X-Ray Fills the Inspection Void for New Board Technologies

Complex, densely populated PCBs are a challenge to manufacturers who must maintain high-yield production levels. Often times, verification of these boards is beyond the capabilities of in-circuit test and visual inspection because of severely limited test-point or visual access.

X-ray equipment fills the gap with its capability to inspect the structural integrity of solder joints for double-sided boards and surface-mount devices such as ball-grid array

(BGA) and fine-pitch gullwing packages. But how do you justify the purchase of an X-ray inspection system for loaded boards?

Several reasons exist for buying X-ray inspection equipment, including reduced process-monitoring costs, higher board yields and reduced repair time, said Christopher McBee, Marketing Manager at Nicolet Imaging Systems. X-ray inspection systems also produce data for use in statistical process control (SPC), which leads to a better understanding of the manufacturing process and reasons for board failures.

Applying a combination of these points helps justify the purchase of an X-ray system, said Arlene Hagio, Product Marketing Engineer at Hewlett-Packard:

· Show the cost savings achieved with an increase in defect coverage. Take into account the cost associated with defects that escape the existing process.

· Assess the savings realized by eliminating fixturing costs required by in-circuit test systems. Also determine the savings when board programming is reduced. These savings can be significant especially if you have a high-mix board environment, such as contract manufacturing.

· Show the cost savings realized by reducing the bone-pile boards—the ones that passed normal inspection but failed functional test.

· Eliminate visual inspectors to reduce costs and improve reliability of process. Visual inspection is often unreliable and unrepeatable, which leads to unnecessary rework of good solder joints and the introduction of defects during the rework process.

· Show the savings achieved by providing the location and identification of a defect. Two of the greatest costs associated with a defect are determining its precise location and labeling the fault accurately. Automated X-ray inspection generates pin-level defect reports along with descriptive labeling of defects.

There are manual X-ray inspection systems to consider as well. In manual systems, an operator visually analyzes an X-ray image to determine what represents a defect. An automated system uses a computer to apply an objective defect criteria to the joint.

The purchase of an X-ray inspection system for loaded boards depends on the type of devices on the board, said Gil Zweig, president of Glenbrook Technologies. For example, surface-mount leaded devices need visual inspection to verify component placement and to detect the presence of lifted leads. But they also need X-ray to find solder balls under packages and solder voids in the bonds.

Surface-mount assembly of BGAs calls for X-ray to inspect bridging between balls, misregistration, missing balls and large-scale solder voids, said Mr. Zweig. Once the solder process is established, only occasional checks are required with the X-ray system.

Manual systems are primarily used in the failure-analysis process and for small sample inspections of a refined manufacturing process, said Mr. McBee. The automated system is best for SPC because it inspects a whole set of boards, devices and solder joints using the objective defect criteria.

Before you buy, the important aspects to consider are the amount of usable SPC data from the X-ray equipment and how the data format suits your needs, continued Mr. McBee. In most cases, the manufacturer can produce a better product with the benefit of process corrections derived from the SPC data.

Accurate SPC data is particularly important for new PCB designs and processes because the time for a new process to reach acceptable first-pass yield is critical to product introduction, added Mr. McBee. Any cost savings associated with an improved manufacturing process depends on the value of the boards, the production volume and typical defect types.

There also are differences in the types of X-ray systems available in the circuit- board inspection market. For example, the Digiray reverse-geometry X-ray system eliminates the need to buy and store film and instantly shows the image on a video monitor. Locating failures in real time can save time in the overall assembly process.

What Does X-Ray Provide?

X-ray provides an additional quality-control tool to check the validity of the fabrication and assembly process from the lamination and drilling of the multilayer board to the reflow of the solder, said Mr. Zweig. The early-warning capability that X-ray provides is essential to maintaining process control because, when a problem occurs, it usually is not an isolated event but the symptom of a process going wrong.

The output of X-ray systems depends on the design and user interface, said Mr. McBee. Some equipment, such as the MV-6000 X-ray product from Nicolet Imaging Systems, provides the defect location for each type of device and solder joint. This allows you to evaluate the board on a separate workstation where a graphic is displayed with the X-ray image of the board. But the real value of X-ray inspection is that it reveals the hidden elements of the soldering process. Other systems, like Hewlett-Packard’s 5DX, provide quantitative data such as solder thickness in the heel region of a gullwing joint and degree of misalignment and paste thickness.

The clarity of image is another important aspect to consider when scrutinizing X-ray equipment. For example, the RGX from Digiray provides a sharp, first-generation image with 2,048 ´ 2,048 pixel resolution with 4 to 5 line pairs/mm-resolution for a 7″ ´ 7″ field of view.

What to Do With the Data

Manufacturers usually use X-ray inspection systems in ways unique to their process. Generally, the data confirms the process works or indicates a process problem, observed Mr. Zweig.

The data must be interpreted by a machine or a person. Skilled personnel with knowledge of the system can interpret an X-ray image, said Mr. Zweig. Machine interpretation must enhance the image of the characteristically grainy real-time X-ray as well as to provide pseudo-color image sharpening and gray-scale equalization.

After obtaining the X-ray, an automated system compares it to a known-good X-ray and automatically rejects or accepts it. Some software, such as the RGX package from Digiray, manipulates the image through a range of 4,096 shades of gray to bring out features not normally visible. By adding color, subtle defects stand out easily. Software

also can produce quantitative values for solder thicknesses and present the X-ray image in an isometric view so the gray scale or solder thickness is presented as 3-D.

The X-ray equipment software makes a series of measurements on each solder joint that is automatically collected and compiled into a lowest-level statistical format for average, standard deviation and min/max results. Defect locations are determined from this format, based on a rule file derived by the manufacturer, said Mr. McBee. You use this list of defects in process correction and board repair. Table 1 lists some typical process correction sites for defects.

Some software packages for X-ray are designed for use after the inspection cycle and aid the manufacturing process, said Mr. McBee. Data is presented in several ways to allow a clear understanding of the defect profile. A graphical and list-based presentation of the defect distribution, the most common defects, and defect frequency per component and pin type is available with SPC software.

X-Ray Inspection Products

X-Ray System Offers

Laminography Module

The B series of the Model RGX is a reverse geometry X-ray system offering a laminography option for inspecting each layer of multilayered boards. It also views devices on either side of a double-sided board. The X-ray exposure captures all layers and reconstructs them three-dimensionally. The acceleration level of the RGX 100B varies from 50 kV to 100 kV, the 120B ranges from 50 kV to 120 kV and the 150B extends from 50 kV to 150 kV. An analog module interfaces between the computer and the electrical systems. A deflection module drives the deflection coils of the X-ray tube. From $204,900. Digiray, (510) 838-1510.

System Provides Simultaneous

Visual and X-Ray Imaging

The Dual-VU™ Real-Time Inspection System provides simultaneous visual and X-ray imaging of multilayer and assembled PCBs. The system includes a black-and-white monitor for X-ray imaging and a color monitor for visual imaging. The color monitor presents a 15X magnified view of a PCB. The system has an 0.8-mm focal spot and a focal spot to image plane distance of 4″ to 6″. A manual X-Y precision slide, image processing and a video micrometer are also available. $61,000. Glenbrook Technologies, (201) 361-8866.

Universal Test Algorithm

Helps Inspect Solder Joints

The Four Pi 5DX X-Ray System offers a new test algorithm to inspect any solder-joint. The system also provides gullwing and chip capacitor/resistor algorithms that adjust acceptance thresholds automatically and find misalignments on the pad. Throughput is 50 joints/s to 150 joints/s. Data, including heel fillet height and average solder thickness, is collected to provide real-time control charts. From $390,000. Hewlett-Packard, (800) 452-4844

Software Used for Real-Time

Process Monitoring and Repair

The X-PERT software package is used for real-time process monitoring and repair verification. It is part of a computer workstation compatible with the company’s automated X-ray inspection systems. The package includes the X-VERT module for board rework and the X-STAT module for data analysis. X-VERT allows you to verify defects without taking the board off-line. X-PERT provides a graphical and list-based presentation of the defect distribution, the defect frequency per component and pin, and the 10 most common defects. Nicolet Imaging Systems, (619) 635-8600.

Table 1 Typical

Copyright 1996 Nelson Publishing Inc.

November 1996