QFN Packaging Solves The Heat Dissipation Problem of LED Display

Currently, almost all LED display manufacturers face the problem of heat dissipation during PCB design, with the thermal effect of drivers disturbing the normal light emitting property of LED and further influencing the color uniformity of the overall display. This article will illustrate how headaches can be prevented by changing the packaging of driving chips.

Quad Flat No Leads Packaging— the technology is named by Japan Electric Device & Equipment Community

QFN is a new surface mounting technology (SMT) encapsulated in plastic and features its small size and dimension, with a welding plate on the bottom. Unlike conventional SOIC packaging, there is no L-shape wire in QFN, thus the conductive channel is shorter and the coefficient of self-induction and resistance of the wire inside the package is lower, so it provides excellent electrical performance. Also, as the elimination of the L-shape wire reduces the antenna effect, the EMC/EMI is reduced in QFN package. Additionally, it provides good heat dissipation via the exposed lead die-pad. The pad has direct heat dissipation passage to release the heat of the chip inside the package. Usually the pad is directly welded to the circuit board and the heat emission holes in the PCB can help dismiss the extra power consumption to the brass-connected floor to absorb the redundant heat as well as reach better common ground effect. The QFN package has already been widely used in handsets and Notebooks, while is on the point of booming in LED display field.

Comparison of QFN and SOP in Heat Dissipation and Size
Generally, the size of SOP is 104 mm2(8X13x1.9mm) while the size of QFN is merely 16mm2 (4X4X0.9mm), which is only 1/6 to 1/7 of the former. As a result, QFN provides more flexibility in design of displays with small intervals.

The resistance (ΘJa) of SOP is 59℃/W and that of QFN is 39℃/W. The resistance represents the temperature difference between the junction and the surface of the chip, and the following is the generally used calculation formula of resistance:

TJ=θja*PD+Ta

TJ=θjc*PD+Tc

θJa=θjc*θca

The symbols and units in the formula:
TJ ℃ : temperature of chip junction
Tc ℃ : actual temperature
Ta ℃ : environmental temperature
PD W : power voltage
θja (℃/W): The thermal impedance from actual to the exterior surface
θjc (℃/W): The thermal impedance from junction to actual
θca (℃/W): The thermal impedance from actual to exterior surface

From the formula we know that even given the same environmental temperature and power consumption, the temperature of junction could differ with different packaging. For example, if the environmental temperature is 85℃ and the power consumption is 0.5W, then the temperature for SOP and QFN are respectively:

The light-and-driver-in-one design
Display manufacturers often choose to separate light and driver in design of outdoor displays of specifications below Pitch16mm, because of the limitation of the routing of PCB and the heat dissipation problem. In this method, LED lighting panel and driving panel are set separately on 2 or 3 different PCBs and then connected together by connector and cables. This design, though solving the problem of heat dissipation, could impact the color definition of the display by the inductive effect generated from connector and cables. And the inductive effect could also increase the possibility of electromagnetic interference. On the other hand, in the QFN design, with its small size and elimination of heat dissipation problem, the chip can be set in the interval of the LED lights, thus no extra PCB or cables are needed, cutting the cost and making the design simpler. At the same time, the heat dissipation of indoor displays can also be largely improved by adopting QFN design.

Fully automated production
Compared with the traditional design which separates the light and the driver, QFN design not only saves cost of materials (PCB, connector and wire), but also save the cost of manpower with fully automated SMT/DIP production.

Conclusion
ENE Technology Inc. is leading the industry in advanced packaging processes through its advantage in the Notebook field. The company began to promote the QFN4X4 packaging technology in the display industry in 2006, expecting to enhance the competitiveness of display makers through the merits and cost advantage of the new packaging product.

by Zheng Wensheng, Manager of Power Products Group of ENE Technology Inc.

Written by :

TLLC LED Tech

 

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