Controlling cleaning inputs is essential to achieving a clean stencil.

There is probably not a fully automatic screen printer sold today that doesn’t include an understencil cleaning option, but the fact is that cleaning is a process fix. If the printing process is well centered, completely in control, and all of the inputs are spot on, cleaning shouldn’t be necessary. We clean because there is something out of control: Maybe you are using poorly produced stencils, poorly manufactured PCBs, or a less-than-ideal solder paste chemistry. Whatever the problem, cleaning is often used to attempt to solve it.

In truth, the more you clean, the more out of control your process can become. We know that in a solder paste printing process, it may take a few boards after a clean for the process to stabilize again. When the stabilization time, yield expense, and the actual time it takes to perform the cleaning are factored in, it becomes clear that valuable machine throughput capacity is being wasted.

OK, I’m stepping off my soapbox. While it’s true that in theory a controlled process shouldn’t require cleaning, the reality is that 99% of manufacturers clean for one reason or another. Given that, let’s examine how this process can be optimized for the best results and least amount of throughput impact.

Understencil cleaning is a sub-process of printing, and that’s exactly how it should be viewed. Like any process, it is essential to control the cleaning inputs so that the output – a clean stencil – is acceptable. Inputs include the cleaning chemistries, the paper rolls and, of course, the cleaning system capabilities. In the case of cleaning chemistries, it’s important to understand the impact these materials have not only on the cleanliness of the stencil and apertures, but their subsequent effect on the solder paste materials. Take IPA. This chemistry definitely cleans the stencil because it is very active on flux and heavily dissolves the flux material. But, if the IPA remains in and around the apertures (and it most likely will because a lot of IPA must be used to get the stencil clean!), the residue will eat at the flux and an undefined paste deposit will result because the chemistry is actually dissolving the flux within the deposit. Following reflow, this may lead to such defects as solder balling or bridging.

There are chemistries designed to clean the stencil by penetrating between the surface of the paste and aperture while still maintaining the flux characteristics’ integrity. Plus, they are more cost-effective because they don’t require nearly the volume that IPA requires for an acceptable clean. Choose the chemistry wisely and it can make all the difference in the world. Likewise, there are varying degrees of effectiveness of the paper used in understencil cleaning. Papers that leave behind a great deal of lint or are substandard and prone to shredding will have a clear and adverse effect on stencil cleanliness and process control. Imagine you’ve got hundreds of little fibers left behind – guess where they’ll likely end up! Not a good scenario.

Last, but certainly not least, are the capabilities of the cleaning system itself. I mentioned earlier that the cleaning process is a non-optimal use of valuable machine throughput capacity. Here’s why: Consider that we are now in a world of extremely high-volume manufacturing where advanced printing systems are pushing sub-five second cycle times. When you factor in a bit of process time for print stroke, separation and board transport, you can probably move a board out of the printer every eight seconds. But wait! You add in a cleaner with a standard cycle of chemistry application (wet), vacuum and drying, and all of a sudden the process takes 45 seconds. So, after investing in this reduced cycle time capability, suddenly you have a 45-second overhead and you’re doing this every 10 prints. The remedy? First, control your inputs. Second, do not clean more frequently than every 10 prints. Third, clean efficiently. New advances in cleaning system technology have streamlined the process to enable a more thorough, yet faster and more effective operation. With cleaning time halved and paper changeover reduced to seconds, newer generation technology helps get back some of that throughput capacity.

I maintain that if you are cleaning more often than every 10 prints – and you know exactly who you are! – there is something wildly wrong with your process, or you are deathly afraid there will be if you don’t clean every chance you get. Step back and take a look at your printing process, evaluate the inputs and get them under control. Cleaning frequency and cycle time – especially with the right chemistries and systems – can be significantly reduced.

Clive Ashmore is global applied process engineering manager at DEK (dek.com); cashmore@dek.com. His column appears bimonthly.

• Prev
• Next