Board Inspection: Solder Paste vs Solder Joint

No one–especially if you’re involved in assembling today’s complex PCBs–will argue that preventing a mistake is far better than correcting one. This lesson is particularly true for the board soldering process.

While solder-joint verification has traditionally been performed at final inspection, many companies are finding it far more cost-effective to inspect solder-paste properties and application prior to reflow to mitigate the possibility of poor solder-joint quality later. After all, solder-paste defects can be remedied in less than five minutes by wiping off the solder paste and recycling a board through the printing process.

Final inspection of PCBs often includes verification of the physical integrity of a solder joint by X-ray inspection methods. Defects found in solder joints using these methods include tombstoning, solder bridging, poor connections, no connection and voids.

The perfect solder joint depends on proper reflow conditions, component placement and solder-paste deposition. Because proper solder-paste deposition and component placement can be assured prior to reflow, the solder-joint inspection should yield information primarily on the reflow process.

In practice, however, solder-joint inspection is often used for finding defects without concern for the source of the defect. While inspection of solder joints following reflow can be of value to assure the integrity and reliability of assembled boards, it may prove prohibitive if the error rate is high.

X-ray or ultrasonic technologies are more than twice the cost of video or laser-based systems, and rework following reflow is unlikely for fine-pitch components. Coupled with the cost of scrapping or reworking, the expense of equipment for identifying faulty product at final inspection can undermine a manufacturer’s competitive edge.

With the current emphasis on process control in the manufacturing process, both bare boards and components should be inspected prior to final assembly. Solder paste should also be measured before reflow. This can be accomplished with an in-line solder-paste inspection system based on a scanning laser which provides a 3-D field of view in real time. Figure 1 shows the field of view from this system for a scan of the solder paste for a 24-lead PLCC.

Solder-paste defects are the result of interrelated errors in printing and solder-paste formulation. Printing parameters such as screen material, thickness, opening geometries and orientations must be addressed along with squeegee material, speed, pressure and angle.

Solder-paste formulation has been the topic of research for some time. Because of the complexity of the solder-paste deposition process, the solder paste can vary across a board or between boards, and solder skips can occur randomly. Solder-paste defects, however, can be remedied easily and more cost-effectively than after-the-fact inspection.

The quality of a solder joint can be directly related to the quality of the solder paste. Defects in solder-paste printing are the cause of more than 80% of the solder-joint defects in fine-pitch production.

Solder-paste quality includes position, volume and coplanarity. Field studies have shown that a variation of ±25% from the nominal solder-paste volume significantly increases the possibility of a solder-joint defect.

Pad coplanarity and volume consistency across the board are also important to solder-joint integrity. Not all solder joints formed with insufficient solder-paste volume are detected at final electrical test or by using X-ray technology at final inspection. The cost of propagating flaws of this nature will be apparent in the cost of warranty replacements due to field failure as well as lost revenues due to customer dissatisfaction.

Specific solder-paste defects lead to identifiable solder-joint defects. Solder-paste position errors can be related to improper screen registration. The end result at the solder joint can be either solder bridging or open or weak solder joints.

Volume and coplanarity errors can be traced to partial blockage, squeegee pressure or solder-paste viscosity problems. Solder-joint bridging occurs when the paste formulation causes solder-paste bridging. Open or weak solder joints occur when the solder-paste volume is too small. Table 1 shows the relative frequency of solder-paste failures leading to solder-joint failures.

The number of leads and the fragility of the leads on fine-pitch components make the lead coplanarity a critical measurement. Lead coplanarity is often measured by the chip manufacturer at final inspection, by the board assembler at incoming inspection, and by the pick-and-place system before placement.

The coplanarity of the solder paste is equally critical; the components are placed gently, without pressure. Ball grid array, in particular, requires consistent paste volume and coplanarity for long-term reliability in the field.

While the solder-paste area and position can be monitored using camera-based systems, such 2-D systems are unable to flag errors in solder-paste volume or coplanarity. When height data is missing, only volume errors resulting from area errors can be detected by 2-D systems. A 3-D laser-based measurement system detects height errors, including scooping, which is typically the cause of insufficient solder paste.

Knowledge of the error types and frequency can help determine the solder-paste control strategy. When the paste-deposition process is consistent, systematic changes of low frequency can be effectively monitored by sampling. However, unpredictable defects such as stencil contamination or clogged holes will contribute the most defects when the paste process is in control. Random defects can be reliably detected only by 100% inspection.

Implementation of an error-detection or error-prevention philosophy should be predicated on an error analysis. Detection alone must suffice either when prevention is not possible or is not cost-effective.

Very high yields on complex boards can be achieved if the total defect rate is maintained below 50 defects per million. Solder-paste inspection can typically find and prevent 80% to 90% of the failures detected by solder-joint inspection. Considering the improvement in quality, a higher yield, defect prevention rather than detection, and the differential cost of repair over rework, solder-paste inspection can provide a cost-effective solution.

About the Author

Chuck Gamble is Vice President of View Engineering and the Director of the SVS Products Group. He has been working in the electronics industry for more than 10 years. View Engineering, Inc., 4750 Venture Dr., Ann Arbor, MI 48106, (313) 665-1140.

Table 1

Copyright 1995 Nelson Publishing Inc.

August 1995