GLOSSARY of Printed Circuit Design and Manufacturing
This glossary has key terminology in use in PCB design and manufacturing, with a smattering of electronics. The definitions were chosen so that their context would likely apply to reading material encountered by a PCB designer. Therefore, many of these terms will have other meanings not given here. It is recommended by scholars that you also clear up the non-technical definitions in regular dictionaries. There are such dictionaries recommended below.
This collection of terms came about as I, a PCB designer, ran across words and acronyms in my field for which meanings were hard to find. As I tracked them down, I made them part of this glossary. If you are a PCB designer, then this glossary could be a good place to start when you find a need to look up the meanings of words related to printed circuits or electronics.
Alphabetizing
Method
Terms that begin with a symbol or a digit are placed in the SYMBOLS page. Terms that contain digits within them are alphabetized as if the numeric
characters were spelled in English.
Terms with two or more words are alphabetized "dictionary style." They are alphabetized as though the spaces between the terms have been removed. If there are other characters in the term, such as a slash (/), dash (-) or plus sign (+), these are treated the same as spaces and ignored for the purpose of alphabetizing.
Modern Dictionary of Electronics
by Rudolf F. Graf
This is the best, most usable dictionary for electronics, because its
definitions help you grasp the terms and therefore the subject. Lesser
dictionaries define electronics terms with even more difficult technical
jargon, leading one into endless"word chains." Not this one.
You can
buy the Modern Dictionary of Electronics new or used
via the Internet.
Citation:
Graf, Rudolf F. Modern Dictionary of Electronics. Newnes, 1999.
The Random House Dictionary of the English Language, Unabridged, 2nd Edition
You need a big, comprehensive dictionary. Get this one. Despite being a big
dictionary,
The Random House
has great definitions, quick to grasp.
Although out of print, as of 2022 you could still buy a great used copy online for $40 including shipping or possibly for much less. Two versions are available of the 2nd Edition, Unabridged:
I have no idea what the difference is for the deluxe edition, but there seem to be fewer copies of it available in 2020 than the regular edition. I'm sure they both have the same set of definitions. My copy has both ISBNs listed in the front matter, and it is the regular edition.
Citation:
Flexner, Stuart Berg, and Leonore Crary Hauck, editors. The Random House Dictionary of the English Language. Unabridged, 2nd Edition, Random House, 1987.
SAC4 — Self-Aligned Controlled Collapse Chip Connect. A variation of
C4 flip-chip technology. . PFEIFFER L, WEST KW, WONG YH ,Journal of the Electrochemical Society (JES) Volume 134, Number 11, November 1987.
SAR — Successive Approximation Register. A type of analog-to-digital converter using a digital-to-analog converter to determine the output word successively, bit by bit. ADC Architectures II: Successive Approximation ADCs
by Walt Kester.
The operating condition of a transistor when an increase in base current produces no further increase in collector current.
A circuit condition whereby an increase in the driving or input signal no longer produces a change in the output.
The condition when a transistor is driven so hard that it becomes biased in the forward direction. In a switching application, the charge stored in the base region prevents the transistor from turning off quickly under saturation conditions.
Generally, that state in which a semiconductor device is conducting most heavily for a given applied voltage. In many devices it is also a state in which the normal amplification mechanisms have become "swamped" and inoperative.
SCM — In the 1980s, the term supply chain management (SCM) was developed to express the need to integrate the key business processes, from end user through original suppliers.[8] Original suppliers are those that provide products, services, and information that add value for customers and other stakeholders. The basic idea behind SCM is that companies and corporations involve themselves in a supply chain by exchanging information about market fluctuations and production capabilities.
schematic — A diagram which shows, by means of graphic symbols, the electrical connections, components and functions of an electrical system. The components are represented by agreed-upon symbols, and the conductors connect them by lines. If two lines connect to each other, a large dot represents a junction, whereas no dot represents no connection.
schematic capture — (ski-MAT-ik KAP-chur) Pronunciation Keynoun[PCB Design]
Schematic capture is the process of creating a graphical representation of an electronic circuit using specialized software tools. It serves as the foundation for PCB design by defining the components, their electrical connections, and logical relationships.
It exists to provide a structured and standardized way to translate a circuit design concept into a format that can be analyzed, simulated, and ultimately transferred into a PCB layout. The process is necessary for ensuring correctness, organization, and manufacturability of electronic designs.
The outcome of schematic capture is a digital schematic file that includes components, nets (connections), and attributes such as reference designators and values. This schematic is then used to generate a netlist, which forms the basis for PCB layout and manufacturing.
Schematic capture can be influenced by using different software tools, following best practices for clarity and organization, and employing design rule checks (DRC) to minimize errors. Changes in libraries, component selection, and connectivity verification also alter the quality and reliability of schematic capture.
The term originates from the words schematic, referring to a diagrammatic representation, and capture, meaning the act of recording or digitizing information. It became widely used with the advent of computer-aided design (CAD) software for electronics in the late 20th century.
SC70 — (aka SC-70) This is a small outline transistor package with a lead pitch of 0.65 or 0.5 mm. I have no clue what the SC stands for, except that it is probably a code deisgnator used by EIAJ. The 70 is probably a sequence number for the application-for-approval as a standard by EIAJ. (The next application would be 71 and the first was 11).
SDRAM — Synchronous Dynamic Random Access Memory, which is a type of solid state computer memory. Other dynamic random access memories (DRAM) have an asynchronous interface which means that it reacts as quickly as possible to changes in control inputs. SDRAM has a synchronous interface, meaning that it waits for a clock signal before responding to its control inputs. It is synchronized with the computer's system bus, and thus with the processor. SDRAM have a more complex pattern of operation than DRAM which does not have synchronizing control circuits. (Compare with DDR SDRAM)
SerDes — (SIR-deez or SIR-dez)
Pronunciation Key noun [Electronics] Short for serializer/deserializer, SerDes is a pair of functional blocks in an integrated circuit transceiver used to convert data between
serial data and parallel interfaces in each direction. It is commonly used in high
speed communications to compensate for limited input/output. SerDes converts data
into a serial data stream and then transmits it over one or more differential lines to transmit data from point A to point B. SerDes reduces the number of data paths and also the number of connecting pins (or wires)
required.
Wikipedia SerDes
In high-speed serial links, the receive signals are quite sensitive to crosstalk, and it doesn’t take much to make them fail.
Citation
Knack, Kella. “Controlling Crosstalk In Connector Pinouts.” Altium, 9 Feb. 2021,
SEU — noun [Outer Space Electronics and Radiation] Single Event Upset. An SEU is caused by a single particle common to the Van Allen radiation belt. A single such particle, the most destructive being either a free proton (from trapped solar wind) or heavy ion such as boron, hits a silicon chip and destroys a small part of it, potentially causing a malfunction or extraneous data bit. These particles are very-difficult or impossible to shield against.
SHDSL — Symmetric High-Speed Digital Subscriber Line. Also has been described (incorrectly?) as "Single-pair high-speed digital subscriber line." It is true that this is a single-pair DSL technology, but that is not what the acronym stands for.
short — Short circuit. An abnormal connection of relatively low resistance between two points of a circuit. The result is excess (often damaging) current between these points. Such a connection is considered to have occurred in a printed wiring CAD database or artwork anytime conductors from different nets touch. If they come closer than the minimum spacing allowed for the design rules being used, an error is flagged. This is because the inadequate spacing can lead to an actual short during manufacture of the PCB.
signal ground — a low-impedance path for current to return to the source (Ott, H.W. "Ground—A Path for Current Flow." IEEE International Symposium on EMC. San Diego, CA, October 9-11, 1979.)
silicon wafer — A thin, iridescent, silvery disk of silicon which contains a set of integrated circuits, prior to their being cut free and packaged. A silicon wafer will diffract reflected light into rainbow patterns and, being a similar size, looks so much like a music CD that it could be mistaken for one (except that it has no label or hole in the middle). On closer inspection, one can see the individual (usually rectangular- or square-shaped) integrated circuits which form a uniform patchwork quite unlike the surface of a music CD. When cut or etched from the wafer these circuits are then called
chips or
dice.
modifier [PCB Manufacturing] (Of PCB manufacturing equipment, such as screen printers) Having a mesh of fine thread stretched across a frame.
Conventional Silkscreen printing requires polyester screens stretched across aluminum frames. Solvent resistant emulsions are used to coat the screens prior to imaging. Both the screen and PCB panel must be in perfect register to ensure proper alignment. This method of screening requires the following equipment: Laser photo plotter to produce the initial film, UV printer, spray developer and curing ovens. Screens can be cleaned and reclaimed to be used again although not for jobs which require high resolutions.
noun [PCB Design] (Also called "silkscreen legend" or just "legend," which is the term preferred by IPC. ) The
decals and
reference designators in epoxy ink on a
printed wiring board, so called because of the method of application—the ink is squeegeed through a silkscreen, very similar to the technique used in the printing of T-shirts.
A silk mesh size commonly used is 6 mils. With this mesh size, the absolute minimum line width of any silkscreen legend artwork is 6 mils, which leaves a very faint line on the finished board. 7 mils works better for a practical minimum line width. A very readable reference designator height to use is 45 mils high with a 9 mil draw.
Newer silkscreening methods allow for sikscreen draws of 5 mils, which come out very clear. Technically, this new method is not silkscreening per se, but ink-jet printing of epoxy-
based ink. A good reference designator size to use is 35 mils high with a 5 mil draw. Legend text can be as little as 25 mils high if needed in a dense design but should only be used if bigger text won't fit. Reading text that small on the finished board would require a hand loupe or stereo microscope.
noun [PCB Manufacturing] A
Gerber file controlling the photoplotting of this legend.
SIM — Subscriber Identity Module. A Subscriber Identity Module (SIM) is a smart card roughly the size of a postage stamp that securely stores the key identifying a mobile phone service subscriber, as well as subscription information, saved telephone numbers, preferences, text messages and other information. The equivalent of a SIM in Universal Mobile Telecommunications System (UMTS) is a Universal Subscriber Identity Module (USIM).
SIMM — Single In-line Memory Module, is a type of memory module used for random access memory in personal computers. It differs from a (DIMM, the most predominant form of memory module today) in that the contacts on a SIMM are redundant on both sides of the module.
SIP — PCB Components Single In-line Package. Through pins commonly at 100-mil pitch in a single row. Typically used for a resistor network.
SiP — Semi-conductor Manufacturing System-in-Package (SiP) assembly The idea behind a SiP is that multiple bare dice and/or chip-scale package (CSP) devices are mounted on a common substrate, which is used to connect them all together. The substrate and its components are then placed in (or built into) a single package
This approach has several advantages over a System-on-Chip (SoC), including the fact that one can include analog, digital, and radio frequency (RF) dice in the same package, where each die is implemented using that domain's most appropriate technology process. Also, designers can employ a number of off-the-shelf dice coupled, perhaps, with a limited number of relatively small, internally-developed components. Furthermore, when it comes to re-spinning the device in the future to evolve existing functionality or add new features, it may well be the case that you need modify only a subset of the dice.
Semiconductor substrates, predominantly silicon, with very fine tracks formed using opto-lithographic processes similar to those used for integrated circuits. Semiconductor substrates are also known as active substrates, because components such as transistors and logic gates can be fabricated directly onto their surface. One additional benefit of using silicon as a substrate is that its coefficient of thermal expansion exactly matches that of any silicon chips that are attached to it.
And before we move on, just to increase the fun and frivolity, it's possible to create a number of small SiPs and then mount these in a larger SiP, in which case we have a scenario known as Package-in-Package (PiP). Also, in some cases, we have a SiP that is mounted on top of another SiP, which some refer to as a Package-on-Package (PoP).
["2D vs. 2.5D vs. 3D ICs 101" Max Maxfield ]
skew — -- The magnitude of the time difference between two events that ideally would occur simultaneously. [ JEDEC Standard 65 (EIA/JESD65)] For good working understandings, see PDF article
Skew definition and jitter analysis by Steve Corrigan, System Specialist, Data Transmission, Texas Instruments Incorporated
SMOBC — Solder mask over bare copper. A PCB finish in which solder mask is applied to a board whose copper is exposed, usually followed by a process known as
HASL (q.v.).
SMS — SMS stands for Short Message Service and is the most widely used type of text messaging. With a SMS, you can send a message of up to 160 characters to another device. Longer messages are normally split up into multiple messages. (Compare with MMS)
SoC — A System on a Chip or System on Chip (SoC or SOC) is an integrated circuit (IC) that integrates all components of a computer or other electronic system into a single chip. It may contain digital, analog, mixed-signal, and often radio-frequency functions—all on a single chip substrate. A typical application is in the area of embedded systems.
The contrast with a microcontroller is one of degree. Microcontrollers typically have under 100 kB of RAM (often just a few kilobytes) and often really are single-chip-systems, whereas the term SoC is typically used for more powerful processors, capable of running software such as the desktop versions of Windows and Linux, which need external memory chips (flash, RAM) to be useful, and which are used with various external peripherals. In short, for larger systems, the term system on a chip is a hyperbole, indicating technical direction more than reality: increasing chip integration to reduce manufacturing costs and to enable smaller systems.
Many interesting systems are too complex to fit on just one chip built with a process optimized for just one of the system's tasks.
When it is not feasible to construct a SoC for a particular application, an alternative is a system in package (SiP) comprising a number of chips in a single package. In large volumes, SoC is believed to be more cost-effective than SiP since it increases the yield of the fabrication and because its packaging is simpler.
Another option, as seen for example in higher end cell phones and on the BeagleBoard, is package on package stacking during board assembly. The SoC chip includes processors and numerous digital peripherals, and comes in a ball grid package with lower and upper connections. The lower balls connect to the board and various peripherals, with the upper balls in a ring holding the memory buses used to access NAND flash and DDR2 RAM. Memory packages could come from multiple vendors.
soft copy — An electronic form of a document; a data file in computer memory or stored on storage media. When one is looking at a soft copy he is viewing the document as displayed on a computer monitor.
software — Programs, data files, procedures, rules, and any associated documentation pertaining to the operation of a computer system or of a computer application.
solder balls — The round solder balls bonded to a transistor contact area and used to make connection to a conductor by face-down bonding techniques.
In IBM's ceramic BGA's, the solder used in solder balls has a higher melting point than that used in soldering the ball to the chip substrate and the BGA to a board. IBM uses 10/90 tin/lead for the solder ball and eutectic solder for the assembly. The high melt balls of the BGA do not melt during PCB assembly and thus create a pre-determined standoff height for the component. PBGA packages use a eutectic solder ball which provides a collapsible joint similar to
C4.
solder coat — A layer of solder that is applied directly from a molten solder bath to a PWB during manufacture, after the application of solder mask. This coats the terminals (copper pads) to be soldered during assembly with solder, providing a non-oxidizing protective layer.
solder mask — A technique wherein everything on a circuit board is coated with a plastic except 1) the contacts to be soldered, 2) the gold-plated terminals of any card-edge connectors and 3)fiducial marks. Sometimes, vias are also left free of solder mask. Much useful information about solder mask can be found in the article by Ian Poole called
PCB Solder Resist.
solder-mask-defined pad — (Abbrv. SMD pad) Encroachment of
solder mask onto a copper land (pad), thus restricting (defining) the area of the copper exposed.
In the case of a land on a PCB, this defines the area exposed to solder. It has been used with BGA lands on PCBs, but has been found to result in high-stress solder joints which are subject to failure.
However, component lands, the pads suppporting the solder balls on the underside of BGA components
(part of the BGA component, not the PCB) typically are solder-mask defined.
In the case of a via under a BGA, an SMD pad is useful in reducing shorts while leaving an opening in the via for gases generated during soldering to escape. (The soldering of BGAs is always by reflow in an oven.)
space transformer — Abbreviated ST. A major component of certain high-density
probe cards (aka ATE (Automatic Test Equipment) systems and DUT boards). It provides pitch reduction, high routing density and localized mid-frequency decoupling. A major developer of
ATE systems which use space transformers is Wentworth Labs.
(SPI) A serial interface in which a master device supplies clock pulses to exchange data serially with a slave over two data wires (Master-Slave and Slave-Master). This term probably originated with Motorola in about 1979 with their first all-in-one microcontroller. (2003-07-13)
verb [PCB Manufacturing and Product Development] Re-build. To manufacture a revision of an electronic product. In the case of PCBs, this means to make bare boards,
stuff them and test them.
noun [Product Development] Spin takes on a negative connotation when used with a number or a word like "multiple," as in "It took multiple spins to get that board working." Obviously, the more spins, the more cost and time are spent to get an electronic product to market. The synonym "build" has a less negative connotation.
SPST — Single-pole, single throw. A switch with two rest positions, one in which it is open, and one in which it closes a single circuit.
Stammtisch Beau Fleuve Acronyms SPST
sputtering — A deposition process wherein a surface, or target, is immersed in an inert-gas
plasma and is bombarded by ionized molecules that eject surface atoms. The process is based on the disintegration of the target material under ion bombardment. Atoms broken away from the target material by gas ions deposit on the part (substrate), forming a thin film.
[Graf, Rudolf F. Modern Dictionary of Electronics. Newnes, 1999]
SQTP — noun [Components] Serial Quick Turn Programming.
A programming service offered by Microchip for factory production orders. Unique, flexible programming option that allows Microchip to program serialized, random or pseudo-random numbers into each device. Serial programming allows each device to have a unique number that can serve as an entry-code, password or identification number.
SRM — Supplier relationship management (SRM) is the discipline of strategically planning for, and managing, all interactions with third party organizations that supply goods and/or services to an organization in order to maximize the value of those interactions. In practice, SRM entails creating closer, more collaborative relationships with key suppliers in order to uncover and realize new value and reduce risk.
SSH — [Networking] Secure Shell. A network communication protocol that enables two computers to communicate and share data. It allows one computer to modify data and code on another computer. It was created in 1995 by Finnish computer scientist Tatu Ylönen in order to make such operations secure and as a replacement for Telnet. (
Telnet is still used these days, despite its being vulnerable to hacks.)
SSH is a much different system than what is used for browsing the Internet (which uses hypertext transfer protocol).
SSO — Simultaneous Switching Outputs. In its Virtex-4 User Guide, Xilinx offers a table which "provides the guidelines for the maximum number of simultaneously switching outputs allowed per output power/ground pair to avoid the effects of ground bounce."
stack-up — The arrangement of layers of materials in the construction of a PCB. The individual layers can be of various thicknesses.
Most commonly, rigid PCBs are 1/16 inch [1.5875 mm] thick in total. This is the thickness of "cards" you can add to your desktop computer. There is a tolerance on this overall 1/16th inch thickness of plus or minus 10%. Of course, PCBs can be thinner or thicker based on the end use for which they are intended.
In the case of the computer card, if the card has a lot of electronic power, such as a gamer's video card, it will be physically large. But also, it will have more layers than a simple Wi-Fi card. Needing to remain 1/16th inch thick [62.5 mils or 1.5875 mm], the layers will be thinner. Especially the center layer.
At the center layer of a PCB stack-up is a core (q.v.). The rest of the layers we can call "outer" layers. These are made thin. The best practice is to keep the outer dielectrics at 4 mils [102 microns] to 8 mils [204 microns] thickness. The use of thin outer layers has to do with electromagnetic compatibility (EMC).
EMC side-note: Thin layers help in electromagnetic field containment when layers with signal wires are sandwiched between ground planes. The thin layers of dielectric between the conducting layers means the electromagnetic fields (EM fields) set up between the signals and ground plane will be small. The smaller the EM fields, the less noise is generated by electromagnetic interference (EMI). This is fundamental to improving EMC.
The core at the center, being a slab of dielectric with layers of copper on top and bottom, provides the two inner-most layers of copper and enough dielectric thickness to make up the difference between the thickness of all the other materials and the 1/16th inch thickness of the card. This provides rigidity. If the core were too thin, the card would be flimsy and not fit snuggly into the motherboard connector. If the core were too thick, the card wouldn't fit at all. And all this might be more than you wanted to know about PCB stack-up. If you're a printed circuit designer, you already know all this. If not, I probably lost you a long time ago.
Stack-up 10 layers in CAD 3D perspective showing thick center core and thin outer layers. Vertical dimension has been exaggerated for illustration.If the layers are reduced from 10 to 6, the center core must be thicker to maintain the overall thickness of 1/16th inch.
starved thermal — A thermal relief whose connection to a negative plane is overlappped by and at least partially obliterated by a negative-image object.
The most common source for this type of design error can happen when dividing a power plane into corridors for handling other voltages:
The fat line (a negative image object) laid down to separate corridors can inadvertantly be drawn across a pad that is supposed to connect to the plane, thus starving its thermals.
Starved Thermals are one of the biggest issues with PCB designers, as most of the CAD system DRC functions do not look to see if a thermal has been choked off or restricted by adjacent data.
This is because most CAD systems do not provide a true representation of the thermal aperture. But Altium Designer is an exception to this.
Altium Designer does allow the designer to explicity define the thermal relief connections, then it helps eliminate the possibility of starved thermals through intelligent negative-plane connectivity checking--checking for broken nets (opens).
STEM — Science, Technology, Engineering and Mathematics (STEM), previously Science, Math, Engineering and Technology (SMET),[1] is a term used to group together these academic disciplines.[2] This term is typically used when addressing education policy and curriculum choices in schools to improve competitiveness in science and technology development.
[Wikipedia.org]
STEP — Standard for the Exchange of Product Model Data, 3D format useful for representing component bodies on PCB designs. Altium Designer can import STEP models. STEP files may have the extension .STP or .STEP. See also IGES. IGES models can be converted to STEP models using 3D engineering drawings software such as Solid Works.
STEP model — (STEP MAHD-dəl)
Pronunciation Key noun [Computer Aided Design] A 3D body in CAD. STEP stands for "Standard for the Exchange of Product model data." Altium Designer 13 and 10 can import STEP models into footprints and PCBs.
3D engineering drawing software such as Solid Works can export STEP models. STEP model files may have the extension .STP or .STEP.
Most connector manufacturers have been very helpful to PCB designers by providing STEP models of their parts.
noun [Electronic Circuit Design] "The time difference of corresponding digital information on separate lines, measured at the rising or falling edges of the digital data." Source: Definition 7 of skew in Modern Dictionary of Electronics by Rudolf F. Graf.
noun [PCB Design] The tolerance in length matching between associated differential pairs with the purpose of meeting the timing skew requirements defined in definition 1 above. This tolerance varies with the application. For example,
Streamlined PCB Design (SLPD) — A system of PCB design authored by John Walt Childers. The verb streamline means "Cause to be quick and efficient." Streamlined design = accuracy plus speed. SLPD is a set of policies that can guide the design of printed circuit boards. The policies have been derived with the aim of simplifying PCB design and systematically eliminating errors from it. This should speed design and manufacturing.
Streamlined PCB Design is not a quick-and-dirty way for a PCB designer to make a fast buck. Some PCB designers who have learned some of its elements have complained that it is more work for them, not less.
It is important to get the viewpoint here: The system is streamlined with highest priority directed at the broad view of the overall electronics project and secondly from the narrower view of each step in the cycle of production. Therefore, part of a PCB designer's job in implementing SLPD is doing extra things in just such a way that help is provided to the downstream actions in the overall production cycle, especially board manufacture and assembly.
Streamlined PCB Design attempts to give value far beyond what is implied by "fair exchange."
A guiding impulse of streamlined design is that one strives to "create final products that really duplicate the client's intentions," our motto.
If one can eliminate whole categories of errors from creeping in, the overall cycle of production gets completed sooner with higher quality and reliability. Design and production become quick, efficient, streamlined.
sub-panel — A group of printed circuits (called modules) arrayed in a panel and handled by both the board house and the assembly house as though it were a single
printed wiring board. The sub-panel is usually prepared at the board house by routing most of the material separating individual modules, leaving small tabs. The tabs are strong enough so that the sub-panel can be assembled as a unit, and weak enough so that final separation of assembled modules is easily done.
Subversion — Apache Subversion is a Version Control Management system widely used by developers and printed circuit designers for collaborating on design projects over a network such as LAN or Internet.
surface mount — Surface mount technology. The technology of creating printed wiring wherein components are soldered to the board without using holes. The result is higher component density, allowing smaller PWB 's. Abbreviated SMT.
SVN commit — [Version Control Management:Apache Subversion] When you execute an SVN commit an upload to a subversion repository will occur if your local copy of a file has been changed in your working directory, and no changes to that file have been committed to the repository since you last updated (since you last got the latest revision). (aka "check in." Contrast with "SVN update.")
The term "upload" is used loosely here, as it can imply that the repository is on a remote server and accessible via WAN such as the Internet, but in fact it could reside locally.
SVN delete — [Version Control Management:Apache Subversion] First, you cannot commit the deletion of a file or directory that isn't fully up to date. If a newer version of the item exists in the repository,
your attempt to delete will be rejected to prevent you from accidentally destroying changes you've not yet seen.
SVN merge — [Version Control Management:Apache Subversion]
This can occur implicity when an SVN update is executed under these conditions: The working copy is locally changed and out of date; in other words, the file has been changed both in the working directory and in the repository. In this case, an SVN commit of the file would fail with an "out-of-date" error. The file should be updated first; an SVN update command would attempt to merge the public changes with the local changes. If Subversion couldn't complete the merge in a plausible way automatically, then a conflict occurs and the users involved should communicate with each other to resolve it.
Beginning in Apache Subversion 1.7, you cannot by default use a mixed-revision working copy as the target of a merge operation. (This new requirement was introduced to prevent common problems which stem from doing so.)
SVN update — [Version Control Management:Apache Subversion] A download from a subversion repository will occur if your (local) working copy is unchanged and out of date when you execute an SVN update. (Contrast with "SVN commit.")
The term "download" is used loosely here, as it can imply that the repository is on a remote server and accessible via WAN such as the Internet, but in fact it could reside locally.
