Issue link: https://iconnect007.uberflip.com/i/550232
August 2015 • SMT Magazine 79 ter than a standard laser-cut stencil, and not as strong as an electroform stencil. Matties: Stencils are part of the process, but it's not what you would call the sexy part of the pro- cess, right? Weissmann: It's certainly not sexy, but it's im- portant. More than half of the defects in assem- bly come at the printing stage—every piece of that needs to be dialed in. Matties: we're focusing our up- coming magazine issue on the 'war on Failure,' and we ran a survey of assemblers. their number one issue, of course, is software, as well as defects like head-in-pillow. when you look at all the contributing factors to this, printing obviously is one of the issues that they have to concern themselves with, along with cleaning and other variables. how do you fit into the solution of that defect? Weissmann: An assembler is looking at a tolerance, so they have a tolerance stack-up with variability that comes from the stencil, the material, and the printer. A cheap stencil uses up way too much of that al- lowable tolerance. It's a question of positional accuracy, dimensional accuracy, as well as hav- ing the ability to get the right amount of paste to transfer through that aperture. A better sten- cil uses a lot less of the variability in the process, and therefore can become less of a critical piece in making that happen. You need all of the piec- es to work well together, but your whole process is only going to be as good as your weakest link. Too often we see that being a low quality stencil. Matties: what's the price difference? what is price difference between high quality and low quality? Weissmann: It might be as much as 4:1 depend- ing on the specific features, but there is a range of different products. Matties: what's the typical price of a stencil? i'm not familiar when you say 4:1, what kind of num- bers are we talking? Weissmann: The cheapest stencils, and we don't do much of these, might be $250 or even a little bit less. Whereas on a single-level stencil, you can go up into the thousands for a really great stencil with really demanding requirements. Most of them are a little less than $1,000 for those kinds of stencils. Matties: Someone who's in auto- motive or high-reliability work, they're going to use the $1,000 models? Weissmann: It depends on their application, and we'll help them match the applica- tion with the right stencil ma- terial for what they're trying to make. Matties: the cost of failure is so high in automotive or aerospace. it's life or death in some cases, and then there's, of course, product liability that comes back to some of the manufacturers. it seems to me for $700–1,000, why risk it? Weissmann: It seems that way to me, too. An- other piece of the process that can make a dif- ference is the blade. We find blades to be the ugly stepchild of the printing process, and cus- tomers should take more care to ensure that their blades are new and well maintained, and that they match the type of stencil that they're using. Matties: how often should they change their blades? Weissmann: They really just have to watch and see, and then they can come up with a time- frame. But it depends on their printing pres- sure, their throughput and their use, and also on the type of stencil that they're using. You STENCILS: WHY THEY STILL MATTER continues IntervIeW you need all of the pieces to work well together, but your whole process is only going to be as good as your weakest link. too often we see that being a low quality stencil. " "