IPC7525A-Stencil Design Guidelines

IPC-7525A Stencil Design Guidelines Working Draft 1 February 2004 ASSOCIATION CONNECTING ELECTRONICS INDUSTRIES 2215 Sanders Road, Northbrook, IL 60062-6135 Tel. 847.509.9700 Fax 847.509.9798 www.ipc.org IPC-7525A 1st Working Draft February 2004 2 1 PURPOSE This document provides guides for the design and fabrication of stencils for solder paste and surface-mount adhesive. It is intended as a guideline only. 1.1 Terms and Definitions All terms and definitions used throughout this handbook are in compliance with IPCT- 50. Definitions denoted with an asterisk (*) below are reprints from IPC-T-50. Other specific terms and definitions, essential for the discussion of the subject, are provided below. 1.1.1 Aperture An opening in the stencil foil. 1.1.2 Aspect Ratio and Area Ratio Aspect Ratio = Width of Aperture / Thickness of Stencil Foil Area Ratio = Area of Aperture Opening /Area of Aperture Walls 1.1.3 Border Peripheral tensioned mesh, either polyester or stainless steel, which keeps the stencil foil flat and taut. The border connects the foil to the frame. 1.1.4 Contained Paste Transfer Head A stencil printer head that holds, in a single replaceable component, the squeegee blades and a pressurized chamber filled with solder paste. 1.1.5 Etch Factor Etch Factor = Etched Depth / Lateral Etch in a chemical etching process 1.1.6 Fiducials Reference marks on the stencil foil (and other board layers) for aligning the board and the stencil when using a vision system in a printer. 1.1.7 Fine-Pitch BGA/Chip Scale Package (CSP) Ball grid array with less than 1 mm [39 mil] pitch. This is also known as Chip Scale Package (CSP) when the package size is no more than 1.2X the area of the original die size. 1.1.8 Fine-Pitch Technology (FPT)* A surface-mount assembly technology with component terminations on centers less than or equal to 0.625 mm [24.61 mil]. 1.1.9 Foil The sheet used to create the stencil. 1.1.10 Frame A device onto which the foil is mounted. This may be tubular or cast aluminum with the border permanently mounted using an adhesive. Some foils can be mounted into a tensioning master case and do not require a border or a permanent fixturing of the foil to the frame. 1.1.11 Intrusive Soldering Intrusive soldering may also be known as paste-in-hole, pin-in-hole, or pin-in-paste soldering. This is a process in which the solder paste for the through-hole components is applied using the stencil, the through-hole components are inserted and reflow-soldered together with the surface-mount components. 1.1.12 Modification The process of changing an aperture in size or shape. 1.1.13 Overprinting The use of stencils with apertures larger than the pads or annular rings on the board. 1.1.14 Pad Metallized shape on the circuit board to which the terminal of a surface mount component is electrically or mechanically attached. 1.1.15 Squeegee A metal or rubber blade used to wipe across the stencil to force solder paste into openings in the stencil. Normally, squeegee is mounted at an angle such that the printing edge of the squeegee trails behind the print head and the face of the squeegee slopes forward. 1.1.16 Standard BGA Ball grid array with 1 mm [39 mil] pitch or larger. 1.1.17 Stencil A tool which may consist of a frame, border, and foil containing apertures through which solder paste, adhesive, or other media is transferred. IPC-7525A 1st Working Draft February 2004 3 1.1.18 Step Stencil A stencil with more than one foil thickness level. 1.1.19 Surface-Mount Technology (SMT)* The electrical connection of components to the surface of a conductive pattern that does not utilize component holes. 1.1.20 Through-Hole Technology (THT)* The electrical connection of components to a conductive pattern by the use of component holes. 1.1.21 Ultra-Fine Pitch Technology A surface-mount assembly technology with component terminations on centers less than or equal to 0.40 mm [15.7 mil] 2 APPLICABLE DOCUMENTS 2.1 IPC1 IPC-T-50 Terms and Definitions for Interconnecting and Packaging Electronic Circuits IPC-A-610 Acceptability of Electronic Assemblies IPC-SM-782 Surface Mount Design and Land Pattern Standard IPC-2511 Generic Requirements for Implementation of Product Manufacturing Description Data and Transfer Methodology IPC-7095 Design and Assembly Process Implementation of BGAs 2.2 Joint Industry Standard1 J-STD-005 Requirements for Soldering Pastes 2.3 Barco/ETS2 Gerber RS-274D Format Reference Guide, Part Number 414-100-002 Gerber RS-274X Format User’s Guide, Part Number 414-100-014 3 STENCIL DESIGN 3.1 Stencil Data 3.1.1 Data Format Regardless of the stencil fabrication method used, Gerber® data is the preferred data format. Possible alternative formats are GenCAM®3, DXF, HP-GL, Barco, etc; however, they may need to be converted to Gerber® format prior to the stencil manufacturing process. Gerber® data describes the file format that provides a language for communicating with the photo plotting system to produce a tool for chemically etched stencils. It is also used to produce the laser cut or electroformed stencils. While the actual data format may vary from file to file depending on the software package or designer, the data format commonly used by photo plotter and laser equipment is known as Gerber®. 3.1.2 Gerber® Format There are two standard Gerber® formats available: • RS-274D - requires a data file listing the X-Y coordinates on the stencil where apertures are to be placed and formed, and a separate Gerber® aperture list that describes the size and shape of the various Gerber® apertures used to prepare the image. • RS-274X - in this format the Gerber® aperture list is embedded in the data file. 3.1.3 Aperture List The aperture list is an ASCII text file containing D codes that define the size and shape for all apertures used within the Gerber® file. Without the aperture list, the software and photo plotting system cannot read the Gerber® data. Only the X-Y coordinates would be available with no size and shape data. 3.1.4 Solder Paste Layer The solder paste layer data is necessary to produce a stencil. If fiducials are required on the stencil, they should also be included in the solder paste layer. 3.1.5 Data Transfer Data can be transmitted to the stencil supplier via modem, FTP (file transfer protocol), e-mail attachment or disk. To ensure data integrity after transmitting and due to the large size of data files, it is suggested that the files be compressed prior to sending data. It is recommended that the full data file (the solder paste, solder mask, silk screen and copper layers) sent to the printed circuit board manufacturer be supplied to the stencil manufacturer. This allows the stencil manufacturer to optimize or make recommendations on aperture sizes based on actual pad sizes for the SMT land. 3.1.6 Panelized Stencils In those cases where it is desired to have more than one image on the stencil, the stencil patterns will be IPC-7525A 1st Working Draft February 2004 4 panelized and included in the data file. In those instances where the data file does not already contain the panelized stencil design, a readme file, panel drawing or order information must specify the location of the two or more designs. This could be a reference from the edge of the frame, distances between patterns, etc. 3.1.7 Step-and-Repeat In those cases where more than one image of the same design is to be printed, the data file for stencil fabrication should contain the stencil design in the step-and-repeat array. In those instances where the data file does not contain the step-and-repeat pattern, a readme file, panel drawing, or order information should specify: • Total number of steps for the final array • Number of steps in the X-direction along with dimensions from a specific feature to corresponding feature (such as fiducials, component pad locations, etc.). • Number of steps in the Y-direction along with dimensions from a specific feature to corresponding feature (such as fiducials, component pad locations, etc.). 3.1.8 Image Orientation/Rotation In those cases where image orientation is not parallel to the frame or the stepand- repeat is not recti-linear (one or more images is rotated), the data for stencil fabrication should contain the oriented image. In those cases where it does not, a readme file, panel drawing or order information should specify this information (X- and Y-offsets) referencing stencil features. 3.1.9 Image Location To accommodate specific printers, the stencil image may have to be located in different positions inside the frame: (a) center image (b) center board/panel – requires board/panel outlines (c) offset board/panel – requires board/panel outlines and reference locations In those cases where this data is not included in the Gerber® data, a read me file, panel drawing or order information should specify this information referencing stencil features. 3.1.10 Identification Stencil should contain identification information such as part number, revision number, thickness, supplier’s name and control number, date and method of manufacture. 3.2 Aperture Design A general aperture design guideline for various SMT components is shown in Table 3-1. Some of the factors influencing stencil aperture design are: component type, pad footprint, solder mask opening, board finish, aspect/area ratio, solder paste type and user process requirement. 3.2.1 Aperture Size The volume of solder paste applied to the board is mainly determined by the aperture size and foil thickness. Solder paste fills the stencil aperture during the squeegee cycle of the print operation. The paste should completely release to the pads on the board during the board/stencil separation cycle of the print operation. From the stencil viewpoint, the ability of the paste to release from the inner aperture walls to the board pad depends primarily on three major factors: (1) the area and aspect ratios for the aperture design (See 3.2.1.1.) (2) the aperture side wall geometry (See Section 4.4) (3) the aperture wall finish (See Section 4.4) 3.2.1.1 Area Ratio/Aspect Ratio Both area ratio and aspect ratio are illustrated in Figure 3-1. A general design guide for acceptable paste release is >1.5 for aspect ratio and >0.66 for area ratio. The aspect ratio is a one dimensional simplification of the area ratio. When the length is much greater than the width, the area ratio ( 2T W ) reduces to a factor of the aspect ratio ( T W ). IPC-7525A 1st Working Draft February 2004 5 When the stencil separates from the board, paste release encounters a competing process: solder paste will either transfer to the pad on the board or stick to the aperture side walls. When the pad area is greater than 0.66 of the inside aperture wall area, a complete paste transfer should occur. Aspect Ratio = Stencil of Thickness Aperture ofWidth = T W Area Ratio = WallsAperture of Area Aperture of Area = T W) (L 2 W L ×+× × 3.2.2 Aperture Size Versus Board Pad Size As a general design guide, the aperture size should be reduced compared to the board pad size. The stencil aperture is commonly modified with respect to the original pad design. Reductions in the area or changes in aperture shape are often desirable to enhance the processes of printing, reflow, or stencil cleaning. For instance, reducing the aperture size will decrease the possibility of stencil aperture to board pad misalignment. This reduces the chance for solder paste to be printed off the pad, which may lead to solder balls or solder bridging. Having a radiused corner for all apertures can promote stencil cleaning. 3.2.2.1 Leaded SMD’s For leaded SMD’s, e.g., J-leaded or gull-wing components with 1.3 - 0.4 mm [51.2 - 15.7 mil] pitch, the reduction is typically 0.03 - 0.08 mm [1.2 - 3.1 mil] in width and 0.05 - 0.13 mm [2.0 - 5.1 mil] in length. 3.2.2.2 Plastic BGA’s Reduce circular aperture diameter by 0.05 mm [2.0 mil]. 3.2.2.3 Ceramic BGA’s Increase circular aperture dimension by 0.05 - 0.08 mm [2.0 - 3.1 mil] when this does not interfere with the solder mask and/or increase the stencil foil thickness to 0.2 mm [7.9 mil] and go one to one with the board pad. Refer to IPC-7095 for solder paste volume requirements. 3.2.2.4 Fine-Pitch BGA and CSP Square aperture with the width of the square equal to, or 0.025 mm [0.98 mil] less than, the diameter of the pad circle on the board. The square should have rounded corners. A guideline is 0.06 mm [2.4 mil] radiused corners for a 0.25 mm [9.8 mil] square and 0.09 [3.5 mil] corners for a 0.35 mm [14 mil] square. 3.2.2.5 Chip Components - Resistors and Capacitors Several aperture geometries are effective in reducing the occurrence of solder balls. All these designs are aimed at reducing excess solder paste trapped under the chip component. The most popular designs are shown in Figures 3-2, 3-3 and 3-4. These designs are commonly used for no-clean processes. Figure 3-1: Cross Sectional View of a Stencil Aperture IPC-7525A 1st Working Draft February 2004 6 Figure 3-4 Oblong Aperture Design 3.2.2.6 MELF, Mini-MELF Components For MELF and Mini-MELF components, ‘‘C’’ shaped apertures are suggested. (See Figure 3-5). Dimensions of these apertures should be designed to match the geometry of component terminals. Figure 3-5: Aperture Design for MELF Components 3.2.3 Glue Aperture Chip Component The glue stencil is typically 0.15 - 0.2 mm [5.9 - 7.9 mil] thick. The glue aperture is placed in the center of the component solder pads. It is 1/3 the spacing between pads and 110% of the component width. (See Figure 6.) Figure 6 Glue Stencil Aperture Design More information about glue stencil will be made available in the next revision of this document. Figure 3-2: Home Plate Aperture Design Figure 3-3: Bow Tie Aperture Design IPC-7525A 1st Working Draft February 2004 7 Table 3-1 General Aperture Design Guidelines for Surface-Mount Devices Part Type Pitch Pad Footprint Width Pad Footprint Length Aperture Width Aperture Length Stencil Thickness Range Aspect Ratio Range Area Ratio Range PLCC 1.25 mm [49.2 mil] 0.65 mm [25.6 mil] 2.00 mm [78.7 mil] 0.60 mm [23.6 mil] 1.95 mm [76.8 mil] 0.15 - 0.25 mm [5.91 - 9.84 mil] 2.3 - 3.8 0.88 - 1.48 QFP 0.65 mm [25.6 mil] 0.35 mm [13.8 mil] 1.50 mm [59.1 mil] 0.30 mm [11.8 mil] 1.45 mm [57.1 mil] 0.15 - 0.175 mm [5.91 - 6.89 mil] 1.7 - 2.0 0.71- 0.83 QFP 0.50 mm [19.7 mil] 0.30 mm [11.8 mil] 1.25 mm [49.2 mil] 0.25 mm [9.84 mil] [1.20 mm] 47.2 mil 0.125 - 0.15 mm [4.92 - 5.91 mil] 1.7 - 2.0 0.69 - 0.83 QFP 0.40 mm [15.7 mil] 0.25 mm [9.84 mil] 1.25 mm [49.2 mil] 0.20 mm [7.87 mil] [1.20 mm] 47.2 mil 0.10 - 0.125 mm [3.94 - 4.92 mil] 1.6 - 2.0 0.68 - 0.86 QFP 0.30 mm [11.8 mil] 0.20 mm [7.87 mil] 1.00 mm [39.4 mil] 0.15 mm [5.91 mil] 0.95 mm [37.4 mil] 0.075 - 0.125 mm [2.95 - 3.94 mil] 1.5 - 2.0 0.65 - 0.86 0402 N/A 0.50 mm [19.7 mil] 0.65 mm [25.6 mil] 0.45 mm [17.7 mil] 0.60 mm [23.6 mil] 0.125 - 0.15 mm [4.92 - 5.91 mil] N/A 0.84 - 1.00 0201 N/A 0.25 mm [9.84 mil] 0.40 mm [15.7 mil] 0.23 mm [9.06 mil] 0.35 mm [13.8 mil] 0.075 - 0.125 mm [2.95 - 3.94 mil] N/A 0.66 - 0.89 BGA 1.25 mm [49.2 mil] CIR 0.80 mm [31.5 mil] CIR 0.80 mm [31.5 mil] CIR 0.75 mm [29.5 mil] CIR 0.75 mm [29.5 mil] 0.15 - 0.20 mm [5.91 - 7.87 mil] N/A 0.93 - 1.25 Fine- pitch BGA 1.00 mm [39.4 mil] CIR 0.38 mm [15.0 mil] CIR 0.38 mm [15.0 mil] SQ 0.35 mm [13.8 mil] SQ 0.35 mm [13.8 mil] 0.115 - 0.135 mm [4.53 -5.31 mil] N/A 0.67 - 0.78 Fine- pitch BGA 0.50 mm [19.7 mil] CIR 0.30 mm [11.8 mil] CIR 0.30 mm [11.8 mil] SQ 0.28 mm [11.0 mil] SQ 0.28 mm [11.0 mil] 0.075 - 0.125 mm [2.95 - 3.94 mil] N/A 0.69 - 0.92 Note: 1. It is assumed that the fine-pitch BGA pads are not solder mask defined. 2. N/A implies that only the area ratio should be considered. 3.3 Mixed Technology Surface-Mount/Through-Hole (Intrusive Reflow) It is desirable to have a process where SMT and THT devices can both be: (1) provided with printed solder paste (2) placed on or in the board (3) reflowed together. The objective of stencil printing of solder paste for the intrusive reflow process is to provide enough solder volume after reflow to fill the hole and create acceptable solder fillets around the pins. Table 3-2 shows process window for a typical intrusive soldering process. Table 3-2 Process Window for Intrusive Soldering - Maximum Limits Desirable Maximum Limits Desirable Hole Diameter 0.65 – 1.60 mm [25.6 – 63.0 mil] 0.75 – 1.25 mm [29.5 – 49.2 mil] Lead Diameter Up to hole diameter minus 0.075 mm [2.95 mil] Hole diameter minus 0.125 mm [4.92 mil] Paste Overprinting 6.35 mm [250 mil] < 4.0 mm [157 mil] Stencil thickness 0.125 - 0.635 mm [4.92 – 25.0 mil] 0.15 mm [7.87 mil], 0.20 mm [5.91 mil] for fine-pitch IPC-7525A 1st Working Draft February 2004 8 3.3.1 Solder Paste Volume A simple equation listed below describes the volume of solder paste required as shown in Figure 3-7. It is desirable to keep the copper pad around the hole as small as possible. It is also desirable to keep the clearance between the pin and the through-hole and the length of the pin as small as possible. By doing this less solder paste volume will be required. Following are three stencil designs used to deliver the through-hole solder paste: (1) Non-step stencil (2) Step stencil (3) Two-print stencil ( ) ( ){ } HPBTpHBS1 OO S V V F F A - AT ) WL(T V −+++= ×= Where: V is volume of solder paste required Vp is the solder volume left on the top and/or bottom board pad S is the solder paste shrink factor AH is the cross sectional area of the through-hole AP is the cross sectional area of the through-hole pin TB is the thickness of the board FT + FB is the total fillet volume required TS is the thickness of the stencil foil LO is the length of the overprint aperture LP is the length of the pad WO is the width of the overprint aperture WP is the width of the pad VH is solder paste filling the hole during the printing Operation Note: Solder paste volume filling the hole can vary from 0% to 100% depending on the print setup. Contained paste transfer heads are effective in achieving close to 100% while metal squeegee blades with a high attack angle and high print speed will deliver minimum paste into the hole. 3.3.1.1 Overprint Without Step This is the stencil of choice when it can deliver enough solder paste to satisfy the through-hole requirement. A cross section of this type stencil is shown in Figure 3-8. An example of when this stencil could be used is a two row connector on 2.5 mm [98.4 mil] pitch with 1.1 mm [43.3 mil] diameter though-holes, 0.9 mm [35.4 mil] diameter pins, 1.2 mm [47.2 mil] thick board and no other components within 3.8 mm [150 mil] of the through-hole openings. An overprint stencil aperture of 2.2 mm [86.6 mil] wide and 5.1 mm [200 mil] long with a stencil foil thickness of 0.15 mm [5.91 mil] can deliver sufficient solder paste. 3.3.1.2 Overprint With Step If the board is thicker, the hole is bigger, or the pin is smaller, more solder paste volume will be required. In this case, a step stencil may be needed to provide sufficient solder paste volume for the THT parts without providing too much paste on the SMT pads. An example of this type stencil is shown in Figure 3-9. K1 and K2 are keep-out distances. K2 is the distance between the through