LED lighting is popular because many governments are banning incandescent bulbs to save energy and carbon dioxide, especially for general lighting. In the automotive industry LEDs are being used because they save energy and costs, especially with the migration to hybrid cars and electric vehicles, and because the flexible LED light shape creates cool designs. These applications have created a need for more precise and accurate fluid dispensing as well as the dispensing of some fluids that require special attention. These dispensing applications must take into account:
- CIE value
- Heat dissipation
- LED life
- Power savings
- Production costs
People think switching to LED lighting is an up-front investment because it is much more expensive than other lights, such as incandescent and fluorescent, but LEDs should last for a much longer time, i.e. more than 10 years, and their power consumption is much less. Therefore, the technologies that enable LEDs to save more power, have a longer life, and lower their production costs are important.
CIE value relates to the clarity and color of the product. People prefer to see a warm color which is similar to an incandescent light rather than a cool color like the fluorescent light in factories. People have a pretty good sense of distinguishing color differences with the naked eye, so the CIE specification is quite tight: many LED manufacturers require “1 bin” for the specification.
Heat dissipation technologies at the package and board assembly levels play a key role in extending LED life. This is because heat dissipation reduces the LED’s junction temperature. The lower the junction temperature the longer the life. In addition, LED designers try to find the best way to extract as much light as possible to save power; the more efficient the light emission the less input power. Reduction in cost also contributes directly to making the investment equation better.
LED packages vary according to the end-application and cost constraints. Typical package types include:
- cavity encapsulation
- flip chip with phosphor attachment
- remote phosphor
Each of these packages has different dispensing applications. Cavity encapsulation is the most popular package because of its long history, fewer patent constraints, simple structure, material availability, and technology, as well as lower cost. The package needs silicone phosphor cavity encapsulation dispensing. The amount of phosphor in the cavity is the key to controlling CIE. The color depends on the consistency of the volume of the dispensed mixture. Changing the volume of the fluid considerably changes the color, so the dispensing operation to fill the cavity with phosphor silicone has to be very precise and consistent.
There are three major ways to get consistent dispensing:
- a jetting valve with controlled process jetting (CPJ+),
- a displacement valve
- an auger valve with mass flow control.
A jetting valve with CPJ+ is a sophisticated method which strikes a balance between getting an outstanding rate of units per hour (UPH) and good dispensing consistency. In CPJ+, the automated dispensing equipment measures the weight of the dot size and compensates for changes by recalibrating and adjusting the fluid volume as the viscosity of the fluid changes or the fluid changes because of expansion. The jetting valve can achieve 18,000 UPH for a 3014 plastic leaded chip carrier (PLCC) package and +/-1.5% weight consistency by dot weight calibration with periodic monitoring. (Figure 1)
A displacement valve has the best weight consistency because its piston and cylinder structure provide volume measurement capability, but its UPH is a slow 8,000. An auger valve provides 8,000 UPH and a little less weight consistency even with mass flow control. Cavity encapsulation packages are produced in billions of units a year, therefore, the balance between consistency and productivity is important.
Flip chip with phosphor plate attachment has a better performance package than cavity encapsulation because it has better heat dissipation, more emission extraction, and a better power supply. Because of its higher performance, it is often used for applications where performance is more critical, such as in automotive head lamps. This package type requires several dispensing operations such as underfill, reflection material dispensing, adhesive dispensing for plate attachment, and silicone casting. Underfill plays an important role in dissipating heat from the LED chip to the package substrate, thus providing a better heat path between the chip and substrate. The underfill material is usually silicone because of its heat resistance. When dispensing underfill, a jetting valve is most commonly used because the fluid has to be placed extremely close to the die, get into very tight, hard-to-reach places, the volume of dispensed fluids is small, and it has to be dispensed accurately.