As device packaging types multiply, so do the sleepless nights for engineers on PCB inspection lines. Each new package type can really compound the factors that must be considered when selecting an inspection platform.
Today, there are single- and double-sided, through-hole and surface-mount technology PCBs. The trend is toward increasingly finer-pitch devices, including the 0.2-mm to 0.4-mm pitch of tape automated bonding and the 0.2-mm to 0.3-mm pitch of the flip-chip devices. Because of the many possible permutations of these packaging types, companies must keep in mind that one or even two inspection technologies won’t work for all PCBs.
Consider, for example, the dilemma of a board-inspection company that has two inspection lines. One is a high-volume, low-mix operation that produces double-sided, through-hole PCBs. The other line assembles boards with surface-mount and through-hole devices in a low-volume, high-mix application.
What inspection methods work best for this company? Available technologies include optical, X-ray laminography, laser triangulation and scanning microscopy. These techniques can identify missing and wrong parts, lead misalignment, lifted leads, cold solder joints, solder voids, poor fillet formation and solder bridging.
For the high-volume, low-mix operation, the primary consideration is performing a first article inspection for each new board setup–verifying the operation of the automatic insertion equipment, said Robert Whitehead of the Electronic Packaging Co. Each component type should be checked for proper part value, correct X-Y placement and missing components on the board. This process is duplicated each time a new reel of components is added to an automatic insertion machine.
The inspection system in this scenario would be networked to the automatic insertion equipment to relay information about repetitive errors and allow operators to promptly correct the problems.
Defects can be identified at key points on the inspection line or assembly line before large quantities of defective boards are produced. It is less costly to stop the line at the earliest point than to produce heaps of scrap or expensive rework, pointed out Paul Kempf, President of Metron Optics.
Visually comparing a known-good master board to a new product allows you to find mismatches or errors. When there is a repetitive error, the repaired boards can be fed into the production line at the same station, which helps avoid costly down time on the line, added Mr. Kempf.
Locating a fault quickly is one feature of the Auto-LocateTM Repair Workstation from Control Automation. It automatically pinpoints the location of inspection failures by projecting a beam of light at the coordinates of the defect. The defect is displayed on a monitor for visual defect analysis and repair facilitation.
A high-volume, low-mix assembly operation producing through-hole PCBs with a wave-solder process can be performed with visual inspection done off-line on a sampling basis, agreed Steve Rooks, a consultant to Four Pi. However, through-hole processes using paste-in-hole screen printing or no-clean fluxes require more thorough inspection and frequent monitoring, especially when the paste-in-hole barrels cannot be examined visually.
Ideally, you should have an inspection system that can accurately determine if the barrel is sufficiently full of solder. A cross-sectional X-ray is one method that can measure the amount of solder fill in the barrels, added Mr. Rooks.
The other operation of the company assembles boards with surface-mount and through-hole devices in a low-volume and high-mix environment. What inspection equipment is best for this line?
Most surface-mount boards cram as many components as possible in minimum space, said Mr. Kempf. Many surface-mount boards have a mixture of through-hole and surface-mount components on both sides of the boards. You could expect problems such as improper rotation of small surface-mount parts and bridging on larger, multilead components.
For vision systems that use golden boards as the inspection standard, the template board must be replaced every time the product changes. Systems like the Circuit Board Comparator from Metron change from one golden board to another very quickly, and require no programming because the board is the program.
The components on the mixed through-hole/surface-mount board usually are loaded by hand and errors average 30% to 35% before inspection, said Mr. Kempf. About 17% of the errors on surface-mount boards are not detected at final visual inspection.
The PCBs with surface-mount and through-hole technology require a more comprehensive inspection and monitoring capability, noted Mr. Rooks. Typical surface-mount defects are opens, lifted leads, insufficient and excessive solder, bridging, solder balls and misalignment. Defect rates range from 100 ppm to 500 ppm, depending on the joint type and component pitch.
The number of plated through-hole defects will be similar to the high-volume assembly operation, but the defect rate will be about an order of magnitude higher, due to the high-mix environment, said Mr. Rooks. Cross-sectional X-ray technology can inspect solder-joint types in-line in a high-mix environment, and can provide immediate feedback of the process quality.
Each packaging technology, combined with board type, volume and product mix, has an effect on the inspection system best suited for you. Focus on these proven inspection technologies and you can zero in on component problems quickly and efficiently.
Board Inspection Products
Video System Offers Measuring
And Marking Capabilities
The VIA-170R Video Image Marker-Measurement System measures video images for microscopic or macroscopic applications. It connects from a camera or VCR to a monitor and generates a graphic overlay on the live or stored image. Graphics and measuring lines are positioned on the overlay to mark or measure the image. Ten measurement capabilities are provided. An align feature for positioning markers in unison over important aspects of the image is supplied. Boeckeler Instruments, Inc., (800) 552-2262.
Inspection Head Uses
Four Miniature Cameras
The InterScanTM 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 degrees apart and angled at 30 degrees 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. Control Automation, Inc., (609) 520-0333.
Vision System Locates
The Model 5400 Automatic Animated Optical Inspection Station locates component faults and detects missing and bent pins. Dual CCD video cameras are provided with lens sizes ranging from 24 mm to 300 mm. The system also detects solder bridges and balls as well as shorts. It has automatic error logging capabilities for repair stations. Board scans take from 10 s to 60 s. Electronic Packaging Co., (214) 484-7671.
Inspects Boards With BGAs
The 5DX Automated Process Test System uses cross-sectional X-ray imaging to inspect and measure solder joints on PCB assemblies with surface-mount or ball grid array components. Throughput is 50 joints/s to 150 joints/s. Solder thicknesses from 0.5 mil to 25 mils are measured with a repeatability standard deviation of <4%. Data, including heel fillet height and average solder thickness, is collected to provide real-time control charts. Four Pi Systems Corp., (619) 485-8551.
Monoscope Offers 3-D Effect
With 8″ Working Distance
The Super 3-D Monoscope provides a resolution of 102 line pairs per millimeter at 5X magnification and a depth-of-field of 15 mm with an 8″ working distance. The working distance allows room for soldering and assembly. Metron Optics, Inc., (619) 755-4477.
System Performs In-Line
Monitoring of Solder Joints
The CXI-3600CS In-Line Process Monitor combines conveyor board handling and variable field of view (FOV) with microfocus real-time X-ray imaging of SMT assemblies. Inspection rates exceed 100 joints/s. The variable FOV allows users to zoom in on fine-pitch components while inspecting standard pitch components. Typical inspection time per view is 400 ms. Nicolet Imaging Systems, Inc., (608) 276-6383.
Copyright 1995 Nelson Publishing Inc.