Stencils for Mixed Flip Chip / SMT Assembly

ABSTRACT The requirement to combing FC (Flip Chip) and SMT assembly on the same substrate has increased dramatically with the demand for smaller and smaller assemblies. Stencils play a major role for this mixed technology. This presentation will review several stencil configurations to satisfy these requirements. 3D Electroform Stencils and Step Electroform Stencils for printing flux or paste for FC and paste for SMT in a “Two Print System” will be shown. The first print is normally a thin stencil for printing FC, the second print stencil has a relief pocket wherever the first print stencil applied material. Both FC and SMT reflow at the same time. Another stencil configuration allows paste for SMT to be printed first and flux for FC second using “flux reservoir printing”. The second stencil has a relief pocket where paste was printed with the first print. Reservoir printing is a process where the stencil has a reservoir with apertures in the bottom of the reservoir. Applying positive pressure to the filled reservoir allows material to be extruded through the stencil apertures. Flux or paste reservoir printing is also useful when the FC is attached in a cavity in the substrate. A 3D Electroform stencil is used for this application and will be demonstrated in this presentation. A relief step Electroform ball drop stencil design for printing flux and then distributing solder balls on a substrate having 01005 chips already mounted on the substrate will be described.