Clive Ashmore
Peak manufacturing efficiency is not the same as maximum printer speed.

When it comes to high-volume assembly, platform speed and throughput rates are critical. How fast high-yield boards can be produced correlates to how quickly the profit line grows, right?

Well, sort of. Let me explain. In the printing world, cycle time – the amount of time it takes for the printer to load, print and index the board to the next process step – is definitely important and a differentiator. Comparisons among suppliers in relation to cycle time are often a determining factor for assemblers when purchasing a printing system. But, if talking about manufacturing efficiency, cycle time is not always about making the printer go as fast as it can. Cycle time should really be centered on line balancing and the speed per PCB, not solely the speed capability of a single platform. There are situations where it might be more productive and efficient to actually slow the print speed to match the line beat rate, and there are cases in which the printer has to work as fast as it can to keep up with the placement process.

Generally speaking, in an SMT production line the placement system is often given the leading role when it comes to determining overall line speed. So, for example, when producing high-volume, multi-part assemblies, the printer easily keeps pace with the beat rate of the line as set by the placement machine. In this case, slowing the print cycle time to match the production rate and optimize the print process is recommended. The converse also occurs in modern-day assembly operations. There are situations where maximizing speed is essential to optimal throughput rates, and the printer has to perform faster to accommodate the whole SMT process. Of course, everything depends on the type of product being built, but cycle time is really all about optimization and line efficiency, whether that’s pushing the printer to its max cycle time capability or slowing it down. It is not always simply about how fast a given platform can go.

In high-volume situations with more complex placement routines, print cycle time optimization is often key. Matching the rate of the line makes for a far more efficient manufacturing operation and also allows time to ensure a well-managed print process. For example, suppose a board is printed every 15 sec., and the printer isn’t set to print again for another 20 sec. Using the idle time for valuable process control to implement activities such as print verification and understencil cleaning will ensure the most stable and effective print process.

Recently, our company helped with an analysis at a customer that was running a process with a line beat rate of 29 sec. per board. The assembler had their print cycle time set to 18 sec. per board. The remaining 11 sec. could be managed in several different ways: 1) Slow the print process to minimize the idle time to maintain material viscosity; 2) increase the process maintenance functions, or 3) implement on-board inspection technology in critical areas. This customer chose to add inspection and slow understencil cleaning speed to ensure the small features were more thoroughly cleaned. This resulted in a more robust process leading to higher yields.

In manufacturing scenarios where throughput is king, print speed maximization becomes the priority. With this situation, it is critical to map the entire print operation and run a design of experiments (DoE) to optimize everything about the process. What should be avoided is simply increasing speed without analyzing the various first and second order interactions, as this can result in managing the beat rate but at the expense of quality. Our company has a customer that manufactures control boards for white goods where the part count is relatively low and the placement machine just cranks out these assemblies. We were asked to run a process audit to find out how we could balance the line. The line beat rate was running at 18 sec., but the printer’s beat rate was 22 sec. After running a DoE framework to optimize the print process, including peripherals such as understencil cleaning and paste dispensing, the assembler was able to reduce the printing beat rate to 18 sec. per board. The extra four seconds of savings were realized by incorporating a higher print speed, greater print pressure and faster separation speed – all of which were achieved without any degradation in quality. In addition, the customer placed a workstation at each printer to aid with setup and maintenance tasks.

Unfortunately, optimizing the printer is often the last resort for some manufacturers. It’s a complex process, and changing parameters without mapping them carefully can produce undesired results. On the flip side, focusing on line balancing and process optimization will yield fantastic print outcomes and, while printer cycle time capability is very important, running an efficient, stable process will result in better assemblies and greater profits.

CLIVE ASHMORE is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.

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