w WAN_0202 Guidelines on How to Use W-CSP Packages and Create Associated PCB Footprints INTRODUCTION The Wolfson Wafer level ChipScale Package (W-CSP) is a die-sized package, which obtains electrical contact via solder bumps on the bottom surface of the device to a Printed Circuit Board (PCB). But it differs from Ball Grid Array (BGA) package by using Redistribution Layer (RDL) instead of bonding wire to connect the die pads. This Application note sets out to explain some of the issues related to the design of a PCB footprint for the W-CSP package and then goes on to describe practical considerations when soldering the device to a PCB. PACKAGE DIMENSIONS The first thing to consider when creating a W-CSP footprint is the package drawing for the device. Wolfson uses a number of different W-CSP packages based on JEDEC specifications as follows in Table 1. Current W-CSP package I/O designs include from sixteen up to 42 solderable terminals but the max terminal count is possibly increasing to 105. See Figures 1 – 4. Package dimensions are shown in Figures 5 – 8. The dimensions of the packages are, at time of document release, as follows. Please refer to the individual datasheets for the most current issue of the package dimensions. SIZE PIN BALL PITCH 1.640mm×1.640mmX0.7mm 16 0.4mm 2.552mmX2.602MMX0.7mm 25 0.5mm 2.590mm×2.500mmX0.7mm 28 0.4mm 3.226mm×3.440mmX0.7mm 42 0.5mm Table 1 WLCSP Package Range Figure 1 16 PIN WLCSP Package WOLFSON MICROELECTRONICS plc To receive regular email updates, sign up at http://www.wolfsonmicro.com/enews/ July 2008, Rev 1.0 Copyright ©2008 Wolfson Microelectronics plc WAN_0202 Figure 2 25 PIN W-CSP Package Figure 3 28 PIN W-CSP Package Figure 4 42 PIN W-CSP Package w July 2008, Rev 1.0 2 WAN_0202 B: 16 BALL W-CSP PACKAGE 1.640 X 1.640 X 0.7mm BODY, 0.40 mm BALL PITCH DM052.A 3 D A DETAIL 1 2 G 3 4 A2 2 1 A1 CORNER 4 A B e E1 5 E C D 2X e D1 DETAIL 2 2X 0.10 Z 0.10 Z TOP VIEW BOTTOM VIEW f SOLDER BALL f bbb Z h 1 Z A1 DETAIL 2 Dimensions (mm) Symbols A A1 A2 D D1 E E1 e f g MIN 0.615 0.195 0.385 0.091 0.035 h NOM 0.7 0.220 0.410 1.640 BSC 1.200 BSC 1.640 BSC 1.200 BSC 0.400 BSC MAX 0.785 0.245 0.435 0.070 0.105 NOTE 5 0.260 BSC NOTES: 1. PRIMARY DATUM -Z- AND SEATING PLANE ARE DEFINED BY THE SPHERICAL CROWNS OF THE SOLDER BALLS. 2. THIS DIMENSION INCLUDES STAND-OFF HEIGHT ‘A1’ AND BACKSIDE COATING. 3. A1 CORNER IS IDENTIFIED BY INK/LASER MARK ON TOP PACKAGE. 4. BILATERAL TOLERANCE ZONE IS APPLIED TO EACH SIDE OF THE PACKAGE BODY. 5. ‘e’ REPRESENTS THE BASIC SOLDER BALL GRID PITCH. 6. THIS DRAWING IS SUBJECT TO CHANGE WITHOUT NOTICE. 7. FOLLOWS JEDEC DESIGN GUIDE MO-211-C. Figure 5 16 pin W-CSP Package w July 2008, Rev 1.0 3 WAN_0202 B: 25 BALL W-CSP PACKAGE 2.552 X 2.602 X 0.7mm BODY, 0.50 mm BALL PITCH DM056.A 6 D DETAIL 1 A 2 g 5 A2 3 4 2 1 A A1 CORNER 4 B C E1 e E 5 D E 2X e DETAIL 2 2X D1 0.10 Z 0.10 Z TOP VIEW BOTTOM VIEW f1 SOLDER BALL f2 bbb Z h 1 Z A1 DETAIL 1 DETAIL 2 Symbols A A1 A2 D D1 E E1 e f1 MIN 0.615 0.225 0.355 Dimensions (mm) NOM MAX 0.7 0.785 0.250 0.275 0.405 0.380 2.552 BSC 2.00 BSC 2.602 BSC 2.00 BSC 0.50 BSC f2 0.266 BSC 0.291 BSC g 0.035 h 0.070 NOTE 5 0.105 0.314 BSC NOTES: 1. PRIMARY DATUM -Z- AND SEATING PLANE ARE DEFINED BY THE SPHERICAL CROWNS OF THE SOLDER BALLS. 2. THIS DIMENSION INCLUDES STAND-OFF HEIGHT ‘A1’ AND BACKSIDE COATING. 3. A1 CORNER IS IDENTIFIED BY INK/LASER MARK ON TOP PACKAGE. 4. BILATERAL TOLERANCE ZONE IS APPLIED TO EACH SIDE OF THE PACKAGE BODY. 5. ‘e’ REPRESENTS THE BASIC SOLDER BALL GRID PITCH. 6. THIS DRAWING IS SUBJECT TO CHANGE WITHOUT NOTICE. 7. FOLLOWS JEDEC DESIGN GUIDE MO-211-C. Figure 6 25 pin W-CSP Package w July 2008, Rev 1.0 4 WAN_0202 B: 28 BALL W-CSP PACKAGE 2.590 X 2.500 X 0.7mm BODY, 0.40 mm BALL PITCH DM054.A 6 D DETAIL 1 A 2 G 6 A2 5 3 4 2 1 A A1 CORNER 4 B e 5 C E1 E D E F 2X e DETAIL 2 2X D1 0.10 Z 0.10 Z TOP VIEW BOTTOM VIEW f1 f2 bbb Z h 1 Z A1 DETAIL 2 Dimensions (mm) Symbols A A1 A2 D D1 E E1 e f1 f2 g MIN 0.615 0.195 0.385 NOM 0.7 0.220 0.410 2.590 BSC 2.000 BSC 2.500 BSC 2.000 BSC 0.400 BSC MAX 0.785 0.245 0.435 0.070 0.105 NOTE 5 0.275 0.230 0.035 h 0.260 BSC NOTES: 1. PRIMARY DATUM -Z- AND SEATING PLANE ARE DEFINED BY THE SPHERICAL CROWNS OF THE SOLDER BALLS. 2. THIS DIMENSION INCLUDES STAND-OFF HEIGHT ‘A1’ AND BACKSIDE COATING. 3. A1 CORNER IS IDENTIFIED BY INK/LASER MARK ON TOP PACKAGE. 4. BILATERAL TOLERANCE ZONE IS APPLIED TO EACH SIDE OF THE PACKAGE BODY. 5. ‘e’ REPRESENTS THE BASIC SOLDER BALL GRID PITCH. 6. THIS DRAWING IS SUBJECT TO CHANGE WITHOUT NOTICE. 7. FOLLOWS JEDEC DESIGN GUIDE MO-211-C. Figure 7 28 pin W-CSP Package w July 2008, Rev 1.0 5 WAN_0202 B: 42 BALL W-CSP PACKAGE 3.226 X 3.440 X 0.7mm BODY, 0.50 mm BALL PITCH DM049.C 6 D DETAIL 1 A 2 G 6 A2 5 3 4 2 1 A A1 CORNER 4 B C e E1 D E 5 E F G 2X e DETAIL 2 2X D1 0.10 Z 0.10 Z TOP VIEW BOTTOM VIEW f SOLDER BALL f bbb Z h 1 Z A1 DETAIL 2 Symbols A A1 A2 D D1 E E1 e f g MIN 0.615 0.225 0.355 Dimensions (mm) NOM MAX 0.7 0.785 0.250 0.275 0.405 0.380 3.226 BSC 2.500 BSC 3.440 BSC 3.00 BSC 0.50 BSC 0.060 BSC 0.035 h 0.070 NOTE 5 0.105 0.315 BSC NOTES: 1. PRIMARY DATUM -Z- AND SEATING PLANE ARE DEFINED BY THE SPHERICAL CROWNS OF THE SOLDER BALLS. 2. THIS DIMENSION INCLUDES STAND-OFF HEIGHT ‘A1’ AND BACKSIDE COATING. 3. A1 CORNER IS IDENTIFIED BY INK/LASER MARK ON TOP PACKAGE. 4. BILATERAL TOLERANCE ZONE IS APPLIED TO EACH SIDE OF THE PACKAGE BODY. 5. ‘e’ REPRESENTS THE BASIC SOLDER BALL GRID PITCH. 6. THIS DRAWING IS SUBJECT TO CHANGE WITHOUT NOTICE. 7. FOLLOWS JEDEC DESIGN GUIDE MO-211-C. Figure 8 42 pin W-CSP Package w July 2008, Rev 1.0 6 WAN_0202 PACKAGE CONSTRUCTION A cross-section of a typical RDL W-CSP is given in Figure 9. RDL W-CSP construction consists of: - Die with bond pad and original passivation - Passivation 1 - Metallization etch (RDL metal) to move solder bumps from peripheral bond pads to any bump array pattern - Passivation 2 - Under Bump Metal (UBM) contact - Ball drop 250/300µm solder sphere (solder composition: SnAg(3%)Cu(0.5%)) Solder ball RDL metal RDL Passivation 2 UBM RDL Passivation 1 Bond pad Si Wafer Die Original Passivation Protective surface coating Figure 9 RDL W-CSP Cross-Section w July 2008, Rev 1.0 7 WAN_0202 PCB DESIGN CONSIDERATIONS The PCB design rules when using the CSP packages are fundamentally no different to those for BGA devices of the same pitch. The most important considerations are those effects that occur during the PCB assembly process, which do affect the PCB land layout. It is recommended that the CSP device is not mounted in an area of the board that is subject to high degrees of mechanical stress. ROUTING OF TRACES Routing strategy for CSP devices will depend upon the PCB technology being used and the volume in which they are being manufactured. Best practice for high assembly yield and minimum component footprint is a “via in pad” approach with a flat, filled microvia to an inner layer where the traces can be fanned out. Where the device pitch and available technology permit a “dog-bone” approach may also be used where the signal is tracked out to an adjacent via down to an inner layer (or opposite side of PCB). The trace-width for connecting to CSP pads (either for dog-bone or tracking directly out on top layer) should be a maximum of half the pad diameter wherever possible. Exceptions may need to be made for power and ground or other signals that are high current or are sensitive to voltage drop. UNDERFILL Underfill can affect the mechanical reliability of the die and substrate assembly, for example, board level mechanical performance during bending, vibration, drop testing and thermal cycling. Underfill material can affect the solder joints thermal cycle life. Wolfson performs die and board level CSP qualification without the use of underfill. CSP FOOTPRINTS DIMENSIONS The actual PCB footprint used for a CSP device will depend on the intended assembly process. It is recommended that footprint design be carried out with reference to IPC-7351 (latest revision) and the assembler’s recommendations. SOLDER MASK AND STENCIL DESIGN CONSIDERATIONS Non-Solder Mask Defined (NSMD) pads are recommended for “via in pad” routing on CSP devices. Solder Mask Defined (SMD) pads may be used as an alternative. The CSP footprint should exclusively use either NSMD or SMD pads, the two types should never be mixed on the same package. For effective solder paste release it is recommended that the stencil aperture be square with rounded corners (25µm radius) and trapezoidal sidewalls (larger opening to bottom side). CSP DEVICE HANDLING Due to CSP devices are silicon level packages rather than plastic encapsulated IC packages, CSP devices are more easily damaged than other IC packages. It is recommended that a vacuum nozzle be used to pick the device up in both manual and automated assembly. If tweezers need to be used to lift or position a device they should always be plastic rather than metal. w July 2008, Rev 1.0 8 WAN_0202 SOLDER PASTE RECOMMENDATIONS AND REFLOW PROFILE Due to the size, pitch and depth of the stencil apertures for the W-CSP package it is recommended that type 4 no-clean solder pastes be used for printing. For reflow it is recommended an IR or Forced Convection system be used or a combination system of IR and Forced Convection. For further information on soldering, please refer to WAN_0158. INSPECTION OF SOLDER JOINTS AFTER REFLOW Due to the pad layout of the W-CSP the solder joints are formed underneath the package and are not visible. It is recommended that to ensure the joints are soldered sufficiently X-ray inspection be utilized whenever possible. Visual inspection may be used for a cursory inspection to ensure that there is no obvious solder bridging. Shown below in Figure 10 is a typical x-ray inspection of the Wolfson 42-pin W-CSP Daisychain part on test board. Figure 10 X-Ray Inspection of 42 pin W-CSP Daisychain Part on Test Board As can be seen the solder has reflowed to form acceptable joints and there is minimal voiding in the solder joints, also there is no bridging visible between the joints. X-Ray inspection can also be useful in highlighting possible process problems such as solder balling and voiding which are often indications of poorly optimized reflow profiles. w July 2008, Rev 1.0 9 WAN_0202 W-CSP REWORK Wolfson does not recommend any rework on W-CSP part itself. APPLICATION SUPPORT If you require further information or require technical support, please contact Wolfson Microelectronics Applications group through the following channels: Email: Telephone: Fax: Mail: [email protected] (+44) 131 272 7070 (+44) 131 272 7001 Applications at the address on the last page. or contact your local Wolfson representative. Additional information may be made available from time to time on our web site at: http://www.wolfsonmicro.com w July 2008, Rev 1.0 10 WAN_0202 IMPORTANT NOTICE Wolfson Microelectronics plc (“Wolfson”) products and services are sold subject to Wolfson’s terms and conditions of sale, delivery and payment supplied at the time of order acknowledgement. Wolfson warrants performance of its products to the specifications in effect at the date of shipment. Wolfson reserves the right to make changes to its products and specifications or to discontinue any product or service without notice. Customers should therefore obtain the latest version of relevant information from Wolfson to verify that the information is current. Testing and other quality control techniques are utilised to the extent Wolfson deems necessary to support its warranty. Specific testing of all parameters of each device is not necessarily performed unless required by law or regulation. In order to minimise risks associated with customer applications, the customer must use adequate design and operating safeguards to minimise inherent or procedural hazards. Wolfson is not liable for applications assistance or customer product design. The customer is solely responsible for its selection and use of Wolfson products. Wolfson is not liable for such selection or use nor for use of any circuitry other than circuitry entirely embodied in a Wolfson product. Wolfson’s products are not intended for use in life support systems, appliances, nuclear systems or systems where malfunction can reasonably be expected to result in personal injury, death or severe property or environmental damage. Any use of products by the customer for such purposes is at the customer’s own risk. Wolfson does not grant any licence (express or implied) under any patent right, copyright, mask work right or other intellectual property right of Wolfson covering or relating to any combination, machine, or process in which its products or services might be or are used. Any provision or publication of any third party’s products or services does not constitute Wolfson’s approval, licence, warranty or endorsement thereof. Any third party trade marks contained in this document belong to the respective third party owner. Reproduction of information from Wolfson datasheets is permissible only if reproduction is without alteration and is accompanied by all associated copyright, proprietary and other notices (including this notice) and conditions. Wolfson is not liable for any unauthorised alteration of such information or for any reliance placed thereon. Any representations made, warranties given, and/or liabilities accepted by any person which differ from those contained in this datasheet or in Wolfson’s standard terms and conditions of sale, delivery and payment are made, given and/or accepted at that person’s own risk. Wolfson is not liable for any such representations, warranties or liabilities or for any reliance placed thereon by any person. ADDRESS: Wolfson Microelectronics plc 26 Westfield Road Edinburgh EH11 2QB United Kingdom Tel :: +44 (0)131 272 7000 Fax :: +44 (0)131 272 7001 Email :: [email protected] w July 2008, Rev 1.0 11