Edited by Martha K. Raymond
Technical Service Manager, SMT
Product Manager, SMT Materials
Thick Film Div.
West Conshohocken, Pa.
Adhesive printing is gaining popularity among engineers for securing components to printed-circuit boards. Although 90% of all adhesive applications still involve dispensing, another technique that’s gaining popularity is printing, because its main benefit is higher throughput. One new printing method uses thick stencils to deposit glue dots with different diameters and heights.
Some benefits of the conventional and thick-stencil printing versus dispensing include the elimination of dispensing machines which leads to lower investment and shorter production lines; larger glue packages which results in less package waste and lower costs for the glue per quantity; using double sided boards populated with SMDcomponents only; and depositing adhesives in shapes.
To ensure optimal results, users should take specific precautions. One is to prevent air from becoming trapped in the glue during the printing process. When using a high-viscosity adhesive, the chance of air entrapment is greater. Therefore, increasing the temperature and using a smaller vertical squeegee can significantly reduce the risk. Another general rule is not to print with stencils considerably thicker than 10 mils because it compromises the consistency of dots less than 20 mils. Some guidelines for designing stencils for each of the printing processes follow.
In conventional printing, the adhesive transfers completely from the stencil to the board. The stencil thickness and aperture control how much adhesive is applied, and adhesive application can be calculated using the formula for the volume of a cylinder:
V = π(d⁄2)2h
where h = stencil thickness ≈ glue-dot height and d = aperture diameter ≈ glue-dot diameter.
The thickness of the stencil must be slightly greater, typically 0.002 in., than the component standoff, which is the distance between the board surface and the bottom of the circuit. Even if all components on the board have similar standoffs, after placement, the standoff can vary for the same component. For example, the thickness of solder resist and copper tracks that may pass between the rails can vary. Pad thickness must also be considered; if the pads are higher than the solder resist, the glue-dot diameter/stencil thickness needs to increase.
It’s easy to select the optimal stencil thickness when all components on the board have a similar standoff. However, on boards with active components, for example, standoff ICs, and passive ones, such as chips, the passive components are the limiting factor because their standoff distance is typically no more than 0.004 in.
Placing chip components in a glue dot that’s too high can spread the glue and contaminate the soldering pads. It can also create an oversized gap between the chip and the PCB after curing, which will generate skips during wave soldering.
Typically, stencil thickness is 0.006 in. for designs with only passive components, and 0.010 in. for designs with both passive and active components. Generally, glue transfers completely from the stencil, if the stencil aperture diameter to stencil thickness is approximately four to one or more.
When specifying aperture diameter for a passive component, a critical measurement to consider is the distance between the inside edges of the solder pads. This distance, less the print alignment tolerance, determines the maximum diameter adhesive deposit.
A new printing process is based on the surface tension between the glue and the stencil openings. This method lets some glue remain in the stencil apertures after separating from the PCB, particularly in stencils with small openings, for example 0.010 in. The surface tension is so strong that the majority of the glue remains in the stencil. As a result, glue dots on the board are shorter.
With bigger stencil openings, close to 0.030 in., most of the glue transfers from the stencil. When the stencil and PCB separate, the stencil drags the excess glue from the stencil holes, increasing the height of the dots.
If the stencil opening is larger than 0.080 in., all of the glue transfers through the stencil opening. The glue dots on the PCB are similar in thickness to the stencil thickness.
The roughness of the stencil aperture also affects the surface tension forces between the glue and the stencil. In addition, glue dot shape and consistency are affected by adhesive rheology, which is the material’s yield point, and plastic viscosity. The latest advances in adhesive formulations have produced resins with excellent adhesion for standard and difficult-to-glue components, such as low-stress, plastic-encapsulated components, and the resins are not hygroscopic.
|PRINTING PARAMETERS |
Engineers can improve the shape and consistency of the glue dots using the following printing parameters guidelines:
• Stencil material: Metal stencils are typically recommended because they are easier to clean and more readily available, though the difference between metal and plastic stencils is minor.
• Stencil thickness: For chips, use a 6-mil-thick stencil; for designs with leadless components a 10-mil-thick stencil works best.
• Squeegee material: A metal blade should be used for metal stencils and a polycarbonate blade for plastic stencils.
• Separation speed: Use rates as slow as 0.2 mm/sec.