Fluid dispensing for the LED assembly process includes various applications such as silicone phosphor cavity encapsulation and phosphor plate attachment as well as applications related to attaching or packaging the LEDs to other substrates.
Silicone Phosphor Cavity Encapsulation
The most common LED assembly application is silicone phosphor cavity encapsulation for the Plastic Leaded Chip Carrier (PLCC) LED package type that makes up the bulk of the market. LED silicone phosphor encapsulation improves production costs and light color consistency with Nordson ASYMTEK high speed jetting and closed-loop process controls.
LED silicone phosphor cavity encapsulation is particularly concerned with:
● Accuracy and consistency of the dispensed fluid volume to achieve the target LED color
● High throughput (units per hour; UPH) to minimize cost of ownership
● Maintaining phosphor suspension and uniformity in the fluid during a production run
In order to achieve a target LED white color temperature, a specific amount of phosphor must be evenly distributed over the LED die. White light LEDs are typically made by combining blue light LEDs with yellow phosphor (new developments may also combine red and green phosphors to enhance the light emission spectrum). It is the secondary emission from the yellow phosphor combined with the correct blue light mixture that makes white light. Variations in the amount of phosphor dispensed over multiple blue LEDs have direct impact to the resulting yield of white LEDs with a target color temperature. As such, it is very critical to maintain good control and consistency of the dispensed fluid volume during the LED assembly process in order to have a high production yield.
A mixture of phosphor and a binder (e.g., silicone or epoxy) is used to encapsulate the LED chips in their PLCC package. One of the technical challenges in this LED assembly process is to ensure uniform mixing and dispersion of the phosphor particles within the binder as the phosphor particles tend to settle out after dispensing and before curing. Less uniformity can adversely affect the color quality.
Nordson ASYMTEK's patented Calibrated Process Jetting (CPJ) and active nozzle technology ensure dispense weight accuracy and consistency, which contribute to tight LED color quality control. Use of jet valves either in single or dual valve configurations on Nordson ASYMTEK platforms provide for the high throughput desirable in the LED market. Likewise, our optional agitation systems help to maintain the phosphor suspension in the silicone prior to dispensing.
LED Packaging Assembly Material Dispensing
LED assembly requires various manufacturing processes that include fluid dispensing. Typical applications are:● Lens attachment
● Underfill for flip chip LEDs
● Conformal coating for LED displays
Lens attachment requires exact adhesive amounts dispensed at specific locations, which are sometimes in the bottom of cavities due to the optics structure. Optics alignment between lens and LED die is critical and the assembly needs to avoid contamination of the lens. The proper amount of adhesive is also critical to ensure the proper parallelism of the lens during bonding.
Flip chip LEDs need tight keep-out-zones and tiny dots dispensed for underfill. Many packages have multiple dies and the spacing of die is tight for optimum performance. The underfill material cannot interfere with the light production and the underfill materials must have high thermal conductivity for good LED heat dissipation; silicone material is often selected.
Outdoor LED displays need protection from environments such as humidity for reliability of electrical circuits.Conformal coating is used to protect electronics from hostile environments with moisture exposure, temperature extremes, and adverse chemical conditions. Many outdoor applications for LEDs, such as signage, traffic lights, automotive and architectural illumination are made reliable with the application of conformal coatings. Equipment from Nordson ASYMTEK is ideal for applying conformal coating to selective areas of the circuits with tight keep-out-zones.