SCREEN printing
When All the Lights are Green
The time to squeeze out more efficiency is when everything is in spec.
What kind of approach do you generally take? Do you follow the “if it ain’t broke, don’t fix it” mantra, or are you more the “it’s good, but it could be better” type? In electronics manufacturing, continuous improvement is often discussed, but how much does your organization adhere to this philosophy when the shop floor is humming and everything is within spec? This is when process engineers should try to squeeze out even more efficiency.

Certainly, there is urgency around a process that is not running as it should. However, when all the lights on the line are green, there is likely opportunity for more improvement than you realize. Consider challenging the process through the lenses of incremental cost reduction and quality enhancement.

In the stencil printing process, there are several possible avenues for lowering cost and raising quality, even when everything is within spec:

Cost down. The first and most obvious area for resource optimization is understencil cleaning. This sub-process of stencil printing has several costs associated with its operation, including time (output reduction) and a fixed cost (fabric and solvent consumables) for every clean. A majority of print platform suppliers ship equipment with relatively liberal default settings for fabric advance (how much is used for each clean) and solvent volume. This is based on assumptions a manufacturer may be cleaning a very dirty stencil with numerous apertures across the entire length of the fabric, so high debris removal must be accommodated. In essence, the defaults are set for worst-case scenarios, as is fairly standard practice. Optimizing these settings based on specific process conditions has the potential to reduce waste through streamlined consumables delivery.

The other understencil cleaning analysis that should be conducted is cleaning frequency. In addition to elevated consumables use, there is an output cost: When you’re not printing, you’re not producing. Leveraging sophisticated software and SPI tools can deliver deep data analysis and a recommended cleaning frequency that allows the process to remain in control. Perhaps you’re cleaning too often, and that is money wasted.

Second, solder paste management is an area a process engineer should examine to further reduce cost. Maximizing paste consumption can have a significant impact on overall process cost. While solder material prices have stabilized in recent years, the move toward Type 5 and even Type 6 pastes to handle miniaturization’s challenges can elevate the solder budget. Leveraging sensor technology that monitors the paste roll height allows operators to take the “Goldilocks” approach: not too much, not too little – just the right amount of paste in front of the blade. On-target paste volume reduces both material expense and the environmental cost of waste.

Quality up. When it comes to improving quality, leveraging Six Sigma tools can further refine an in-specification process and move conditions on the very edge of acceptable to the center of the range. Data mining carried out through Excel or any other statistics correlation method – including my favorites XBar and Range – can provide a view into process centering and spread. Then, Cp and Cpk analysis will reveal the repeatability of the process. Perhaps there’s a green tick in the box, but only on the border of the “within spec” range.

Using an SPI tool to data mine, one can pinpoint certain areas of the board that may be on the verge of moving from green to amber. Looking at elements like aperture design, processes and material sets (squeegees, tooling, etc.) and making slight changes may deliver more bandwidth to the process and quality to the product.

Last, using design of experiments (DoE) methodology offers continuous improvement opportunities. This is a bit of a forgotten tool, but for process engineers striving for quality utopia, it is essential. Many SPI software packages contain DoE functionality; some are more user-friendly and thorough than others. DoE is not something to undertake before the process is in control. Rather, DoE is about finding the best optimal solution for the centered process, pushing Cp/Cpk higher than where the process began. Engaging in DoE is also quite revealing and identifies the most and least important elements of the printing process. We often talk about stencil printing factors such as speeds and pressures as if they are all equally important; they’re not. Some elements, like separation speeds, can have a massive amount of leeway in certain applications. And some, such as print pressure, simply do not. Contemporary DoE software tools are very slick, taking the areas you are interested in evaluating and running countless sequences to find the optimal conditions and inputs.

Extra time is a rarity these days, but when time is on your side, and the stencil printing operation is all systems go, use the opportunity to make the process even tighter and the output quality superior.

Clive Ashmore headshot
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.