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108 SMT Magazine • March 2014 of laser-cut stencils has had a dramatic posi- tive effect on the amount companies spend on stencils. Depending on your needs, there are also frameless stencil options available that can lower prices even further. While the addition of coatings, steps in the stencil, or fabrication of electroformed stencils come with an increased price, the overall benefit of the added cost may be more than made up for with improved pro- cess yield. 3. how long will this stencil last? What is the life of a stencil? That is the golden question almost every customer wants answered, but the answer is not terribly straightforward. Because there are so many variables that have to be taken into consider- ation, there truly is no average actual lifespan of a stencil. Paste type, blade pressure, blade type, board layout, stencil thickness, cleaning chemistry, and cleaning frequency all affect the stencil's life. I am aware of customers running stencils for over 100,000 print cycles, and then I know of other customers who re- place stencils as soon as they cross the 40,000-print cycle mark. In the most optimized printing environments there should be no struggle with a good stencil printing success- fully for 75,000 prints. 4. how does area ratio impact the stencil type? Paste transfer is a compet- itive process. The side walls of the aperture hold the sol- der paste inside the aperture while the area under the aper- ture (pad on PCB) pulls the sol- der paste out of the aperture away from the aperture walls. The larger the wall area compared to the pad area beneath the aperture, the more difficult it is for the paste to release from the aperture walls. Because of this, identifying area ratio ranges for the stencil and pairing them with the appropriate stencil tech- nology is key. Doing so helps lessen rework and aids in the success of stencil printing. 5. Do I really need a coating on my stencil? Many companies today use a nano-coating of some type to enhance the properties of the stencil. These benefit the printing process by providing slight paste release and under-screen cleaning improvements. Since most of these coatings are not permanent, it can be difficult to determine how long the coating will last in relation to the life expectancy of the stencil. Of- ten a change in the coating becomes apparent only after process yields start to shift or clean- ing becomes more difficult. More permanent coatings are hitting the market and some are colored for a visual indication of their presence on the stencil. 6. What type of frame do I need? Due to the countless types of printers used by manufac- turers, the variety of frame types needed to service the industry is in the hun- dreds. Fortunately, high-vol- ume stencil sizes are fairly regular in size, typically com- ing in as 8x10, 12x12, 20x20, 23x23, and 29x29 inch frames. Many of these frames are available as standard frames, or as frames with space-saving options. Cus- tomers who can move to standard frame types usually enjoy cost savings compared to those using less common frame types. 7. can you fix the stencil I have? Yes, many stencil providers will add apertures and remount exist- ing stencils into different frames. In some cases, the time, effort, and costs associated with ship- ping the stencil back and forth and remounting or adding apertures to an existing stencil make procuring a new stencil a more appealing choice. a ShorT SCoop Because there are so many variables that have to be taken into consideration, there truly is no average actual lifespan of a stencil. Paste type, blade pressure, blade type, board layout, stencil thickness, cleaning chemistry, and cleaning frequency all affect the stencil's life. " " 10 cOmmON STENcIL QUESTIONS continues