'A machine is not a process': it is the poor craftsman who blames his tools.

Over the past several months I have visited a number of customers. The primary purpose of my visits is to attempt to help them improve their solder paste printing process quality and cycle time performance.

One thing I have observed during my travels, especially during these recent visits, is that there is a dramatic difference in how folks think the performance of the solder paste printing process should be monitored and measured. The process of printing solder paste is as old as the surface-mount technology process itself, so one might think there would be a consensus on how the process's performance should be monitored and measured. However, no such consensus has formed.

In practice, I have seen the whole spectrum. I have seen automated optical inspection equipment used to inspect every single solder paste deposit immediately after printing, with the board wiped clean if just one print deposit does not meet the specified solder volume requirement. And I have seen operations with no inspection or monitoring at all because no correlation between actual end-of-line solder defects can be made with solder paste printing defects discovered immediately after printing.

What is the correct method to monitor and measure the performance of a solder paste printing process? Many customers feel that purchasing the "best" printing equipment will ensure the "best" printing process. Although I strongly believe that optimizing the printing process requires the best equipment (more on this momentarily), a printing machine--or any other machine on the line--is not in itself a "process."

Indeed, the equipment put into use on the line is just piece of a company's manufacturing capability. Likewise, the purchase of a machine to print solder paste does not buy a solder paste printing process.

You need well-trained operators, technicians and process engineers to identify and optimize all the other factors that will maximize the performance of that solder paste printing process. In previous columns we discussed the factors that contribute to printing process performance, noting how they are dependent on each other. Even the most sophisticated equipment will produce poor results if not operating in a well-designed, controlled and optimized process.

For example, if you hire a craftsman to build a custom piece of furniture for your home and are not satisfied with the results of his work, is it the fault of the hammer he used? Only the tool (in our case, the process equipment) in the hands of the skilled craftsman (read: the operator, technicians and engineers) can produce optimum results.

Having said that, printing equipment that provides the best possible performance specifications (such as accuracy and repeatability) is a significant contributor to optimizing the printing process.

Also, it is important to understand the structural design of the equipment in regards to maintaining the initial performance specifications over its useful life. This is especially true for operations that run equipment 24 hours a day, 7 days a week. Likewise, less-than-optimum printing equipment will result in a less-than-optimum printing process. The creation of the printing process must include evaluation of the printing equipment specifications and the formal engineering work required to optimize all factors in the printing process.

Call me old fashioned, but I am staunch believer in process monitoring using statistical process control (SPC). The proven control chart that periodically measures a key critical process output is effective in measuring process performance. In the case of solder paste printing, the critical process output most often measured is solder paste volume or, in some cases, paste height. In an SPC program for solder paste printing, paste volume is a more robust and preferred output to measure.

I have observed a much more stable, repeatable, high performance printing process in operations that focus on preventing defects than I see in operations that are experts in finding defects. SPC is the primary tool used by the former. Monitoring and measuring the process--rather than monitoring and measuring the product--supports a defect prevention manufacturing culture.

SPC will continually "take the pulse" of the process and provide an early warning signal when the process is going out of control and defects are likely to occur. Once an out-of-control situation is experienced, the process should be stopped, the situation investigated and the cause corrected before the process resumes. The data from this event should be collected to determine if any defect trends are occurring.

AOI certainly has a role. The optimum use of AOI is as an automated SPC tool and to identify random defects that are created within normal process variation.

Evaluating printing process equipment and process performance are each vital elements to implementing an optimized solder paste printing process. Neither can be ignored.

Joe Belmonte is project manager, advanced process development, at Speedline Technologies (speedlinetech.com); [e-mail]. His column appears semimonthly.

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