One of the largest challenges for manufacturers of printed circuit boards (PCBs) is improving first-pass yield at functional test, particularly when solder-joint defects are escaping from post-assembly inspection. If inspection or in-circuit test (ICT) is not entirely effective in isolating assembly defects, functional-test yield suffers.
To compound the problem, the effort and cost of troubleshooting functional-test fallout due to assembly defects are at least two to three times greater than reworking defects screened at post-assembly inspection. So to improve functional test yield and reduce costs, manufacturers must improve the effectiveness of post-assembly inspection.
The U.S. division of Mitsubishi that manufactures cellular phones is a good example of such improvement efforts. This site found that X-ray laminography inspection systems provided an effective solution to the problem of monitoring quality and screening defects before functional test on complex PCBs used in its cellular phones. Neither visual inspection nor ICT was effective in screening defects before functional test, resulting in a significant portion of functional-test fallout due to solder-joint defects.
Manufacturing Test Problem
The options available for testing the solder-joint quality on Mitsubishi’s cellular-phone boards were limited because of flux residue and the high component density of the PCBs. These complex, double-sided PCBs were entirely surface-mount (SM) with approximately 1,300 SM joints, including more than 400 0.5-mm pitch gullwing joints and many small chip components.
Three major issues inhibited reducing assembly defects and improving first-pass yield at functional test:
ICT was not effective for testing these complex PCBs because of flux residue, limited test access and the potential for de-tuning the high-frequency analog circuits.
Troubleshooting functional-test fallout due to assembly defects was at least two to three times the cost of reworking assembly defects screened at post-assembly inspection.
Visual inspection was not effective for screening assembly defects.
ICT Limitations
The flux residue of the solder paste resulted in very poor pin contact at ICT. The high density of the PCBs also limited the real estate available to route the necessary test points away from the solder joints to avoid the flux residue. Even if real estate was available, routing test points could de-tune the high-frequency analog circuits. For these reasons, ICT could not cost-effectively screen assembly defects before functional test.
High Cost of Functional-Test Fallout
Isolating defects at functional test required skilled troubleshooters. It also took much longer because functional test could not pinpoint the specific cause of the failure, particularly if it was an assembly defect such as insufficient solder or lifted leads. Consequently, it was crucial to minimize the number of assembly defects reaching functional test. The process-test flow initially consisted of visual inspection followed by functional test, with rework after each step (Figure 1).
Unreliable Visual Inspection
Visual inspection was not very effective for consistently and accurately identifying process defects. A significant number of assembly defects escaped from visual inspection to functional test, reducing first-pass yields.
The assembly defects that escaped to functional test included gullwing joints with insufficient solder, missing and tombstoned chip components, and misaligned small-outline transistors. Also, the production manager was convinced that good joints were being identified as defective by visual inspectors. As a result, many PCBs were unnecessarily touched up, reducing the reliability of otherwise good solder joints. Because of the complexity of the boards, the visual inspection process was also relatively slow, often taking at least 3 to 5 minutes per PCB.
Test Objectives
So began the search for a new post-assembly process to inspect the mix of fine-pitch gullwing joints and chip components that would:
Reduce or eliminate visual inspection.
Reduce unnecessary debugging and rework.
Reduce troubleshooters by increasing first-pass yield at functional test.
X-Ray Inspection Solution
After evaluating different inspection systems, Mitsubishi installed Hewlett-Packard 5DX X-ray systems to replace visual inspection (Figure 2). The systems use X-ray laminography to generate cross-sectional images of solder joints. This technique images solder joints on either side of a PCB without interference from solder joints on the opposite side. In contrast, standard transmission X-ray systems show both sides of the PCB at once, resulting in overlapping images of solder joints that are difficult to analyze.
The systems use image-analysis algorithms to automatically process the cross-sectional X-ray images by first locating each solder joint using CAD data. Then the algorithms compare calibrated measurements of critical solder-joint features, such as lead-to-pad alignment, and fillet thickness and length, with acceptance criteria to identify defective solder joints. These measurements are also analyzed with statistical process control tools in real time to support process control and improvement efforts.
Measurable Benefits of X-Ray Inspection
With X-ray laminography, Mitsubishi reduced inspectors and troubleshooters, improved process yields and eliminated unnecessary touch-up. The systems provided at least 95% fault coverage of the PCB joints for defects including missing components, shorts, opens, misaligned and marginal joints, such as those with insufficient solder.
The implementation of a more effective post-assembly inspection process resulted in these benefits within the first year of operation:
First-pass yields at functional test increased by seven percentage points.
The combined number of visual inspectors and touch-up operators decreased by 25%.
The number of functional-test troubleshooters similarly decreased.
Post-assembly inspection times decreased by more than 65% to less than 1.5 minutes per board.
These improvements and manpower reductions translated into combined annual savings of more than $750,000, which resulted in a payback period of a little more than one year. Mitsubishi also used the solder-joint measurement data from the inspection systems to analyze and control each SM assembly process step. These efforts led to the implementation of new and more effective process equipment and an order of magnitude improvement in process yield.
Figure 1.
Figure 2.
Copyright 1996 Nelson Publishing Inc.
November 1996