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Mixed technology packages that have both flip chip (FC) and surface mount devices (SMD) on the same substrate are now being assembled in a normal SMT assembly process.
Instead of dipping the FC die in flux, both flux and paste are printed on the substrate. Then the FC and SMD are placed into the paste/flux with pick-and-place equipment and sent to a reflow oven where both are soldered to the substrate.
This assembly process requires a "two-print stencil" printing procedure and requires two inline stencil printing machines. Normally, flux is printed first using a thin electroform stencil (40–50 μm thick) to print flux on the FC die pads. This is referred to as the "first print stencil."
Immediately after the first print, the substrate is advanced to the second printer. The "second print stencil" is used to print solder paste for the SMDs. This stencil is normally an electroformed 75 μm thick stencil for the solder paste apertures but has a relief step pocket 50μm thick on the contact side of the stencil in the area where flux was printed with the first stencil. This relief pocket provides sufficient clearance to avoid contact with the previously printed flux.
Editor's Note: This article originally appeared in the February 2016 issue of SMT Magazine.