Complete PCB Design. Using OrCad Capture and Layout. By. Kraig Mitzner. Amsterdam • Boston • Heidelberg • London. New York • Oxford • Paris • San Diego. CHAPTER 1. Introduction to PCB Design and CAD. Computer-Aided Design and the OrCAD Design Suite. Printed Circuit Board Fabrication. Download Citation on ResearchGate | Complete PCB Design Using OrCad Capture and Layout / K. Mitzner. | This book provides instruction on how to use the.
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Library of Congress Cataloging-in-Publication Data. Mitzner, Kraig. Complete PCB design using OrCad capture and layout / Kraig Mitzner. p. cm. Includes. Function of Allegro PCB Editor in the PCB Design Process . .. 9. .. Generating a Regular PDF of the Schematic. .. For more information , read Complete PCB Design Using OrCAD. Capture and PCB. Complete PCB Design Using OrCAD® Capture and PCB Editor PCB Editor Users Guide: Transferring Logic Design Data (caite.info, p. 18) which is located.
Allegro PCB Editor is a powerful, fullfeatured design tool. Published on Jan 5, If a board is machine soldered, through-hole components should be mounted on a single side that is opposite the solder whenever possible. To stop routing, right click and select Done from the pop-up menu. The dielectric constant of the prepreg also varies by manufacturer and is discussed in Chapter 6. Click Yes.
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Start on. Show related SlideShares at end. WordPress Shortcode. Published in: Full Name Comment goes here. Are you sure you want to Yes No. Add Rectangle. Add Text. Edit Text. Edit text is used to change the text characters. If you need to change the text properties size and spacing etc. Control Panel with Foldable Window Panes The control panel is an area containing three tabs that show or hide collapsible window panes. The panes are shown in Figure in their default condition no tools or commands active.
The panes are dynamic, so what is displayed depends on what tool is active or what type of object you selected at that moment. From these panes, you can control visibility and selectability of objects and what particular tools can do extent of effect, etc.
The panes are collapsible, so that they do not take up design window space, but you can pin them in place by toggling the stick pin icon. You can also close them altogether by clicking the X in the upper right corner. Each pane is described briefly. You see examples of them in action during the design examples. Visibility Pane The Visibility pane is a shortened version of the Color dialog box described later.
It provides a handy way to turn on and off routing and plane layers or specific elements on those layers. Find Filter Pane The Find pane acts as a selection filter for tools and commands. By selectively checking object boxes, you can restrict which objects will be selected when you perform mouse picks in your design.
The order of the object types indicates a level of hierarchy, so if all objects are enabled and you attempt to perform a mouse pick in a congested area of your board, the top-down order indicates which object type will likely be picked.
Unchecking an object prevents that type of object from being selected. If you attempt to perform a command on an object and PCB Editor does not let you do it, check the Find pane; chances are, the box for that type of object has been unchecked. You can further narrow searches and selections by using the Find By Name area. Select the type of thing you are looking for from the dropdown list then click the More… button. A dialog box will pop up, which will allow you to pick specific objects from a list by name or property.
Options Pane The Options pane is very dynamic and you will use it often. Its appearance is determined by what tool you have selected or what command you are running. Figure gives an example, where left shows what the Options pane looks like when the Delete tool is active and right shows what the Options pane looks like when the Add Connect tool is active. When you first start out learning PCB Editor, it is easy to forget about the Options pane, but you want to keep it in mind, as it gives you significant control over your tools.
You might want to pin this one up until you get used to relying on it. Command Window Pane The Command window Figure provides you with information, gives instructions, and allows you to enter commands at the Command prompt.
Most of the commands you often use are also located on the toolbar and in the menus, but the Command window can give you greater control of the tools if you know the commands. The documentation folder in the OrCAD directory has manuals called command references for these commands. The manuals are listed alphabetically by the first letter of the command followed by coms. So, for example, if Figure Command window pane.
WorldView is interactive. If you use the highlight tool and select a part or net whether by clicking on it or by selecting it from a list using the Find pane , the object will be displayed in the WorldView.
If you right click inside of it, a pop-up menu is displayed that lets you change the size and location of the view in the design area. Figure WorldView window pane. Status Bar The status bar shown in Figure is located along the bottom of the design window, below the Command window and WorldView window. At the far left of the bar, the Command Status section lets you know if a command is active and running.
If a tool has been selected e. When the command has finished executing, the box turns green again. Figure Status bar. The x, y coordinates indicate the location of your cursor. You can toggle between the two at any time using the A or R button, as described later. The P button is interactive. Clicking it produces a Pick dialog box, which you can use to enter coordinate points in the work area using the keyboard rather than selecting a point with the mouse. This is useful for drawing outlines and so forth on large designs, so that you need not pan around the design trying to find and select a particular point.
The x and y coordinates in the Pick box are entered with space between them, not a comma. The A or R button determines whether the coordinates are absolute or relative.
Absolute coordinates show the cursor position relative to the design origin. Relative coordinates show the cursor x and y distance relative to the last pick point you made whether a tool was active or not. The DRC Status box lets you know at a glance if the design rule check is up to date and if any errors exist by the color of the box.
If the box is any color other than green see Figure , then you need to update the DRC and use a DRC report to locate any errors if they exist. Color and Visibility Dialog Box Color button The Color dialog box Figure is used to define custom colors for classes and subclasses and allows you to control the visibility of specific objects belonging to those classes Xs indicate the object is visible, open boxes indicate they are invisible.
The Color dialog box can be displayed by selecting the Color button, , on the toolbar. As mentioned previously, the Color dialog box performs some of the same functions as the colored and check boxes in the Visibility and Options panes, but as the figure shows, you have much greater control of objects and layers. From this dialog box, you can add or delete layers, define their physical and electrical properties if you want to , and define positive and negative properties to routing and plane layers, respectively.
The Layout Cross Section dialog box is displayed using the Xsection button,. Constraint Manager Cmgr button The Constraint Manager, shown in Figure , is where you set routing and component placement rules for your board design.
The Constraint Manager has four tabs and each tab has several views indicated by folders and icons. From these tabs and views, you have very precise control over routing characteristics for every net e. Figure Constraint Manager dialog box. The Constraint Manager takes a moment to load, but if it is taking a long time to load, check the Command window.
This tool will not load if a command is still active, and the Command window and status bar will tell you if that is the case. If so, you need to stop the current command by right clicking in the work space and selecting Done from the pop-up menu. Figure Padstack Designer. The information in the board design is separated into specific data files Gerber files , which are used by the board manufacturer to make the different parts of your board. The Artwork Control Form Figure is used to specify all the different types of layers for which Gerber files will be created and the format of the files.
You can add as many layers as necessary to fully define your board design.
Chapter 10 describes how to set up the Artwork Control Form based on a board layout example. The drill files are generated from drill settings that you specify in the NC Parameters dialog box shown in Figure The use of this dialog box is discussed further in Chapter 9. The number and types of files generated by the Artwork Control Form depends on the complexity of your board and the requirements of your board manufacturer. Most of the artwork files correspond to the layers with which you work in the layout environment, but some of the files do not.
For example, files are generated for etch and silkscreen layers and drill files, all of which depend on the output format you chose. The Excellon format is typically used, but your board manufacturer should tell you which specific files it needs and which format it prefers. Drill tapes and apertures are such terms. Nowadays, a drill tape is just an electronic file that describes drill holes and sizes just like any of the other files describes its data.
However, originally the drill tape was actually a role of paper or Mylar with holes punched into it that described hole sizes and locations for early computer numerical control CNC machines, but it is still sometimes called a drill tape.
Here is another example. In Chapter 1, the current technology of photoplotting and laser direct imaging was discussed. In contrast, the older technology used a xenon flash lamp and a shutter to expose photosensitive film or glass plates. The shutter controlled the exposure and an aperture controlled the size of the exposed area.
Any shape or drawing could be made by having apertures of different sizes and shapes. Opening and closing the shutter in one area to make a pad was called a flash, while holding the shutter open and moving the light source or film in the x and y directions to make a trace was called a draw.
The technology today is more advanced, but the concept is the same, and the same terminology is used. Figure shows a list of Figure Edit Aperture dialog box. A Gerber D code is a just a chronological number that specifies the size and shape of the aperture used on a given layer. Flash symbols e. The procedure for drawing thermal flash symbols is described in Chapter 8 and the use of them is described in the PCB Design Examples.
We have not, as of yet, taken a look at how to design the PCB itself. Chapters 4 through 6 provide an introduction to PCB design. Chapter 4 introduces industry standards related to PCB design. Not every PCB that is manufactured makes it into service. The higher the yield the better because failed boards cost time and money and produce waste. There are several failure points that can be addressed to increase yield.
To have high yield we need three things. First the board has to be manufacturable, second it has to perform properly signal integrity and quality , and third it has to be reliable it has to work for the full length of its expected life span. Being manufacturable means two things. The bare board has to be able to be fabricated given standard fabrication allowances SFA and the board also has to be able to be assembled; that is, parts need to be able to be attached to the board with proper solder joints without damaging the parts or the board.
Performance refers to both mechanical and electrical considerations. Mechanically, the board must physically fit into its enclosure and it must be able to handle its environment with respect to ambient temperature, vibration, and humidity. Being reliable means it meets the above considerations over the expected life of the device. If it is designed correctly it should not fail before the expected end of life unless the user exceeds specified operational design criteria.
If reliability problems exist then a failure analysis is conducted and the board is redesigned. Introduction to the Standards Organizations When you begin a new PCB design you may be asking how big and what shape should the board outline be, where should the parts be placed and in what order, what kind of layer stack-up should be used, how wide and far apart should the traces be routed, and what grounding and shielding techniques should be used?
There are several standards related to PCB design. The organizations below set standards that may be guides, rules for certification, or even laws. To cover all aspects of these standards would fill an entire book by itself.
The discussion in this chapter is limited to the basic standards for PCB design. A listing of applicable design standards is presented in Appendix A.
It is an organization made up of contributors from industry and includes designers, board manufacturers, assembly companies, suppliers, and original equipment manufacturers. Contributing members bring lessons learned and known good practices to the table, and they document and disseminate the knowledge base through industry-accepted standards.
You can also visit the IPC Web site www. Its primary focus is promoting the market development and competitiveness of the U. It has influence on design standards set by contributing groups, which include the following: It is an association of several hundred organizations that represents all areas of the electronics industry. Its primary focus with regard to PCB design is in standardizing discrete and integrated circuit semiconductor devices and packages.
You need to know the package specifications in order to design footprints for your PCB. You can access many of the standards online at www. A list of package specifications is provided in Appendix B. It conducts research and reports findings in publications and conferences to address industry opportunities and challenges to aid industry and academia.
You can find out more about the IEC including a free online educational program at www. The Department of Defense develops and procures an incredible amount of material and engineering services through private contractors. MIL-STD set and communicate standards on how things are to be designed, built, and tested in a controlled, known, and acceptable manner so that all who bid on contracts know exactly what is expected of them, so that they can be successful and competitive.
You can find out more about ANSI at www. IEEE is a central source of standardization in fields such as telecommunications and power generation.
Visit IEEE at www. Classes and Types of PCBs The design approach for a PCB depends on many factors including its intended end use, design and fabrication complexity, acceptable fabrication allowances, and type of component and attachment technology. Standard classifications have been established to aid designers, fabricators, and consumers in communicating with each other on these issues.
The classifications include performance class, producibility level, and type of construction. Performance Classes PCBs can fall into any of three end-use performance classes. Performance classes are based on things like allowed variation in copper-plating thickness, feature location tolerance, and hole diameter tolerance plated and unplated , to name a few.
The three classes are as follows: Class 1, General Electronic Products, includes general consumer products like televisions, electronic games, and personal computers that are not expected to have extended service lives and are not likely to be subjected to extensive test or repairability requirements. Since these items usually have a higher cost they are usually repairable and must meet stricter testing requirements.
Examples include critical medical equipment and weapons systems. They typically have more stringent test specifications and possess greater environmental robustness and reworkability. The levels are not a set of explicit requirements but a way of describing how complex a design is and the precision required to produce the particular features of a PCB or PCB assembly.
Smaller features trace widths, etc. For example, issues such as tolerances for interconnecting lands and conductor width tolerances are described in the standards. The three producibility levels are Level A, general design—preferred complexity. Level B, moderate design—standard complexity. Issues that are related to the fabrication type are the number of copper layers e.
The six fabrication types defined by IPC are Type 1, single-sided printed board. Type 2, double-sided printed board. The subclasses are as follows: Subclass A, through-hole devices THD only. Subclass B, surface-mounted devices SMD only. Higher performance class boards are made more reliable by using stricter producibility levels and lower easier fabrication types and assembly classes.
Density Level B, nominal land protrusion median courtyard and median density.
PCB manufacturing is no exception. Design tolerances include drill-hole location and diameter, copper plating and etching, and soldermask resolution, to name a few. Manufacturing tolerance becomes increasingly important as the number of layers increases and line widths and spacing decrease.
The tolerance errors can add up at each manufacturing step and result in a scrapped board. Industry standards exist to set minimum performance and process guidelines. Just because your design meets certain minimum industry standards does not mean that every manufacturer has the ability to manufacture or assemble the PCB as designed.
The following discussion covers the major design issues to look out for and references to the appropriate standards. Registration Tolerances As described in Chapter 1 see also Coombs , p. The design parameters in each step have to line up with the next, or misregistration can occur, which can result in manufacturing defects and a nonoperational board.
Breakout and Annular Ring Control Figure shows how fabrication allowances result in a final hole tolerance. Figure a shows the ideal hole, which has a specified diameter and location. Figure b shows the uncertainty of the final hole diameter due to drilling Introduction to Industry Standards a b Chapter 4 d c? Figure c shows the uncertainty of the hole location. Figure d shows the desired hole compared to the possible hole after considering the combined uncertainties. Feature dimensions on each layer of a PCB have uncertainty, which, when combined, can result in a bad board if allowances are not designed into the board.
The combination of these uncertainties can result in problems such as loss of annular ring control and subsequent land breakout as shown in Figure Coombs , p. Plated through holes can often function with breakout, but reliability is greatly reduced.
By knowing limitations of fabrication processes and following design guides you can greatly reduce the occurrence of defects and increase yield. The boards are identified by a letter and a number, which represent the x and y dimensions. Sizes range from AI to D4 as shown in Table An example of a size C2 panel is listed in the table, where C2 is 7.
If you have flexibility in specifying the size of your board, you can do so in a way that will allow you to maximize the number of boards on one panel. This helps reduce cost by minimizing the number of parts being handled and the amount of waste generated. This is not always an option as PCB size is often driven by design constraints that are out of your control.
Small boards may have to be panelized, though, if they are to be assembled and soldered by automated processes since automated machinery has minimum size limitations and maximums as well on the size of PCBs that they can process. This area is called a tooling area. The required tooling area ranges from 0. On panelized designs the distance between board outlines is typically 0. The goal is to utilize as much of a panel as possible without having to go to the next larger size IPC-D Section 2, Table , p.
Most of the time these considerations are handled by the board manufacturer and are transparent to the board designer, but being aware of the issues may allow you to optimize the board layout and reduce production costs.
Standard finished PCB thickness As described in Chapter l, a PCB is an assembly of one or more cores joined together by sheets of partially cured epoxy called prepreg.
By stacking combinations of various core thicknesses and sheets of prepreg, a wide variety of finished board thicknesses can be achieved.
The following sections discuss standard core, prepreg and copper thickness, and tolerances. Unless you are designing controlled impedance PCBs you may not be immediately concerned about the discussion of each of the thicknesses described below.
Core thickness Cores are made up of substrates i. Table shows typical laminate epoxy thicknesses without copper cladding or foil see also Coombs , Table , p. Copper foil and plated cladding thicknesses are described below. Table shows the various prepreg types and their sheet thicknesses before curing Coombs , Table , p. Board manufacturers stack up combinations of sheets to obtain the desired board thickness.
The actual thickness of a sheet once it is in a board and cured depends on whether it is between plane layers or signal layers, because signal layers tend to sink into the prepreg, which results in a thinner end thickness.
The dielectric constant of the prepreg also varies by manufacturer and is discussed in Chapter 6. While drilled holes are being plated some of the external surfaces are also plated. After finishing processes are completed, the external surfaces of a PCB can be much thicker, but the wall thickness of a PTH is usually 1 mil or less.
Most of the time it does not enter into the drill size calculations unless the hole is very small most finished hole sizes are 8 mils or larger , but the information is presented here for completeness. IPCA, Table partial data.
The thickness of the copper depends on how much current the trace will be required to carry and the required impedance of the traces for Introduction to Industry Standards controlled-impedance PCBs.
The thickness also plays a role in how narrow the traces can be because thicker copper takes longer to etch and can result in variations in trace width and etchback effects as described below. As mentioned above, various finishing processes etching and plating alter the final cladding or foil thickness. The processes are described in detail in the IPC standards and Coombs and are not described here. The values listed here are for reference only. The wall will have a slight angle to it because, as the acid begins to work its way into the exposed copper, a sidewall begins to form, which also is attacked by the acid.
As Figure shows, the copper near the etch resist begins to be removed under the mask. This effect is called etchback or undercutting. If the etching process is stopped as soon as the last bit Etchback of copper cladding or foil is removed from the surface of the board, the trace width at the botW tom will be the initial size of the mask width which is defined by the Gerber files.
The designer should be aware of the width variations when calculating trace widths for controlled impedances and for current handling ability. For general design considerations traces should be made as wide as practical.
Per IPCA the minimum trace width and spacing is 3. Individual board manufacturers may have their own etching and spacing tolerances. Typical minimum trace widths are 4 to 8 mils.
It is a good idea to call and ask what their capabilities are or check their Web sites. Standard Hole Dimensions Holes are drilled into PCBs by various techniques including twist bits, router bits, lasers, and plasmas. The capabilities with regard to placement and size accuracy and the speed can vary considerably. But the board designer needs to know what size of drill hole to specify in a padstack.
However, not all board manufacturers carry every size bit. Some may round up or down to the nearest drill size available or they may always round up to the next largest drill bit to make sure that the hole is never too small. This affects annular ring width, which can lead to breakout as described earlier. When laminate materials are drilled they become soft due to frictional heating.
The softened laminate then smears during the drilling process, which coats the surface of the copper and can prohibit good plating Coombs , p. To solve this problem the laminate inside the hole is etched back after being drilled to desmear the hole.
Etchback enhances plating of PTHs Coombs , p. This is one reason why adequate clearance is required between the plated through hole and the ground plane Coombs , p. After the hole is drilled and desmeared it is plated. In most cases the variance in available drill bit size and plating is not a problem. Padstack calculations that account for plating widths and tolerances are described in the next chapter. Another problem that can occur is that, if the hole size is very small compared to the thickness of the board, the plating process may not be satisfactory.
It is recommended that the AR be between 3: The AR for Level B boards is 6: Check with your board manufacturer to know what its capabilities are, because if the aspect ratio is too high, plating problems can occur inside the hole, which can lead to open circuits from incomplete plating and barrel cracking. Padstack design, which includes all of the issues described above, is covered in detail in Chapters 5 and 8.
Soldermask Tolerance Due to photolithography misregistration and swell of the soldermasks, lands can be partially covered. Soldermasks that are patterned on solder-plated lands may be damaged when the solder reflows during soldering operations and can adversely affect solderability.
This may be especially troublesome on very small parts SOT To reduce these risks, the soldermask openings are often larger than the lands oversized.
There are two basic categories of soldermask materials: The recommended oversizing for liquid-screen-printed coatings is 16 to 20 mils, and the recommended oversize of photoimageable masks is 0 to 5 mils IPCA, 4. Many of the PCB Editor footprints have clearances of between 0 and 10 mils. Many fabricators will adjust the soldermask as necessary for their processes. However, some do not and expect or assume that you will do it. Check with your board house to find out if it requires oversizing and, if so, how much and who it expects to do it.
Lincolnwood, IL: Institute for Interconnecting and Packaging Electronic Circuits. New York: Graphic Symbols for Electrical and Electronics Diagrams. May Generic Standard on Printed Board Design.
Northbrook, IL: June February January Guidelines for Printed Board Component Mounting.
A more thorough list is given in Appendix B. Other Items of Interest 1. Chapter 5 Introduction to Design for Manufacturing 71 Introduction to PCB Assembly and Soldering Processes For a PCB to be manufacturable the bare board has to be able to be fabricated within standard fabrication allowances SFAs , and the board has to be able to be assembled given the different component technologies.
As discussed in the previous chapters, there are many steps to designing and fabricating a PCB. When a design is complete and submitted to a board house, the house must be able to perform the manufacturing steps that the design calls for.
Whether a PCB is manufacturable or not really begins with and includes parts creation and schematic entry in Capture, padstack and footprint design, parts placement and trace routing, and artwork production in PCB Editor. But it does not end there. Once the board has been fabricated, it is of little use without the functional parts. Those parts need to be able to be attached to the board without them or the board being damaged. This chapter provides the information necessary for padstack and footprint design and component placement for the design of manufacturable PCBs.
Assembly Processes Printed circuit boards may be manually assembled or assembled by automated machinery. Assembly processes depend on the class of component technology Classes A through Z as described earlier and the number of boards to be assembled at one time.
Some companies may both fabricate and assemble boards under one roof, while some companies may specialize in PCB fabrication only and others in PCB assembly only. The method of assembly plays a role in how you lay out your PCB because of clearance and orientation issues and soldering processes. A brief summary of assembly processes is given here along with component placement, orientation, and spacing considerations. In low-volume work an assembly line of several assemblers may be used, in which each person is responsible for attaching specific components.
The assembly processes may be interrupted several times to functionally test sections of the PCB as it is assembled. Manual assembly may involve both manual placement and soldering or a mixture of manual placement and automated soldering described below.
Manual assembly can be tedious work. Consistent component placement and orientation can aid manual assemblers. For example, orienting polarized components capacitors and diodes in the same direction and orienting integrated circuits ICs so that pin 1 on all ICs is located in the same direction can significantly reduce assembly defects and increase yield. Automated machines are programmed to extract parts from reels or bins and place the components on the PCB in the correct location and orientation.
Through-hole devices are usually packaged as roles or strips of components, which are taped together by their leads. Through-hole components are usually placed only on the top side of the board so that the leads can be wave soldered and the components themselves are not exposed to molten solder. Soldering processes are described below. The typical automated process steps for throughhole devices is insertion of dual inline packages first, then axial-leaded devices, then radial-leaded devices, and finally odd-form devices.
After the components have been inserted, the board is most often wave soldered wave soldering is described below , but can be reflow soldered intrusive reflow; see Coombs , p.
Surface-mounted devices are commonly packaged in tubes, matrix trays, tape and reel, and bulk. Surface-mounted devices may be mounted on one or both sides of a PCB. The parts are then placed onto the board by the pick-and-place equipment with the component lead terminations set into the paste, temporarily holding the parts in place. The board is then run through a reflow oven, which melts and then cools the solder, thereby attaching the part to the board. Surface-mounted devices can typically be populated at rates from 10, to , CPH Coombs , Section First the top-side surface-mounted devices are attached to the board using the solder paste and reflow process described below.
Next, through-hole devices are inserted from the top and held in place by clinching bending the leads on the bottom, by gluing the part to the top, or in some cases by friction between the lead and the hole. The board is flipped over and adhesive dots are applied to the bottom of the board by an automated dispenser.
The bottom-mounted surface-mounted devices SMDs are then positioned manually or by pick-and-place machines onto the glue dots. The adhesive holds the bottom-side surface-mounted devices in place until the solder joint has been completed. The assembly is run through an oven to cure the adhesive. The board, with through-hole components on the top and SMDs on the top and bottom, is then run through a wave-soldering station, which solders the through-leads and the bottom-mounted SMDs.
The previously soldered top-mounted SMDs remain soldered on the top. The top-mounted SMDs are attached to the board first using a high-temperature solder paste, and then the board is run through a high temperature reflow oven.
With the top-side components securely in place, the board is flipped over and the bottom-mounted SMDs are attached with the lower temperature paste and reflow process as described below. For the soldering process to be successful an intermetallic compound, or alloy, must be formed between the solder and the base material the leads and traces. To protect the solder joint areas from oxidation, contact areas on new PCBs receive a surface finish by being dipped in a solder bath and hot-air solder leveled or are plated by some other plating process such as electroless nickel or palladium Coombs , p.
Just prior to or during soldering, the surfaces to be joined are cleaned deoxidized with flux so that the solder can flow over and wet the surfaces. There are two general soldering methods: Only manual, wave soldering, and ovenreflow soldering are discussed here and only briefly.
Manual Soldering Manual soldering is used for a wide variety of applications from complete PCB assembly to simple repair work and touch-up. Other than slower soldering speed, the biggest drawbacks to manual soldering are the increased risk of electrostatic discharge during handling and thermal gradients caused by localized heating of the board and parts. Parts placed on the board that will be manually placed and soldered require no special layout consideration as far as spacing and orientation other than the basics described below.
However, it is helpful to the assembler if parts placement affords room to work and similar parts are aligned and oriented in a consistent manner as described above. Wave Soldering During wave soldering the board is held by its edges on a conveyor, fluxed, and preheated as shown in Figures and The conveyor moves the board past a standing wave of molten solder so that only the bottom side of the board is exposed to the solder.
The through-hole components are placed on the top with the leads protruding out the bottom of board, which is prefluxed. As the conveyer moves the PCB into the solder wave, solder wicks up the barrel and creates fillets on the top and the bottom. SMDs are glued to the board, fluxed, and run through the wave.
Very small components or large tantalum caps can be problematic with wave soldering. The small parts cause problems during the gluing process because in some cases the glue dot is larger than the component and the glue can ooze over onto the solder pads.
Large SMDs cause problems because of thermal stresses that can lead to component cracking. When surface-mounted devices are wave soldered i. Solder bridging can occur because of the fine lead pitch of some SMDs. As each pin leaves the solder wave, it tends to draw some of the solder from the pin beside it as the molten solder attempts to reduce its surface tension for the Direction of board travel Solder thieves Figure SMD component orientation for wave soldering.
This causes a problem at the last two rows of pins as there are no pins that follow to draw the excess solder away from them, which can result in the last two pins on a side being bridged by the excess solder. The solder thieves are extra pads placed after the last pads that pull the excess solder away from the pads. Solder thieves can also be made by simply making the last pads a little larger and extending farther back than the typical pads on that device.
If SMD parts are on top of the board and are reflow soldered prior to wave soldering, fan-out vias need to be located away from lands to prevent solder migration away from the SMD lands and down into the via.
This can occur as heat conducts from the solder wave up via barrels and re-reflows the solder, which then draws the solder down the via by capillary action.
Some boards may be too large or too small to be wave soldered. Large boards sag as they are heated unless special holding fixtures are used. Very small boards may need to be panelized with breakaways tab routes or V scores so that they can be handled by automated equipment without having to make specialized board holders.
Reflow Soldering There are various types of reflow soldering, but the discussion here is limited to oven-type reflow soldering. Reflow soldering is most often used for surfacemount devices, but through-hole components can also be soldered this way called pin-in-paste or intrusive reflow soldering.
A schematic diagram of a reflow oven is shown in Figure Solder paste which contains solder and flux in a viscous paste is applied to a bare board with a stencil. The components are placed into position usually by pick-and-place machines so that the leads sit in the solder paste. As the assembly heats up the flux is activated and the solder reflows melts.
Surface tension of the molten solder tends to self-align the parts. However, if parts are not in proper alignment and thermal gradients do not melt the solder on all pads at the same time, the component may stand up on one end called tombstoning.
TCH Choosing a strategy file. Kraig Mitzner has done a wonderful job of covering the full spectrum of printed circuit board fabrication.
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