Placement of LEDs and other surface mount devices onto printed circuits has been done for years in a variety of ways. Companies have developed processes using a wide variety of adhesives, application methods, components, and placement methods. Although initially most of the small and medium sized companies placed LEDs primarily by hand in a very labor intensive process, which also led to significant quality issues due to the lack of repeatability and consistency.
At the request of our customers, who were searching for new ways to justify automating their metal dome placement, Nicomatic, began searching for a process to place LEDs with their metal dome placement equipment. In conjunction, with their automation partner, Assembly Automation Alternatives, Nicomatic developed the equipment and a process for successfully automating the placement of LEDs and other SMD components to printed circuits with conductive adhesives. The equipment and process developed is now widely used throughout North America, and has also been successfully introduced to numerous locations in Europe and Asia. Nicomatic North America has also become the largest sub-contractor for LED placement in the industry, serving a wide variety of customers.
Based on input from our customers, Nicomatic has also developed an MembraneThin LED for use in membrane switch products. Nicomatic’s UltraThin LED is a 1206 package, which is easy to place because of its large width and length, but is as thin, 0.35 mm (0.020”) as virtually any LED on the market. While you can find 0805 and 0603 LEDs that are as thin as the Nicomatic UltraThin LED, the smaller length and width of these LEDs makes them much more difficult to place without shorting for example. Nicomatic offers a wide range of colors from stock and sells broken reels as well.
Based on these years of experience, Nicomatic has developed guidelines for the pad design and the dispensing of adhesives to improve bond strength and reduce shorts. The most significant process improvement is the use of center drop of non-conductive adhesive in between the pads in order to improve the bond strength of the joint and to serve as a dielectric to prevent shorting of the two conductive adhesive drops that are dispensed over the pads.
The suggested design of the pad is shown in Figure 1 below. The biggest improvement of this pad design is the use of the 1 mm diameter circle in the center of the pad. The purpose of this circle is to create a bond directly between the conductive adhesive and the substrate, which is stronger than the bond between the printed ink and the substrate that is typically the failure point of any flexing of the joint.
Figure 1: Pad Design for LED Placement
With the reduction in price of White and Blue LEDs over the last few years, these products have become more and more common in membrane switch designs. Unfortunately and frequently, unknown to the manufacturer, these LEDs also typically are very sensitive to ESD. Even for manufacturers that have been placing LEDs for years, placement of these ESD sensitive devices can be challenging and frequently results in costly quality issues that are difficult to diagnose. Nicomatic has extensive experience in diagnosing ESD problems and in the successful placement of these sensitive devices. If you suspect you might have an ESD problem, which can occur during placement or even after your products are sent into the field, please contact us for assistance.