To meet the world-class quality standard of no more than 3.4 defects/million, PCB manufacturers must employ superior inspection techniques and successful feedback mechanisms to fine-tune their production processes. For resolving today’s soldering anomalies, many companies are turning to X-ray inspection which offers the right capabilities for finding manufacturing problems.
X-ray helps uncover many soldering problems, such as voids and insufficient solder, that are difficult to find or are hidden on the newer packaging interconnect technologies, including ultra fine-pitch, ball grid array (BGA) and tape automated bonding. With X-ray, a beam projects directly through a circuit board. As the board moves through the beam, selected areas of the board are captured and an image of that area is collected by a computer.
“The computerized parts of the X-ray manipulator system are programmed to move the X, Y and Z stages to predetermined locations and zoom to the required magnification,” said Joe Plevak of Lixi. “Using a golden part to establish both the parameters of acceptance and rejection along with the inspection procedure allows an operator to establish an automated routine. If the part is defective, a marking device will identify the rejected part. The manipulator then moves to the next preset position and continues with the programmed inspection.”
As the X-rays pass through the board, more dense areas, such as solder, attenuate more of the beam while less dense areas, such as the substrate, absorb less of the beam. After passing through the board, the beam is projected onto a detector, converting the X-ray into a visible light image that is viewed by a high-definition video camera. The camera transmits the signal to a computer where the X-ray image is digitized and automatically analyzed.
The automated X-ray inspection process uncovers many of the problems previously considered difficult or impossible to find. For example, the X-ray provides images on heel fillets directly behind and under the leads as well as images of partially lifted leads that may be present in optically hidden portions of the assembly. The images also supply information about solder mass and solder distribution for SMT connections.
One variation of the automated X-ray process, called laminography, uses cross-sectional imaging for detecting solder-joint defects on single- and double-sided PCB assemblies. The technique synchronously moves a combination of the X-ray source, a detector and the object being inspected.
Electronic controls synchronize the scanned-beam motion with the detector. The detector is a fluorescent screen that converts the X-rays into a visible light image viewed through a rotating periscope by a stationary video camera.
An example of a laminography system is the Model 5DX from Hewlett-Packard’s Four Pi product line. The system provides separate views of the solder joints on the top of a PCB from those on the bottom, and detects shorts, opens, insufficient/excess solder, excessive voiding and missing or misaligned components. It also can determine the quality of a plated through-hole and of the solder balls on a BGA.
Because of the thickness of the solder balls, connections on a BGA are difficult to image with transmissive X-rays. Even the 160 kV from the X-ray source may not be enough to penetrate the solder balls. A conventional X-ray system using a beam perpendicular to the plane of the balls provides little information about the soldered connection. A cross-sectional X-ray technique surmounts the problem by imaging different levels of the solder ball, including the ball center and slices close to the carrier or pad.
Trends
The demand for portable electronic products accentuates the need for complex PCBs that have more joints per board, use finer pitch components, utilize double-sided boards and are populated with new types of devices such as BGAs, said Bruce Bolliger of Four Pi/Hewlett-Packard. The new board and component requirements also limit physical test access, calling for noncontact testing such as X-ray inspection, continued Mr. Bolliger.
With the trend toward more complex PCBs, there is a need for faster inspection throughput to verify all solder joints within a shortened manufacturing cycle. To accommodate this, X-ray systems must have higher resolution to inspect the finer-pitch components and cross- sectional imaging for double-sided boards.
Products requiring 100% inspection need inspection systems that have the capability to verify more parts per minute, said Mr. Plevak. The logical selection is an in-line inspection system on the production floor. Even though the cost may be perceived as higher than the random-sample procedure, it is actually lower, he added.
Another trend is driven by customers who want to purchase only one piece of equipment that supports several departments, said Wai-Yee Lim of FeinFocus USA. The system must perform inspection during various stages of the production cycle, including product development, reliability and post-environmental test evaluation, as well as field and warranty inspections.
Economic pressure caused by the shorter life cycle of electronic products is also affecting the X-ray inspection industry, observed Mr. Bolliger. This trend requires manufacturers to attain higher process yields earlier in the product’s useful life to achieve acceptable profit margins.
To minimize this pressure, manufacturers are looking for ways to quickly locate the sources of their process variations and then fine-tune the process to achieve higher yields. One such method is quantitative measurement of solder joints.
Quantitative measurements are obtained from computer analyses of cross-sectional images. When these measurements are collected for a single process line, the variations will depict a drift in the line parameters. It is essential, then, that the process mechanism causing the drift be corrected and the manufacturing line be brought back into control before a true defect is observed.
The trend toward more complex PCBs is sure to continue, and one way to keep your manufacturing process pumping out world-class quality products is to use the best inspection technique for your parts. X-ray inspection is the solution of choice for minimizing the soldering defect rate and may lower costs for many products.
EE provides a comparison chart of X-ray equipment offered by the leading manufacturers. It lists the X-ray source and technology used, the voltage rating, imaging resolution and geometric magnification as well as helpful information describing where the equipment is used.
Copyright 1995 Nelson Publishing Inc.
November 1995