Top Considerations for Solder Paste Stencils for SMT

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This month I had asked Kenny and Andrew, my lucky manufacturing engineers who are also (albeit, forced to be) my technical writing assistants, about their thoughts for the next blog topic. Kenny suggested top considerations for solder paste stencils for su… They both felt this was a relative topic as fine pitch components and alternative solders/pastes have generated recent conversation and questions surrounding best practices for solder paste stencils.

For starters, to ensure solder joint reliability and consistency during the surface mount assembly process, solder paste stencil manufacturers have had to develop innovative ways to battle the shrinking size of electronics and components. The most common processes used in the manufacture of solder paste stencils are chemical etching, laser cutting, and electroforming.  Depending on the type of process used, the surface of the stencil aperture walls will obtain a different finish. Not only is it important that the stencil assists in providing the correct placement of the paste for the components, but it must also release the correct amount of solder paste.  The smoother the finish, the more easily the solder paste will be released from the stencil. Though electroforming is a more recent option, it has not fully caught on due to the accuracy levels it provides during the formation of the apertures, as well as the costs for engineering and manufacturing them.

Our three top considerations when picking a stencil for the surface mount assembly process have to do with stencil materials, paste volumes and stencil coatings. As the beginning and very critical stage of the circuit card assembly process, it is very important that the stencil provides both the correct placement and the correct amount of solder paste to the boards.  Insufficient or incorrectly placed solder paste can set the entire project up for failure right from the start. Therefore, our first two considerations go hand in and are driven by the materials the stencils are made of, and how they are formed. 

Our manufacturing engineers recommend stainless steel as the top stencil material.  Stainless steel provides more accurate apertures (effective area ratio) compared to nickel due to how they are made. (OK, I could go on to say something about the nickel stencils are made via ‘adding’ materials and the steel stencils are made via ‘subtraction’ but that is a WHOLE other blog in itself.) The molecular structure of the stainless steel, as well as the laser process used in their creation, provides for a smoother aperture wall and therefore a more precise release of solder paste. 

Solder paste contains alloys, flux, that assists with the reflow process where solder is applied.  If there is not enough solder paste released, there would be insufficient flux, preventing the solder from adhering components properly to the circuit board.  Keeping the correct ratio of flux in the solder paste has become increasingly difficult as smaller amounts of solder paste are required for smaller apertures due to the demand for smaller components. 

As designs call for increasingly fine pitched components, adjustments must be made to the stencils to ensure that the correct amount of solder paste is applied.  The decreased circumference of the apertures leads to an increase in stencil thickness in order to hold the required amount of paste.  This can make the release of the paste more difficult, as well as requiring modification of the rate in which paste is applied onto the stencils.  

Our third consideration assists in combating the issue of fine pitch components in varying production run volumes.  Different types of coatings are applied to stainless steel stencils to account for various problems that can occur.  In prototype and low volume runs, typically the stencil is wiped from the underside to rid of excess solder that could transfer to the next board.  In larger production runs, it is not ideal for the stencil to be wiped in between each board/pallet.  The solution for this is to apply coatings, such as FCT Assembly’s NanoSlic and UltraSlic.   to the stencils which decrease the grain in the aperture walls, thus eliminating residual solder that could throw off the required ratio of solder paste/flux. 

Proper stencil construction – both by way of materials and design – is a critical step in the surface mount assembly process. All and all, it boils down to math and physics. And, while that’s more than enough stencil science for me, there’s a lot of resources available on the topic, like     IPC, IEEE,  Or, you can just give Kenny a shout!

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