Stencil printing process

The main advantage of stencil printing over screen printing occurs in applications where very small areas of paste have to be deposited. For components with pitches equal to or smaller than 0.65 mm, the stencil printing process is the only viable way for printing solder paste. Therefore, stencil printing has replaced screen printing in most cases. 

Figure 4.5. Schematic diagram of plastic-stencil printing process. (Source M. Whitmore, DEK Printing Machines Ltd.)...

Fig. 12. Schematic depicting the ICA paste stencil printing process.

Amalu, E.H., Ekere, N.N. Mallik, S. (2011). Evaluation of Rheological Properties of Lead-Free Solder Pastes and their Relationship with Transfer Efficiency During Stencil Printing Process. Materials and Design, Vol. 32, (February 2011), pp. 3189-3197. 

The lead-free transition will have some impact on the stencil printing process. Printability of solder paste depends on flux formulation, but not for all alloy types. Experience has proven that there are no differences when printing lead-free solder pastes than when printing tin/lead solder pastes. Formal testing has verified that the printed volume of several lead-free solder pastes and tin/lead solder pastes were the same statistically when using the same stendl, printing equipment and boards (Figure 2). 

The process parameters key to the stencil printing process include the squeegee type and hardness, print speed, print pressure, and print gap. An optimal stencil printer setup provides a clean sweep on the stencil surface and a repeatable solder paste deposition process. Shore A scale polyurethane squeegee hardness level between 85 to 95 is appropriate to achieve repeatable solder deposits for lead-free solder pastes. Print parameters such as the print speed, print pressure, and the print gap are adjusted to accommodate the variety of solder paste rheologies available. There are no specific equipment modifications necessary to stencil printer lead-free solder pastes. 

Of the mass-transfer dispensing methods, screen printing and stencil printing are the oldest and most widely used. Screen printing has been used for over 40 years in the electronics industry to apply thick-film conductors, resistors, and dielectrics in fabricating circuits on ceramic and plastic-laminate substrates. Screen printing is also used as a batch process for depositing electrically conductive and insulative adhesives to interconnect devices on thin-film and thick-film hybrid microcircuits. 

Stencil printing is a fast efficient process for long production runs and is widely used in surface mounting of components. In surface mounting, the solder paste is generally dispensed first by stenciling, followed by needle dispensing the adhesive however, these processes may be reversed or both the solder and adhesive can be selectively dispensed by needle. 

As with most processes, numerous variables affect the final product. For solder paste deposition using stencils, Chouta and Heck identified 39 variables relating to materials, 32 variables to processes and equipment, and 10 variables to personnel and environment. Among variables critical for the successful stencil dispensing of adhesives are stencil design, the adhesive material and its rheological properties, and the printing process. 

Although stencil printing is widely used for both adhesives and solder paste, and has many advantages over other dispensing processes, there are several limitations that must be addressed as follows ... 

In a similar process, known as polymer-film interconnect (PFI), an insulative thermoplastic film is laminated over the devices at the wafer stage, and vias are opened over the bonding pads using a laser. At that point, either the normal solder bumps can be formed or a silver-filled conductive adhesive can be stencil printed into the vias to form polymer bumps. After printing, the epoxy is B-staged and the flip-chip devices are diced. In assembly, the devices are heated to a temperature that completes the cure of the B-staged bumps and simultaneously reflows the thermoplastic underfill material. 

The screen printing process uses a porous mesh stretched tightly over a frame made of wood or metal. Fig. 9.1 The mesh is made of porous fabric or stainless steel. A stencil is produced on the screen either manually or photochemically defining the image to be printed. Thus, the design of the stencil allows to obtain a wide range of screen-printed electrodes in which the electrodic configuration, as well as the size and form of these electrodes can be controlled. 

Stencils and other tools, such as squeegees, are cleaned in order to assure a reproductive and qualitative satisfying printing process. ... 

Although cleaning of stencils improves the printing results drastically, the production of misprints during the printing process cannot be avoided completely. However, the cleaning of misprinted assemblies is an application that is still frequently ignored. This involves the removal of misprinted or smeared solder... 

Screen printing of conductive andinsulative adhesives has been used for decades in the assembly ofhybridmicrocircuit and multichip modules. Since screen-printing processes are also used to deposit thick-film conductors, resistors, and dielectrics, the processes are fully compatible and cost effective in the production of hybrid microcircuits. Screen printing and stencil printing are both batch processes that are usually less expensive and... 

Stencil printing is a fast efficient process for long production runs and is widely used in surface mounting of components. In surface mounting,... 

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