Putting Pb-free to the test: why AOI and SPI are vital to upfront process control and verification.

Solder paste inspection (SPI) and AOI, integrated into the manufacturing process, can ease the transition to lead-free. SPI and AOI provide early defect detection and prevention specifically by identifying new types of defects that are expected to appear with the use of lead-free solder. An added benefit--and perhaps the most valuable one--is the insight gained by evaluating the results from SPI and AOI systems as lead-free materials are adopted.

Control of solder paste printing process is necessary for the quality and efficiency of any manufacturing line. The solder paste process has more variables than most other steps in SMT manufacturing. Typical paste printing processes require hourly human intervention, which provides the opportunity for error. Any errors in the paste process are compounded later, at component placement and reflow. Rework costs increase as components are added. SPI provides actionable information immediately with a cost per correction that is lower than anywhere else along the manufacturing line. (1,2)

With the transition to lead-free pastes, print parameters and other process steps must be set up to function in the most repeatable and robust fashion possible. Using 3-D paste inspection will help to establish optimized parameters more quickly. By implementing SPI initially, as lead-free processes are established, manufacturers have a unique opportunity to save time, money and materials. Verifying the individual deposit measurements can assist with generating information about the process, and as such can facilitate process characterization and eventually process control.

With the use of lead-free materials, process characterization and process control becomes more important. Lead-free processing has a new set of issues and unknowns, including increased reliability concerns. 3-D inspection permits viewing of solder paste volume, a key process indicator for joint reliability--especially for smaller components and pad sizes. In addition, a system with 3-D capability for 100% of solder paste deposits is useful for both defect detection and process control.

AOI methods are also well established, both in- and offline, for the verification of screen print, component assembly and solder joint integrity. AOI systems typically use visible light and cameras to acquire images of the (partly) assembled PCB and then deploy algorithms to verify the correctness of the product.

Layout designs for lead-free components have changed with smaller solder pads that have shifted closer to the components to avoid tombstoning. These smaller solder joints pose challenges to an AOI system including increased resolution and visibility requirements, but AOI systems can be used to verify potential resulting problems such as excess solder which can result from the smaller pads. If an AOI system is installed post-reflow with a lead-free process, the defect detection does not change significantly. However, if the AOI system is used in the pre-reflow position for process verification with lead-free, it has to be able to detect the offset component with limits as small as 70 [micro]m for an 0402 component.

An accurate pre-reflow AOI returns precise measurements of assembly quality and process variation which can be used as statistical feedback on the process effectiveness. Monitoring these data will allow the process engineer to manage and control the manufacturing process, which, again, will be extremely important during the initial phases of the lead-free process implementation.

AOI and SPI systems are well established as valuable test and inspection tools. As new defects emerge due to the use of lead-free materials, AOI and SPI will not only perform the critical function of identifying defects but will facilitate prevention as well. They will be used to help maintain and control the process by utilizing a system's data record and measuring that relative to the process parameters. Since the biggest change with the introduction of lead-free soldering is expected in the quality of the solder joint--i.e., the electrical and mechanical connection between the PCB and the component--it makes sense to spend as much effort on the test equipment as on the manufacturing machines.

References

1. D. Burr, "Printing Guidelines for BGA and CSP Assemblies," Surface Mount International Proceedings, 1998.

2. M. Owen and J. Hawthorne, "Process Control for Solder Paste Deposition," SMTA International 1999.

Stig Oresjo is a senior test strategy consultant, Thorsten Niermeyer is senior product manager, SMT AOI systems and Stacy Johnson is product marketing engineer with Agilent Technologies (agilent.com); 970-679-3215.

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