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1 Introduction
Stencil printing is used in the electronic packaging industry primarily to generate the mechanical and electrically conductive interconnects between two devices. The devices themselves can be pre-packaged chips, individual die, wafers, discrete devices and substrates such as PCBs, flex substrates or ceramic multi-layer circuits.
Stencil printing uses an angled blade, called a squeegee, to press a viscous material through pre-defined open apertures in a solid foil onto a substrate. The configuration of the stencil apertures determines the basic layout of the deposits. In a modern stencil printing machine for electronic packaging the stencil is normally located in the front of the machine, with the squeegees positioned above the stencil, as shown in Figure 1 [Figure omitted. See Article Image.]. The substrate carrier is then passed through the side of the machine and subsequently aligned to the stencil. During the printing process the substrate is brought into contact or proximity with the stencil. The squeegees are placed onto the stencil with a set pressure and driven across the surface of the stencil at a pre-defined speed. This action causes the paste material to roll across the stencil and the apertures on the stencil are filled with a viscous material, usually solder paste or an isotropic conductive adhesive (ICA). When the stencil is released from the substrate the resulting contents of the filled apertures are ideally transferred to the bond pads, thereby forming deposits that will create the interconnects. With one print stroke, millions of deposits can be placed simultaneously onto the substrate surface. This process, that takes place within seconds, can be repeated thousands of times with the same stencil onto subsequent substrates, thereby creating a low cost, high throughput process. Photographs of a modern stencil printing machine are shown in Figure 2 [Figure omitted. See Article Image.].
The stencil printing process was originally adapted from the screen printing process, whose main difference lies in the actual artwork through which the printing material is transferred. The screen is a woven mesh, which has been photopatterned to create a defined image, whereas a stencil is a solid foil with suitably placed holes.
Records indicate that the screen printing process was developed long before stencil printing, by the Chinese and Egyptians for the production of consistent...