STEP 5: Adhesives/Epoxies and Dispensing
December 31, 1969 |Estimated reading time: 8 minutes
Stencils used to print adhesives must work in three dimensions, therefore requiring more extensive input data than a conventional steel stencil would to print solder paste. Most of this data is already held in CAD files that describe the product, and can be extracted automatically to streamline stencil design for adhesive printing applications.
By Clive Ashmore
Surface mount adhesives (SMAs) can be deposited with high X/Y accuracy, and in finely metered volumes, using dispensing or screen printing equipment and processes. Screen printing is potentially faster, especially when large numbers of dots are required, because the total cycle time is constant, regardless of the number of deposits required. For example, the total cycle time to print adhesive dots onto a test board measuring 200 × 200 mm may be about 30 seconds, including transport time in and out of the machine. A board of this size may easily require up to 18,000 or more components to be mounted on the underside of the board, including ICs, SMT decoupling capacitors, bus termination resistors, other small passives, and hardware components such as connectors.
The screen printer completes its cycle within 30 seconds, whether the number of adhesive dots required is 180 or 18,000. Consider a high-specification dispenser, which may have a maximum speed of up to 140,000 dots per hour. Within 30 seconds, such a machine can produce less than 1,200 dots. The time to complete a board requiring 18,000 dots can be calculated as 7.7 minutes. If large dots or complex shapes are required, this will further slow the dispenser. With a cycle time of 30 seconds, regardless of the number, type, and shape of dots required, the print process delivers progressively greater benefits as the number of dots per board increases.
In addition, the equipment required to host the process is relatively inexpensive. In fact, a standard solder paste printer can easily be changed over to perform adhesive printing, and then changed back when printing with paste is required, saving additional capital investment.
Stencil Design
Taking a few minutes to understand how an adhesive-printing stencil works can streamline process implementation and eliminate many defect sources. Most adhesive-printing defects can be traced to avoidable errors in the stencil design. Fortunately, computer-aided design (CAD) tools can generate most additional information from base product files to prevent these defects. It is critical to understand that an adhesive-printing stencil is a 3-D stencil. Unlike a metal stencil for solder paste applications, its design must take into account the existing board-surface topology, so as not to interfere with features such as pre-mounted components or cut-and-clinched thru-hole leads protruding above the underside surface. Therefore, the basic input data for the stencil-design process must include information about the assembly and the basic description of aperture sizes and position, if the resulting stencil is to produce high-quality results.
The stencil material is typically a thermoplastic resin such as a Polyethylene Terephthalate (PETE), displaying low coefficient of thermal expansion (CTE) and good machining properties. The stencils usually are of uniform thickness, typically ranging from a minimum of 3 mm to a maximum of 8 mm. This is thicker than a laser-cut steel stencil for solder paste printing, and allows the bottom side to be rebated so as not to come into contact with existing topology.
Figure 1. MELF diode (top) and 1206 chip resistor (bottom).
Generally, when depositing adhesives, a variety of shapes and sizes are required. Unlike solder paste deposits, which usually must match standard pad sizes for established component outlines, production or process engineers can optimize adhesive deposits to maximize adhesion of the component to the board. This often is the case with large components such as ICs, where depositing a square, cross, or bar, for example, beneath the center of the device, may deliver the best results. In addition to depositing single dots, double and triple dots can be deposited to maximize the volume of adhesive deposited. Using a stencil of suitable design, these optimized shapes can be printed in a single pass, whereas a more complex sequence may be required with a dispenser. However, it is important to know the type of component to be placed at each location where an adhesive deposit is required. For example, a Gerber file describes nearly identical pad patterns for metal electrode face (MELF) diodes and 1206 chip resistors. But the adhesive deposit required to bond the MELF to the board is much taller than that required for a 1206 resistor (Figure 1). There is no single set of deposit characteristics suitable for both component types.
Figure 2. To be used if position of clinched lead is not known and space allows (left). To be used if position of clinched lead is known, or if space around lead is limited (right).
Where the stencil meets the board, it is usually routed on the underside to clear components such as surface mount devices (SMDs) that will be attached before the adhesive-printing process occurs. The type and location of any such component must be known for the stencil to be routed correctly to clear these components. Another important fact that the stencil designer needs to know is the nature and position of any thru-hole components. Where leads protrude, the stencil underside must also be routed to clear these features. In some cases, the components may be cut and clinched. The stencil designer must know the length of the cut lead and the direction of clinching. If the leads are always clinched into the same position, the routing required can be minimized. Figure 2 illustrates how the stencil is routed to clear cut-and-clinched leads.
Data to Drive Design
Although a board Gerber file contains sufficient information to generate a suitable program for laser cutting, Gerber data alone is inadequate when designing a 3-D stencil, such a bottom-side-routed PETE stencil for adhesive printing. The additional information required is available, and exists mostly within supporting product CAD files. This allows much of the input data to the stencil design process to be generated automatically, with minimal manual intervention. The key to meeting data requirements is to identify SMT components at each location, and assess clearance requirements in general so that the stencil can be routed correctly to ensure close contact with the board throughout the print stroke.
There are a number of sources for this information to achieve the best possible results when printing with adhesives. The bill of materials (BOM), or component list, generated by the CAD system, for example, describes SMT components used, allowing the appropriate adhesive deposit to be assessed for each site. Suitable apertures can be designed. Suitable component data can be extracted from most CAD systems, just as the Gerber layers can be generated automatically. Figure 3 shows electronic data, including pad dimensions and locations, as well as component outlines and additional details for printing.
Figure 3. Electronic data containing solder paste pads, step and repeat, fiducials, silk screen identification, orientation, board and panel edge, and front edge/print direction.
Additional information required includes the length of cut leads for thru-hole components. This may be specified in documentation supporting the board, or from program or machine settings for automatic or semi-automatic thru-hole assembly. Ideally, a completed assembly should also be provided to aid stencil design. If such an assembly is available, it must be a perfect example that complies with latest revisions, and is not damaged. It must also be suitably packaged prior to transit. If the board arrives damaged; for example, with components detached from the underside or missing, then the stencil design process risks beginning with an incomplete set of data. If a perfect assembly cannot be provided, a clear digital image of the board may suffice.
Stencil Refinements
It is possible to fine-tune the height of each individual adhesive deposit, without varying stencil thickness. Due to adhesive properties, increasing the aperture size tends to increase deposit height after stencil separation. By exploiting this behavior, careful stencil design can achieve deposits ranging in height from 50 to more than 1,000 µm.
The arrival of JEDEC SMD outlines, such as 0402, have helped advance stencil design further. The pad layout for 0402 components requires the size of the adhesive deposit to be so small that is was initially beyond known practical limits for the aspect ratio of aperture diameter to stencil thickness. With a stencil thickness of 3 mm, apertures less than 0.6-mm diameter typically do not permit adequate paste transfer to create a repeatable deposit.
Figure 4. To help glue penetrate through small holes on thicker stencils, a countersink is added to the squeegee side of the stencil, forming funnel-shaped apertures.
Machining a countersink in the upper surface of the stencil (Figure 4) enhances adhesive entry into the aperture while allowing the aperture at the surface of the board to be below 0.6 mm. Countersunk apertures are now known to effectively enable adhesive printing for 0402 components.
Further Guidelines
Further valuable guidelines for screen printing with adhesives include taking precautions when handling and using the stencils. For example, if the acrylic material used to produce the stencil does not possess anti-static properties, measures must be taken to protect the board and its components against static discharge.
Cleaning also is critical for any stencil. Adhesive stencils are no exception. If solvents are to be used, it is recommended to use only the solvent approved by the adhesive manufacturer. Automatic cleaning using ultrasonic techniques or submerged spray jets also produce satisfactory cleaning results.
Conclusion
Understanding the differences between adhesive-printing stencils and stencils developed for solder paste printing may unlock cost of ownership savings and throughput enhancements for many SMT assemblers - allowing printing of adhesives instead of dispensing. While the equipment hosting the processes may be identical, stencil philosophies for each are subtly, but crucially, different. More input data is required to generate an adhesive stencil, and most of this can be generated automatically from existing product files. Awareness is key, which allows stencils for adhesive applications to be designed quickly, and will deliver high yield in volume production.
Clive Ashmore, global applied process engineer, DEK International, may be contacted via e-mail: cashmore@dek.com.