PCB & PCBA Glossary
62 terms, each defined in plain engineering language rather than restated from a dictionary. Where a term has a practical limit attached to it, the limit is given.
A–Zterminology
PCB & PCBAterms
Jump to: A B C D E F G H I L M O P Q R S T V W
Each entry is written so that it can be quoted on its own. Terms that carry a manufacturing limit — aspect ratio, annular ring, voiding — state the limit we actually work to. Figures match the capabilities page, which is authoritative.
A
American Wire Gauge
A wire diameter standard. Used in electrical tables, rarely on PCB stackups — PCB work normally uses mil, mm and ounces of copper.
Annular Ring
The width of copper left around a drilled hole after the hole is made. Too small a ring risks a breakout; typically 0.05 mm minimum on a Class 2 board.
Aspect Ratio
Board thickness divided by the smallest drilled hole diameter. Governs how difficult it is to plate copper evenly through a via barrel. 8:1 is standard, up to 25:1 for backplanes.
Automated Optical Inspection
Camera-based inspection of copper features or solder joints against the design data. Runs on 100% of inner and outer layers at Lingxin.
B
Ball Grid Array
A package whose connections are solder balls on the underside of the body rather than leads around the edge. Requires X-ray inspection because the joints are not visible.
Blind Via
A via that connects an outer layer to one or more inner layers without passing all the way through the board. Requires sequential lamination.
Buried Via
A via that connects inner layers only and is not visible on either outer surface. Requires sequential lamination and is the most expensive via type.
C
Coefficient of Thermal Expansion
How much a material expands per degree of temperature rise. The mismatch between laminate CTE and copper CTE drives stress on plated barrels during reflow and thermal cycling.
Composite Epoxy Material
Laminate grades using a paper or glass composite core. Cheaper than FR-4, used in single and double-sided consumer boards.
Controlled Impedance
Fabrication where the characteristic impedance of specified traces is held within a stated tolerance, verified by measuring coupons built onto the production panel. Lingxin holds ±8%.
Core
A fully cured, copper-clad sheet of laminate. The rigid middle of a multilayer stackup, as opposed to prepreg.
Coverlay
The protective film laminated over a flexible circuit in place of a solder mask, usually polyimide. Flexible, unlike solder mask.
D
Design for Manufacturability
Review of a design against the fabricator's process window before production. Included on every Lingxin order.
Dielectric Constant / Relative Permittivity
The ratio of a material's permittivity to that of vacuum. Determines propagation velocity and trace impedance. Standard FR-4 is about 4.3; Rogers materials run 3.3–3.6.
Differential Pair
Two traces routed together carrying equal and opposite signals. The impedance is a property of the pair and its coupling, not of either trace alone.
Dry Film Resist
A photosensitive polymer film laminated onto copper, exposed and developed to define the etch or plating pattern.
Dual In-line Package
A through-hole package with two rows of pins. Still common for connectors, relays and power devices.
E
Edge Coupling
Differential routing where the two traces run side by side on the same layer. The alternative is broadside coupling, where they sit on adjacent layers.
Electroless Nickel Electroless Palladium Immersion Gold
An ENIG variant with a palladium layer that allows both soldering and aluminium wire bonding on the same pad.
Electroless Nickel Immersion Gold
A surface finish of gold over nickel. Flat, good for fine pitch and BGA, and the most common finish for complex boards. The nickel barrier prevents copper diffusion.
F
Flame Retardant 4
The standard woven glass-epoxy laminate. The default substrate for rigid PCBs.
Flexible Printed Circuit
A circuit built on a flexible polyimide substrate. Can be single-sided, double-sided or multilayer.
Flying Probe Test
Electrical test using moving probes rather than a fixed fixture. No tooling cost, so it suits prototypes and small batches, but slower per board.
Functional Circuit Test
Powering a board and exercising its actual functions. The last electrical test before shipment, and the only one that proves the board works rather than merely connects.
G
Gerber (RS-274X)
The standard data format describing PCB layers. Usually delivered alongside a drill file (Excellon) and a stackup drawing.
Glass Transition Temperature
The temperature at which a laminate softens and its expansion behaviour changes. Standard FR-4 is 130–140 °C; high-Tg is 170–180 °C.
H
Half Ounce Copper
0.5 oz copper, about 17 µm thick. Used for very fine inner-layer features where thin copper etches more cleanly.
Hard Gold Plating
Gold with a small amount of cobalt or nickel, plated over nickel. Much harder than soft gold, so it is used on edge connectors and keypads subject to wear.
Heavy Copper
Boards with copper heavier than the standard 1–2 oz, typically 3 oz and above. Requires different etch and often copper fill on inner layers.
High Density Interconnect
Boards using laser-drilled microvias and, often, sequential lamination to achieve finer features than mechanical drilling allows. Measured in stages (1–3) by how many lamination cycles are needed.
Hot Air Solder Levelling
A surface finish applied by dipping the board in molten solder then blowing excess off with hot air. Cheap and robust; lead-free HASL is now standard.
I
IPC-2221
The generic design standard for printed boards. Source of the widely used trace width versus current and temperature rise charts.
IPC-A-600
The acceptability standard for finished printed boards. Class 2 is the general industrial default; Class 3 is for high-reliability.
IPC-A-610
The acceptability standard for electronic assemblies. Covers solder joint appearance, cleaning, and component mounting criteria.
Impedance
The frequency-dependent resistance a signal sees as it travels along a trace. Determined by geometry and dielectric, and the reason stackups matter.
In-Circuit Test
Electrical test of an assembled board using a bed-of-nails fixture. Fast and thorough; requires tooling.
L
Laminate
Copper-clad dielectric sheet. In a stackup, laminates appear as either cured core or partially cured prepreg.
Liquid Photo-Imageable (Solder Mask)
The standard solder mask type: applied as a liquid, imaged photographically, then cured. More accurate than screen-printed mask.
Loss Tangent
How lossy a dielectric is at a given frequency. Dominates insertion loss above roughly 1 GHz. Standard FR-4 is around 0.02; low-loss materials are 0.002–0.004.
M
Metal Core PCB
A board built on an aluminium or copper base rather than FR-4, for thermal and current capacity. Typically single or double-sided because the core must be insulated.
Microstrip
A trace on an outer layer over a reference plane. The simplest controlled-impedance geometry.
Microvia
A laser-drilled via, typically 0.10–0.15 mm, connecting adjacent layers. The defining feature of HDI.
mil (thou)
One thousandth of an inch, 0.0254 mm. The traditional PCB unit; still standard for trace and space on drawings.
O
Organic Solderability Preservative
A thin organic coating protecting bare copper. Flat and cheap; shelf life is limited compared with ENIG.
P
PTH
A drilled hole with plated copper on its wall, connecting layers electrically.
Panel
The sheet of laminate that enters production, containing multiple copies of the customer board. Panelisation determines material utilisation and therefore price.
Parylene
A conformal coating deposited in vacuum, giving a pinhole-free film. More expensive and harder to rework than sprayed coatings.
Prepreg
Partially cured resin-impregnated glass. It flows and cures during lamination, bonding the layers together and filling the space between copper features.
Q
Quad Flat No-leads
A package with pads under the body rather than leads. Needs a thermal pad and, usually, a stencil aperture calculation rather than a guess.
R
Reference Plane
A continuous copper layer that the signal traces reference for impedance and return current. Its continuity matters as much as its presence.
Rigid-Flex
A board combining rigid and flexible sections in one construction, laminated together. Eliminates connectors and increases reliability but has a strict bend-radius requirement.
S
Silkscreen
Printed legends and reference designators on the board surface. Minimum readable line width is about 0.15 mm.
Solder Mask
The protective polymer layer covering everything except the pads. Prevents solder bridging and protects traces.
Solder Paste Inspection
Measurement of printed solder paste volume before reflow. Catches stencil and printer problems before they become solder defects.
Stackup
The ordered list of copper and dielectric layers in a board, with their materials and thicknesses. Determines total thickness, impedance and the fabrication process.
Stripline
A trace buried between two reference planes. Better shielded than microstrip, but wider for the same impedance.
T
Tented Via
A via covered by solder mask, preventing solder from wicking down the hole. Standard on most designs.
Thick Film
A resistor or conductor pattern printed as a paste and fired, used on ceramic substrates.
Time Domain Reflectometry
Measuring impedance along a trace by sending a fast edge and observing reflections. The practical way to verify a finished coupon.
V
Via
A plated hole used to connect layers electrically and carry no component.
Voiding
Gas bubbles trapped in solder or in a plated barrel. In BGA joints, voiding above roughly 25% is normally rejected.
W
Wave Solder
Mass soldering of through-hole components by passing the board over a standing wave of molten solder.