AN032 PCB Technologies for LED Applications

1 Product DocumentPublished by ams OSRAM Group1 / 152018-10-18 | Document No.: AN032 NotePCB Technologies for LED applicationsAbstractThis application note provides a general survey of the various available printed CircuitBoard (PCB) Technologies for the use in LED for:all OSRAM Opto Semiconductors LEDsAuthors: Huber Rainer / Lang Kurt-J rgenApplication Note No. / 152018-10-18 | Document No.: AN032 Table of contentsA. Introduction ..2B. PCB Technologies commonly used for LED Applications ..3FR4 printed circuit board (PCB) ..3FR4 PCB with thermal via array ..4 Thick copper PCB ..6 Integrated copper wires and profiles.

2 7 Insulated Metal Substrate (IMS) ..7 IMS with exposed copper ..8 Aluminum with thin insulation layer ..8 Thick / thin film ceramics ..9C. PCB pad finishes ..9 Second level reliability ..11D. Thermal comparison of PCBs ..12E. Conclusion ..13A. IntroductionDue to the direct conversion of electrical current to light (optical radiation) in thesemiconductor, LEDs are highly efficient more efficient than most traditionallight sources. However, with LEDs as well most of the electrical power isconverted to heat rather than light. This thermal loss must be dissipated toensure reliable and stable LED operation for exploiting the completeperformance and efficiency of the LEDs.

3 Because of the trend towardsminiaturization, thermal output per surface unit is increasing, which means thatever increasing heat is emitted onto an ever smaller surface area for high-power Applications , sufficient thermal management is of centralimportance. In general, many options exist to achieve this, although these depend on thespecific Applications and ambient conditions. An universal, optimal design orconcept is not realizable for this reason, but has to be individually designedaccording to the specific assembly, the specific requirements and the completesystem. A principal overview of PCB Technologies available on the market andtheir suitability for LED Applications is outlined / 152018-10-18 | Document No.

4 : AN032 B. PCB Technologies commonly used for LED applicationsAs a support element with direct contact to the components, the PCB is incertain respects the primary component for achieving thermal mistakes here with insufficient optimization means that resultantdisadvantages at other locations in the complete system must be compensatedfor by measures that increase the this reason the following fundamental considerations must be madebeforehand: Which thermal quantity (power loss) must be dissipated where? How are the dimensions and performance data of the components? Where are the LEDs positioned on the PCB (position of heat sources)?

5 Available space and assembly periphery? Application temperatures and ambient temperatures? What mechanisms are used to cool the system? (free or forced convection) How should the heat be conducted to the heat sinks? Do specific reliability requirements exist ( cycle stability)? Do special cost factors have to be observed?The selection of appropriate materials for the circuit board is therefore of printed circuit board (PCB)Classic composite PCB of glass fiber and epoxy resin, available in variouscompounds, versions (single-/ multilayer) and thickness ( mm).Table 1: FR4 PCB versionsDesignationMaterialNoteFR4 standardEpoxy + glass fabricsTg= 130 140 C, RoHS/WEEE compliantFR4 non-halogenEpoxy + glass fabricsTg 150 C, CAF-resistantFR4 mid TgEpoxy + glass fabricsTg~ 140 160 C, increased thermal stability, low elongation in z-axis, high steadiness at lead-free soldering, often CAF-resistantFR4 high TgEpoxy + glass fabricsTg~160 190 C, increased thermal stability, low elongation in z-axis, high steadiness at lead-free soldering, often / 152018-10-18 | Document No.

6 : AN032 Thermal distribution occurs here in the copper layer (typical thickness 35 m) onthe PCB surface via heat distribution. Thicker copper layers ( 70 m &105 m) improve thermal dissipation properties and thus enable heat distributionover larger 1: Schematic diagram of a single-sided FR4 PCBPure standard substrates without improvements such as FR4 are normally notsuitable for use with high-power LEDs due to their low thermal conductivity(~ Wm-1K-1).FR4 PCB with thermal via arrayThin double-sided FR4 material with plated through-holes (= thermal vias) toensure a thermal path through the FR4 substrate and to improve thickness of the PCB is typically in the range of mm t < 2: Example of an FR4 PCB with thermal via arraysThe vias take on the heat transport function here, thus significantly improvingvertical thermal resistance of the FR4 material in a targeted and localizedmanner.

7 The thermal conduction capability of the vias themselves is determinedby the plating, via diameter and via pitch. Standard copper thicknesses of 20 25 m have become established in the industry, but greater wall platingthicknesses are also used. As a general rule, the thicker the copper layer, thebetter the performance but also costs increase with types of thermal via are possible: Simple open PTH (plated through hole) vias (Figure 3) Vias filled with epoxy and then capped with copper (Figure 4)Copper layerDielectric( FR4 - epoxy / glas fabrics) / 152018-10-18 | Document No.: AN032 Figure 3: Schematic layout of a thermal viaThe use of thermally conductive silver pastes for filling in order to improve thethermal conductivity of the vias has in practice only a minor thermal effect butincreases costs.

8 For economic reasons it is thus better to slightly enlarge thecopper thickness in the 4: Schematic diagram of an OSLON LED on an FR4 PCB with plugged and filled viasThe filled, copper-capped vias have the advantage that they can be arrangeddirectly under the thermal solder pad of the LED ( OSLON , OSLON BlackFlat, etc.), meaning they are able to directly conduct heat. The copper pluggingprevents uncontrolled solder wicking / solder voiding during reflow soldering atopen vias (solder run-off).Figure 5: Unfilled thermal vias have solder wicking if used in solder padst 25 mOSLON LEDFR4 PCB withplugged & filled viasThermal Interface Material (TIM)Heat / 152018-10-18 | Document No.

9 : AN032 Uncontrolled solder wicking generates solder balls on the backside of the PCB(Figure 5), leading to insufficient thermal connection between the PCB and theheat sink. In many cases however, simple thermal vias are sufficient to achievea clear reduction in thermal resistance down to the targeted value. The level ofresultant thermal resistance is affected by the number and position of the closer the vias are located to the heat source, the better and faster the heatcan be dissipated and the lower the thermal resistance (Figure 6).Figure 6: Comparison of the effect of two different configurations of thermal vias (simulation)As a general rule it is advisable to design the copper surface around the solderpads, especially at the thermal pad if existing, to be as large as possible in orderto achieve sufficient heat distribution across the copper PCBAre PCBs with copper layer thicknesses of 105 m to 400 m, and up to 500 min special cases.

10 According to requirements and technology used, layers areapplied in a planar way as external and/or internal layer. The basis material usedis FR4 or a metal substrate (Al, Cu) with a dielectric. The primary applicationsector consists of Applications with high power (high current) and for the coolingof components with large thermal power losses. Constructions are mainly multi-layer or double-sided 7: Schematic diagram of a copper thick-layer PCBA special form is the so-called iceberg technology. Here a thin-layer copperlayout is combined with selectively applied thick copper layers. The thick layerareas are recessed into the basis material (FR4) to achieve a uniform surfacelevel.