Abstract
This paper address two significant applications of stencils in advance packaging field: 1. Ultra-Thin stencils for miniature component (0201m) assembly; 2. Deep Cavity stencils for embedded (open cavity) packaging.
As the world of electronics continues to evolve with focus on smaller, lighter, faster, and feature-enhanced high-performing electronic products, so are the requirement for complex stencils to assemble such components. These stencil thicknesses start from less than 25um with apertures as small as 60um (or less). Step stencils are used when varying stencil thicknesses are required to print into cavities or on elevated surfaces or to provide relief for certain features on a board. In the early days of SMT assembly, step stencils were used to reduce the stencil thickness for 25 mil pitch leaded device apertures. Thick metal stencils that have both relief-etch pockets and reservoir step pockets are very useful for paste reservoir printing. Electroform Step-Up Stencils for ceramic BGA's and RF Shields are a good solution to achieve additional solder paste height on the pads of these components as well as providing exceptional paste transfer for smaller components like uBGAs and 0201s. As the components are getting smaller, for example 0201m, or as the available real estate for component placement on a board is getting smaller – finer is the aperture size and the pitch on the stencils. Aggressive distances from step wall to aperture are also required. Ultra-thin stencils with thicknesses in the order of 15um–40um with steps of 15um are used to obtain desired print volumes. Stencils with thickness to this order can be potential tools even to print for RDLs in the package.
Cavity technology can be an effective solution in reducing the total assembled PCB thickness (Z-height), most importantly, on designs utilizing taller - stacked devices. Traditionally, dipping process or dispensing process was used to deposit solder paste, flux, or glue on uneven surfaces. However, this takes a longer time when compared to printing using a stencil printer. Reservoir printing using a stencil printer has greater potential in such application. Extensive work has been done in the past to print glue, solder paste and/or flux into cavities using reservoir printing. This paper focuses on printing solder paste into multiple cavities (stencil pockets) with depths ranging from 355 microns to 450 microns, and with varying cavity size, wall angles and various stencil thicknesses ranging from 100 microns to 150 microns. Apertures varying in area ratio were placed in these cavities and experiments were conducted to analyze the print performance of the stencils. As the size of the components and boards/substrates gets smaller - closer placement of components to the cavity (stencil pocket) walls needed to be assessed as well.
These applications, the associated stencil design and print results were discussed in detail in this paper.
CONTRIBUTION OF DRIFT AND DIFFUSION TO WATER CAPACITY DEIONIZATION
