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December 2017 • SMT Magazine 19 EQUIPMENT MATTERS IN SOLDER PASTE PRINTING pect that the solder paste may not have as long a stencil life or storage time," adds O'Neill. The mesh size has the same problem, ac- cording to him. Dropping the mesh size dra- matically increases the surface area available for the same solder deposit volume. "I use this analogy when I give my talks: you've got a shoe- box filled with Vaseline and marbles, and that is Type 4 solder paste. You take out all the mar- bles, and you fill it full of BBs. Now, you still have a shoebox full of metal, 90% by weight. The volume of metal has not changed, but the surface area that's available in that same vol- ume has exploded when you think about the difference between a BB and a marble, or a ping pong ball and a grapefruit," explains O'Neill. "You just have all this additional surface area that's interacting with the flux, and that does a number of things. As I mentioned about re - activity, that oxide is interacting with the flux medium, and the flux medium is beginning to change as a result. When you add energy dur- ing the printing process, there's friction in- volved, so the temperature of the solder paste rises, and that increases the reactivity even more. Add some moisture from a humid envi- ronment, and now more chemical reactions are starting to be initiated. All those things are im- pacted when you drop this mesh size down. It just becomes much more sensitive to any type of input, whether it' s temperature and mois- ture, and the solder paste's characteristics start to drift. I mentioned at the beginning, con- sistency is the key to the whole thing. Even if you're consistently bad, it's a process that you can control. It's inconsistency that's probably the number one problem." "These are all the things we are doing. I mean, I am going along and I'm dropping mesh sizes down, and we have made Type 4 a stan- dard now. We don't supply Type 3 as a stan- dard mesh size anymore, as part of addressing the challenges that fine pitch introduces. We have people that are insisting on Type 5, which, as you can imagine, if I'm going from marbles to BBs, I'm going to dust when I go to Type 5, and we're even seeing in applications outside of traditional stencil printing and getting into jetting these Type 6 and finer powders being used," says O'Neill. "That's the direction that we're headed. I think there is some theoretical limit. So, if we get the print process sorted and we're getting the solder paste through the aper- ture we expect, and it's being done consistently and repeatedly, I can still encounter problems in reflow. All of this additional surface area that we've introduced to solve the transfer efficiency issue needs to be contended in the reflow oven. You end up with graping and wetting-related defects because now, I've got a deposit that's so small that I have very little flux available, and I've exploded the available surface area conse- quently. It's all bad and it's all counter to what is friendly to what we do. It's forcing us to con- tinue to innovate, both in terms of our flux chemistries, but as well as our powder manu- facturing capabilities as well. We're continual- ly improving the quality of materials as it re- lates to our paste chemistries. On top of that, it seems like I'm singing all my woes here, but it's voiding on bottom terminated devices. There are all these other ancillary defects that have come along as the package and componentry has evolved. When I got into the business, ev- erything was a QFP, and then came along BGA, which introduced some new challenges. Now, bottom terminated devices have just been ad- opted so quickly. One day a couple guys were using them, and within 18 months they were pretty much everywhere. They've introduced a whole bunch of new challenges for us, as I men- tioned, voiding being the biggest one probably. I don't see that problem going away, either. You can't extricate the printing process from every- thing downstream. It's all affected along the way."

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