LUXEON C Series. Proper handling, assembly, board design and thermal management are required for high optical output and reliability.

Transcription

1 LUXEON C Series Assembly and Handling Information Application Brief AB36 LUXEON C Series Assembly and Handling Information Introduction LUXEON C series is the power LED that enables manufacturers of appliances, portable lighting solutions, power tools and more, to economically incorporate high quality, high performance LEDs in their applications. Proper handling, assembly, board design and thermal management are required for high optical output and reliability.

2 Table of Contents 1. Component Reference Documents Description Optical Center Handling Precautions Cleaning Electrical Isolation Mechanical 3-D Board Design Rules PCB Requirements FR4 PCB Design Considerations Examples of LUXEON C Footprint Assembly Process Recommendation Stencil Design Solder Paste Pick and Place FR4 Board Handling Product Packaging Consideration Chemical Compatibility LUXEON C Application Brief AB

3 1. Component 1.1. Reference Documents LUXEON C DS41 datasheet. Application Brief AB32 LUXEON LED Assembly and Handling Information. Documents are available from the Phillips Lumileds website Description LUXEON C LEDs consists of a miniature rectangular package with dimensions of 1.64 mm x 2.04 mm x 0.70 mm. There are two electrode pads (anode and cathode) at the base of the ceramic substrate. Heat is dissipated through both electrode pads. The top is coated with silicone. Figure 1 shows the cross section of the package. There is a back-to-back zener diode (TVS Transient Voltage Suppressor) connected in parallel with the InGaN chip to provide ESD protection as shown in the electrical schematic in Figure 2. InGaN LED chip & phosphor Silicone overcoat TVS ESD chip Ceramic substrate cathode anode Figure 1. Cross section of LUXEON C. - + TVS chip Figure 2. Electrical schematics. LUXEON C Application Brief AB

4 1.3. Optical Center LUXEON C has no lens (primary optics). The theoretical filament center is at the center location of the LED chip as indicated by a red dot in Figure 3. The centering tolerance is ± 0.10 mm with respect to the red dot. LED chip Theore cal op cal center Typical Top view Typical Anode Bo om view Figure 3. Theoretical optical center Handling Precautions Like all LEDs, there are handling precautions that need to be considered. Minimize all mechanical force exerted onto the silicone overcoat layer as much as possible. For manual handling with tweezers, always pick up from the sides of the ceramic substrate and never from the sides of the silicone overcoat. An L shaped tweezers (e.g. Knipex precision tweezers ) with the pointed tip in parallel to the LED s ceramics is more appropriate and easier to use than straight tweezers. To reduce the chance of any damage to the LED part and provide stability during pick up and manual placement on the board, it is recommended that the thickness height of the tweezers tip and the thickness of the ceramic sides are the same (0.4 mm) as shown in Figure 4. LUXEON C Application Brief AB

5 Silicone layer Figure 4. Examples of picking-up LEDs. LUXEON C Application Brief AB

6 Alternatively, a vacuum pen can be used in place of a tweezers. The suction tip should be made of a soft material such as rubber to minimize the mechanical force exerted onto the top surface of the silicone overcoat layer. When handling finished board containing LUXEON C, do not to touch the top surface with fingers or apply any pressure to it (Figure 5a). Also, do not turn over the board (if electrode pads are at the back of board) for probing as shown in Figure 5b. The surface of a workstation may be rough or contaminated and may damage the silicone overcoat layer. Any pressure applied during probing may potentially damage this layer. Figure 5a. Do not touch the top surface of LED package when handling a finished board. Figure 5b. Electrical probing of a finished board by turning over the board (LED face down), with electrode pads on the back. probes Silicone overcoat layer Rough surface Figure 5c. Do not probe LUXEON C upside down Cleaning The surface of the LED chip array should not be exposed to dust and debris. Excessive dust and debris on the LED chip array may cause a decrease in optical output. In the event that the surface requires cleaning, a compressed gas duster at a distance of 6 away from the LED will be sufficient to remove the dust and debris or an air gun with 20 psi (at nozzle) held a distance of 6 from the LED. Make sure the parts are secured before the air is applied and follow safety precautions. One can also use a lint-free swab and optional isopropyl alcohol to gently swab the surface. Extra care should be taken to prevent damage to the silicone coating. Make sure there are no large particles or debris on the LED before swabbing. Do not use other solvents as they may adversely react with the LED assembly. LUXEON C Application Brief AB

7 1.6. Electrical Isolation The bottom pads are not electrically isolated. They provide dual functions of removing heat and conducting electrical current. There is an exposed copper trace (cathode) on the long side (both edges) of the LED package as shown in Figure 6. cathode Figure 6. Side view of package Mechanical 3-D The 3-D *.igs and *.stp file formats are available from Philips Lumileds website 2. Board Design Rules 2.1. PCB Requirements LUXEON C can be mounted on FR4, IMS (insulated metal substrate) or flex circuit board. In this report, we will focus on FR4 design since this is an industry standard PCB technology and offers a low cost economical solution. A good reference to FR4 board design consideration can be found in Philips Lumileds application brief AB32 document LUXEON LED Assembly and Handling Information which describes this in greater detail. Heat is dissipated from the chip via both the anode and cathode pads. Having an optimal design to remove heat from both pads is required. The package thermal resistance as defined in the datasheet refers to heat flow out from both electrodes. As a note, the FR4 LUXEON C basic footprint can be used on IMS or flex circuit board if desired. LUXEON C Application Brief AB

8 2.2. FR4 PCB Design Considerations A typical thermal load of one watt may be dissipated by LUXEON C when operating at 350 ma DC current. The FR4 designs below consider several board design rules that balance performance and cost needs. Ultimately, it is the customer s responsibility to determine the final design. Below is a list of considerations when designing FR4 boards with dual copper layers. Effect of thermal vias: FR4 board with no thermal vias will have the highest thermal resistance. Adding more vias will reduce the thermal resistance but may increases the FR4 board cost. At some point, the additional thermal vias added will only marginally improve the thermal resistance. Figure 7 shows the simulation data for the thermal resistance of junction-to-heatsink (Rth j-hs ) of a 0.8 mm thick FR4 board with 20 um copper plating inside the via holes (open via type) and 50 um top and bottom copper plating (Figure 8). Solderability indicator pads Open vias Rth j-hs = 50 K/W 30 K/W 25 K/W WITH solderability indicator pads Rth j-hs = 51 K/W 27 K/W 26 K/W WITHOUT Solderability indicator pads Figure 7. Simulated thermal resistance of thermal vias (open type). LUXEON C Application Brief AB

9 The solderability indicator pads provide visual proof of effective solder reflow on all pads. They can also be used to electrically probe the LED for analysis. In addition, they allow one to attach a thermocouple and indirectly estimate the LED junction temperature. The reflow placement accuracy and thermal resistance are not affected if the solderability indicator pads are not present. The copper area around the LUXEON C electrodes has a significant contribution to the thermal spreading. We recommend that the minimum copper area extends 4 mm beyond the electrodes. Any extension beyond 4 mm will not significantly lower the thermal resistance. Type of thermal vias: The filled and capped vias as described in AB32 will yield the best thermal performance. However the FR4 cost will be higher than with the open vias design. The recommended open vias design is shown in Figure 8 below with finished via diameter of 0.5 mm. Copper plated hole Figure 8. Cross section of FR4 with open vias. Effect of plating thickness: As described in AB32, having thicker copper plating on the top and bottom of FR4 and inside the vias improves the thermal resistance performance. LUXEON C Application Brief AB

10 2.3. Examples of LUXEON C Footprint Figure 9 below shows the various FR4 board designs for LUXEON C. For boards with open vias, the bottom copper layers of FR4 must be divided into two sections to prevent electrical shorting via the electrically (and thermally) conducting vias. Note: a white solder mask may be used instead of the usual green solder mask. The white layer enhances reflectivity, but is optional. Copper layers Outline of package LED chip Solder mask Stencil mask Anode pad Cathode pad Thermal vias Copper layers at bo om of the FR4 board must be separated into two sec ons as indicated by the pink Front Back Photos of front and back of a completed FR4 board with white solder mask. Note that the copper layers at the back of the FR4 are separated to prevent shorting. Figure 9. LUXEON C footprint. LUXEON C Application Brief AB

11 Figure 9, continued. LUXEON C footprint. The FR4 footprint drawings are available from Philips Lumileds website, LUXEON C Application Brief AB

12 3. Assembly Process Recommendation 3.1. Stencil Design Figure 10 shows the outline drawing of LUXEON C electrodes. Figure 10. LUXEON C footprint. Figure 11 shows the recommended stencil layout of the solder pads for both with and without the solderability indicators. The drawings are available from Philips Lumileds website, Legend red line - copper (top) green line - solder mask blue line - stencil mask Figure 11. Stencil layout. LUXEON C Application Brief AB

13 3.2. Solder Paste Philips Lumileds recommends no-clean lead-free solder paste of SnAgCu (SAC 305) metal alloy. Some examples of solder pastes that Philips Lumileds has tested are Alpha Metals OM338 grade 3 and OM 325 grade Pick and Place LUXEON C series devices are compatible with high speed pick and place operations. For minimal risk of damage to the top silicone overcoat surfaces of these devices we recommend the use of a rubber tip pick-up tool. Below are the rubber material properties provided by Micro- Mechanics, Malaysia. High Temperature Rubber 810 Spec No. : Part No. : HTR1 and HTR3 Series Description : High Temp Rubber Pick-Up-Tool (Black and Grey Color Respectively) Physical Properties: Table 1. Physical Properties of High Temperature Rubber 810. a) Hardness : 80 Shore A at 23 C ASTM K-6301 Method b) Heat Resistance : 280 C to 300 C Maximum ASTM K-6301 Method c) Low Temperature Resistance : -40 C to -50 C Minimum ASTM K-6301 Method d) Tensile Strength : 165 Kgf/cm 2 at 23 C ASTM K-6301 Method e) Elongation : 210% at 23 C ASTM K-6301 Method f) Thermal Conductivity : cal/(cm.sec. C) ASTM K-6301 Method Electrical Properties: g) Surface Resistivity : 2.3 x ohm at 23 C ASTM D-257 Method Published date: Sep 20, 2001 Below is the Micro-Mechanic Malaysia contact information: mmmalaysia@micro-mechanics.com Telephone: Website: LUXEON C Application Brief AB

14 Pick and place capabilities have been successfully demonstrated with the use of a metal tip tweezers using the following parameters as shown in Table 2. Note that this was derived using the Universal Instruments Corporation HSP4796 chip placement machine. For other chip placement machines, Philips Lumileds recommends process optimization to be carried out first using these initial setup parameters. Table 2. Pick and Place Parameters. Tip Material Nozzle External Placement Speed Placement Force Diameter (mm) (sec) (grams) Rubber Metal Figure 12 below illustrates the pick and place mechanism. We recommend a vacuum force of 0.7 bar when the pick-up (collet) tip touches the top silicone surface. For placement of LEDs, a purge force between 1.0 bar to 1.5 bar when downward force reaches 50 grams to 100 grams maximum is recommend. Picking Specifica on Placing Specifica on Lower collet un l it just touches the device surface. Do not apply any pressure on device surface when lowering the collet Reel Collet with rubber p Solder paste Lower collet un l it pushes the device into the solder paste with about 50g (100g max) force PCB Vacuum force: create 0.7 bar vacuum when collet touches device surface Purge force: create 1.0 to 1.5 bar when downward force reaches 50g (100g max) Figure 12. Pick and place mechanism. LUXEON C Application Brief AB

15 Below is the recommended collet tip design from Micro-Mechanics. NOT USED Figure 13. Recommended collet tips. LUXEON C Application Brief AB

16 To prevent fallout of units after the cover tape is removed, add a shutter as shown in Figure 14 below to your feeder section of the pick and place machine. We recommend a gap of 0.25 to 0.30 mm to avoid damage to the top surface of the parts during indexing and also at the same time preventing the parts from tilting over. The pick-up nozzle area is the located a few pockets away from the cover tape peeling point. Figure 14. Feeder optimization to prevent fallout of units FR4 Board Handling The substrate of LUXEON C is made of ceramic which is a relatively brittle material. Excessive bending will lead to cracks in the ceramic substrate or cause the solder joint to break. Even though this product has a small form factor and is unlikely to cause any problems, it is worth noting that any bending force should be kept to minimum. Figure 15 below shows what forces are applied to the package when a flat assembled board is bent. This can happen for example when punchingoff or breaking-off LED strips of a PCB panel (Figure 16). Figure 15. Forces acting onto package when board is bent. LUXEON C Application Brief AB

17 Board panel LED strips Figure 16. PCB panel consisting of several strips of LEDs.. Board warping after reflow may occur when boards with different CTE (coefficient of thermal expansion) layers are applied to the top and bottom. When the bent board is secured to a flat surface, a vertical upward force is applied to the ceramic, if this force is high enough may cause the ceramic to crack. Figure 17. Securing a bent board may cause ceramic cracking. Board warpage can be minimized by understanding how different CTE materials are being stacked up. For example, when a FR4 board is sandwiched between two full copper sheets, adding copper isolation (to create islands of copper) can minimize board warping as shown in Figure 18. To minimize LED ceramic package cracking when the board does warp, orientate the package such that the long side of the package is perpendicular to the warpage. Copper isola on Figure 18. Reducing board warpage by creating copper islands. Side note: when handling flex circuit boards, extra care must be taken as not to break the solder joints or package due to excessive bending of the board. LUXEON C Application Brief AB

18 4. Product Packaging Consideration Chemical Compatibility LUXEON C contains a silicone overcoat to protect the LED chip. As with most silicones used in LED optics, care must be taken to prevent any incompatible chemicals from directly or indirectly reacting with the silicone. The silicone overcoat in the LUXEON C is gas permeable. Consequently, oxygen and volatile organic compound (VOC) gas molecules can diffuse into the silicone overcoat. VOCs may originate from adhesives, solder fluxes, conformal coating materials, potting materials and even some of the inks that are used to print the PCBs. Some VOCs and chemicals react with silicone and produce discoloration and surface damage. Other VOCs do not chemically react with the silicone material directly but diffuse into the silicone and oxidize during the presence of heat or light. Regardless of the physical mechanism, both cases may affect the total LED light output. Since silicone permeability increases with temperature, more VOCs may diffuse into and/or evaporate out from the silicone. Careful consideration must be given to whether LUXEON C are enclosed in an air tight environment or not. In an air tight environment, some VOCs that were introduced during assembly may permeate and remain in the silicone lens. Under heat and blue light, the VOCs inside the silicone lens may partially oxidize and create a silicone discoloration, particularly on the surface of the LED where the flux energy is the highest. In an air rich or open air environment, VOCs have a chance to leave the area (driven by the normal air flow). Transferring the devices which were discolored in the enclosed environment back to open air may allow the oxidized VOCs to diffuse out of the silicone lens and may restore the original optical properties of the LED. Determining suitable threshold limits for the presence of VOCs is very difficult since these limits depend on the type of enclosure used to house the LEDs and the operating temperatures. Also, some VOCs can photo-degrade over time. Table 3 provides a list of commonly used chemicals that should be avoided as they may react with the silicone material. Note that Philips Lumileds does not warrant that this list is exhaustive since it is impossible to determine all chemicals which may affect the performance of LUXEON C. Figure 19 below illustrates the silicone permeability in an unenclosed system of LUXEON C. VOC molecule evaporates VOC molecule Silicone permeability allows the VOC gas molecules to diffuse into and out the dome.. Figure 19. Illustration of gas permeation into and out of silicone. LUXEON C Application Brief AB

19 Table 3. List of commonly used chemicals that will damage the silicone overcoat layer of a LUXEON C. Avoid using any of these chemicals in the housing that contains the LED package. Chemical Name hydrochloric acid sulfuric acid nitric acid acetic acid sodium hydroxide potassium hydroxide ammonia MEK (Methyl Ethyl Ketone) MIBK (Methyl Isobutyl Ketone) Toluene Xylene Benzene Gasoline Mineral spirits dichloromethane tetracholorometane Castor oil lard linseed oil petroleum silicone oil halogenated hydrocarbons (containing F, Cl, Br elements) rosin flux acrylic tape Normally used as acid acid acid acid alkali alkali alkali solvent solvent solvent solvent solvent solvent solvent solvent solvent oil oil oil oil oil misc solder flux adhesive The chemicals in Table 3 are typically not directly used in the final products that are built around LUXEON LEDs. However, some of these chemicals may be used in intermediate manufacturing steps (e.g. cleaning agents). Consequently, trace amounts of these chemicals may remain on (sub) components, such as PCBs. Philips Lumileds, therefore, recommends the following precautions when designing your application: - When designing secondary lenses to be used over a single LED, provide a sufficiently large air-pocket and allow for ventilation of this air away from the immediate vicinity of the LED. - Use mechanical means of attaching lenses and circuit boards as much as possible. When using adhesives, potting compounds and coatings, carefully analyze its material composition and do thorough testing of the entire fixture under High Temperature over Life (HTOL) conditions. LUXEON C Application Brief AB

20 Company Information Philips Lumileds is a leading provider of LEDs for everyday lighting applications. The company s records for light output, efficacy and thermal management are direct results of the ongoing commitment to advancing solid-state lighting technology and enabling lighting solutions that are more environmentally friendly, help reduce CO 2 emissions and reduce the need for power plant expansion. Philips Lumileds LUXEON LEDs are enabling never before possible applications in outdoor lighting, shop lighting, home lighting, consumer electronics, and automotive lighting. Philips Lumileds is a fully integrated supplier, producing core LED material in all three base colors, (Red, Green, Blue) and white. Philips Lumileds has R&D centers in San Jose, California and in the Netherlands, and production capabilities in San Jose, Singapore and Penang, Malaysia. Founded in 1999, Philips Lumileds is the high flux LED technology leader and is dedicated to bridging the gap between solid-state technology and the lighting world. More information about the company s LUXEON LED products and solid-state lighting technologies can be found at Although PHILIPS LUMILEDS LIGHTING COMPANY has attempted to provide the most accurate information and materials and services data (hereinafter Data ), the Data is provided as is and may contain errors. The entire risk of use of the data shall be with the user. PHILIPS LUMILEDS LIGHTING COMPANY makes no warranty, express or implied, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose, regarding the contents or correctness of the Data provided or the ability of the Data to meet the user s needs or expectations. PHILIPS LUMILEDS LIGHTING COMPANY reserves the right to make changes without notice. You as user agree to this disclaimer and user agreement with the download or use of the provided materials and Data. In no event shall PHILIPS LUMILEDS LIGHTING COMPANY be liable for any direct, indirect, special, incidental, exemplary, or consequential damages arising out of or related to the use of the Data, however caused, regardless of theory of liability, and whether or not PHILIPS LUMILEDS LIGHTING COMPANY has been advised of the possibility of such damage. This limitation shall apply notwithstanding any failure of essential purpose or any exclusive remedy Philips Lumileds Lighting Company. All rights reserved. Product specifications are subject to change without notice.