What are Sink-Pad Copper Substrates?

PCB Substrates 
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Editorial Team - PCB Directory

Mar 13, 2022

The presence of metal helps in managing the heat dissipated by high power LEDs when operating, thereby keeping them cool and working longer.

The next generation LED solution is the sink-pad copper substrate PCBs for a wide array of products in LED modules. The sink-pad technology offers a printed circuit board with a high efficiency thermal management capability. This technology allows conducting heat from operating LEDs into the surrounding air far more effectively than conventional MCPCBs can. Sink-pad PCBs are, therefore, superior in providing thermal performance for medium and high power LEDs. Among all MCPCB solutions currently available in the industry, sink-pad technology offers the most economical and scalable direct thermal path solution.

Advantages of Sink-Pad Technology

Sink-pad technology offers very high thermal efficiency compared to what the best MCPCBs offer presently. Sink-pad MCPCBs are available with both aluminum and copper bases. Whereas conventional MCPCBs offer a heat transfer rate of about 1 to 5 W/mK, sink-pad PCBs with aluminum base offer 210 W/mK, and sink-pad PCBs with copper base offer 385 W/mK. To accomplish this dramatic improvement, manufacturers of sink-pad MCPCBs create a direct thermal path from the LED to the base metal.

Thermal Path in LED PCBs

The primary thermal path in LED applications passes through several mediums before it reaches the surrounding air. For incandescent bulbs the heat originates in the filament, and passes through the glass bulb to reach the surrounding air. In LEDs, heat originates in the die of the chip. This heat must pass through the plastic encapsulation of the LED before it can reach the surrounding air.

If the thermal conduction rate of the intervening medium is high, the LED can run cooler. But if the thermal conduction rate is low, the LED may grow hot while operating and has the potential of self-destruction. To improve the heat conduction, manufacturers provide a direct path to the die of the LED in the form of a thermal pad. This thermal pad is under the LED and has a high heat conductivity rate.

Conventional MCPCBs use this direct heat conducting path under the LED to channel the heat to a heat sink. However, they use a thermally conducting dielectric layer for bonding the circuit layer with a base metal such as aluminum or copper. In conventional MCPCBs, this dielectric layer is the greatest impediment to its thermal efficiency. Although the thermally conductive dielectric layer offers a better thermal performance compared to that from regular dielectrics PCBs use, it still remains the weakest link in the thermal path in MCPCBs.

By eliminating the thermally conductive dielectric from the heat flow path, sink-pad technology improves the thermal efficiency of MCPCBs significantly.

Sink-Pad Technology

Sink-pad technology allows manufacturers to build special types of metal core printed circuit boards. The main difference between conventional MCPCBs and sink-pad MCPCBs is the latter has a metal core that protrudes through the substrate and the dielectric up to the top copper layer. The thermal pad of the LED can touch these protrusions, and is therefore in direct contact with the metal core, rather than through a dielectric. This allows a heat transfer directly from the die of the LED to the metal core of the PCB with an extremely high efficiency. The net result is a superior thermal performance for any heat producing chip.

As it offers the highest thermal conductivity, most sink-pad MCPCBs commonly use copper. This has given rise to a common name for such PCBs—Sink-Pad Copper Core PCBs. As the sink-pad MCPCBs do not have a dielectric layer between the thermal pad of the LED and the metal core, its thermal conductivity can be very close to that of copper, 400 W/mK. However, at present, sink-pad technology is useful only for LEDs with electrically neutral thermal pads.

While the sink-pad provides excellent heat transfers from the LED to the metal base, they also offer excellent electrical isolation. The copper base offers mechanical integrity to the board, while at the same time, distributing and transferring heat to a heat sink.

The sink-pad technology gets its name from the circuit area or copper tracks and pads of the PCB being routed in between the protrusions of the copper core. As the heat from the LEDs passes directly into the copper core, these sink-pad MCPCBs are also known as direct thermal path boards or DTP boards.

Sink-Pad MCPCB Classification

Although single-sided sink-pad MCPCBs are more common, double-sided sink-pad MCPCBs are also available. While the single sided sink-pad MCPCBs have the copper core in one side of the board, in double-sided boards the copper core is in the center, and it is possible to mount LEDs on both sides of the board. Another form of the double-sided PCB can have the copper core on one of the sides.

Board thickness of sink-pad MCPCBs can vary from 1 to 2 mm. And their copper weight can vary from 0.5 to 3 oz. Manufacturers can provide copper base of thickness varying from 0.8 to 1.5 mm.

A variety of surface finishing is available for sink-pad MCPCBs. These include ENIG, OPS, Immersion Tin, ENEPIG, and HAL along with excellent reliability of the surface treatment layer.

Applications of sink-pad MCPCBs

The metal core, being copper of high density, has a strong thermal conductivity and carries heat effectively. That means, it is possible to dissipate the same heat using a smaller sink-pad MCPCB, as compared to regular MCPCBs.

Very popular with high power LED mounting, sink-pad MCPCBs with their thermoelectric separation structure, can minimize the lumens depreciation of the LED, thereby prolonging the life of the LED lamp. Sink-pad MCPCBs are suitable for mounting high-power LED lamps and COB packaged LEDs. Moreover, manufacturers can tailor the sink-pad MCPCB to match the different design requirement needs of any LED.

Conclusion

Although conventional MCPCBs are common in LED lighting applications, sink-pad MCPCBs have their unique advantages like higher thermal efficiency, smaller size, and lower weight.

Author Bio:

RUSH PCB Inc has 20 years of experience of electronic contract manufacturing. RUSHPCB Inc provides detailed printed-circuit-board development resources.

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