Issue link: https://iconnect007.uberflip.com/i/686649
96 SMT Magazine • June 2016 lenging issue is the complexity—you are mixing small components, and larger components that require more solder paste, in a board. So, it will be more difficult. There are several design con- siderations that we do. We can do electroform step stencils, or sometimes, we have to overprint the solder paste—you have the solder paste out- side the pad, you will have enough volume for those larger components. We can deposit small volume for the small components and enough volume for the larger components. Las Marias: How does the solder paste impact the printing process? Apolinario: The solder paste has a big impact on printing as it is the main ingredient of the pro- cess and it has a direct impact on reliability. The choice of the right type and composition of sol- der paste may help achieve good printing quality. But, the key here is having the right pa- rameter to use, for instance, the design of the pad and the stencil. We need to select the ap- propriate type of solder to match the stencil design and pad design to have a good printing result. Another factor is the machine. We need to use the appropriate machine to get good re- sults. We have different types of solder printing machines—we have the screen printing as our typical printing method; we are now also using jet printing process. So, that's the factor. Apart from the multi-step stencil design to accommo- date those complex PCBAs, we are now trying electroform nano-coated stencil. This is some- thing new in our process—nano-coating by immersion coating process. The design of the stencil is electroformed, then it goes through a nano-coating process. This process makes the stencil smoother and non-sticky, to have a good transfer efficiency and to reduce cleaning sequence in the process. Rodney Bebe: The solder powder size varies from type 1 to type 8—the higher the type num- ber, the smaller the solder powder size. Techni- cally, the smaller the balls, the more alloy you can deposit onto a pad. Another thing is the alloy content itself. Solder alloy varies, majority of our current solder are SAC (tin, silver and copper) as this is widely used for industrial and automotive and cost effective. Another consideration is cus- tomer requirements. If the product is automotive grade, definitely you must have high reliability solder pastes. So solder pastes go through evalu- ation and tests to compare which is the best to use. For instance, we use a certain type of solder paste for automotive applications. For commer- cial or industrial PCBs, we use another type. Currently, we are exploring the use of high- er-grade solder pastes. One is Innolot alloy which is touted to have top-level reliability; it's an SAC-based alloy with small amount of anti- mony, bismuth, and nickel. Las Marias: Is there a need to collaborate with sol- der suppliers? Apolinario: Yes, we need to collaborate, espe- cially when there are tighter requirements, or if there are requirements we were not able to prove from our current solder. We need to work with solder paste makers to study or come up with the best formulation of alloys that can be a solution for certain problems. But it will be done through a series of evaluation, including temp-cycle process, manufacturing tests, and also functional test. We have to prove first be- fore we proceed to use a particular solder paste. Bebe: We have a generic formulation—for exam- ple, we are using SAC solder for automotive elec- tronics applications, which requires higher reli- ability—and we have a specialized solder, which also needs to be approved by the customer. Las Marias: How does the trend towards finer pitches and tighter tolerances affect the solder paste printing process? " The choice of the right type and composition of solder paste may help achieve good printing quality. " SOLDER PASTE PRINTING: A USER'S PERSPECTIVE