Stencil printing of adhesives
A set of Mylar wipers contains the glue within the head assembly. As the head moves over the stencil, the paste within the transfer head rolls and the wipers guide the adhesive back into the head, leaving no adhesive on the stencil.
As a high-throughput alternative to serial adhesive dispensing, high-volume surface mount assembly facilities are using stencil printing platforms equipped with relatively thick, lightweight acrylic stencils and enclosed print heads to deposit large numbers of glue dots of varying heights in a single print cycle.
The enabling factor for adhesive printing is that aperture diameter controls deposit height. To produce glue dots with heights of 75mm to 1 mm, apertures of different sizes are machined into a single stencil. In a 1-mm-thick stencil, apertures of 0.6, 0.8 and 1.0 mm in diameter produce dots for small 0603, 0805 and 1206 components. For larger components, two to three apertures of 1.5 to 2 mm diameter each produce dots for SOICs, and five apertures of 2 mm diameter each produce dots for QFPs.
When glue is deposited onto a stencil that has protruding leads from components previously placed on its underside, a 3-mm-thick stencil with clearance pockets routed into its underside, as well as different-sized apertures, is used. The pockets may be rectangular, circular or channel-like in shape and require 0.2 to 0.24 mm of clearance from fully drilled holes. To reduce the length of the narrow section of smaller holes (typically those that are less than 1 mm in diameter), the top of the aperture may be counterbored.
Stencils for adhesive deposition may be generated using Gerber, HPGL, DXF and ODB++ data, or designs can be scanned directly from a PCB. With CAD/CAM processing, manufacture can be completed in a few hours and, in many regions, stencils can be shipped to users by way of overnight delivery services.
Depositing the adhesive through the stencil may be done with a traditional squeegee; however, process control is optimized when an enclosed print head system is used. The enclosed print head, with its direct imaging system, encases the adhesive within an air-tight cassette, isolating it from the atmosphere. An air piston applies constant, pre-set pressure to deposit the material independently of print speed, ensuring rapid, consistent application of any number of glue dots in a typical eight-second print cycle.
The transfer head uses an air piston to provide controlled pressure on the material (paste or glue) cassette, for an accurate material flow through the stencil apertures; the head is enclosed to keep out contamination.
Factors involved in the decision to use a stencil-printing platform for adhesive deposition include production line speed, equipment cost and manufacturing flexibility. When an assembly line includes one or more placement systems processing 60,000 to 80,000 components an hour, rapid board delivery is crucial to eliminating assembly line bottlenecks.
Machining a 3-mm thick stencil to accommodate component leads allows the stencil to sit flush on the board and apply glue in a controlled fashion.
The high throughput achieved by a stencil printer depositing adhesives can help manufacturers realize the productivity benefits of high-speed placement systems without increasing the number of serial dispensers, each of which requires an initial capital outlay twice that of a stencil printer. The accessories - stencils machined for each glue deposition pattern - have a modest impact on the overall cost of equipment ownership. The stencil printing platform may also be used to deposit a range of surface-mount assembly materials - including solder paste, adhesives, epoxies, fluxes, dielectrics and high-value alloys - in response to changing or varying production requirements.
For more information on stencil printing, contact DEK, e-mail firstname.lastname@example.org
or visit http://www.dek.com