Stencil dispensing

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. 

Figure 30 Alternative Principles for Application of Solder Paste, (a) Stencil printing. (i>) Dispensing.

In surface mounting of components, both solder paste and adhesive need to be dispensed. The solder paste is usually screen printed or stenciled first, followed by selective dispensing of the adhesive between the solder pads. The prime function of... 

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. 

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 ... 

Zarrow P. Adhesive deposition pin, syringe, stencil. Circuits Assembly. Dec. 1995. Piracci AF. Ultra high-speed dispensing of surface mount materials to enhance production throughput, Proc. APEX Conf. Long Beach, CA Mar. 2000. 

First, in terms of their physical form, adhesives may be either pastes or preforms (also known as films or tapes). Pastes are semisolid materials, easily dispensed through a needle or applied by screen or stencil printing. Film adhesives are solid sheets of thermoplastic or partially cured (B-staged) thermosetting polymers that can be cut to size and generally used to attach large-area components, substrates, and lids. 

Chipbonder 3616/ Loctite High speed stencil printing adhesive compatible with DEK Proflow dispense system 32,000/NA 300-550 Rounded High-speed stencil printing up to 152.4 mm/sec (6 in/sec) squeegee pressure (4-6 N/cm)... 

Staychip 2078E/ Cookson Epoxy anhydride, fluxing imderfiU N/A Ceramics, laminates Automatic dispense or stencil Area array on PWBs using solder-reflow cycle for underfill curing... 

Amicon E1330LV/ Emerson Cuming Unfilled, no-flow fluxing underfill (reworkable) Not applicable Nickel-gold and OSP finishes Syringe, jet dispense, or stencil print Flip-chip, CSP, EGA modest post-reflow cure (offline) required. Reworkable, 300 second reflow cycles 225°C peak. Variable fi-equency microwave curable. 

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