AN2053 PCB Mounting Practices to Obtain Optimum Performance for High-Voltage BGA and LGA Packages Author: Rohit Tirumala Microchip Technology Inc. INTRODUCTION The plastic, or over-molded, Ball Grid Array (BGA) is a lead-less, low-profile surface-mount package. It offers a high number of potential input and output connections. Lead-less packages provide short, heavy copper (Cu) paths with low inductance (L) and resistance (R) connections from packages to Printed Circuit Board (PCB), enabling high-speed operation with low switching and conduction losses. Furthermore, the short vertical structure provides very low RJC (junction-to-case or die-to-ball thermal impedance) for high-power dissipation. The BGA package was developed for high density and high-speed digital devices, but has been increasingly used for packaging power devices, both single and multi-chip. Figure 1 shows an example of a 36-terminal BGA package outline. Power packages typically require fewer connections, but do require higher current connections. Therefore, power BGA packages have larger copper land areas with multiple solder balls for higher current connections and lower RJC. Even lower package RJC, L and R are achieved with Land Grid Array (LGA) packages. The LGA replaces solder balls/pads with larger area solder pads. BGA ball arrays are usually standard ball sizes and pitch, whereas LGA pads are almost always custom to maximize package-to-PCB contact areas for each product. Figure 2 shows an example of a 3-terminal LGA package outline. Side View Side View Bottom View Bottom View FIGURE 2: 3-Terminal LGA Package Outline Drawing Example. FIGURE 1: 36-Terminal BGA Package Outline Drawing Example. 2015 Microchip Technology Inc. DS00002053A-page 1 AN2053 Plastic BGA and LGA devices are molded packages with an internal PCB substrate for die/component assembly. The PCB substrate is typically multilayer, with copper trace layers and interconnecting copper vias optimized for electrical and thermal performance by the BGA and LGA device manufacturers. Figure 3 shows an example of a 36-terminal BGA Digital IC package structure, and Figure 4 shows an example of a 3-terminal LGA power transistor package. Molding Compund Die and Wire Bonds BGA Substrate BGA AND LGA ASSEMBLY MATERIALS, TECHNIQUES AND CONFIGURATIONS The BGA package can accommodate many solder ball materials, size and pitch, but in practice there are only a few common/standard ball solders, sizes and pitches. Solder balls are usually Lead-Free (Tin/Silver or Tin/Silver/Copper) and Eutectic Tin/Lead. Pb-Free solders are generally preferred, but Sn/Pb solders are still common for compatibility with older processes and components and for some lower temperature devices. The same or similar type solder pastes are typically used for BGA and LGA assembly to the PCB. Standard ball pitch/diameters and associated ball separations are given in Table 1. TABLE 1: Cross Section FIGURE 3: 36-Terminal BGA Digital IC Package Structure Example. Molding Compund Die and Wire Bonds BGA Substrate Cross Section FIGURE 4: 3-Terminal LGA Power Transistor Package Structure Example. STANDARD BALL PITCH, DIAMETERS AND SEPARATIONS FOR BGA PACKAGES Ball Pitch (mm) Ball Diameter (mm) Ball Separation (mm) 0.75 0.40 0.35 1.00 0.40 0.60 1.00 0.50 0.50 1.00 0.60 0.40 1.27 (50 mil) 0.51 (20 mil) 0.76 (30 mil) 1.27 (50 mil) 0.64 (25 mil) 0.63 (~25 mil) The larger LGA pads are usually coated with the same solders (Pb-free or Sn/Pb), but the lower profile solder on the pads does not contain solder balls. LGA pads do not have standard sizes, and are designed/optimized to accommodate specific products and applications. BGA and LGA users must build on these electrical and thermal advantages with suitable Solder Mask Defined (SMD) solder connections to the PCBs and optimum electrical and thermal PCB design. This application note will provide recommendations for BGA and LGA PCB design and assembly techniques. DS00002053A-page 2 2015 Microchip Technology Inc. AN2053 PCB Solder Pads for BGA and LGA Solder and terminal pads located on BGA/LGA PCB are either SMD or Non-Solder Mask Defined (NSMD), and sometimes both SMD and NSMD on the same PCB. NSMD are generally preferred and more accurate/reliable, but SMD accommodate larger land areas for multi-ball connections, and can accommodate higher currents and power dissipation. Figure 5 shows a PCB SMD Terminal Pad/Resist layout and cross-section. Figure 6 shows a PCB NSMD Terminal Pad/Resist layout and cross section. Cu Resist Opening Top View Resist Resist Cu PCB Cross Section FIGURE 5: PCB SMD Terminal Pad/Resist Layout. • Recommended SMD PCB resist opening diameters are 0.075 – 0.10 mm less than ball diameters. • Recommended NSMD PCB pad and resist openings are given in Table 2. • Recommended LGA solder resists opening are 0.05 mm larger than the LGA pads, but custom pad sizes and shapes for LGA often require different rules, which are usually provided by the LGA device manufacturer. TABLE 2: RECOMMENDED NSMD PCB PAD AND RESIST OPENINGS Ball Pitch/Diameter (mm) Solder Resist Opening Diameter (mm) Solder Pad Diameter (mm) 0.75/0.40 0.45 0.30 1.00/0.40 0.45 0.30 1.00/0.50 0.50 0.35 1.00/0.60 0.60 0.40 1.27/0.51 0.75 0.60 1.27/0.64 0.80 0.65 Standard recommendations are generally suited for lower voltages between pads, and larger spacing may be required at higher voltages. Higher voltage (HV) minimum pad spacing guidelines should be compliant with UL Pollution degree 1 minimum creepage paths, assuming a suitable HV underfill (see Table 3 for a condensed list of creepage distances). The HV device manufacturer will typically recommend optimum PCB pads for attaching BGA and LGA products, but these may vary for some user applications. TABLE 3: Operating Voltage (Volts, AC RMS or DC) Cu Resist Opening Top View Resist Cu Resist PCB Cross Section FIGURE 6: Resist Layout. PCB NSMD Terminal Pad 2015 Microchip Technology Inc. CREEPAGE DISTANCES FOR POLLUTION LEVEL 1 FROM UL840 Minimum Creepage Distance (mm) 80 0.22 100 0.25 160 0.32 200 0.42 250 0.56 320 0.75 400 1.00 Example of underfill materials are: • Loctite® 3153™ for BGA packages • Hysol® E1926 for LGA packages DS00002053A-page 3 AN2053 Printed Circuit Boards for BGA and LGA STENCIL AND SOLDER PASTE PCBs are typically multilayer to accommodate interconnects for higher density balls and pads. The copper pads and traces on different layers are interconnected with copper vias. These vias create both electrical and thermal connections between the layers. Vias at pads and under balls must be filled or tented to provide flat and solderable pad surfaces. This is especially important for BGA pads, which must ensure accurate ball locations. Solder pastes are usually Pb-Free (Sn/Ag/Cu), are Eutectic Sn/Pb, and it is recommended that Pb-Free solder paste be used with Pb-Free solder balls and that Sn/Pb solder paste be used with Sn/Pb solder balls. Mixing paste and ball solder types is possible, but should be done cautiously. High glass transition (Tg) FR-4 laminate materials are most common and economical, but other higher performance, high-Tg laminates may be used to meet special electrical, mechanical or thermal requirements. In general, the laminate Tg should be greater than +170°C. The BGA and LGA solder pads are copper, and a number of different finishes are available with differences in cost, shelf life and long-term reliability. Common finishes are Organic Solderability Preservatives (OSP), Hot Air Leveling, Sn/Pb (HASL), Electroless Sn, Electroless Ag and Electroless Ni/Au. The choice of pad finish must be made based on specific products, applications and the PCB fabricators capabilities. Soldering BGA and LGA Packages to Printed Circuit Boards The key processes are: 1. 2. 3. 4. 5. Stencil solder paste onto PCB pads Place BGA/LGA into solder paste Reflow the solder Clean and remove flux Add underfill. There are variations and options for specific materials, equipment, devices and assemblers, but these processes are used for almost all BGA and LGA assembly. DS00002053A-page 4 Solder pastes should be stencil grade with high solids/solder (typically 90% by weight). No-clean solder pastes are preferred, but flux removal/cleaning is possible. There are many types of flux systems including aqueous, semi-aqueous and solvent based. New and complex cleaning chemistries are very specialized, and the cleaning chemistry recommended by the solder paste manufacturers is usually the best option. Stainless steel stencils are recommended, with the stencil apertures and thicknesses shown in Table 4. These may vary based on the solder paste and print parameters. TABLE 4: STENCIL APERTURES AND THICKNESSES FOR BGA PACKAGES Ball Pitch/Diameter (mm) Stencil Aperture Diameter (mm) Stencil Thickness (mm) 0.75/0.40 0.35 0.125 1.00/0.40 0.35 0.125 1.00/0.50 0.40 0.150 1.00/0.60 0.45 0.150 1.27/0.51 0.55 0.150 1.27/0.64 0.60 0.150 PLACEMENT OF BGA/LGA INTO SOLDER PASTE The placement of BGA and LGA must be automatic to ensure the typical requirements of ±0.10 mm, which is within the ±0.05 mm accuracy of common pick and place equipment. 2015 Microchip Technology Inc. AN2053 REFLOW SOLDERING The recommended solder reflow equipment is Forced Air Convection (FAC), or a combination of FAC and Infrared (IR). Other types of equipment can achieve the required profile, but are less common and require special care to control. There are two standard profiles solder reflow profiles: one for Pb-Free (Sn/Ag and Sn/Ag/Cu) solder, and the other for Eutectic Sn/Pb solder. The recommended profiles are shown in Figure 7 and Figure 8, respectively. In general, it is recommended to begin with the solder paste manufacturers’ recommended profile, which addresses their solder composition, solder particle size/shape, and organic system. Any recommended profiles are a starting point, and must be adjusted to match the requirements of specific products and assembly processes. Factors to consider include the product size and mass, the type of assembly equipment, carriers and more. The nominal peak temperatures are +245°C for Pb-Free solder and +225°C for Eutectic Sn/Pb solder. 300 Temperature (C) 250 Typical Minimum Maximum 200 150 100 50 0 Time (s) FIGURE 7: Typical Pb-Free (Sn/Ag and Sn/Ag/Cu) Solder Reflow Profile. 300 Temperature (C) 250 Typical Minimum Maximum 200 150 100 50 0 Time (s) FIGURE 8: Typical Eutectic Sn/Pb Solder Reflow Profile. 2015 Microchip Technology Inc. DS00002053A-page 5 AN2053 CLEAN AND REMOVE FLUX BGA AND LGA SOLDERING REWORK Cleaning and baking are recommended, and may be advantageous even for no-clean solder pastes. Solder pastes containing fluxes require more aggressive cleaning materials and processes. The specific cleaning chemistry and equipment required is dependent on the solder paste flux and organic chemistry, and the solder paste manufacturers’ recommendations are the best starting point for determining cleaning chemistry and associated cleaning equipment. Again, any cleaning process will need to be optimized for specific solder reflow processes and products. Although rework should not be required in a well-controlled assembly process, it is possible to remove and replace BGA and LGA packages. However, it is not possible to rework individual solder joints. Removal is usually accomplished by heating locally below the device to be removed and removing the device after the solder melts. Before replacing the device with a new BGA/LGA, the pads must be clean and flat. The replacement device will typically need added flux to hold during automatic placement, and should also be heated locally from the back of the PCB. Any rework should be performed before underfill. UNDERFILL FOR BGA AND LGA Underfill is recommended for all BGA and LGA assemblies, and it should be used automatically unless there is a good reason not to use it. It is even more important with larger packages, higher power dissipation and higher voltages. It can: • Provide mechanical strength against vibration, coefficient of thermal expansion (CTE) mismatch and other stresses • Improve heat transfer to the PCB • Eliminate dirt, moisture and other contaminates that can collect under the BGA and LGA packages initially and over the life of the product • Eliminate high-voltage arcing or leakage between pads, and allow reduced spacing between pads. The minimum spacing with underfill should meet UL Pollution degree 1 minimum creepage, but specific products and application conditions may allow tighter spacing or require larger spacing. BGA AND LGA RECOMMENDATIONS DISCLAIMER The BGA and LGA recommendations are based on typical products, materials, applications and assembly processes/equipment. Although these recommendations will often be sufficient, they should always be validated for the specific products and product requirements. This is especially important for high-power and high-voltage products, which can have unique requirements. Underfills are very specialized with specific formulations for BGA and LGA. Their viscosity is adjusted for specific space height to make underfill application easy and virtually void free. It is generally applied along one or two adjacent edges of the BGA and LGA package, where the surface tension pulls the underfill under the package while pushing air out the opposite edges. INSPECTION OF BGA/LGA PACKAGES The BGA and LGA solder pads are hidden under the packages and cannot be visually inspected. However, current solders pastes, stencils and placement equipment are well controlled and can provide very high assembly yields. The yields are generally verified with 100% testing, and sometimes supported by X-ray inspection. X-rays are well suited for sample checking and analysis of solder defects. DS00002053A-page 6 2015 Microchip Technology Inc. Note the following details of the code protection feature on Microchip devices: • Microchip products meet the specification contained in their particular Microchip Data Sheet. • Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. • There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. • Microchip is willing to work with the customer who is concerned about the integrity of their code. • Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.” Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. Information contained in this publication regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. Microchip disclaims all liability arising from this information and its use. Use of Microchip devices in life support and/or safety applications is entirely at the buyer’s risk, and the buyer agrees to defend, indemnify and hold harmless Microchip from any and all damages, claims, suits, or expenses resulting from such use. No licenses are conveyed, implicitly or otherwise, under any Microchip intellectual property rights unless otherwise stated. Trademarks The Microchip name and logo, the Microchip logo, dsPIC, FlashFlex, flexPWR, JukeBlox, KEELOQ, KEELOQ logo, Kleer, LANCheck, MediaLB, MOST, MOST logo, MPLAB, OptoLyzer, PIC, PICSTART, PIC32 logo, RightTouch, SpyNIC, SST, SST Logo, SuperFlash and UNI/O are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. The Embedded Control Solutions Company and mTouch are registered trademarks of Microchip Technology Incorporated in the U.S.A. Analog-for-the-Digital Age, BodyCom, chipKIT, chipKIT logo, CodeGuard, dsPICDEM, dsPICDEM.net, ECAN, In-Circuit Serial Programming, ICSP, Inter-Chip Connectivity, KleerNet, KleerNet logo, MiWi, motorBench, MPASM, MPF, MPLAB Certified logo, MPLIB, MPLINK, MultiTRAK, NetDetach, Omniscient Code Generation, PICDEM, PICDEM.net, PICkit, PICtail, RightTouch logo, REAL ICE, SQI, Serial Quad I/O, Total Endurance, TSHARC, USBCheck, VariSense, ViewSpan, WiperLock, Wireless DNA, and ZENA are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. SQTP is a service mark of Microchip Technology Incorporated in the U.S.A. Silicon Storage Technology is a registered trademark of Microchip Technology Inc. in other countries. GestIC is a registered trademark of Microchip Technology Germany II GmbH & Co. KG, a subsidiary of Microchip Technology Inc., in other countries. All other trademarks mentioned herein are property of their respective companies. © 2015, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. ISBN: 978-1-5224-0128-5 QUALITY MANAGEMENT SYSTEM CERTIFIED BY DNV == ISO/TS 16949 == 2015 Microchip Technology Inc. Microchip received ISO/TS-16949:2009 certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona; Gresham, Oregon and design centers in California and India. The Company’s quality system processes and procedures are for its PIC® MCUs and dsPIC® DSCs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory and analog products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001:2000 certified. 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