Most of today's LED screen ( LED display ) manufacturers will face heat dissipation problems in PCB design, especially because the heat generated by the driver chip affects the normal illumination characteristics of the LED; thus affecting the color uniformity of the entire display. . How to solve the heat problem really makes the designer headache. This article will further explain how to change the package of the driver chip to solve the problem of heat dissipation of the driver chip. QFN Package (Quad Flat No Leads) - QFN is a name specified by the Japan Electromechanical Industry Association (JEDEC).
One of the four-sided leadless package, surface mount package, is an emerging surface mount chip package technology with a pad at the bottom, small size, small size, and plastic as a sealing material. Since the QFN package does not have a gull-wing lead like the conventional SOIC package, the conductive path between the internal leads and the pad is short, so the self-inductance coefficient and the wiring resistance in the package are low, so it can provide excellent electrical performance. Also, because there is no gull-wing lead, the so-called antenna effect can be reduced to reduce the overall electromagnetic interference (EMC/EMI). In addition, it provides excellent thermal performance through exposed leadframe pads, which have direct thermal pathways to dissipate heat from the chips inside the package. The thermal pad is usually soldered directly to the board, and the thermal vias in the PCB help to dissipate excess power dissipation into the copper ground plane to absorb excess heat; this also allows for better ground sharing. effect. At present, QFN packages have been widely used in general mobile phones and notebook computers, but are experiencing vigorous development in LED displays.
QFN and SOP heat dissipation and size comparison
Generally, the size of the SOP used is 104 mm2 (8X13x1.9mm), while the relative size of the QFN is only 16mm2 (4X4X0.9mm). It is only 6~7 of the size of the SOP; in the design of some small pitch display More flexibility.
The thermal resistance (ΘJa) has a coefficient of SOP = 59 ° C / W, and QFN = 39 ° C / W, that is, the power at one Watt, the junction temperature of the chip to the surface. The following are commonly used thermal resistance calculation formulas in the industry:
TJ=θja*PD+Ta
TJ=θjc*PD+Tc
θJa=θjc*θca
The symbol, unit used in the formula
TJ °C: node (chip) temperature
Tc °C : actual temperature
Ta °C : ambient temperature
PD W : power supply voltage θja (°C/W): heat transfer impedance from actual to outer surface θjc (°C/W): from node to actual heat transfer impedance θca (°C/W): from actual to outside Surface heat transfer impedance
That is to say, if the temperature and power consumption are the same at the same temperature, the temperature of the node staying on the chip will be different due to the difference of the package. For example: If the ambient temperature is 85 ° C and the power consumption of the chip is 0.5 W, the temperatures of SOP and QFN are as follows:
Light drive unity design
Due to the small size and good heat dissipation of the QFN, the specifications of the Pid 16mm or less in the outdoor display are limited by the size of the PCB board and the heat dissipation. Therefore, the general display factory will choose the design of the separate light source; The light board and the driving chip board are respectively placed on two to three different PCB boards, and then connected to each other through a connector (connector) and a transmission line (Cable). Although this design can solve the heat dissipation problem, the inductance effect generated by the connector and the wire may greatly reduce the color resolution of the display, and the inductance effect also increases the chance of electromagnetic interference. When using the QFN design; because of its small size and no heat dissipation, the chip can be placed in the gap of the LED lamp, so that it is not necessary to use more PCB boards and transmission lines, and the cost can be reduced. If the QFN design is used in the display of the same household, the heat dissipation problem can be greatly improved.
Fully automated production
The design of the traditional lamp-drive separation is higher than the cost of the design material of the lamp-driver (more PCB, connector and wire costs), which can save the labor cost of assembly and achieve fully automated production of SMT/DIP.
in conclusion
Due to its advantages in notebook computers, Xunjie Technology has led the industry in some advanced packaging processes. Two years ago (2006), the QFN4X4 package was introduced in the display industry. It is expected to bring the advantages and cost advantages of this package to the display. Screen manufacturers are more competitive.
By Xunjie Technology Co., Ltd. Zheng Wensheng, Manager of Power Products Division
Xunjie Technology is a professional R&D manufacturer of notebook computer chips, and currently has a market share of over 30% in notebook computers. In the LED is a new recruit; with the laptop's strong R & D, production advantages, hope to bring a new concept to the LED display industry, such as the concept of Xunjie technology to provide customers with "excellent performance, good quality, fast service" cost-effective products To create a higher competitive advantage for customers.
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