of the systems used a motherboard and four daughterboards with components varying from simple wirewound resistors to QFPs and pin grid arrays.
Control Automation provided the Model 5515B for optical inspection of every card going through the wave-solder operation to ensure all the plated-through hole and bottom-side SMT components were present and properly placed. The system performed 1,800 inspections for 504 components in 45 s with a 99.5% detection rate. The resulting defect information was used for process control and root-cause analysis.
Inspection Products
System Uses Image Processing
And Motion Control Software
The Powervision™/VGM family of inspection and control systems integrates motion control into the Powervision™ line of machine vision systems. It is based on an 80-MHz PowerPC RISC processor and includes a four-camera frame grabber, image analysis software and the company’s Vision Guided Motion extension. The system combines machine vision and motion control of up to eight axes of DC servo motors or six axes of stepper motors. The imaging software enables viewing up to 18,000 parts/h with accuracy of 0.0002″. Starts at $26,150. Acuity Imaging, Inc., (603) 598-8400.
Inspection Head Uses
Four Miniature Cameras
The InterScan™ 5519 ZOOM Automated Inspection System combines four miniature CCD video cameras with variable fields of view and lighting provided by 684 LEDs formed in an elliptical pattern. The cameras are located 90° apart and angled at 30° to the perpendicular. The LEDs are placed inside the lighting fixture and are individually programmable. The cameras offer optional 1″, 1/2″ and 1/4″ fields of view. $225,000. Control Automation, Inc., (609) 520-0333.
Lower-Cost Model Added to
Cross-Sectional Imaging Series
The Four Pi Model 5000 is new to the 5DX Series of automatic cross-sectional X-ray inspection systems. The lower-cost system is for PCBA manufacturers who do not inspect fine-pitch devices. It detects solder-joint defects on 20-mil-pitch components and on hidden solder joints of BGA packages. The 5000 can be upgraded for fine-pitch technology. $300,000. Hewlett-Packard Co. (970) 679-5700.
System Links Data
To SPC Software
The OPTEK V-Series Video Inspection and Measurement Systems is comprised of a multitasking computer, a hard drive and a 17″ color monitor. The series can perform real-time linking of dimensional data to an optional statistical control program or spreadsheet. A servo driven transport system is available in sizes ranging from 12″ x 18″ to 22″ x 26″. Automatic skew alignment, distance, radius, diameter, roundness and angle measurement capabilities are provided. Field of view and table-motion measurements are integrated at speeds >200 in./min. Video probe: $22,500 to $80,000; video/X-ray probe: $125,000. Operations Technology, Inc., (908) 362-6200.
Laser-Based Lead Scanner
Uses Two Inspection Areas
The LS-3700 DB Lead Scanner is a laser-based, dual-bed in-tray inspection system. The assembly holds one tray for scanning while the other is loaded with devices. This arrangement enables BGA and QFP inspection on the same system. The system features a coplanarity accuracy of 0.0002″, a lead angle accuracy of 1.0° and automatic calibration. Throughput of QFP 208 is >2,000 uph. Software packages are available for QFP, TSOP, SOIC, PLCC and BGA devices. Robotic Vision Systems, Inc. (RVSI), (516) 273-9700.
Mobile Inspection System
Checks Solder Paste for BGAs
The CyberSentryTM is an over-the-line solder-paste inspection system for BGA, discrete and fine-pitch devices. The mobile unit positions over the existing SMT production line to monitor wet solder paste, measuring height, area and volume at multiple sites. The system permits installation in front or in back of the line. System accommodates board sizes of 18″ x 22″. A touch-screen interface features on-line help. $70,000. CyberOptics Corp., (612) 331-5702.
Copyright 1995 Nelson Publishing Inc.
December 1995