PEC for LED interconnection
18 January 2010
PCB mounting of LEDs has so far been limited to mechanical interconnection or the use of PCBs built on thermally conductive copper clad aluminum substrates. Mike DuBois of Caledon Controls looks into the subject.
Higher wattage LEDs and other power components in higher densities require a larger dissipation of heat in a more effective way.
These laminated substrates, available from several specialty laminate manufacturers, typically consist of a copper foil, which is laminated to the aluminum using a glass cloth impregnated with heat conducting additive filled epoxies. The cloth offers small window openings between the weave patterns where the filled epoxies can make contact with both the copper foil and the aluminum to transfer heat.
The glass cloth, which comprises 50% of the prepreg, reduces the thermal conductivity. Additionally the existing available thermal prepregs are thicker at 100 microns, which increases the effective thermal resistance limiting the maximum watts per square inch dissipation.
A PCB manufacturer would use this laminate material, laminating photo-imageable etch resist, exposing with UV light, then developing to form a circuit image, before etching away unwanted copper to produce circuit traces. All exposed aluminum must be well masked off, which is a time and material consuming task.
Printed electronic inks
Enhanced Thermally Conductive Inks are versatile as they can be used to make a more efficient Thermally Conductive copper clad aluminum laminate substrate by the PCB fabricator, as shown in figure.1. Alternatively, they could used by selectively printing both the dielectric and conductive traces to make a thermally conductive circuit by anyone with a silk screen printer and a thermal oven, (figure.3).
Using CCD-120A Thermally Conductive Dielectric ink with its nano technology enhanced formulation in making PCB boards or even making copper clad aluminum circuit laminate, will deliver lower thermal resistance than that of other currently available copper clad aluminum substrates.
Copper clad aluminum
The thermally conductive B stage ink is applied to either the surface of the copper foil or of the aluminum, and is semi cured (B-Stage) using a thermal oven. Application is typically a silk screening operation to deposit a thin film of thermal ink approximately .001in thick. The ink is then baked at 250°F for 20-30 minutes, at which time the material reaches the desired B stage cure.
The coated B Stage aluminum is laminated with the copper foil of choice using a typical vacuum assist laminating press and fully cured under pressure. The end result is a thin layer only 25 microns thick of the thermal material sandwiched between the copper and aluminum. With this version of a copper clad aluminum, the B stage thermal ink offers lower thermal resistance, because of the surface area contact and the thickness of the deposit.
As well as this, the pressure of the lamination cycle constricts the ink, allowing a closer contact between thermally conductive ceramic particles increasing the thermal conductance. A one mil thick layer of the thermal ink properly applied has a 2500 volt rating.
Processing the panel to make a simple single sided circuit is done by a time honoured print and etch method requiring PCB fabrication equipment and waste water treatment facilities because of the copper etching process.
Figure 2 shows the etched circuit trace paths on the Thermally Conductive dielectric, which has a hard surface due to its high ceramic content, but can accept a coat of soldermask before the final cure. A compatible solder mask could later be applied.
Print only circuits
A selectively deposited thermal conductive ink on the aluminum offers the benefit of being deposited only where thermal heat transfer is needed, reducing consumables costs when compared to the currently popular copper clad aluminum substrate, which has full coverage of the underlying surface with the glass cloth impregnated with heat conductive material.
The circuit traces are screened using new Nano silver solderable inks. The net result is that by using a thermal ink selectively applied under the silver traces, less of the thermal material is used for the same overall area, yet it will deliver better heat transfer capability because it is thinly applied.
Building a printed circuit utilising the print only process can not only simplify the manufacturing, but it will also reduce costs. Print only is manufactured with a full additive green technology that will deliver better heat transmission performance, and no etching equipment is required.
Screen printing a slightly wider surface area than that of the conductors, then curing the dielectric at a temperature of 250°F for 30 minutes establishes a non-tacky but yet uncured surface, which is then overprinted with the silver conductive traces. The silver ink is then cured at 150°C for 30 minutes which fuses the silver trace into a solid metal track. A compatible solder mask can be applied and tack dried, followed by an overall full encompassing cure of 300°F for 60 minutes which forms the final circuit. Using thermal/silver ink combinations offers many more build options and cost concession possibilities. As there is no requirement to etch, and therefore no need to mask off the aluminum when etching or processing, labour is reduced. When using the silver ink trace technology, assembling and soldering the components requires a lower temperature solder, which is typically used in assembly houses.
The thermal characteristics are vastly improved over that of a normal copper clad aluminum etched type circuit. The area of the aluminum which is not covered by the dielectric material and silver traces has a larger thermal conductance (380w/mc) than the aluminum with a layer of 2w/mc thermal material.
Figure 4 shows a comparison between three heat conducting and dissipating constructions. Clearly evident is the larger amount of exposed aluminum made possible with an all printed version of the board and the resulting superior heat dissipation available.
Printed Electronics versatility
Using ink technology, more complex circuit boards can be manufactured by using the more advanced and green PEC (Printed Electronic Circuit) technologies.
By silk screen coating the aluminum with thermally conductive dielectric, a second layer of a specially formulated LPI dielectric ink is deposited at some 1 to 1.2ml thickness depending on screen mesh selections. If desired, extra layers of the dielectric will provide thicker conductor lines. Tack cured and then exposed using ordinary UV exposure units, the LPI is spray developed on typical developing equipment and chemistry. What remains is a coating on the substrate with trenches and pad openings. (See figure 5).
A thermally conductive PEC’s third component is a nano technology silver conductive ink which is applied onto the surface of the LPI. Filling of the trenches and pads is accomplished and the conductive ink is cured. The ink fuses into a solid mass with a slight deposit on the surface of the LPI, which is easily removed. (See figure 6).
The next step is a repeat of the second, except that this interconnection layer will be a via formation layer only. Coat, tack, expose, develop and silver fill. Because the LPI is never stripped off and the entrenched silver conductive ink is level with the top of the LPI, everything conductive is flush to the dielectric. The same process is repeated again for the second conductive layer. The result is that the interconnect vias are buried and is a 100% metal connection to the first layer. In the end, anything reliability wise to a PCB PTH interconnect has been eliminated. The process is repeated for as many layers as is required by design. The last top layer has pads left exposed for component mounting. All circuitry and interconnections are buried. Versatile, boards can be configured to use the PEC full additive process even on two sides of the aluminum if desired, or build multilayer circuits on one side of the aluminum backing.
Figure 7 shows a typical PEC build configuration; a multilayered single sided thermally conductive PEC technology PCB board. Designs could be altered a number of ways allowing for many new interconnect possibilities.
Using printed electronic ink products and processes to manufacture high performance thermally conductive circuit boards offers clear advantages in manufacturing flexibility, performance and cost. When using these inks in the full additive print only process or the PEC method, these high performance heat dissipating circuits are also environmentally friendly.
Figure 1: Copper Clad Aluminum Substrate
Figure 2: Etched Copper Circuit trace on Thermal Dielectric & actual etched circuit; 60X
Figure 3: Silver Trace on selective print Thermal Dielectric
Figure 4: Heat radiating comparison
Figure 5: PEC’s developed LPI Dielectric
Figure 6: PEC 4 mil lines/Space Flush Silver
Figure 7: Single side Multi Layer construction
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