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Partnering July, 2014
PCB Design Considerations for High Power LEDs (Part 2)
By Stanley L Bentley, P.E., Senior Technical Advisor, DIVSYS International LLC
for a light emitting diode (LED) is primarily a thermo-electric problem rather than strictly a cir- cuit design problem. The required circuitry is usu- ally fairly simple but heat must be removed from this circuit and the required structure can be com- plex. Also, in refining the circuitry for good ther- mal efficiency, a number of variables must be con- sidered, including:
A l l
The thermal conductivity of each material in the heat path (W/m°K).
The thickness of each material in the heat path (m).
l The surface area of the heat path (m2).
l The thermal resistance of the heat path (°K/W). l The heat generated across the surface area (W).
l
The desired temperatures for top side (components), thermal path (the PCB and support structure), and the heat-sinking surface (usually the maximum ambient temperature).
l
The maximum acceptable cost for the thermal structure.
For example, the thermal conductivity of
some materials commonly used in electronic man- ufacturing are 0.024W/m°K for air, 385W/m°K for copper, 205W/m°K for aluminum, 1W/m°K for di- electric adhesive, and 0.2W/m°K for epoxy-glass material. A number of approaches are available for re-
moving heat from an LED circuit. The techniques are quite varied, but subject to the same physical constraints. For one thing, any selected thermal insulating material (TIM) must remove sufficient heat to maintain the circuit elements within their safe operating temperature ranges. Also, the se- lected material and system must have sufficient mechanical strength for a target application, and the system/material must be cost-effective. Unfortunately, these simple constraints are
mutually exclusive. Present technology does not allow all three to be achieved at the same time. The design process is therefore iterative instead of linear and a hierarchal design approach (outlined in Part 1) must be followed to control system costs.
Any selected thermal insulating material (TIM) must remove
sufficient heat to maintain the circuit elements within their safe operating temperature ranges.
Usually, this cost is prohibitive, requiring a redef- inition of system variables, followed by another system design, and final cost estimation. To simplify this process, the author has creat-
ed an LED PCB standard approach that can be used to quickly determine a design solution. The approach starts with the assumption that all ma- terials have a thermal conductivity, and the ther- mal conductivities of the insulating materials used in PCB fabrication vary from 0.1 to 7.5W/m°K. The thermal conductivity value must be converted into a thermal resistance value for the TIM, which is accomplished by converting several variables into constants. For this standard design approach, a circle with radius of 1-in. was chosen, which yields
s detailed in the first part of this article se- ries (see U.S.Tech March, 2014, page 52), the design of a printed circuit board (PCB)
a working area of 3.14-in.2. The circular pattern has the property of removing placement geometry from the calculation and providing a uniform dis- tribution of heat. As the design evolves into an ac- tual product profile (outline), it is recommended that the calculations still work in increments of the 1-in. radius, which can help reveal potential thermal “hot spots” in a design.
Maximum Temperature Drop The next step in finding a thermal solution for
a design is to determine the maximum allowable temperature drop across the TIM. This decision
Thcond
0.12
TIMth Inch
(0K/W) Rth 25
0.0020 0.656167 38.10 0.0030 0.98425
25.40
0.0040 1.312333 19.05 0.0100 3.280833
7.62
T(0C) 50
76.20 50.80 38.10 15.24
WATTS Table 1. Convert Watts to Thickness for FR4.
must be made with the understanding that compo- nent temperatures and operating lifetimes are in- versely related, so that the lowest component tem- peratures are most desirable (for the longest operat- ing lifetimes). But this must also be a design trade- off that considers a targeted product cost. The author has developed an LED PCB stan-
dard thickness table to assist with LED PCB ther- mal design, based on the use of one constant and four variables. The constant is 3.14-in.2 of surface area . The variables are the thermal conductance of the system, which can be obtained from a data sheet for the selected dielectric insulation materi- al; the total watts of power channeled by the cir- cuit elements (detailed in Part 1 of this article se- ries and, for larger surfaces, the total watts must be divided by the ratio of the area to 3.14-in. 2); the thickness of the thermal structure (thinner mate- rials with high thermal conductance must be rein- forced with some form of backing material, such as aluminum or copper); and the temperature drop that can be allowed across the thermal structure. From this temperature drop, a designer can deter- mine the maximum component temperature by adding the maximum ambient temperature to the temperature drop across the TIM. The standard thickness table was generated by calculating the thermal resistance for each
structure, Rth(k/W), and then using the thickness of each row to calculate the heat input required (W) to produce a temperature drop of 25°C, 50°C, or 75°C across the thermal structure. With a tem- perature drop of 75°C across the TIM and an am- bient temperature of 25°C, the topside tempera- ture of the PCB design would be 100°C. A circuit designer can use the standard thick-
ness tables by performing the following actions: calculate the total amount of power (W) consumed by the system, dividing the total area of the circuit design by 3.14-in.2; dividing the total power (W) by this result to create a value for “normalized watts”; determining the desired temperature drop across the TIM in one of the three temperature columns of the table (25, 50, or 75°C) and subtract the max- imum anticipated ambient temperature from the maximum allowable device temperature for the LED that was selected; find the normalized watts
75
114.30 76.20 57.15 22.86
in the table column for temperature drop across the TIM; look to the row to the left for the maxi- mum thickness for a given material; and compare the available materials by their thicknesses and thermal conductances. The tables enable fast com- parisons of different materials for cost and per- formance.
Size Constraints As an example, consider an LED design that
must fit within an area of 3.14-in.2 and produce 3000mLumens. If a super-bright LED is chosen, 3000mLumens will equate to 45W power at 12VDC. The “normalized watts” for this area will be 14.3W. With an ambient temperature of 35°C, a maximum device temperature of 85°C can be allowed; as a re- sult, the temperature difference across the thermal structure must be less than 50°C. Based on FR4 cir- cuit material and the FR4 chart, with a normalized power input of 14.3W, the maximum thickness of FR4 substrate (with thermal conductance of 0.2W/m°K) must be less than 10mils (0.010in.). Un- fortunately, FR4 lacks sufficient rigidity for this par- ticular design, and the designer determines that the desired rigidity can be added by means of an alu- minum backing plate of 40mils thickness. Bonding a 10mil PCB to a 40mil aluminum backing plate will require a 2mil adhesive layer, and this adhesive ma- terial has a thermal conductance of 1.0W/m°K. The designer must now determine the total thermal re- sistance of this hybrid design because there will be an additional temperature drop across the adhesive
Thcond
1
TIMth Inch
(0K/W)
Rth 0.0020 0.07874
Table 2. Convert Adhesive thickness to Thermal Resistance.
layer. The designer calculates that a 2mil layer of di- electric thermal material with thermal conductance of 1.0W/m°K will have a thermal resistance of 0.078°K/W. From the charts, the designer determines
that the thermal resistance of the 10mil PCB will be 1.96°K/W. Since thermal resistances are addi- tive in nature, the designer totals the thermal re- sistance for the TIM structure as 2.038°K/W. This thermal resistance can be converted to tempera- ture by multiplying by the normalized input power (14.3W). The temperature drop across the TIM is calculated as 29.14°C, or well below the maximum allowable temperature of 50°C, so that this is con- sidered valid for a first design effort. Suppose that during this exercise, the design-
er had been advised that the available area has been reduced, and that the same 3000mLumens must be achieved in one-half the circuit area. Such a change would eliminate FR4 from consideration as the PCB substrate. The designer must now se- lect a substrate with higher thermal conductance than FR4, and restart the calculations based on those new figures. Such a circular exercise is com-
mon in the design of thermal structures for LEDs. Contact: DIVSYS International LLC, 8110
Zionsville Road, Indianapolis, IN 46268 % 317-405- 9427 fax: 317-663-0729 Web:
www.divsys.com r
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