Indium Corporation’s Miloš Lazić, Technical Support Engineer, and Sze Pei Lim, Regional Manager, Semiconductor, discuss new technologies available for the semiconductor industry’s fine-pitch assembly processes.
Miloš Lazić: Sze Pei, as we all know features are getting smaller and demands become more stringent. What is going wrong in our customer's fine feature assembly?
Sze Pei Lim: I wouldn't say that there is something wrong, but I think that one thing that we need to place a lot of emphasis—one process—is the printing process. It includes the design of the stencil—how you design the stencil opening, and the board support—as well as how you set the process parameters. So, all this actually plays a very important role, especially when you go down to the fine pitch assembly process. We really have to take care of each process step and make sure to achieve good printing performance.
Miloš Lazić: How does smaller powder size effect flux used in a solder paste?
Sze Pei Lim: As the powder size becomes smaller and smaller, so the surface area of the powder also increases. Especially, when you can go from Type 5 to Type 6 and Type 7—and maybe some are thinking about Type 8, so the surface area literally increases exponentially. As this area increases, the oxidation also increases, and the function of the flux is actually to remove the oxides. So, if there is more oxidation, the loading to the flux actually increases and you need a stronger flux to effectively clean off all the oxides and create a good solder joint.
Miloš Lazić: How do you choose the right solder paste?
Sze Pei Lim: To choose the right solder paste: First, you look at the powder size. What is the opening that you want to print? The general industry guideline is to have at least five to six particles across the opening of the stencil, so that is how you choose the correct powder size. Then you have to choose the correct alloy that you are looking at—is it a standard set alloy or other alloys. And of course, the last thing is to choose the correct flux system. Whether you want to use a no-clean flux or a water-wash flux. Generally, most of the industry is still using the water-wash flux because the subsequent process would be the molding process or the underfill process, where it needs a good adhesion to encapsulate all the components. That is why a majority is still using the water-wash solder paste.
Miloš Lazić: And at the end, Sze Pei, what would be your general recommendation for fine-feature solder paste printing?
Sze Pei Lim: Water-wash is what the majority of our customers are using, but in the future, when density is getting high with a lot of fine-pitch components and even fine-pitch copper-pillars where cleaning will be difficult and where water will be difficult to penetrate and clean off all the residue, there will be a point that they have to look at some other solutions that we can offer. Currently, that includes what we call the ultra-low residue solder paste. This range of solder paste leaves a really minimal amount of flux residue behind, and even though there is this small amount of flux residue, it is compatible with the subsequent process of the molding material or the underfill material.
Miloš Lazić: Thank you, Sze Pei.