190
9 Packaging of Group-III Nitride LED
At present, the LED package substrate mainly includes printed circuit board
(PCB), metal-core printed circuit board (MCPCB), direct-bonding copper substrate,
ceramic substrate, copper-plated substrate, aluminum and silicon substrate.
PCB is widely used in LED packaging. However, thermal conductivity of the material is relatively low. PCB with vertical structure for better heat dissipation channel
is the development trend. MCPCB consists of metal plate, insulation layer and
copper. Thermal conductivity of the structure can be further improved by replacing
the organic insulating layer with high thermal conductive material. Direct bonding
copper substrate is ceramic plate with copper-cladding layer with good electrical and
thermal conductivity. Thermal expansion coefficient of such material is very similar
to that of sapphire substrate, which enables high reliability. However, application of
the material is limited by circuit fabrication accuracy and production cost.
The low temperature co-fired ceramic (LTCC) substrate has simple structure and
little thermal interface. Direct copper plating substrate has the advantages of high
thermal conductivity, low process temperature, low cost, fine line and high reliability,
which is very suitable for high-power LED packaging requirements with high alignment accuracy. Aluminum substrate surface is insulated by applying anodic oxidation
treatment and the substrate has advantages such as simple process, low cost, high
thermal conductivity, high temperature resistance, good impact resistance performance. Disadvantage of the substrate is that the anodic oxidation layer strength is
insufficient, which limits the practical application. Silicon substrate has high thermal
conductivity and good thermal mismatch with LED chip material. Most importantly,
processing technology of the material is mature. Especially through-silicon via (TSV)
technology can greatly improve the integration and the cooling capacity of the device.
In recent years, transparent ceramic material technology is developed. It can achieve
high heat dissipation efficiency, better matching of thermal expansion coefficient,
excellent electrostatic protection and other excellent thermal and electrical properties.
It is expected to make breakthroughs in optical properties.
9.2 Group III Nitride LED Encapsulation Process
Although LED packaging forms may vary according to their application situations,
shapes, heat dissipation design and luminescence mode, their packaging processes
basically are the same, mainly comprising raw material preparation, die bonding,
sealing, testing and other processes.
(1) Raw materials preparation: as illustrated in Fig. 9.3, LED chips are examined
by microscope. During the process, any chip surface damage, chip size, electrode
size can be confirmed. As far as the lead frame of LED is concerned, it is necessary
to check the size and surface of the lead frame for abnormalities.
(2) Die bonding: as shown in Fig. 9.4, adhesive is placed onto the LED lead frame
for the following die attach process. The adhesive generally consists of silver paste
and insulating paste. For GaAs, SiC and other conductive substrates, silver paste is
generally used for die bonding while enables good electrical conductivity. On the
9 Packaging of Group-III Nitride LED
At present, the LED package substrate mainly includes printed circuit board
(PCB), metal-core printed circuit board (MCPCB), direct-bonding copper substrate,
ceramic substrate, copper-plated substrate, aluminum and silicon substrate.
PCB is widely used in LED packaging. However, thermal conductivity of the material is relatively low. PCB with vertical structure for better heat dissipation channel
is the development trend. MCPCB consists of metal plate, insulation layer and
copper. Thermal conductivity of the structure can be further improved by replacing
the organic insulating layer with high thermal conductive material. Direct bonding
copper substrate is ceramic plate with copper-cladding layer with good electrical and
thermal conductivity. Thermal expansion coefficient of such material is very similar
to that of sapphire substrate, which enables high reliability. However, application of
the material is limited by circuit fabrication accuracy and production cost.
The low temperature co-fired ceramic (LTCC) substrate has simple structure and
little thermal interface. Direct copper plating substrate has the advantages of high
thermal conductivity, low process temperature, low cost, fine line and high reliability,
which is very suitable for high-power LED packaging requirements with high alignment accuracy. Aluminum substrate surface is insulated by applying anodic oxidation
treatment and the substrate has advantages such as simple process, low cost, high
thermal conductivity, high temperature resistance, good impact resistance performance. Disadvantage of the substrate is that the anodic oxidation layer strength is
insufficient, which limits the practical application. Silicon substrate has high thermal
conductivity and good thermal mismatch with LED chip material. Most importantly,
processing technology of the material is mature. Especially through-silicon via (TSV)
technology can greatly improve the integration and the cooling capacity of the device.
In recent years, transparent ceramic material technology is developed. It can achieve
high heat dissipation efficiency, better matching of thermal expansion coefficient,
excellent electrostatic protection and other excellent thermal and electrical properties.
It is expected to make breakthroughs in optical properties.
9.2 Group III Nitride LED Encapsulation Process
Although LED packaging forms may vary according to their application situations,
shapes, heat dissipation design and luminescence mode, their packaging processes
basically are the same, mainly comprising raw material preparation, die bonding,
sealing, testing and other processes.
(1) Raw materials preparation: as illustrated in Fig. 9.3, LED chips are examined
by microscope. During the process, any chip surface damage, chip size, electrode
size can be confirmed. As far as the lead frame of LED is concerned, it is necessary
to check the size and surface of the lead frame for abnormalities.
(2) Die bonding: as shown in Fig. 9.4, adhesive is placed onto the LED lead frame
for the following die attach process. The adhesive generally consists of silver paste
and insulating paste. For GaAs, SiC and other conductive substrates, silver paste is
generally used for die bonding while enables good electrical conductivity. On the