Stencils printing

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

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. 

Screen, stencil printing Surface mount, hybrid, or MCM/ stencil or screen printer Fast, multiple dot size. Dots as small as 3-mils diameter on 5-mil pitch can be printed Stencil printing requires a new stencil per design complex cleaning procedures for small aperture stencils cannot be used with other components on boards... 

Adhesives used for screen or stencil printing in surface-mount applications are generally electrically insulative types whose functions are mechanical attachment and thermal dissipation. However, electrically conductive, silver-filled epoxies have been used for many years as ohmic contact adhesives to interconnect bare-chip devices in hybrid microcircuits and are used as solder replacements for surface mounting of components on printed-circuit boards. Regardless of their... 

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 ... 

Table 4.16 Examples of adhesives used for stencil printing... 

Loctite 3612/Henkel Metal-stencil print applications for small parts bonding 15-55 350 700 Print speed 60 mm/s Squeegee pressure 3-4 N/cm Separation speed 0.1 3 mm/s Stencil and printed-circuit board gap none (on contact)... 

Breed, SR, Pemice, RE. Stencil printing SMT adhesives a technology review, www. alphametals.com/products/techarticle 199914002.pdf. Aug. 26, 2002. 

Chouta P, Eleck I. A new dimension in stencil print optimization. Cookson Electronics. 2003. 

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. 

Fill the vias by screen printing or stencil printing sufficient silver-filled epoxy to form bumps or leave the vias open for wire bonding. 

The spread and slump for surface-mount adhesives may be measured according to IPC-SM-817, Method 4.5.5. According to this method, sets of adhesive dots (minimum three per set), 0.65 cm in diameter and 0.25-mm thick, are deposited (for example, by screen or stencil printing) on each of two frosted glass microscope slides. One sample is stored 50-70 minutes at 25 5°C and 50 5% relative humidity and then measured for spreading of the uncured adhesive. The second sample is cured and measured for the increase in pattern from the original dimensions. The acceptance criterion is an increase of less than 10% of the original diameter of the dot or pattern. 

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. 

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... 

---