Printing Solder Paste and Flux for Tomorrow’s Electronics, Parts 1 and 2
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In the early days of surface mount technology (SMT), step stencils were used to reduce the stencil thickness for 25-mil-pitch leaded device apertures. However, as SMT requirements became more complex and, consequently, more demanding, so did the requirements for complex step stencils. Mixed technology applications of solder-paste printing for through-hole/SMT, as well as solder-paste/flux printing for flip-chip/SMT, require special step-stencil designs. Keeping up with changes in technology is always a challenge for doing business in a fast-paced high-tech field.
Thick metal stencils that have both relief etch pockets and reservoir step pockets are very useful for glue and paste reservoir printing. Electroform and laser-cut step-up stencils for ceramic BGAs and RF shields are a good solution to achieve additional solder-paste height on the pads of these components. Special 3-D electroform stencils are a workable solution for odd PCBs that have some raised areas on the board (up to 0.120 in. high). These applications, and the stencil design to achieve a solution, will be discussed in detail along with some other unique applications.
Step-stencil applications and solutions
Ceramic BGAs present a challenge to the SMT-assembly process. Since the high melting temperature prevents solder balls from melting at normal reflow temperatures, any slight coplanarity problem can result in an open contact to the CBGA balls. It is desirable to print higher solder bricks on the CBGA pads to prevent this problem. Normally, a solder-paste brick height of 7-8 mils is desirable.
On the other hand, SMT components like 0.5-mm-pitch QFPs, 0402 chip components, and R-Packs will not tolerate an 8-mil-thick stencil. Their aperture sizes are too small to achieve good paste release with a stencil this thick. Therefore, a step-up stencil is required for this application. An example of this stencil is shown in Figure 1. This stencil starts out as an 8-mil-thick foil, and the foil is etched back to 5 mils in all areas except in the CBGA area. The foil then has all the apertures, including the CBGA apertures, laser-cut into the foil. Electropolish and nickel plating are recommended for this stencil to achieve good paste release for the small apertures.
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