rajpurkar/squad huggingface dataset:
Title: Printed_circuit_board

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?
Answer: conductive

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

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

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

Question: Etch-back removes glass fibers and what other material?
Answer: resin
Title: Printed_circuit_board

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?
Answer: oxidizes

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

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

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

Question: What alloy can no longer be used in HASL because of restrictions on the use of one of its metal components?
Answer: tin-lead
Title: Printed_circuit_board

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?
Answer: organic surface protectant

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

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

Question: Ag3Cu is one intermetallic that tin forms; what's the other one?
Answer: Cu5Sn6

Question: What might the rapid diffusion of coating metal into tin solder leave in the surface coating?
Answer: voids
Title: Printed_circuit_board

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?
Answer: Electrochemical migration

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

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

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

Question: What's present in plated surfaces that causes tin to grow metal filaments?
Answer: tension
Title: Printed_circuit_board

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?
Answer: solder resist

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

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

Question: What kind of coating gets applied to the PWB before light exposure?
Answer: photo-sensitive

Question: What solder resist is so inaccurate that few choose it anymore?
Answer: screen print epoxy ink
Title: Printed_circuit_board

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"?
Answer: "shorts"

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

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

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

Question: What system tells the flying probe where to go and what to do?
Answer: CAM system
Title: Printed_circuit_board

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?
Answer: through-hole

Question: Which type of construction makes components that use more space?
Answer: through-hole

Question: Which of the two kinds of construction is weaker under strain?
Answer: surface-mount

Question: If you're building a circuit board that has a lot of components that won't be used, which construction would be better?
Answer: surface-mount
Title: Printed_circuit_board

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?
Answer: Joint Test Action Group

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

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

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

Question: What would you avoid by using the Joint Test Action Group standard?
Answer: physical test probes
Title: Printed_circuit_board

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?
Answer: conformal coating

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

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

Question: What material was first used for conformal coating?
Answer: wax

Question: What action is made much more difficult for boards that have a conformal coating?
Answer: servicing
Title: Printed_circuit_board

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?
Answer: antistatic bags

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

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

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

Question: What class of PCBs are even more susceptible to static than standard ones?
Answer: non-traditional
Title: Printed_circuit_board

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?
Answer: wave soldering machine

Question: Which type of PCB construction technique was used first?
Answer: through-hole

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

Question: For boards with two sides, to what angle are leads bent to in order to install them horizontally?
Answer: 90 degrees

Question: What is improved by bending leads in opposite directions on the back of double-sided PCBs?
Answer: mechanical strength
Title: Printed_circuit_board

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?
Answer: Through-hole

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

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

Question: What components can be surface mounted to save space?
Answer: small-sized SMD components

Question: A component might not be able to be made any smaller because of its mechanical limitations or what other need?
Answer: power requirements
Title: Printed_circuit_board

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?
Answer: mid-1990s

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

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

Question: What are SMDs?
Answer: surface mount devices

Question: Is it the chip or the package that influences the price of an SMD more?
Answer: chip
Title: Printed_circuit_board

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?
Answer: etching

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

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

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

Question: What type of circuits use a stripline and microstrip format?
Answer: microwave circuits
Title: Printed_circuit_board

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?
Answer: type designation

Question: What's the name for the measurement of a laminate's change in thickness as its temperature changes?
Answer: Z-axis expansion coefficient

Question: What's the abbreviation for the glass transition temperature of a laminate?
Answer: Tg
Title: Printed_circuit_board

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?
Answer: Teflon

Question: What pre-preg material is designated by "FR-2"?
Answer: phenolic cotton paper

Question: To what type of packaging is thermal expansion particularly critical?
Answer: ball grid array

Question: What material delivers optimum dimensional stability?
Answer: glass fiber

Question: What dielectric is matte glass and polyester?
Answer: FR-6
Title: Printed_circuit_board

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?
Answer: woven

Question: Which is the more expensive reinforcement type?
Answer: non-woven

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

Question: Which ratio varies because of the structure of woven reinforcements?
Answer: resin/glass

Question: For what kind of applications are non-woven reinforcement materials better than woven ones?
Answer: RF/analog
Title: Printed_circuit_board

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?
Answer: glass transition temperature

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

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

Question: What do thick boards require to resist plating failure?
Answer: high Tg matrix

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

Moisture absorption occurs when the material is exposed to high humidity or water. Both the resin and the reinforcement may absorb water; water may be also soaked by capillary forces through voids in the materials and along the reinforcement. Epoxies of the FR-4 materials aren't too susceptible, with absorption of only 0.15%. Teflon has very low absorption of 0.01%. Polyimides and cyanate esters, on the other side, suffer from high water absorption. Absorbed water can lead to significant degradation of key parameters; it impairs tracking resistance, breakdown voltage, and dielectric parameters. Relative dielectric constant of water is about 73, compared to about 4 for common circuitboard materials. Absorbed moisture can also vaporize on heating and cause cracking and delamination, the same effect responsible for "popcorning" damage on wet packaging of electronic parts. Careful baking of the substrates may be required.

Question: What will happen to the materials when a board gets wet or is in a humid area?
Answer: Moisture absorption

Question: If materials absorb moisture and then are heated enough to vaporize the water, the board might show delamination or what other type of damage?
Answer: cracking

Question: What percentage absorption do F-4 epoxies have?
Answer: 0.15%

Question: What's the approximate relative dielectric constant of most materials in circuit boards?
Answer: 4

Question: What material has an absorption rate of only 0.01%?
Answer: Teflon