sentence-transformers/squad huggingface dataset:
Passage: The simplest method, used for small-scale production and often by hobbyists, is immersion etching, in which the board is submerged in etching solution such as ferric chloride. Compared with methods used for mass production, the etching time is long. Heat and agitation can be applied to the bath to speed the etching rate. In bubble etching, air is passed through the etchant bath to agitate the solution and speed up etching. Splash etching uses a motor-driven paddle to splash boards with etchant; the process has become commercially obsolete since it is not as fast as spray etching. In spray etching, the etchant solution is distributed over the boards by nozzles, and recirculated by pumps. Adjustment of the nozzle pattern, flow rate, temperature, and etchant composition gives predictable control of etching rates and high production rates.

Question: What non-professionals might use immersion etching, since it's the easiest way to make a PCB?

Question: If you wanted to speed up the etching process, you could agitate the bath up or add what to it?

Question: Is the immersion etching process long or short compared to other production methods?

Question: What funny name is used for the etching process where air is blown through the etching solution to get it moving around?

Question: What's the fastest and most easily controlled process for etching?
Passage: Multi-layer printed circuit boards have trace layers inside the board. This is achieved by laminating a stack of materials in a press by applying pressure and heat for a period of time. This results in an inseparable one piece product. For example, a four-layer PCB can be fabricated by starting from a two-sided copper-clad laminate, etch the circuitry on both sides, then laminate to the top and bottom pre-preg and copper foil. It is then drilled, plated, and etched again to get traces on top and bottom layers.

Question: If you started with a two-sided laminate etched on either side, laminated to the top and bottom, and processed it again to get traces on the outside layers, how many layers would your final PCB have?

Question: What makes up the inside of a multi-layer PCB?

Question: Pressure is one thing you need to apply to make a multi-layer PCB; what's the other thing?

Question: What tool is used to push the trace layers in a multi-layer PCB together?
Passage: Holes through a PCB are typically drilled with small-diameter drill bits made of solid coated tungsten carbide. Coated tungsten carbide is recommended since many board materials are very abrasive and drilling must be high RPM and high feed to be cost effective. Drill bits must also remain sharp so as not to mar or tear the traces. Drilling with high-speed-steel is simply not feasible since the drill bits will dull quickly and thus tear the copper and ruin the boards. The drilling is performed by automated drilling machines with placement controlled by a drill tape or drill file. These computer-generated files are also called numerically controlled drill (NCD) files or "Excellon files". The drill file describes the location and size of each drilled hole.

Question: What kind of material sometimes used for drilling will lose sharpness quickly if used for PCBs?

Question: What property do PCBs have that makes them tough to drill through?

Question: What kind of drill bit do you need to drill holes in a PCB?

Question: What's another name for NCD files that has an "x" in it?

Question: What does the "NCD" in "NCD files" mean?
Passage: The hole walls for boards with two or more layers can be made conductive and then electroplated with copper to form plated-through holes. These holes electrically connect the conducting layers of the PCB. For multi-layer boards, those with three layers or more, drilling typically produces a smear of the high temperature decomposition products of bonding agent in the laminate system. Before the holes can be plated through, this smear must be removed by a chemical de-smear process, or by plasma-etch. The de-smear process ensures that a good connection is made to the copper layers when the hole is plated through. On high reliability boards a process called etch-back is performed chemically with a potassium permanganate based etchant or plasma. The etch-back removes resin and the glass fibers so that the copper layers extend into the hole and as the hole is plated become integral with the deposited copper.

Question: What do the hole walls in two-layer PCBs have to be before they can be electroplated?

Question: What's the minimum number of layers a PCB can have to be considered "multi-layer"?

Question: What process does a multi-layer PCB usually undergo before plating to make sure the layers have good connectivity with each other?

Question: What type of boards undergo the etch-back process?

Question: Etch-back removes glass fibers and what other material?
Passage: Matte solder is usually fused to provide a better bonding surface or stripped to bare copper. Treatments, such as benzimidazolethiol, prevent surface oxidation of bare copper. The places to which components will be mounted are typically plated, because untreated bare copper oxidizes quickly, and therefore is not readily solderable. Traditionally, any exposed copper was coated with solder by hot air solder levelling (HASL). The HASL finish prevents oxidation from the underlying copper, thereby guaranteeing a solderable surface. This solder was a tin-lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the EU and US, which restricts the use of lead. One of these lead-free compounds is SN100CL, made up of 99.3% tin, 0.7% copper, 0.05% nickel, and a nominal of 60ppm germanium.

Question: What happens to untreated copper that makes it difficult to solder?

Question: What chemical treatment will prevent bare copper from oxidizing?

Question: What's the process by which bare copper gets covered in solder?

Question: What does hot air solder leveling ensure the coated copper will have?

Question: What alloy can no longer be used in HASL because of restrictions on the use of one of its metal components?
Passage: Other platings used are OSP (organic surface protectant), immersion silver (IAg), immersion tin, electroless nickel with immersion gold coating (ENIG), electroless nickel electroless palladium immersion gold (ENEPIG) and direct gold plating (over nickel). Edge connectors, placed along one edge of some boards, are often nickel plated then gold plated. Another coating consideration is rapid diffusion of coating metal into Tin solder. Tin forms intermetallics such as Cu5Sn6 and Ag3Cu that dissolve into the Tin liquidus or solidus(@50C), stripping surface coating or leaving voids.

Question: What does OSP stand for?

Question: What metal is often under the gold plating on edge connectors?

Question: What's the abbreviation for immersion silver plating?

Question: Ag3Cu is one intermetallic that tin forms; what's the other one?

Question: What might the rapid diffusion of coating metal into tin solder leave in the surface coating?
Passage: Electrochemical migration (ECM) is the growth of conductive metal filaments on or in a printed circuit board (PCB) under the influence of a DC voltage bias. Silver, zinc, and aluminum are known to grow whiskers under the influence of an electric field. Silver also grows conducting surface paths in the presence of halide and other ions, making it a poor choice for electronics use. Tin will grow "whiskers" due to tension in the plated surface. Tin-Lead or solder plating also grows whiskers, only reduced by the percentage Tin replaced. Reflow to melt solder or tin plate to relieve surface stress lowers whisker incidence. Another coating issue is tin pest, the transformation of tin to a powdery allotrope at low temperature.

Question: What's the process whereby metal filaments are grown on or in a PCB via DC voltage?

Question: Along with silver and aluminum, what metal grows metal filaments when exposed to an electric field?

Question: In addition to "whiskers," what does silver sprout around ions like halide?

Question: What do scientist call the function whereby tin changes when it's very cold?

Question: What's present in plated surfaces that causes tin to grow metal filaments?
Passage: Areas that should not be soldered may be covered with solder resist (solder mask). One of the most common solder resists used today is called "LPI" (liquid photoimageable solder mask). A photo-sensitive coating is applied to the surface of the PWB, then exposed to light through the solder mask image film, and finally developed where the unexposed areas are washed away. Dry film solder mask is similar to the dry film used to image the PWB for plating or etching. After being laminated to the PWB surface it is imaged and develop as LPI. Once common but no longer commonly used because of its low accuracy and resolution is to screen print epoxy ink. Solder resist also provides protection from the environment.

Question: If part of a PCB isn't solderable, what can be used instead?

Question: What's another term used for "solder resist"?

Question: What solder resist with a four-word name is a popular choice in the industry?

Question: What kind of coating gets applied to the PWB before light exposure?

Question: What solder resist is so inaccurate that few choose it anymore?
Passage: Unpopulated boards are usually bare-board tested for "shorts" and "opens". A short is a connection between two points that should not be connected. An open is a missing connection between points that should be connected. For high-volume production a fixture or a rigid needle adapter is used to make contact with copper lands on the board. Building the adapter is a significant fixed cost and is only economical for high-volume or high-value production. For small or medium volume production flying probe testers are used where test probes are moved over the board by an XY drive to make contact with the copper lands. The CAM system instructs the electrical tester to apply a voltage to each contact point as required and to check that this voltage appears on the appropriate contact points and only on these.

Question: What are bare-board tests looking for besides "opens"?

Question: What conductive metal are the lands on a circuit board made out of?

Question: What's an absent connection that needs to be linked up on an unpopulated board called?

Question: What does a flying probe tester deliver to the contact points during testing?

Question: What system tells the flying probe where to go and what to do?
Passage: Often, through-hole and surface-mount construction must be combined in a single assembly because some required components are available only in surface-mount packages, while others are available only in through-hole packages. Another reason to use both methods is that through-hole mounting can provide needed strength for components likely to endure physical stress, while components that are expected to go untouched will take up less space using surface-mount techniques. For further comparison, see the SMT page.

Question: Surface-mount is one type of construction used in PCB assembly; what's the other one?

Question: Which type of construction makes components that use more space?

Question: Which of the two kinds of construction is weaker under strain?

Question: If you're building a circuit board that has a lot of components that won't be used, which construction would be better?
Passage: In boundary scan testing, test circuits integrated into various ICs on the board form temporary connections between the PCB traces to test that the ICs are mounted correctly. Boundary scan testing requires that all the ICs to be tested use a standard test configuration procedure, the most common one being the Joint Test Action Group (JTAG) standard. The JTAG test architecture provides a means to test interconnects between integrated circuits on a board without using physical test probes. JTAG tool vendors provide various types of stimulus and sophisticated algorithms, not only to detect the failing nets, but also to isolate the faults to specific nets, devices, and pins.

Question: What's the most frequently used test configuration procedure for ICs?

Question: Do test circuits create permanent or temporary connections between traces?

Question: What component of the printed circuit board is appraised with boundary scan testing?

Question: To whom would you go to acquire the algorithms you'd use for the Joint Test Action Group procedures?

Question: What would you avoid by using the Joint Test Action Group standard?
Passage: PCBs intended for extreme environments often have a conformal coating, which is applied by dipping or spraying after the components have been soldered. The coat prevents corrosion and leakage currents or shorting due to condensation. The earliest conformal coats were wax; modern conformal coats are usually dips of dilute solutions of silicone rubber, polyurethane, acrylic, or epoxy. Another technique for applying a conformal coating is for plastic to be sputtered onto the PCB in a vacuum chamber. The chief disadvantage of conformal coatings is that servicing of the board is rendered extremely difficult.

Question: If a PCB is intended for use in an setting with extreme conditions, what would probably be applied?

Question: The conformal coating can prevent leaking, shorts, and what other type of moisture damage?

Question: A conformal coating can be applied by spraying the PCB or by doing what else to it?

Question: What material was first used for conformal coating?

Question: What action is made much more difficult for boards that have a conformal coating?
Passage: Many assembled PCBs are static sensitive, and therefore must be placed in antistatic bags during transport. When handling these boards, the user must be grounded (earthed). Improper handling techniques might transmit an accumulated static charge through the board, damaging or destroying components. Even bare boards are sometimes static sensitive. Traces have become so fine that it's quite possible to blow an etch off the board (or change its characteristics) with a static charge. This is especially true on non-traditional PCBs such as MCMs and microwave PCBs.

Question: What special protection are most PCBs shipped in?

Question: What force can easily change or even completely destroy an etch on a PCB?

Question: What's another term for "grounded" in the context of PCBs?

Question: Non-traditional PCBs include microwave PCBs and what other type of board?

Question: What class of PCBs are even more susceptible to static than standard ones?
Passage: The first PCBs used through-hole technology, mounting electronic components by leads inserted through holes on one side of the board and soldered onto copper traces on the other side. Boards may be single-sided, with an unplated component side, or more compact double-sided boards, with components soldered on both sides. Horizontal installation of through-hole parts with two axial leads (such as resistors, capacitors, and diodes) is done by bending the leads 90 degrees in the same direction, inserting the part in the board (often bending leads located on the back of the board in opposite directions to improve the part's mechanical strength), soldering the leads, and trimming off the ends. Leads may be soldered either manually or by a wave soldering machine.

Question: If a lead wasn't soldered manually, what device was used?

Question: Which type of PCB construction technique was used first?

Question: In through-hole construction, what are the leads on the front attached to on the back?

Question: For boards with two sides, to what angle are leads bent to in order to install them horizontally?

Question: What is improved by bending leads in opposite directions on the back of double-sided PCBs?
Passage: Through-hole manufacture adds to board cost by requiring many holes to be drilled accurately, and limits the available routing area for signal traces on layers immediately below the top layer on multi-layer boards since the holes must pass through all layers to the opposite side. Once surface-mounting came into use, small-sized SMD components were used where possible, with through-hole mounting only of components unsuitably large for surface-mounting due to power requirements or mechanical limitations, or subject to mechanical stress which might damage the PCB.

Question: Which type of construction is more expensive?

Question: What type of boards end up with limited routing space in through-hole manufacture?

Question: What force might damage a PCB if large components are surface mounted?

Question: What components can be surface mounted to save space?

Question: A component might not be able to be made any smaller because of its mechanical limitations or what other need?
Passage: Surface-mount technology emerged in the 1960s, gained momentum in the early 1980s and became widely used by the mid-1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto the PCB surface, instead of wire leads to pass through holes. Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much smaller PCB assemblies with much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labor costs and greatly increasing production rates. Components can be supplied mounted on carrier tapes. Surface mount components can be about one-quarter to one-tenth of the size and weight of through-hole components, and passive components much cheaper; prices of semiconductor surface mount devices (SMDs) are determined more by the chip itself than the package, with little price advantage over larger packages. Some wire-ended components, such as 1N4148 small-signal switch diodes, are actually significantly cheaper than SMD equivalents.

Question: When did surface-mount become a commonly used manufacturing process?

Question: What did the new components with end tabs replace?

Question: What improvement is possible with surface-mounting that enables higher production and lower labor costs?

Question: What are SMDs?

Question: Is it the chip or the package that influences the price of an SMD more?
Passage: Each trace consists of a flat, narrow part of the copper foil that remains after etching. The resistance, determined by width and thickness, of the traces must be sufficiently low for the current the conductor will carry. Power and ground traces may need to be wider than signal traces. In a multi-layer board one entire layer may be mostly solid copper to act as a ground plane for shielding and power return. For microwave circuits, transmission lines can be laid out in the form of stripline and microstrip with carefully controlled dimensions to assure a consistent impedance. In radio-frequency and fast switching circuits the inductance and capacitance of the printed circuit board conductors become significant circuit elements, usually undesired; but they can be used as a deliberate part of the circuit design, obviating the need for additional discrete components.

Question: What process is a trace left over from?

Question: Along with thickness, what determines the resistance of a trace?

Question: What type of trace is often narrower than a ground trace?

Question: In a multi-layer board, what metal would a ground plane be mostly made out of?

Question: What type of circuits use a stripline and microstrip format?
Passage: The cloth or fiber material used, resin material, and the cloth to resin ratio determine the laminate's type designation (FR-4, CEM-1, G-10, etc.) and therefore the characteristics of the laminate produced. Important characteristics are the level to which the laminate is fire retardant, the dielectric constant (er), the loss factor (tδ), the tensile strength, the shear strength, the glass transition temperature (Tg), and the Z-axis expansion coefficient (how much the thickness changes with temperature).

Question: What element of a laminate is designated by the cloth to resin ratio, among other things?

Question: What's the name for the measurement of a laminate's change in thickness as its temperature changes?

Question: What's the abbreviation for the glass transition temperature of a laminate?
Passage: There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the requirements of the circuit. Some of these dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well known pre-preg materials used in the PCB industry are FR-2 (phenolic cotton paper), FR-3 (cotton paper and epoxy), FR-4 (woven glass and epoxy), FR-5 (woven glass and epoxy), FR-6 (matte glass and polyester), G-10 (woven glass and epoxy), CEM-1 (cotton paper and epoxy), CEM-2 (cotton paper and epoxy), CEM-3 (non-woven glass and epoxy), CEM-4 (woven glass and epoxy), CEM-5 (woven glass and polyester). Thermal expansion is an important consideration especially with ball grid array (BGA) and naked die technologies, and glass fiber offers the best dimensional stability.

Question: What's the common name of polytetrafluoroethylene?

Question: What pre-preg material is designated by "FR-2"?

Question: To what type of packaging is thermal expansion particularly critical?

Question: What material delivers optimum dimensional stability?

Question: What dielectric is matte glass and polyester?
Passage: The reinforcement type defines two major classes of materials - woven and non-woven. Woven reinforcements are cheaper, but the high dielectric constant of glass may not be favorable for many higher-frequency applications. The spatially nonhomogeneous structure also introduces local variations in electrical parameters, due to different resin/glass ratio at different areas of the weave pattern. Nonwoven reinforcements, or materials with low or no reinforcement, are more expensive but more suitable for some RF/analog applications.

Question: Along with the non-woven class, what makes up the reinforcement type materials?

Question: Which is the more expensive reinforcement type?

Question: What type of structure do woven reinforcements have that cause them to have variation in their electrical parameters?

Question: Which ratio varies because of the structure of woven reinforcements?

Question: For what kind of applications are non-woven reinforcement materials better than woven ones?
Passage: At the glass transition temperature the resin in the composite softens and significantly increases thermal expansion; exceeding Tg then exerts mechanical overload on the board components - e.g. the joints and the vias. Below Tg the thermal expansion of the resin roughly matches copper and glass, above it gets significantly higher. As the reinforcement and copper confine the board along the plane, virtually all volume expansion projects to the thickness and stresses the plated-through holes. Repeated soldering or other exposition to higher temperatures can cause failure of the plating, especially with thicker boards; thick boards therefore require high Tg matrix.

Question: What's the temperature at which resin softens and expands called?

Question: Along with vias, what makes up the components of a board?

Question: What will happen to the components when the temperature of resin goes above that of glass transition?

Question: What do thick boards require to resist plating failure?

Question: When resin stays below Tg, its thermal expansion is similar to that of copper and what other material?