A via (Latin for ''path'' or ''way'') is an
electrical connection between copper layers in a
printed circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
. Essentially a via is a small drilled hole that goes through two or more adjacent layers; the hole is plated with copper that forms electrical connection through the insulation that separates the copper layers.
Vias are important for PCB manufacturing.
This is because the vias are drilled with certain tolerances and may be fabricated off their designated locations, so some allowance for errors in drill position must be made prior to manufacturing or else the manufacturing yield can decrease due to non-conforming boards (according to some reference standard) or even due to failing boards. In addition, regular through hole vias are considered fragile structures as they are long and narrow; the manufacturer must ensure that the vias are plated properly throughout the barrel and this in turn causes several processing steps.
In printed circuit boards
In
printed circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
(PCB) design, a via consists of two pads in corresponding positions on different copper layers of the board, that are electrically connected by a hole through the board. The hole is made conductive by
electroplating
Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be ...
, or is lined with a tube or a
rivet. High-density multilayer PCBs may have
microvias: blind vias are exposed only on one side of the board, while buried vias connect internal layers without being exposed on either surface. Thermal vias carry heat away from power devices and are typically used in arrays of about a dozen.
A via consists of:
# Barrel — conductive tube filling the drilled hole
# Pad — connects each end of the barrel to the component, plane, or trace
# Antipad — clearance hole between barrel and metal layer to which it is not connected
A via, sometimes called PTV or plated-through-via, should not be confused with a plated through hole (PTH). Via is used as an interconnection between copper layers on a PCB while the PTH is generally made larger than vias and is used as a plated hole for acceptance of component leads - such as non-SMT resistors, capacitors, and DIP package IC. PTH can also be used as holes for mechanical connection while vias may not. Another usage of PTH is known as a castellated hole where the PTH is aligned at the edge of the board so that it is cut in half when the board is milled out of the panel - the main usage is for allowing one PCB to be soldered to another in a stack - thus acting both as a fastener and also as a connector.
Three major kinds of vias are shown in right figure. The basic steps of making a PCB are: making the substrate material and stacking it in layers; through-drilling of plating the vias; and copper trace patterning using photolithography and etching. With this standard procedure, possible via configurations are limited to through-holes. Depth-controlled drilling techniques such as using lasers can allow for more varied via types. (Laser drills can also be used for smaller and more precisely positioned holes than mechanical drills produce.) PCB manufacturing typically starts with a so-called core, a basic double-sided PCB. Layers beyond the first two are stacked from this basic building block. If two more layers are consecutively stacked from bottom of core, you can have a 1-2 via, a 1-3 via and a
through hole. Each type of via is made by drilling at each stacking stage. If one layer is stacked on top of the core and other is stacked from the bottom, the possible via configurations are 1-3, 2-3 and through hole. The user must gather information about the PCB manufacturer's allowed methods of stacking and possible vias. For cheaper boards, only through holes are made and antipad (or clearance) is placed on layers which are supposed not to be contacted to vias.
IPC 4761
IPC 4761 defines the following via types:
* Type I: Tented via
* Type II: Tented & covered via
* Type III-a: Plugged via, sealed with non-conductive material on one side
* Type III-b: Plugged via, sealed with non-conductive material on both sides
* Type IV-a: Plugged & covered via, sealed with non-conductive material and covered with wet solder mask on one side
* Type IV-b: Plugged & covered via, sealed with non-conductive material and covered with wet solder mask on both sides
* Type V: Filled via, filled with non-conductive paste
* Type VI-a: Filled & covered via, covered with dry film or wet solder mask on one side
* Type VI-b: Filled & covered via, covered with dry film or wet solder mask on both sides
* Type VII: Filled & capped via, filled with non-conductive paste and overplated on both sides
Failure behavior
If well made, PCB vias will primarily fail due to differential expansion and contraction between the copper plating and the PCB in the out of plane direction (Z). This differential expansion and contraction will induce cyclic fatigue in the copper plating, eventually resulting in crack propagation and an electrical open circuit. Various design, material, and environmental parameters will influence the rate of this degradation.
To ensure via robustness,
IPC
IPC may refer to:
Computing
* Infrastructure protection centre or information security operations center
* Instructions per cycle or instructions per clock, an aspect of central-processing performance
* Inter-process communication, the sharin ...
sponsored a round-robin exercise that developed a time to failure calculator.
Vias in integrated circuits
In
integrated circuit
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
(IC) design, a via is a small opening in an insulating oxide layer that allows a conductive connection between different layers. A via on an integrated circuit that passes completely through a
silicon wafer or
die
Die, as a verb, refers to death, the cessation of life.
Die may also refer to:
Games
* Die, singular of dice, small throwable objects used for producing random numbers
Manufacturing
* Die (integrated circuit), a rectangular piece of a semicondu ...
is called a
through-chip via
In electronic engineering, a through-silicon via (TSV) or through-chip via is a vertical electrical connection (via) that passes completely through a silicon wafer or die. TSVs are high-performance interconnect techniques used as an alternativ ...
or
through-silicon via (TSV). Through-glass vias (TGV) have been studied by
Corning Glass
Corning Incorporated is an American multinational technology company that specializes in specialty glass, ceramics, and related materials and technologies including advanced optics, primarily for industrial and scientific applications. The co ...
for semiconductor packaging, due to the reduced electrical loss of glass versus silicon packaging.
A via connecting the lowest layer of metal to diffusion or poly is typically called a "contact".
Gallery
See also
*
Through-hole technology (THT)
*
Surface-mount technology
Surface-mount technology (SMT), originally called planar mounting, is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An electrical component mounted in this manner is referred ...
(SMT)
*
Through-silicon via (TSV)
*
Via fence
*
Feedthrough
Notes
References
Further reading
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External links
Online Via Calculator(Ampacity, Capacitance, Impedance, Power Dissipation Calculation).
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Electronic design
Electronics manufacturing
Printed circuit board manufacturing