Metal Core Printed Circuit Board Research Articles

The influence of a soldering manner on thermal properties of LED modules

Purpose This paper aims to present the results of the influence of a manner of soldering light emitting diodes (LEDs) to the metal core printed circuit board on thermal parameters of the module LED containing these diodes. Design/methodology/approach Using the authors’ elaborated measuring method and the dedicated measurement set-up, transient thermal impedances of LED modules, mounted using different soldering processes and mounted to the heat-sink with different values of the moment of force, are measured. The obtained results of measurements are discussed. Findings It was shown experimentally that the manner of soldering could strongly influence efficiency of dissipation of heat generated in the module. The best thermal properties were obtained for soldering using vapour phase technology with vacuum and paste LFS-216LT. It was also proved that the moment of force used while mounting the considered modules on the heat-sink can result in a change of the value of thermal resistance of this module exceeding even 12 per cent. Research limitations/implications The investigations were performed for five LED modules operating at one, arbitrarily selected value of power dissipated in these modules mounted on the heat-sink of arbitrarily selected dimensions. Practical implications The obtained results of measurements could be usable for designers of mounting processes of power LED modules. Originality/value This paper presents the results of investigations of thermal properties of LED modules, in which different techniques of soldering are used. It was shown experimentally that the manner of soldering could strongly influence efficiency of dissipation of heat generated in the module. It was also proved that the moment of force used while mounting the considered modules on the heat-sink is important.

RGB-Stack Light Emitting Diode Modules with Transparent Glass Circuit Board and Oil Encapsulation

The light emitting diode (LED) is widely used in modern solid-state lighting applications, and its output efficiency is closely related to the submounts’ material properties. Most submounts used today, such as low-power printed circuit boards (PCBs) or high-power metal core printed circuit boards (MCPCBs), are not transparent and seriously decrease the output light extraction. To meet the requirements of high light output and better color mixing, a three-dimensional (3-D) stacked flip-chip (FC) LED module is proposed and demonstrated. To realize light penetration and mixing, the mentioned 3-D vertically stacking RGB LEDs use transparent glass as FC package submounts called glass circuit boards (GCB). Light emitted from each GCB stacked LEDs passes through each other and thus exhibits good output efficiency and homogeneous light-mixing characteristics. In this work, the parasitic problem of heat accumulation, which caused by the poor thermal conductivity of GCB and leads to a serious decrease in output efficiency, is solved by a proposed transparent cooling oil encapsulation (OCP) method.

Etching Behaviors of Cu and Invar for Metal Core PCB Applications

Etching Behaviors of Cu and Invar for Metal Core PCB Applications

Thermal Performance of a Light Emitting Diode Light Engine for a Multipurpose Automotive Exterior Lighting System With Competing Board Technologies

In recent years, light emitting diodes (LEDs) have become an attractive technology for general and automotive illumination systems replacing old-fashioned incandescent and halogen systems. LEDs are preferable for automobile lighting applications due to its numerous advantages such as low power consumption and precise optical control. Although these solid state lighting (SSL) products offer unique advantages, thermal management is one of the main issues due to severe ambient conditions and compact volume. Conventionally, tightly packaged double-sided FR4-based printed circuit boards (PCBs) are utilized for both driver electronic components and LEDs. In fact, this approach will be a leading trend for advanced internet of things applications embedded LED systems in the near future. Therefore, automotive lighting systems are already facing with tight-packaging issues. To evaluate thermal issues, a hybrid study of experimental and computational models is developed to determine the local temperature distribution on both sides of a three-purpose automotive light engine for three different PCB approaches having different materials but the same geometry. Both results showed that FR4 PCB has a temperature gradient (TMaxBoard to TAmbient) of over 63 °C. Moreover, a number of local hotspots occurred over FR4 PCB due to low thermal conductivity. Later, a metal core PCB is investigated to abate local hot spots. A further study has been performed with an advanced heat spreader board based on vapor chamber technology. Results showed that a thermal enhancement of 7.4% and 25.8% over Al metal core and FR4-based boards with the advanced vapor chamber substrate is observed. In addition to superior thermal performance, a significant amount of lumen extraction in excess of 15% is measured, and a higher reliability rate is expected.

Numerical Thermal Analysis and Optimization of Multi-Chip LED Module Using Response Surface Methodology and Genetic Algorithm

In this paper, the heat transfer performance of the multi-chip (MC) LED module is investigated numerically by using a general analytical solution. The configuration of the module is optimized with genetic algorithm (GA) combined with a response surface methodology. The space between chips, the thickness of the metal core printed circuit board (MCPCB), and the thickness of the base plate are considered as three optimal parameters, while the total thermal resistance ( $R_{\mathrm {tot}})$ is considered as a single objective function. After optimizing objectives with GA, the optimal design parameters of three types of MC LED modules are determined. The results show that the thickness of MCPCB has a stronger influence on the total thermal resistance than other parameters. In addition, the sensitivity analysis is performed based on the optimum data. It reveals that $R_{\mathrm {tot}}$ increases with the increased thickness of MCPCB, and reduces as the space between chips increases. The effect of the thickness of base plate is far less than that of the thickness of MCPCB. After optimization, three types of MC LED modules obtain lower $T_{j}$ and $R_{\mathrm {tot}}$ . Moreover, the optimized modules can emit large luminous energy under high-power input conditions. Therefore, the optimization results are of great significance in the selection of configuration parameters to improve the performance of the MC LED module.

Testing and Analysis of Boron-Doped Aluminum Nitride Thin-Film-Coated Al as Thermal Substrates in PCB Fabrication for LED Application

Chemical vapor deposition was used to coat boron-doped AlN thin film on Al substrates and used as thermal substrates in the fabrication of printed circuit boards (PCBs). Conventional screen printing method was used to deposit dielectric, conductor, and protective coating on thin-film-coated Al substrate for the printed metal PCBs (MPCBs). Commercial light emitting diode (LED) was fixed on MPCB and tested for thermal and optical properties under various driving currents. The observed results are compared with those of commercial metal-core printed circuit board (MCPCB). The measured total thermal resistance ( $R_{\mathrm {th-tot}})$ of the LED was low for MPCB and recorded good difference in $R_{\mathrm {th-tot}}$ ( $\Delta R_{\mathrm {th-tot}}= 7.55$ K/W) when compared with MCPCB. An impressive difference on rise in junction temperature ( $T_{\mathrm {J}})$ was noticed ( $\Delta T_{\mathrm {J}}= 13.72$ ) with MPCB at 700 mA as compared with MCPCB. As a result of reduced $T_{J}$ , an improved light output was recorded for MPCB in all driving currents and a higher value also with MPCB. Hence, B-AlN thin-film-coated Al substrate could be used as thermal substrates in PCB fabrication for efficient electronic packaging in LED industries.

A single unit cooling fins aluminum flat heat pipe for 100 W socket type COB LED lamp

Although the LED light has higher energy conversion efficiency compared with the conventional metal halide lamp, it is well known that there is a thermal burden due to the poor photoelectric conversion efficiency of the semiconductor diode. SMD (Surface Mount Device) LED lamp which has been mainly used has induced a problem of poor optical efficiency though it has relatively large heat dissipation area. Much effort has been put to improve the optical efficiency through COB (Chip on Board) LED lamp, but the thermal problem particularly in the over 100W application is still exist. As the studies for applying the heat pipe to the high power LED, an embedding technology the heat pipe in the MCPCB (Metal Core Printed Circuit Boards), loop heat pipe, and a columnar heat pipe for 90W, etc. were developed and may apply to COB LED lamp. In this paper, a single unit cooling fins aluminum flat heat pipe (AFHP) with U shape is developed for the 100W COB LED lamp with 120∗120∗170mm3 of small volume and socket type electric connection. The U shape AFHP module has less than 900g, low price by extrusion method and socket type lamp, therefore replacement and maintenance of the lamp for factory-light or street-light could be easily conducted. Through the study, a single unit cooling fins AFHP is designed and fabricated. And the heat transfer characteristics of the AFHP with two condensers have been investigated, and the junction temperature of the COB LED module was also evaluated within 85°C at 100W of input power.

Evaluation of a Lead-Free Ultra Low Temperature Co-Fire Ceramic (ULTCC) Tape Designed for Lamination on Aluminum Substrates and a Compatible Co-Fireable Silver Conductor

Abstract Light-emitting diodes (LEDs) are the product of choice for many commercial and industrial lighting applications. Due to increasing power densities, alternative mounting solutions are being evaluated in order to achieve the lowest junction temperatures and lowest thermal resistances. Previous papers have explored the benefits of using a thick film system as opposed to a traditional Metal Core Printed Circuit Board (MCPCB). The advantage of being able to directly insulate a heat sink eliminates additional MCPCB layers and mechanical attachment that can decrease the thermal conductivity of the module and increase cost. Lower thermal resistance results in higher performing LED modules at a lower cost and a longer lifetime. A process has been developed and evaluated where an ultra-low temperature co-fired ceramic (ULTCC) tape will be laminsated directly on to an aluminum heat sink in order to electrically isolate the substrate. The green tape will then have a silver conductor applied to it and fired in one step. Using a tape system as opposed to an ink system creates many advantages. The tape system allows for processing in cavities, which is beneficial for recessed lighting and three-dimensional substrates. The co-firing silver conductor reduces the number of firing steps, leading to less dimensional variation of the heat sink and lower processing costs. This paper will discuss the material solutions, processing requirements, and reliability data for a completely co-fired ULTCC system on aluminum.

Microstructural and thermal characterizations of light-emitting diode employing a low-temperature die-bonding material

A Sn/Bi bilayer was deposited on a hot air solder leveling (HASL)-treated metal-core printed circuit board (MCPCB) using electroplating as a low-temperature die-bonding material for light-emitting diode (LED). The eutectic feature of the Sn/Bi contact enabled the die-bonding process to accomplish through a liquid/solid reaction at 185°C with a proper compression force. A high-temperature die-bonding structure composed of a Bi layer sandwiched by two intermetallic compounds (IMCs) formed after thermocompression. Employment of the Sn/Bi bilayer for low-temperature die-bonding prevented the LEDs from thermal stress problems, and the resulting high-temperature IMC/Bi/IMC die-bonding structure was capable of withstanding multiple bonding reactions and high temperature/current operation environment. Durability tests including mechanical, thermal, and optical performance were systematically performed and compared with other commercially available die-bonding materials (Ag paste and solder alloys).

High-Power LED Chip-on-Board Packages With Diamond-Like Carbon Heat-Spreading Layers

The thermal properties of a flip-chip light-emitting diode (FCLED) chip-on-board (COB) with diamond-like carbon (DLC) heat-spreading layers are investigated. The temperature-dependent performance of the COB packages with and without DLC heat-spreading layers at the LED and metal core printed circuit board sites are studied. Results show that the device junction temperature and thermal conductivity of the COB package with DLC heat-spreading layers (DLC package) are less than those of the COB package without DLC heat-spreading layers (regular package). For the steady state of light intensity, the mean of light intensity drop for 12 tested packages of DLC package was improved by 6.1% after 3 hours of burn-in compared with that of the regular package.

Thermal resistance of high power LED influenced by ZnO thickness and surface roughness parameter

Purpose – The purpose of this research is to study the effect of thickness and surface properties of ZnO solid thin film for heat dissipation application in LED. Heat dissipation in electronic packaging can be improved by applying a thermally conductive interface material (TIM) and hence the junction temperature will be maintained. ZnO is one of the oxide materials and used as a filler to increase the thermal conductivity of thermal paste. The thickness of these paste-type material cannot be controlled which restricts the heat flow from the LED junction to ambient. The controlled thickness is only possible by using a solid thin-film interface material. Design/methodology/approach – Radio Frequency (RF)-sputtered ZnO thin film on Cu substrates were used as a heat sink for high-power LED and the thermal performance of various ZnO thin film thickness on changing total thermal resistance (R th-tot) and rise in junction temperature were tested. Thermal transient analysis was used to study the performance of the given LED. The influence of surface roughness profile was also tested on the LED performance. Findings – The junction temperature was high (6.35°C) for 200 nm thickness of ZnO thin film boundary condition when compared with bare Cu substrates. Consecutively, low R th-tot values were noticed with the same boundary condition. The 600 nm thickness of ZnO thin film exhibited high R th-tot and interface resistance than the other thicknesses. Bond Line Thickness of the interface material was influenced on the interface thermal resistance which was decreased with increased BLT. Surface roughness parameter showed an immense effect on thermal transport, and hence, low R th (47.6 K/W) value was noticed with low film roughness (7 nm) as compared with bare Cu substrate (50.8 K/W) where the surface roughness was 20.5 nm. Originality/value – Instead of using thermal paste, solid thin film ZnO is used as TIM and coated Cu substrates were used as a heat sink. The thickness can be controlled, and it is a new approach for reducing the BLT between the metal core printed circuit board and heat sink.

Heat Transfer Characteristics in High Power LED Packaging

This study uses the T3Ster transient thermal resistance measuring device to investigate the effects to heat transfer performances from different LED crystal grains, packaging methods and heat-sink substrates through the experimental method. The experimental parameters are six different types of LED modules that are made alternatively with the crystal grain structure, the die attach method and the carrying substrate. The crystal grain structure includes the lateral type, flip chip type and vertical type. The die attach method includes silver paste and the eutectic structure. The carrying substrates are aluminum oxide (Alumina) and aluminum nitride (AIN) ceramic substrates and metal core PCB (MCPCB). The experimental results show that, under the conditions of the same crystal grain and die attach method, the thermal resistance values for the AIN substrate and the Alumina substrate are 2.1K/W and 5.1K/W, respectively and the total thermal resistance values are 7.3K/W and 10.8K/W. Compared to the Alumina substrate, the AIN substrate can effectively lower the total thermal resistance value by 32.4%. This is because the heat transfer coefficient of the AIN substrate is higher than that of the Alumina substrate, thus effectively increasing its thermal conductivity. In addition, under the conditions of the same crystal grain and the same substrate, the packaging methods are using silver paste and the eutectic structure for die attach. Their thermal resistance values are 5.7K/W and 2.7K/W, respectively, with a variance of 3K/W. Comparisons of the crystal grain structure show that the thermal resistance for the flip chip type is lower than that of the traditional lateral type by 0.9K/W. This is because the light emitting layer of the flip chip crystal grain is closer to the heat-sink substrate, shortening the heat dissipation route, and thus lowering the thermal resistance value. For the total thermal resistance, the crystal grain structure has a lesser effect to the heat dissipation performance whereas the packaging method and the choice of the substrate have a relatively higher effect to the LED heat dissipation performance. Combining the above results, we see that selecting the flip chip crystal grain with a closer light emitting layer to the substrate, the eutectic structure that has better thermal conductivity and the AIN substrate can effectively increase the heat dissipation performance of the LED module.

Effect of aluminum anodizing in phosphoric acid electrolyte on adhesion strength and thermal performance

This study examined the adhesive bond strength and thermal performance of the anodized aluminum 6061 in phosphoric acid electrolyte to improve the adhesive bond strength and thermal performance for use in metal core printed circuit boards (MCPCB). The electrolyte temperature and applied voltage were altered to generate varied pore structures. The thickness, porosity and pore diameter of the anodized layer were measured. The pore morphologies were affected most by temperature, which was the driving force for ion transportation. The mechanism of adhesive bond was penetration of the epoxy into the pores. The optimal anodization conditions for maximum adhesive bond strength, 27 MPa, were 293 K and 100V. The maximum thermal conductivity of the epoxy-treated anodized layer was 1.6 W/m·K at 273 K. Compared with the epoxy-treated Al layer used for conventional MCPCBs, the epoxy-treated anodized layer showed advanced thermal performance due to a low difference of thermal resistance and high heat dissipation.

Thermal distribution of multiple LED module

Light-emitting diode (LED) is an electronic device with high heat flux. The simulation and analysis of the heat dissipation play an important role in the improvement of performance and design of LED module. In this paper, a thermal model was developed using ANSYS software to simulate the temperature distribution of LED module. The effect of thermal conductivity of MCPCB (metal-core printed circuit board) substrate on the heat dissipation of LED module in the vertical direction and MCPCB substrate in the length and width direction was evaluated. A mathematical formula to describe the vertical temperature distribution of LED module was developed. The results showed that the maximum temperature of LED module and the temperature gradients near the outer edge of the contact surfaces between cooling blocks and MCPCB substrate decreased with the increase of thermal conductivity of MCPCB substrate.

Print Copper on Ceramic for High Reliability Electronics

In the power electronics world, Direct Bonded Copper (DBC) is the primary substrate technology. In this paper, we will discuss an alternative technology utilizing screen printable copper pastes (Thick Printed Copper - TPC) on a variety of substrate technologies including Alumina (Al2O3) and Aluminum Nitride (AlN). These materials when processed, look and perform similar to DBC, but exhibit superior reliability and excellent design flexibility. DBC has drawbacks when it comes to thermal mechanical reliability and lacks the flexibility to have multiple copper thicknesses for power and signal circuits within the same design, which is easily achieved via screen printing. The benefits of this TPC system will be demonstrated through data generated on passive thermal shock tests in comparison to high end DBC. Furthermore, this Thick Print Copper technology has the excellent potential for replacing high end Metal Core Printed Circuit Board (MCPCB) technology due to utilization of higher thermal conductive dielectric materials like Al2O3 and AlN. This will allow for designers to drive their LED's harder and effectively producing LED modules with higher power densities.

Optimization of heat flow analysis for exceeding hundred watts in HI-LEDs projectors

This study conducted a heat flow-lighting performance test to evaluate and analyze an exceeding hundred watts of HI-LEDs (higher-power LEDs) projector. In addition, a Windows-based optimization program was developed to optimize the heat sink of the HI-LEDs projector. The results indicated that the maximal LED temperature of the projector was 108°C at a steady temperature state (approximately 1h of operation). When the metal-core printed circuit board (MCPCB) of the projector was replaced with a vapor-chamber PCB (VCPCB), the LED temperature decreased to 87.6°C. Iterative calculations were subsequently performed using the self-developed Windows-based optimization program for determining the optimal fin interval (8.94mm) and thickness (2.56mm). These optimal parameters were simulated using CFD (computational fluid dynamics), and the results were compared with those of the original 150-W HI-LEDs projector. A Rayleigh number of 6.1304×105 was obtained, indicating a laminar flow model. When the optimized heat sink was used, the LED temperatures for the MCPCB and VCPCB HI-LEDs projector samples decreased by 14 and 9.74°C, respectively. The h value of the optimized HI-LEDs projector (8.094W/m2k) was higher than that of the original HI-LEDs projector (7.154W/m2k), and its weight was reduced by approximately 2%.

Influence of thermal cycling on reliability of solder joints executed on long and metal core PCBs

Purpose – The purpose of this paper is to evaluate the reliability of solder joints made on long FR-4 and metal core printed circuit boards using the accelerated thermal cycling. Design/methodology/approach – Solder joints of diodes and resistors samples made on long FR-4 and aluminum (Al) core printed circuit boards were examined. Two kinds of solder pastes were used for the samples preparation. All samples were subjected to temperature aging cycles (−40°C – 3 hours/+85°C – 3 hours). Solder joints resistance, X-Ray inspection and metallographic cross-sections for samples as received and after 100, 500 and 1,000 hours of thermal cycles were utilized for solder joints assessment. Findings – It was stated that 1,000 hours of thermal cycles were enough to show reliability problems in solder joints on long and/or AL core printed circuit board assembly (PCBA). The solder joints of R1206 components were the most sensitive reliability elements. The solder joints of LED diodes are more reliable than solder joints of R1206 resistors. Solder joints made on FR-4 substrate were about two times more reliable than ones on AL core substrate. Cracks in solder joints were the visible reason of solder joints failures. Originality/value – The influence of thermal cycles on the reliability of solder joints on long, FR-4 and metal core printed circuit boards were presented. Findings from this paper can be used for planning of reliability trials during validation of reflow processes of products containing long or long metal core printed circuit boards (PCBs).

Thermal and optical investigations of high power LEDs with metal embedded printed circuit boards

To improve the thermal and optical performances for high power LEDs, a metal embedded printed circuit board (MEPCB), with a columnar copper slug inside the low cost FR4 material, was introduced in this paper. This novel structure makes it possible to connect the LED component directly to the outside environment. The thermal performances, including thermal resistances and temperature distributions of the MEPCB were evaluated by employing the finite element analysis method (FEA) and transient thermal measurement. The luminous and chromatic characteristics were also measured and analyzed at transient state and steady state. Results showed that the maximum reductions in the total thermal resistance and junction temperature were 65.60% and 56.25% by using the MEPCB compared with the conventional metal core printed circuit board (MCPCB). Benefiting from the excellent thermal performance of the MEPCB, up to 24.14% increase in steady state luminous flux was obtained compared with the MCPCB. Moreover, the correlated color temperature (CCT) shift value between 150mA and 900mA could be controlled within 93K. This work demonstrates that the MEPCB is more suitable for LEDs working at wide current ranges.

Thermal investigation of LED array with multiple packages based on the superposition method

In this paper, the superposition method is used to investigate the complete temperature field of a light-emitting diode (LED) packaging substrate, based on the results of transient temperature rise measurements and the thermal resistance coupling matrix. The feasibility of use of the superposition method in an LED array with multiple packages has been proved first by temperature comparisons with the simultaneous operation of an array (5×5) of 25 high power LEDs mounted on a metal core printed circuit board (MCPCB). Compared with existing approaches, the superposition method will measure the internal temperature of chip directly, accurately and nondestructively. According to the relatively accurate and reliable self-heating and coupling temperature rise data, optimization scheme of LED lamp with multiple packages is proposed. The results show that increasing the heat source separation distance and improving the thermal conductivity of thermal interface materials will reduce the temperature rise and thermal non-uniformity.

Development of Epoxy/BN Composites with High Thermal Conductivity for Metal-Core Printed Circuit Board (MCPCB)

The epoxy composites with high thermal conductivity for metal-core printed circuit board (MCPCB) can be prepared by varnish coating and a hot press method. Alumina filler of plate-like shape was used as primary micro-filler, while plate-like alumina filler, h-BN, a-BN and s-BN filler were used for blending into the plate-like alumina filler as the secondary filler. Results showed that the secondary fillers a-BN and s-BN loaded epoxy composites have higher thermal conductivity than alumina filler single-loaded composites. Also, BN filler has high thermal conductivity, but h-BN filled epoxy composite has lower thermal conductivity than alumina filled epoxy composite. The decrease of voids in epoxy composite are very important, and the filler shape and surface modification is also necessary to achieve high thermal conductivity in epoxy composite for MCPCB