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50 SMT007 MAGAZINE I DECEMBER 2019 to have good tack time and to be malleable, so they can print it, which makes raw paste a little trickier to clean. Now, it's not as hard to clean as reflowed materials. But the equip- ment that's used to do it isn't what is often used to do reflowed materials, so that's the whole challenge there. Again, it's one of those areas where everyone has to keep their stencils clean and wants their stencils to last a long time. You don't want materials that are going to have an adverse impact on stencil life or performance. You don't want those aluminum frames to turn dark or be attacked. Certainly, you don't want the mesh or the adhesive that holds the mesh onto the frame to be attacked. It's per- ceived quite negatively, not unexpectedly (laughs). Because of that, there are generally temperature limitations. Remember that those stencil frames are expensive, and they like to reuse the frames. You want a thermally sen- sitive adhesive putting it all together so you can get them off when you want to get them off, but that means you don't have tempera- ture available to clean; otherwise, the stencil falls out, and everyone gets angry. This is an undesirable outcome. Matties: Is the cleaning method in-line or batch? Forsythe: Generally, batch and mostly spray systems. Stencils tend to be spray systems. There are some ultrasonic ones around, but they're usually spray systems. Stencils are bigger than just about any assembly, so they're always used in a fairly big, open area. When you're cleaning stencils, you're cleaning raw paste, which means you're cleaning a bunch of solder balls connected with other stuff. It has been recommended for many years that you not clean assemblies or misprints in the same tool. Everyone tries to have good filtration and whatnot, but solder balls are small and perni- cious, and you really don't want them on your assembly tomorrow. Matties: Are you recommending a secondary line for that process? Forsythe: A second cleaning device. These tend to be offline devices. They're batched and offline. That's been the best practice for a long time. Matties: And the investment to have that sec- ond really is minimal, I would think. Forsythe: It's more than lunch, but it's less than a house. Matties: But it's less than defects. Forsythe: Yes, these kinds of things pay for themselves a million times over, so those are the best practices, and that's why it is the com- mon practice in the industry. Matties: You're driving this new product out. Were there problems that people were facing, or is this something that you're showing peo- ple who had problems? Forsythe: There have been challenges all along. We started doing some renewed research on screen cleaning and stencil cleaning because we realized that it had been forever. We were dealing with all the constant evolution of no- cleans, and we finally reached the point where that's largely under control, but maybe we hadn't paid attention to stencil and mainte- nance cleaning in a while. A lot of the popular products were 15–20 years old, and the world had changed a lot in that time.

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