Nano

Step StencilS

Step stencils
In the early days of SMT assembly, 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 solderpaste/flux printing for flip-chip/SMT require special step stencil designs. Thick metal stencils, having both relief etch pockets and reservoir step pockets, are very useful for glue and paste reservoir printing. EFAB®* 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 to cope with non-coplanarityissues. Special 3-D E-FAB stencils are an excellent solution for unique PCBs having some raised areas on the board (up to .120” high). These applications and the stencil design to achieve solutions will be described in detail in this paper.

Introduction Eight different applications for step stencils will be described. These include the following: 1) Step-up stencils for ceramic BGAs 2) Step-upstencils for intrusive reflow of through-hole components. 3) Step stencils with a relief etch pocket on the contact side (board side) for: a. Raised vias b. Bar codes c. Board hold-down clips d. Additive traces 4) Step stencils for two print applications for mixed technology a. SMT/through-hole b. Flip chip/SMT 5) Thick stencils with etched reservoir pockets for printing glue for a variety ofcomponents with different standoff heights. 6) Two print stencils with etched relief pockets for printing glue for component attachment after SMT solder paste has been printed. 7) Thick stencils with etched relief pockets for printing on and over jumper wires on a PCB. 8) Three dimensional E-FAB Stencils with a formed relief pocket for a high flex connector that connects two PCBs. Step stencilapplications and solutions Ceramic BGAs Ceramic BGAs present a challenge to SMT assembly process. Since the high melting temperature prevents the 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 brickheight of 7 to 8 mils is desirable. On the other hand, SMT components like .5mm 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 8mil 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. In cases where 0201 chip components and .5mm pitch µBGAs are present, a stepped EGlobal SMT &Packaging - October 2006

Figure 1. Laser-Cut Step-Up Stencil for CBGAs.

Figure 2. AMTX Electroform Step-Up Stencil for CBGAs.

William E. Coleman Ph.D. and Michael R. Burgess, Photo Stencil, Colorado Springs, CO Keywords: Step stencils, step-up stencils, etched relief pockets, E-FAB stencils, mixed technology

FAB stencil is recommended to achieve good paste release for these smallapertures. A picture of a step-up E-FAB stencil is shown in Figure 2. The stencil is made by plating up to 5 mils in all areas and then continuing to plate up to 8 mils in the CBGA areas. Customers usually prefer the step on the squeegee side of the stencil and this is the case for the prior two examples. A general rule for spacing between the step ledge and first aperture in the lower thickness area...