Thermal Management of the ACULED VHL

Thermal Management of the ACULED® VHL Introduction Excelitas’ new ACULED® VHLTM, with its superior four-chip design and smallest footprint, gives cus...
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Thermal Management of the ACULED® VHL

Introduction Excelitas’ new ACULED® VHLTM, with its superior four-chip design and smallest footprint, gives customers the most flexible multi-chip LED on the market. The product family contains various products from UV via VIS to IR with a variety of chip configurations, including sensors and thermistors. Excelitas’ ACULED® DYOTM even enables customers to put together their own configuration. Please refer to the Custom Design Guide, “ACULED DYO Design-Your-Own,” for more details on this product. Preventing the ACULED from overheating by taking away the heat from the package is a key point when designing the ACULED into your product. This application note describes the thermal management of the ACULED and further considerations in heat-sink design.

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Features and Benefits of the ACULED® VHL and DYO High power light, UV and IR source Ultra compact footprint Excellent color mixing due to high packaging density Separate anode and cathode for each color and pad Various standard configurations available Combination of LED with sensors Design-Your-Own (DYO)

Applications General illumination Entertainment and shop design Furniture lighting Architectural and landscape lighting Mood lighting Vision systems Backlighting Medical lighting Display and signs Customized chip configuration

Author • Jörg Hannig Excelitas Technologies Luitpoldstrasse 6 85276 Pfaffenhofen Germany Phone: +49 8441 8917 0 Fax: +49 8441 71910 Email: [email protected]

Technical Support • For additional technical support, please contact us at: [email protected]

Table of Contents

General Remarks and Construction of the ACULED VHL

3

Efficiency and Efficacy

4

Influence of Thermal Management

4

Heat Generation in the ACULED

8

Heat Transportation and Thermal Path

9

Calculating with Thermal Resistances

11

Thermal Resistances of the ACULED VHL

13

Thermal Resistances of the ACULED DYO

15

Heat-Sink Calculation

19

Heat-Sink Types

22

Heat-Sink Mounting

24

Symbols and Units

28

Abbreviations

30

Notes

32

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Thermal Management of the ACULED® VHL

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General Remarks and Construction of the ACULED VHL The ACULED VHL board is based on an insulated metal core substrate (IMS) made from copper and a highly-sophisticated isolation material with a low thermal resistance between the copper and chip pads. This package provides excellent heat dissipation and thermal management from the chip to the substrate’s backside. The thermal resistance Rth,JB of the entire package is quite low, depending on the chip configuration. To dissipate the heat, adequate cooling must be considered. To avoid damaging the LED chips by overheating when equipped with at least one high-power LED chip, the ACULED must not run without appropriate cooling - even at lower currents! Figure 1 shows the typical layout of the ACULED VHL. The chips are placed in the middle of the board, protected by a PPA-based ring and silicone resin encapsulation. The latter is transparent and suitable for a wide range of radiation - from ultraviolet (UV) to infrared (IR). It is also more resistant to heat than epoxy resin, and its heat expansion characteristics are closer to those of chips. With the ACULED’s high-power LEDs, silicone achieves superior resistance to light radiation, mitigating degradation, and maintaining LED color purity over the LED’s lifetime. The mechanical stress applied to the chips is lowest with silicone, when compared to other standard encapsulation materials. Due to its softness, pressure to the silicone area within the ring must be avoided. Please refer to the application note, “Handling of LED and Sensor Products Encapsulated by Silicone Resin,” to learn more about handling silicone-based products such as the ACULED. The clockwise numbered pads C1 - C4 inside the encapsulation ring show the pads where the LED chips are placed. The distance between the chips is typically 0.2 mm with a pitch of 1.2 mm. Therefore, the lighting area is approximately 2.2 x 2.2 mm², depending on the particular chip configuration for the specific ACULED VHL or ACULED DYO product. The numbers of the soldering pads run counter-clockwise from pin 1 to pin 8. The pad numbers 4 and 5 are printed on the board. Pin 1 is easily located by the small gold dot, which can be used as a reference for mounting. Whether ESD protection diodes can be used depends on the ESD sensitivity of the LED chips, referenced in the specific datasheets of the products. The tracks on the ACULED are made from copper with a thin gold layer to achieve better bonding results.

Figure 1 Layout and Dimensions of the ACULED VHL

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Thermal Management of the ACULED® VHL

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Efficiency and Efficacy Modern high-power and high-brightness LED chips used with the ACULED product family can be driven at 1 to 5 W, depending on the chip material (i.e. color) and cooling. The efficiency of these chips is up to 20% and more in practice, depending on the current, chip material, cooling etc. In laboratory scale and at low current, the efficiency is even much higher. Efficiency η means how much optical power (radio metrical, measured in watts) comes out of the LED chip when a certain amount of electrical power is applied, usually measured in mW/W or simply percentage. Besides the efficiency, the efficacy ηopt is also often used. This is the output power in lumens, i.e. the photometrical value giving the amount of visible light that leaves the LED per Watt of electrical input power. Efficacy is measured in lm/W and only suitable when comparing LEDs of the same color or wavelength and within the visible range. For example, a 950 nm IR LED does not have any lumens, since it emits only non-visible radiation and no visible light. Thus, for heat management, efficiency rather than efficacy must be dealt with. The following equations show the general calculations: Calculation of electrical power The electrical power Ptot is forward current multiplied by forward voltage: Ptot

= IF — VF

(1)

The forward voltage VF at a certain forward current IF can be taken from the specific diagram in the ACULED datasheet. The electrical power of the particular ACULED at defined operating conditions is also provided by its datasheet. Calculation of the efficiency Efficiency is optical power Popt over electrical power: η = Popt / Ptot

(2)

The optical power is given in the specific ACULED datasheet for non-visible chips and chips with colors close to the limit of the human eye’s spectral sensitivity. The efficiency of the particular ACULED at defined operating conditions is also provided by its datasheet for the nonvisible LED-chips. Calculation of the efficacy Efficacy is luminous flux Φ V over electrical power: ηopt = ΦV / Pel

(3)

The luminous flux at a certain forward current IF can be taken from the specific diagram in the ACULED datasheet. The efficacy of the particular ACULED at defined operating conditions is also provided by its datasheet for the visible LED-chips.

Influence of Thermal Management As we have seen in the previous section, modern high-power LED chips used with the ACULED product family have efficiencies of 10% - 20% of radiation output at common operation conditions. Thus, 80% to 90% of the electrical energy is transformed into another kind of energy, specifically heat. Unlike with incandescent lights, almost no heat is radiated into the LED’s environment. Hence, the LED light is often referred to as “cold” due to the absence of IR

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Thermal Management of the ACULED® VHL

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radiation. With standard light bulbs, a high amount of IR radiation is emitted, warming up the environment or illuminated subject. A big advantage of LEDs is not radiating heat around heatsensitive subjects requiring illumination. However, heat is still present in LED chips, particular in the pn-junction where the light is generated. This heat must be mitigated by thermal conduction to avoid destruction of the LED chip and other unwanted effects. Besides helping to avoid chip damage by overheating, good thermal management helps us to get a grip on all parameters impacted by temperature, which include: • • • •

life time [tLife] forward voltage [VF] flux [Φ e and Φ V] wavelength [λ] resp. color [x2°/ y2°] and color temperature (TCT).

A big advantage of the ACULED VHL and DYO versus other similar products on the market is minimized thermal crosstalk between the pads. We will learn more about this in a later chapter of this application note. As a result of this low thermal crosstalk, the warming of one LED has minimal affect on neighboring chips, resulting in excellent constancy in the parameters described above. Influence on lifetime Overheating an LED chip, i.e. exceeding its junction temperature TJ over the allowable maximum, will damage the chips within a short time. But long term temperature affects also influence a decreased lifetime. During operation, the lower the temperature, the longer the lifetime for the chip and the whole ACULED product. Some degeneration processes require a minimum temperature to get started. Thus, a low TJ will dramatically increase the product’s lifetime. Since these processes are very complex and not fully understood today, it’s virtually impossible to get reliable curves of tLife versus TJ today for a longer periode of time. Influence on forward voltage The forward voltage VF usually decreases in the range of several mV per Kelvin with increases in temperature. Since this change in approximation is linear over the typical small temperature changes, it is provided in the temperature coefficient of forward voltage TCVF found in the ACULED VHL datasheet or the chip datasheets of the ACULED DYO chips. Figure 2 shows typical curves for red, green, and blue chips. The change ∆VF of the forward voltage is calculated by the following equation: ∆VF = TCVF — ∆TJ VF1

= TCVF — (TJ1 - TJ0) + VF0

(4) (4a)

VF1 is the forward voltage that we want to calculate at a temperature TJ1, whereas VF0 is a known forward voltage at a known temperature TJ0, e.g. values given by the datasheets. In a steady state, the junction temperature TJ and the substrate temperature TB are interchangeable in the equation: VF1

= TCVF — (TB1 - TB0) + VF0

(4b)

With a decrease of the forward voltage, power consumption drops as well at a given current. But due to the small change, it is of no practical significance.

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Figure 2

Relative Forward Voltage

Relative forward voltage versus ACULED VHL

1,06

board temperature TB for red, green and blue chips

1,04 1,02

V/V

1 0,98 0,96 0,94 0,92 10

20

30

40

50

60

70

80

T B [°C]

Influence on flux and intensity The flux, Φ e and Φ V , and their deducted values, such as luminance, radiance, luminous intensity or radiant intensity, decreases with increasing temperature. Generally speaking, the intensity drop of blue and green chips is usually small, whereas the drop with yellow, amber and red chips is larger. Figure 3 shows typical curves representing the relative luminous drift for the chips of the RGYB ACULED VHL. These charts can be found in the specific datasheets of the ACULED VHL products. For the ACULED DYO, an approximation is given by the luminous or radiant flux temperature coefficient (TCΦV resp. TCΦe) in the specific datasheets of the chips. The change ∆Φ of the luminous or radiant flux can be calculated analogous to the calculation of the forward voltage drift over temperature by the following equation: ∆Φ

= TCΦ — ∆TJ

Φ1

= TCΦ — (TJ1 - TJ0) + Φ0

(5) (5a)

Φ 0 is the known flux at a known temperature TJ0 given by the datasheets. In a steady state, the junction temperature TJ can be interchanged with the substrate temperature TB in the equation: Φ1

= TCΦ — (TB1 - TB0) + Φ0

(5b)

If a certain flux is necessary in your application, it’s important to level out the intensity drop. A good thermal management will also help you to keep the drift as low as possible. The balancing of the drift over temperature is important, particularly when having chips of different colors on your ACULED like RGGB or RGYB, to keep the same intensity ratio and, therefore, the same color appearance. With the RGYB for example, the color mix drifts to a blue-greenish light with increasing temperature, since yellow and red fade out much more than blue and green, as shown in Figure 3. Due to the excellent suppression of any thermal crosstalk, each chip can be levelled out individually without regard for how its temperature and heating change influence its neighbors.

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Besides a changing mixed color ratio due to the different intensity changes, each chip also changes color as a result of wavelength drift caused by temperature.

Figure 3

Relative Luminous Flux = f(T B)

Change of relative luminous flux vs. board temperature TB for the RGYB chips of the ACULED VHL

140,00 130,00 120,00

in %

90,00

/

100,00

25°C

110,00

80,00 70,00 60,00 50,00 40,00 10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

T B [°C]

Influence on wavelength and color The change of the wavelength (λpeak resp. λdom) and, therefore, the color versus the temperature, is linear in first approximation at small temperature changes and can be determined by the temperature coefficients TCλ peak and/or TCλ dom. These are found in the ACULED VHL datasheets or the specific chip datasheets of the ACULED DYO. Figure 4 shows a typical curve of the IR ACULED VHL. This curve is shown in each individual ACULED VHL datasheet. The change ∆λ of the peak or dominant wavelength can be calculated analogous to the calculation of the forward voltage drift over temperature by the following equation: ∆λ

= TCλ — ∆TJ

λ1

= TCλ — (TJ1 - TJ0) + λ0

(6) (6a)

λ0 is the known wavelength at a known temperature TJ0 given by the datasheets. In a steady state, the junction temperature TJ is interchangeable with the substrate temperature TB in the equation: λ1

= TCλ — (TB1 - TB0) + λ0

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(6b)

Thermal Management of the ACULED® VHL

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Figure 4

Peak wavelength λ peak = f(T B)

Change of peak wavelength vs. substrate temperature TB for the IR ACULED VHL

875,0

870,0

peak

[nm]

865,0

860,0

855,0

850,0

845,0 10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

T B [°C]

Heat Generation in the ACULED The amount of heat or thermal power Pth that is generated in an operating ACULED is estimated by the following equation: Pth

= Ptot - Popt

(7a)

= (1 - η) Ptot

(7b)

For an easier calculation with regard to safety in heat management, we recommend calculating with the electrical power consumption Ptot as the total heat dissipation. This gives 10% - 20% of “safety margin” for any unexpected thermal conditions in your application. Hence, all further calculations shown in this application note are done with Ptot. The electrical power of the particular ACULED product can be found in its datasheet. Typical values of ACULED products equipped with four high-power LEDs are 5 W at 350 nm and 10 W at 700 nm, respectively. This requires up to 10 W of heat to be drawn away from the ACULED board. The limiting value for prevention of overheating is the junction temperature TJ at the pn junction of the chip. Typically, it must not ever exceed TJ max = 125 °C. The specific values of your ACULED VHL or chips used for your ACULED DYO can be found in the datasheets. The maximum substrate temperature1 TB max is calculated as follows: TB max = TJ max - ∆TJB = TJ max - Rth JB — Ptot

(8a) (8b)

1

The symbol TB often is explained as board temperature or base temperature. It indicates the temperature at the backside of the substrate and, hence, at the interface to the subsequenting board or heat sink. In this application note, “board.” “base,“ and “substrate“ temperature are used synonymously for the TB on the back side of the ACULED.

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∆TJB represents the temperature difference between junction and substrate temperature, and Rth JB represents the thermal resistance between junction temperature and substrate, which is typically 5 K/W for the ACULED VHL. Taking a Ptot = 5 W ACULED, for example, will lead to an absolute maximum substrate temperature according to Equation 8b of: TB max =

125 °C - 5 K/W — 5 W =

100 °C

(Remember, that the Kelvin [K] scale equals the Degree-Celsius [°C] scale when only looking at difference values like ∆TJB in this case.) In this example, the substrate temperature must not exceed 100 °C. The thermal resistance for a particular ACULED product, as well as the maximum allowable junction temperature TJ , can be found in its datasheet. The junction temperature is mainly dependent on the following parameters: • • •

power consumption Ptot of the ACULED, as described in this chapter ambient temperature TA thermal path from chip’s pn-junction to ambient surrounding (resistance Rth JA)

Table 1 Material

Thermal Conductivity λth [W / (mK)]

copper

400

gold

320

wire bonds, chip pads

aluminum

230

heat sinks

AlN: Al2O3: GaN: silicon: sapphire:

ceramic chip substrates

used for ACULED board (IMS), heat pipes, PCB tracks and pads

170 30 160 150 42

Thermal conductivity λth (typically) for different materials

PCB substrate chip substrate

tin

67

solder

TIM (thermal grease)

2 - 4.5

water (no convection)

0.6

cooling (convection)

FR4

0.23

PCB substrate

silicone

0.1

encapsulation

air (no convection)

0.025

cooling (convection)

interface between ACULED and heat sink

Heat Transportation and Thermal Path Generally, there are three different ways of heat transportation: • • •

Convection Radiation Conduction

Unlike with incandescent lights where a significant amount of the generated heat is transported by radiation, high-power LEDs have less than 5% of their heat transported by convection and radiation. Hence, more than 95% of the generated heat must be drawn away from the LED chip

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by conduction. The highly sophisticated copper substrate (IMS) of the ACULED VHL was designed to provide excellent heat transport from the chip to the backside of the substrate. However, it’s also important to dissipate the heat from the ACULED’s substrate by adequate heat-sinks and cooling.

temperature

thermal resistance

Figure 5

assembly

Principle of thermal paths in the ACULED VHL/DYO LED-Chip TJ TC

red: pn-junction (J) chip substrate

Rth C

Rth JB Rth P

IMS-PCB

TIM

TB Rth JA Rth BA

TA

Heatsink

Ambient Surrounding (Air)

Figure 5 shows the characteristic of the ACULED construction regarding the conductive thermal path. Depending on the cooling and heat-sink, the heating situation will achieve a steady state within a few tens of milliseconds for the ACULED VHL package without an additional heat-sink (see Figure 6) and up to several minutes when cooled properly, as shown later in Figure 16 on page 22 of this application note. Since the silicone filling material has very low thermal conductivity, it does not extract much heat from the chips. Also, the heat transportation by the bond wires is marginal and, therefore, not included in the thermal path model of the ACULED. The thermal resistances of the ACULED, as shown in Figure 5, are explained in Table 2. Table 1 shows the thermal conductivity λth for different materials, showing which are good for heat transfer or heat spreading and which are not.

Figure 6 Turn-on (left) and turn-off (right) decay of the ACULED VHL when operated without cooling

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In the ACULED VHL datasheet, the thermal resistance Rth JB between the chip’s junction layer and the back of the substrate is given for all chips running with the same nominal current. In the previous section, the use of this value to calculate the temperature drop was shown in Equation 8. When configuring your own ACULED DYO, this critical resistance is computed by combining the “package” resistance Rth P for the IMS-PCB with the individual chip’s thermal resistances Rth C , provided in the appropriate chip datasheets. The next sections describe how to achieve this calculation. The thermal resistant Rth P , of course, includes the whole ACULED VHL package - from the chip pad layer, including the die junction, down to the substrate’s back side. Besides calculating Rth JB for your own ACULED DYO configuration, calculating Rth BA from the ACULED to the ambient surrounding is important for selecting an appropriate heat-sink. This is calculated according to Equation 8b as follows: Rth BA = ∆TBA / Ptot =

(TB - TA) / Ptot

(8c)

With the ambient temperature TA and the given maximum temperature TB of the ACULED VHL substrate (see datasheet), we can calculate the maximum thermal resistance allowable for the heatsink. Of course, heat-sinks with better thermal resistances, as calculated by Equation 8c, are also working properly. The last step from the heat-sink to the ambient surrounding (e.g. air) is usually a heat transfer by convection. The air or liquid in water-cooled systems flows through the heat-sink and heat is removed. Therefore, along with the ambient temperature TA , the flow parameters of the air or liquid, such as velocity and density, are also very important to consider. With a fan, for example, the air flow velocity and, in turn, the heat transfer from the heat sink to the ambient air can be increased dramatically. Figure 7 shows the difference made by a cooling fan - up to 30 °C in this case.

120

Figure 7

100

TJ vs. time t of operation of the ACULED VHL on the same heat-sink with free convection (red) and with additional fan (green)

TJ [°C]

80

60

40 free convection

20

with fan

0 0

200

400

600

800

1000

1200

t [s]

Calculating with Thermal Resistances When calculating the thermal resistance of your own ACULED DYO configuration, the necessary heat-sink, or the temperature drops, it is important to know how all three individual

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values work together. In this section, we will learn how to use the various formulas and equations. In these calculations, the thermal resistances Rth are analogous to the electrical resistances. The formula required to calculate the temperature drop ∆T through a thermal resistance has already been used in previous sections: ∆T = Ptot — Rth

(8)

Serial resistances can simply be added: Rth, all = Rth, 1 + Rth, 2 + Rth, 3 + T

(9)

For example, this was done with Rth JB as an addition of the internal chip and the package resistance in Table 2. Parallel resistances are calculated by the following equation:

1

Rth, all =

1 R th,1

+

1 R th,1

(10)

1 + +L R th,1

Figure 8 shows the corresponding schematic diagrams. When more than one ACULED is mounted to a single heat-sink, it results in parallel working heat resistances. The individual thermal resistances Rth JB of each ACULED must be calculated according to Equation 10 to provide the combined thermal resistance of all the ACULEDs. Though the resulting Rth JB all is lower than the individual resistances, it does not indicate a lower temperature drop from TJ to TB all since the power consumption Ptot all is also increasing.

Thermal Resistance

identifies internal resistance of LED chip

Rth C

package (board)

Rth P Rth JB = Rth C + Rth P

whole ACULED heat sink including thermal grease and heat transfer to air overall module resistance

Rth BA Rth JA = Rth JB + Rth BA

between Temperatures chip’s pn-junction TJ and chip substrate backside TC

ACULED VHL

not published

TC and ACULED backside TB TJ and TB TB and ambient temperature TA

ACULED DYO

Table 2

shown in DYO

Explanation of the thermal resistances of the

chip datasheets

ACULED VHL/DYO as shown in Figure 5

5 K/W shown in datasheets

depends on chip configuration

depending on application specific heat sink

TJ and TA

Let us look on an example. Let’s assume putting a single RGYB ACULED VHL (product code ACL01-MC-RGYB-E08-C01-L) on a heat sink. From the datasheet, we get the following values: • • •

Rth JB Ptot TJ max

= 5 K/W = 11.2 W (@ 700 mA) = 125 °C

According to Equation 8, we get a temperature drop of ∆TJB = 11.2 W — 5 K/W = 56 K, or a maximum allowable substrate temperature TB max = 69 °C. The heat-sink must assure this maximum temperature to avoid heat damage of the LED chip on this RGYB ACULED when

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driven at 700 mA. Let’s now assume we have 10 of these ACULEDs on one heat-sink. What is the maximum temperature TB all the heat-sink must cool to? According to Equation 10, the overall thermal resistance is:

Rth, all =

1 10 5 KW

= 0.5 K/W

The resistance of this configuration now is much lower, but the total power consumption is 10 times higher, or Ptot all = 10 — Ptot = 112 W. According to Equation 8, the temperature drop is still ∆TJB tot = 112 W — 0.5 K/W = 56 K or TB all = 69 °C. Hence, the heat sink still must cool the substrates down to 69 °C - meaning it must draw away 10 times more heat (112 W instead of 11.2 W). The lesson learned is that it’s always very important to look at the whole situation when calculating thermal resistance, particularly in terms of the total heat generated or power consumed. The value Rth by itself can be misleading. This is also very important to know when comparing different packages. However, all these equations apply strictly to vertical heat transportation. The more ACULEDs that are mounted on a heat-sink, the higher the influence of the horizontal heat spreading in the different layers of the heat-sink. It’s also very important to keep in mind the total amount of heat (given by Equation 7b) that must be drawn away. We will learn in the next sections that the ACULED can be considered as a mixture between parallel and serial thermal resistances, which is especially important when designing your ACULED DYO for your particular application. But even if you use standard ACULED VHLs, it is recommended to read the next section and learn more about the ACULED package and its internal thermal management.

a)

Figure 8

b) Rth, 1

Rth, 2

Rth, 3

Rth, 1

c)

Schematic diagrams of thermal resistance principals: a) serial b) parallel c) mixture

Rth, 2 Rth, 1 Rth, 3

Rth, 3

Rth, 2

Thermal Resistances of the ACULED VHL Figure 9 shows the thermal schematic diagram of the ACULED VHL and ACULED DYO together with a heat-sink. The ACULED VHL has four electrically and thermally separated pads (see Figure 1) whose resistances Rth Cn are “connected” in parallel. Due to its superior internal thermal concept, there is almost no thermal crosstalk (typically Rth JB ACL

Assuming a typical Rth JB of 5 K/W for the ACULED VHL, we get a safety power of approx. 200 mW. Of course the ACULED also transports some heat to the air on the front side, which can be considered when used without heat-sink. Therefore, in practice, it can be operated at 50 mA and 600 mW without additional heat-sinks. However, all these formulas are only very rough estimations since α is hard to determine in reality. The most accurate approach is simulating the heat transfer processes with modern thermal simulation software (see www.nusod.org) that can handle complex structures showing the critical thermal paths. Figure 15 shows the ACULED VHL in this thermal simulation process done by finite element simulation software. The results of this process, confirmed by measurements of the real product, enabled us to improve the internal thermal management of the ACULED, evidenced, for example, at the low thermal crosstalk.

Figure 15 Thermal simulation of the ACULED (detail of the chip area) Upper left: finite element mesh model Upper right: temperature simulation at all chip operation

When calculating heat-sinks, we must consider the maximum current IF at which the ACULED will be driven. This is included in Ptot in the previous formulas (see Equation 1). Figure 16 shows that chips can be damaged by exceeding the maximum current of the heat-sink in operation. In this figure, the curves of the pn-junction temperature TJ of a RGYB-ACULED are shown when driven at IF = 350 mA, respectively, 700 mA on a small heat-sink of 25 x 25 x 18.5 mm³. Though this heat-sink works properly at lower currents and power consumptions up to approx. Ptot = 6 W, it will fail with the ACULED VHL’s highest current because the junction temperature exceeds the critical maximum after two minutes of operation.

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Figure 16

200 180

TJ at different currents IF with the same heat-sink vs. time t of operation Though the heat-sink works properly at 350 mA it is not suitable at higher current.

700 mA 350 mA

160 140 TJ [°C]

120 100 80 60 40 20 0 0

50

100

150

200

250

300

t [s]

Heat-Sink Types The task of the heat-sink is to transport heat generated by the chips of the ACULED to the environment or cooling medium. This is typically done by convection of air, but can also be accomplished by a water-cooled system. Water cooling can be a good solution, particularly when driving several ACULEDs at high current and, therefore, at high total power consumption (see Equation 1). In some applications like machine vision, water is already used for other cooling processes and can, therefore, be easily adapted. Also, the use of Peltier-coolers (thermoelectric cooling) is an option for specific applications. Figure 19 shows examples of different heat-sink types. The main parameters that determine the type and size of heat-sink are the temperature difference (e.g. ∆TBA), the surface area Ahs , and the flow rate v of the cooling medium (water or fluid). Heat-sinks against air Standard heat-sinks made from a metal with good thermal conductivity, such as aluminium or copper, are available to the market in a variety of dimensions and profiles. To enlarge the surface area against air, these heat-sinks are equipped with fins or fingers. To support the convection by heat radiation, most heat-sinks are black anodized. The principle of these heatsinks is quite easy - the heat-sink is heated by the ACULED and, therefore, is warming the surrounding air. This warm air is rising, and cooler air follows. The cool air is again warmed by the heat-sink, and so on. This is known as natural convection. As well as its area Ahs , the properties of the surface (e.g. material, color) are also important for heat transfer to the air since they have an influence on the surface emission factor and heat transfer coefficient α. Heat-sinks and fans If the natural convection of the air is not enough for cooling, a fan can force the air and strongly decrease the thermal resistance. Heat-sinks with fans, such as those used with modern microprocessors are also available. See Figure 7 for an example of additional cooling by a fan with the ACULED VHL.

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Figure 17 shows how a fan can strongly decrease the thermal resistance Rth hs of a heat-sink by as much as five times, depending on the air flow velocity vA. The curves indicate this for different aluminium-made pin fin heat-sinks from the company PINBLOC, all on a 32 x 32 mm² base with 81 pins at different heights (10, 15 and 20 mm). The power dissipation of the heatsinks dramatically increases when the air flow is forced by a fan.

Figure 17 Power dissipation P (rising curves) and heat-sink resistance Rth hs (falling curves) vs. air flow velocity vA for different pin fin heatsinks (small picture). picture and graphic courtesy of PINBLOC (Cologne, Germany)

Heat pipes Heat pipes provide a very effective way of cooling by using evaporation and condensation of fluids in a self-contained system. Figure 18 shows the principle of a heat pipe. The heat source (e.g. the ACULED) heats up the fluid (e.g. water or alcohol) inside the heat pipe on its “warm” side or end. The fluid, which is in a relative small volume inside the heat pipe, evaporates due to the heat and is cooled down on the “cold” side through condensation. The cold side is connected to another external heat-sink or designed for a good cooling by the surrounding ambient. Since the temperature depends on the low pressure inside the heat pipe, the temperature drop from the warm to the cold side can be defined by the vacuum. Heat pipes do not require external water or electrical connections, do not have any moving parts, and do not need maintenance. They are widely used for cooling high-end microprocessors, for example, and have a thermal resistance or Rth hs < 1 K/W. Heat-sinks with liquid cooling Unlike heat pipes, which use the mechanism of evaporation and condensation of a liquid in a contained system, a convection system uses the good thermal capacity of water or liquids. The principle of these coolers is similar to air cooled systems, but now a liquid, rather than air, transports the heat. Therefore, theses systems require water connection, pumps, and usually a condenser to continuously cool down the liquid. They are used in applications where water cooling already exists (for instance at machines) and high amounts of heat are generated such as heat generated by a large number of ACULEDs with a high packaging density. Some companies provide micro-channel coolers that allow high heat flux capability of approx. 1000 W/cm² (remember, the ACULED VHL has up to approx. 10 W/cm² heat density). These

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liquid-cooled heat-sinks can take away a high amount of heat in a small footprint and are also used for laser diode bars.

Figure 18 The principle mechanism of heat pipes

Figure 19 Examples of different heatsink types for air cooling: Top: standard heat-sinks Bottom left: ceramic heatsink Bottom right: heat-sink with fan

Heat-Sink Mounting When mounting the ACULED to a heat-sink, the most important issue is a very good thermal contact between the ACULED’s substrate back side and the heat-sink itself. Figure 9 previously pointed out the additional thermal resistance Rth TIM for the junction material between the ACULED and the heat-sink, known as thermal interface material (TIM). This resistance should be kept as small as possible. Depending on the kind of mechanical connection (clamping,

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screwing, gluing etc.), the TIM must also provide a good mechanical connection/adhesion. For general mounting recommendations, please refer to the application note, “Mounting of the ACULED® Product Family.” Thermal grease and similar materials Without any TIM, there can be a risk of having bad thermal conduction between the ACULED and heat-sink due to air gaps resulting from micro-roughness of the materials. The use of thermal grease helps avoid these gaps. However, since the thermal conductivity of thermal grease though much better than air - is in the range of 2 - 5 W / (m—K), we must apply it as thinly as possible. Its task is to displace only the air gaps and any additional (thick) layer between the ACULED and the heat-sink will increase the thermal resistance. A few 10 micrometers of thermal grease is usually sufficient. If good adhesion is necessary, a kind of thermal glue should be used. Generally speaking, you could say that the better the adhesion of the glue, the lower the thermal conductivity. Therefore, the best heat transfer is achieved by highly-conductive thermal grease and screwing or clamping the ACULED to the heat-sink. Since the ACULED board is electrically isolated from the solder pads and from the chip pads, there is no need to use insolating material, which also has lower thermal conductivity. Thermal conductivity tapes Thermal tapes are typically double-sided adhesive films that are easy to handle, but bear the risk of delamination. Due to their thickness of some 100 s micrometers and a relatively low thermal conductivity of 1 - 2 W / (m—K), they have a much higher thermal resistance than thermal grease. Therefore, they should be used only at lower power or less critical applications. Table 6 shows the properties of typical TIMs between the ACULED and heat-sink. The heat transition resistance Rα (reciprocal value of the heat transfer coefficient α) can be used to calculate the thermal resistance Rth TIM of the material when the area ATIM is known (usually the area of the ACULED AACL): Rth TIM

TIM thermal grease elastomer tapes adhesive tapes phase change materials thermal glue

= Rα / ATIM

heat transistion resistance Rα [cm²—K/W]

(15)

Table 6 operating temperature

Properties of different remarks

thermal interface materials

1-4

up to 250 °C

high thermal conductivity; high pressure helpful, no mechanical adhesion lower thermal conductivity, high pressure required, suited for mass production lower thermal conductivity, easy mechanical mounting

0.3 - 0.7

up to 200 °C

Wax-like material with low glass transition temperature, available as tape or paste

0.3 - 2

- 60 - + 250 °C

thermal grease with glue; easy mechanical mounting, curing necessary

0.3 - 2

- 60 - + 200 °C

1-3

- 40 - + 200 °C

During mounting, it is important to put some uniform pressure on the ACULED to achieve a good mechanical and thermal contact and to help the interface material displace the air. However, be careful not to put pressure on the silicone (refer to application note, “Handling of LED and Sensor Products Encapsulated by Silicone Resin”). Also consider the “cool” side of the heat-sink. For example, in an air-cooled system, ensure that the air can flow fast and easily through the rims and fins of the heat-sink. When using natural convection, the rims of the heat-

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sink should be vertical for optimized air flow, since warm air flows to the top (see Figure 20). Also ensure that other heat sources, such as radiators, sunlight, microprocessors, and other power consumers that can produce heat, are kept away from the LED or heat-sink - or that they are considered in your thermal calculations. As described before, it is highly recommended to control the heat by a thermistor on the ACULED, the board, or the heat-sink to avoid overheating due to damaged fans, broken water tubes (with water cooled systems), or additional heat sources. Unlike other SMD-LEDs on the market, it’s not necessary to draw away the heat via the solder pads. Therefore, the ACULED can be upside-down mounted on a simple FR4-PCB for electrical connection and have a heat-sink attached to its back side. With this concept, the use of highpriced IMS or ceramic PCBs is not necessary and multi-layer PCBs can even be used. This is not possible with the previously-mentioned high thermal conductive materials (refer to Table 1 for λth of different PCB materials). However, if possible, the solder pads of the ACULED can be used to assist the heat transportation (e.g. by designing big and thick copper pads and tracks or even thermal vias on your PCB where the ACULED is mounted). Refer to Figure 21 for examples on the recommended method for mounting the ACULED as a through-looking device with the best thermal and electrical connectivity.

Figure 20 Orientation of the heatsink rims when used with natural convection: best (right) and less effective orientations (left, middle)

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Figure 21 Clockwise from upper left: (1) The ACULED mounted as through looker on a FR4 board, small heat-sink (for 350 mA operation) and PMMA standard optics. (2) All parts mounted together. (3) 3 assemblies of (2) connected together (4) 3 blue ACULED VHL under operation

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Symbols and Units The following terms and their typical units are used in the application notes and datasheets of the ACULED. Please note that not all of these are used in this particular note. A

[m²]

area, surface

AACL

[cm²]

surface area of ACULED substrate back side

Ahs

[cm²]

surface area of heat-sink

Arad

[mm²]

radiating surface

ATIM

[cm²]

surface area of TIM with usually ATIM = AACL

α

[W/(m²—K)]

heat transfer coefficient

Cnm

[K/W]

thermal crosstalk coefficient between ACULED pads n and m

Ee

[W/m²]

irradiance

EV

[lx]

illuminance [lux]

Φe

[mW]

radiant flux

ΦV

[lm]

luminous flux [lumen]

Ie

[W/sr]

radiant intensity

IF

[mA]

forward current

IFM

[mA]

surge current

IR

[µA]

reverse current

IV

[cd]

luminous intensity [candela]

Le

[W/(m²—sr)]

radiance

LV

[cd/m²]

luminance

λdom

[nm]

dominant wavelength

λpeak

[nm]

peak wavelength

λth

[W/(m—K)]

thermal conductivity

∆λ

[nm]

spectral half bandwidth

η

[%]

efficiency

ηopt

[lm/W]

optical (luminous) efficacy

PCn

[W]

power consumption of chip placed on pad n of the ACULED

Popt

[mW]

output power (optical)

Pth

[W]

thermal power (i.e. the amount of electrical power consumption that is transformed into heat)

Ptot

[W]

power consumption (electrical) [Watt]

R

[Ω]

(electric) resistance



[(K—cm²)/W]

heat transition resistance; reciprocal value of the heat transfer coefficient α

Rth

[K/W]

thermal resistance (general) [Kelvin per Watt]

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Rth BA

[K/W]

thermal resistance from base (B) backside to ambient surrounding (A)

Rth C

[K/W]

thermal resistance of chip from junction (J) to chip substrate back side

Rth Cn

[K/W]

thermal resistance of chip from junction (J) to chip substrate back side at chip placed on pad n of the ACULED

Rth hs

[K/W]

thermal resistance of heat-sink

Rth JA

[K/W]

thermal resistance from junction (J) to ambient air or surrounding (A)

Rth JB

[K/W]

thermal resistance from junction (J) to base (B) back side

Rth JBn

[K/W]

thermal resistance from junction (J) to base (B) back side assigned to pad n of the ACULED

Rth P

[K/W]

thermal resistance of package from chip pad to base (B) back side, indicates the mount without chip

Rth Pn

[K/W]

thermal resistance of package from chip pad to base (B) back side assigned to pad n of the ACULED

Rth TIM

[K/W]

thermal resistance of TIM, usually between ACULED substrate (back side) and heat-sink

RH

[%]

relative humidity

t

[s]

time

tLife

[h]

life time of LED chip or module

T

[°C] or [K]

temperature (general)

TA

[°C]

ambient temperature

[°C]

maximum allowed ambient temperature

TB

[°C]

base temperature on back side of package (substrate)

TB max

[°C]

maximum allowed base temperature on back side of package (substrate)

TC

[°C]

temperature on back side of chip substrate

TCn

[°C]

temperature on back side of chip substrate placed on pad n of the ACULED

TCT

[K]

(correlated) color temperature

TJ

[°C]

temperature at pn-junction of the LED chip; usually referred to the maximum allowable junction temperature

TJn

[°C]

temperature at pn-junction of the LED chip placed on pad n of the ACULED. Usually the maximum allowable temperature is meant.

TNTC

[°C]

temperature inside NTC chip

Top

[°C]

operating temperature

Tsold

[°C]

soldering temperature (at backside of the ACULED VHL)

Tst

[°C]

storage temperature

∆TB NTC

[K]

difference between base and temperature and NTC chip temperature

TA

max

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∆TBA

[K]

difference between base and ambient temperature

∆TJA

[K]

difference between junction and ambient temperature

∆TJB

[K]

difference between junction and base temperature [Kelvin]

∆TJBn

[K]

difference between junction temperature of chip placed on pad n of the ACULED and base temperature

∆TSafety

[K]

additional “safety” temperature to be subtracted from the TB max to be on the safe side when calculating heat-sink dimensions

TCΦe

[mW/K]

temperature coefficient of radiant flux

TCΦV

[mlm/K]

temperature coefficient of luminous flux

TCλ dom

[nm/K]

temperature coefficient of dominant wavelength

TCλ peak

[nm/K]

temperature coefficient of peak wavelength

TCVF

[mV/K]

temperature coefficient of forward voltage

vA

[m/s]

(cooling) air flow velocity

VF or UF

[V]

forward voltage

VR or UR

[V]

reverse voltage

xn°

[-]

x coordinate in CIE color space for n-degree observer (usually n = 2 is used with light sources like LEDs: x2°)

yn°

[-]

y coordinate in CIE color space for n-degree observer (usually n = 2 is used with light sources like LEDs: y2°)



[°]

viewing angle (usually at half of maximum intensity)

Abbreviations The following abbreviations are used in the application notes. Please note that not all of these abbreviations are used in this particular note. ACULED®

The trademarked name for Excelitas’ range of All Color Ultrabright LEDs.

BOM

Bill of material

ccw

Counter clockwise

CCT

Correlated color temperature

CIE

Commission Internationale de l'Eclairage = International Commission on Illumination

COB

Chip-on-board

CRI

Color rendering index, value to measure the quality of light used for illumination purposes. 100% means best natural appearance of illuminated colors by the light source.

DYOTM

Design-Your-Own, indicates an ACULED with customized chip configuration

DUT

Device under test

ESD

Electro-static discharge

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FR4

Flame resistant 4, low cost PCB material made from epoxy resin and fiberglass mat

FWHM

Full width at half maximum

IMS

Insulated metal substrate, PCB substrate made from aluminum or copper to provide excellent heat management

IR

Infra-red, radiation above 700 nm within the scope of this application note

LED

Light-emitting diode

NTC

Negative temperature coefficient, used as acronym for an NTC resistant. Thermistor to control (LED-) temperature

PCB

Printed circuit board

PD

Photo-diode

PMMA

Polymethyl methacrylate, transparent thermoplastic; in optical grade used for lenses

pn junction

Layer in the LED chip, where positive (p) and negative (n) charged carriers recombine to light respectively radiation.

PPA

Polyphtalamide (plastic)

PT100

Thermistor made from platin with 100 Ω at 0 °C. Has a positive temperature coefficient (PTC).

SMD

Surface mount device

TIM

Thermal interface material

UV

Ultra-violet, with LEDs radiation below 405 nm within the scope of this application note

VHLTM

Very high lumen. This is the name for the newest generation of standard monochromatic and multi-colored four-chip ACULEDs.

VIS

Visible light, radiation between 405 and 700 nm within the scope of this application note

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Notes 1. 2. 3. 4. 5.

6. 7. 8. 9.

Excelitas maintains a tolerance of ± 5% on flux and power measurements. Excelitas maintains a tolerance of ± 2 nm for dominant wavelength measurements. Excelitas maintains a tolerance of ± 1 nm for peak wavelength measurements. Excelitas maintains a tolerance of ± 2 K/W for thermal resistance measurements depending on chip properties. Due to the special conditions of the manufacturing processes of LEDs, the typical data or calculated correlations of technical parameters can only reflect statistical figures. These do not necessarily correspond to the actual parameters of each single product, which could differ from the typical data and calculated correlations or the typical characteristic line. If requested, e.g. because of technical improvements, these typ. data will be changed without any further notice. Proper current derating must be observed to maintain junction temperature below the maximum. LEDs are not designed to be driven in reverse bias. All drawings are not to scale. All dimensions are specified in [mm] if not otherwise noticed.

North American Sales Office Excelitas Technologies 35 Congress Street Salem, MA 01970, USA Telephone: +1 978-745-3200 Toll free: (North America) +1 800-950-3441 Fax: +1 978-745-0894 [email protected] www.excelitas.com

European Headquarters Excelitas Technologies Wenzel-Jaksch-Str. 31 65199 Wiesbaden, Germany Telephone: (+49) 611-492-269 Fax: (+49) 611-492-170

Asia Headquarters Excelitas Technologies 47 Ayer Rajah Crescent #06-12 Singapore 139947 Telephone: (+65) 6775-2022 Fax: (+65) 6775-1008

For a complete listing of our global offices, visit www.excelitas.com ©2011 Excelitas Technologies Corp. All rights reserved. The Excelitas logo and design are registered trademarks of Excelitas Technologies Corp. ACULED®, VHL™, and DYO™ are trademarks of Excelitas Technologies Corp. or its subsidiaries, in the United States and other countries. All other trademarks not owned by Excelitas Technologies Corp. or its subsidiaries that are depicted herein are the property of their respective owners. Excelitas reserves the right to change this document at any time without notice and disclaims liability for editorial, pictorial or typographical errors. 600195_01 APP0707

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