Via (electronics)

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A via (Latin, 'path' or 'way') is an electrical connection between two or more metal layers, and are commonly used in printed circuit boards (PCB). Essentially a via is a small drilled hole that goes through two or more adjacent layers; the hole is plated with metal (often copper) that forms an electrical connection through the insulating layers.

Contents

Vias are important for PCB manufacturing. [1] 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

Different types of vias:
(1) Through hole.
(2) Blind via.
(3) Buried via.
The gray and green layers are nonconducting, while the thin orange layers and red vias are conductive. Via Types.svg
Different types of vias:
(1) Through hole.
(2) Blind via.
(3) Buried via.
The gray and green layers are nonconducting, while the thin orange layers and red vias are conductive.
PCB via current capacity chart showing 1 mil plating via current capacity & resistance versus diameter on a 1.6 mm PCB. ViaCurrentCapacity.png
PCB via current capacity chart showing 1 mil plating via current capacity & resistance versus diameter on a 1.6 mm PCB.

In printed circuit board (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.[ citation needed ] The hole is made conductive by electroplating, or is lined with a tube or a rivet.[ citation needed ] 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. [2] [3]

A via consists of:

  1. Barrel — conductive tube filling the drilled hole
  2. Pad — connects each end of the barrel to the component, plane, or trace
  3. 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. [4]

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. [lower-alpha 1] 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:

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. [5] [6] To ensure via robustness, IPC sponsored a round-robin exercise that developed a time to failure calculator. [7]

Vias in integrated circuits

In integrated circuit (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 is called a through-chip via or through-silicon via (TSV). Through-glass vias (TGV) have been studied by Corning Glass for semiconductor packaging, due to the reduced electrical loss of glass versus silicon packaging. [8] A via connecting the lowest layer of metal to diffusion or poly is typically called a "contact".

See also

Notes

  1. Through-holes per core. It is possible, though more expensive, to create blind or buried vias by using additional cores and lamination steps. It is also possible to backdrill and remove the plating from one side through to the desired layer, which leaves the physical hole as a through-hole, but creates the electrical equivalent of a blind via. If a PCB needs enough layers to justify blind and buried vias, it is probably also using small enough traces packed tightly enough to require (laser-drilled) microvias.

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<span class="mw-page-title-main">Printed circuit board</span> Board to support and connect electronic components

A printed circuit board (PCB), also called printed wiring board (PWB), is a medium used to connect or "wire" components to one another in a circuit. It takes the form of a laminated sandwich structure of conductive and insulating layers: each of the conductive layers is designed with an artwork pattern of traces, planes and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Electrical components may be fixed to conductive pads on the outer layers in the shape designed to accept the component's terminals, generally by means of soldering, to both electrically connect and mechanically fasten them to it. Another manufacturing process adds vias, plated-through holes that allow interconnections between layers.

<span class="mw-page-title-main">Surface-mount technology</span> Method for producing electronic circuits

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 to as a surface-mount device (SMD). In industry, this approach has largely replaced the through-hole technology construction method of fitting components, in large part because SMT allows for increased manufacturing automation which reduces cost and improves quality. It also allows for more components to fit on a given area of substrate. Both technologies can be used on the same board, with the through-hole technology often used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.

<span class="mw-page-title-main">Flexible electronics</span> Mounting of electronic devices on flexible plastic substrates

Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide, PEEK or transparent conductive polyester film. Additionally, flex circuits can be screen printed silver circuits on polyester. Flexible electronic assemblies may be manufactured using identical components used for rigid printed circuit boards, allowing the board to conform to a desired shape, or to flex during its use.

<span class="mw-page-title-main">Stripboard</span>

Stripboard is the generic name for a widely used type of electronics prototyping material for circuit boards characterized by a pre-formed 0.1 inches (2.54 mm) regular (rectangular) grid of holes, with wide parallel strips of copper cladding running in one direction all the way across one side of on an insulating bonded paper board. It is commonly also known by the name of the original product Veroboard, which is a trademark, in the UK, of British company Vero Technologies Ltd and Canadian company Pixel Print Ltd. It was originated and developed in the early 1960s by the Electronics Department of Vero Precision Engineering Ltd (VPE). It was introduced as a general-purpose material for use in constructing electronic circuits - differing from purpose-designed printed circuit boards (PCBs) in that a variety of electronics circuits may be constructed using a standard wiring board.

<span class="mw-page-title-main">Printed circuit board milling</span>

Printed circuit board milling is the milling process used for removing areas of copper from a sheet of printed circuit board (PCB) material to recreate the pads, signal traces and structures according to patterns from a digital circuit board plan known as a layout file. Similar to the more common and well known chemical PCB etch process, the PCB milling process is subtractive: material is removed to create the electrical isolation and ground planes required. However, unlike the chemical etch process, PCB milling is typically a non-chemical process and as such it can be completed in a typical office or lab environment without exposure to hazardous chemicals. High quality circuit boards can be produced using either process. In the case of PCB milling, the quality of a circuit board is chiefly determined by the system's true, or weighted, milling accuracy and control as well as the condition of the milling bits and their respective feed/rotational speeds. By contrast, in the chemical etch process, the quality of a circuit board depends on the accuracy and/or quality of the mask used to protect the copper from the chemicals and the state of the etching chemicals.

<span class="mw-page-title-main">Gold plating</span> Coating an object with a thin layer of gold

Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver, by chemical or electrochemical plating. This article covers plating methods used in the modern electronics industry; for more traditional methods, often used for much larger objects, see gilding.

<span class="mw-page-title-main">Through-hole technology</span> Circuit board manufacturing technique

In electronics, through-hole technology is a manufacturing scheme in which leads on the components are inserted through holes drilled in printed circuit boards (PCB) and soldered to pads on the opposite side, either by manual assembly or by the use of automated insertion mount machines.

<span class="mw-page-title-main">Electroless nickel-phosphorus plating</span> Chemical-induced nickel coating of a surface

Electroless nickel-phosphorus plating, also referred to as E-nickel, is a chemical process that deposits an even layer of nickel-phosphorus alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing nickel salt and a phosphorus-containing reducing agent, usually a hypophosphite salt. It is the most common version of electroless nickel plating and is often referred by that name. A similar process uses a borohydride reducing agent, yielding a nickel-boron coating instead.

<span class="mw-page-title-main">Perfboard</span> Material for prototyping electronic circuits

Perfboard is a material for prototyping electronic circuits. It is a thin, rigid sheet with holes pre-drilled at standard intervals across a grid, usually a square grid of 0.1 inches (2.54 mm) spacing. These holes are ringed by round or square copper pads, though bare boards are also available. Inexpensive perfboard may have pads on only one side of the board, while better quality perfboard can have pads on both sides. Since each pad is electrically isolated, the builder makes all connections with either wire wrap or miniature point to point wiring techniques. Discrete components are soldered to the prototype board such as resistors, capacitors, and integrated circuits. The substrate is typically made of paper laminated with phenolic resin or a fiberglass-reinforced epoxy laminate (FR-4).

Automated optical inspection (AOI) is an automated visual inspection of printed circuit board (PCB) manufacture where a camera autonomously scans the device under test for both catastrophic failure and quality defects. It is commonly used in the manufacturing process because it is a non-contact test method. It is implemented at many stages through the manufacturing process including bare board inspection, solder paste inspection (SPI), pre-reflow and post-re-flow as well as other stages.

<span class="mw-page-title-main">Solder mask</span> Layer of polymer applied to printed circuit boards

Solder mask, solder stop mask or solder resist is a thin lacquer-like layer of polymer that is usually applied to the copper traces of a printed circuit board (PCB) for protection against oxidation and to prevent solder bridges from forming between closely spaced solder pads. A solder bridge is an unintended electrical connection between two conductors by means of a small blob of solder. PCBs use solder masks to prevent this from happening. Solder mask is not always used for hand soldered assemblies, but is essential for mass-produced boards that are soldered automatically using reflow or wave soldering techniques. Once applied, openings must be made in the solder mask wherever components are soldered, which is accomplished using photolithography. Solder mask is traditionally green, but is also available in many other colors.

Electroless nickel immersion gold (ENIG or ENi/IAu), also known as immersion gold (Au), chemical Ni/Au or soft gold, is a metal plating process used in the manufacture of printed circuit boards (PCBs), to avoid oxidation and improve the solderability of copper contacts and plated through-holes. It consists of an electroless nickel plating, covered with a thin layer of gold, which protects the nickel from oxidation. The gold is typically applied by quick immersion in a solution containing gold salts. Some of the nickel is oxidized to Ni2+ while the gold is reduced to metallic state. A variant of this process adds a thin layer of electroless palladium over the nickel, a process known by the acronym ENEPIG.

Board-to-board (BTB) connectors are used to connect printed circuit boards (PCB), electronic components that contain a conductive pattern printed on the surface of the insulating base in an accurate and repeatable manner. Each terminal on a BTB connector is connected to a PCB. A BTB connector includes housing and a specific number of terminals. The terminal is made from a conductive material, and plated to improve conductivity and antirust. Terminals transmit the current/signal between PCBs connected by BTB; the housing is made of insulating material.

Microvias are used as the interconnects between layers in high density interconnect (HDI) substrates and printed circuit boards (PCBs) to accommodate the high input/output (I/O) density of advanced packages. Driven by portability and wireless communications, the electronics industry strives to produce affordable, light, and reliable products with increased functionality. At the electronic component level, this translates to components with increased I/Os with smaller footprint areas, and on the printed circuit board and package substrate level, to the use of high density interconnects (HDIs).

The Occam process is a solder-free, Restriction of Hazardous Substances Directive (RoHS)-compliant method for use in the manufacturing of electronic circuit boards developed by Verdant Electronics. It combines the usual two steps of the construction of printed circuit boards (PCBs) followed by the population process of placing various leaded and non-leaded electronic components into one process. The name "Occam" comes from a quotation by William of Ockham.

Stencil printing is the process of depositing solder paste on the printed wiring boards (PWBs) to establish electrical connections. It is immediately followed by the component placement stage. The equipment and materials used in this stage are a stencil, solder paste, and a printer.

<span class="mw-page-title-main">Teardrop (electronics)</span> Printed circuit board feature

A teardrop is typically drop-shaped feature on a printed circuit board and can be found on the junction of vias or contact pads.

Conductive anodic filament, also called CAF, is a metallic filament that forms from an electrochemical migration process and is known to cause printed circuit board (PCB) failures.

<span class="mw-page-title-main">Electronics prototyping</span>

In electronics, prototyping means building an actual circuit to a theoretical design to verify that it works, and to provide a physical platform for debugging it if it does not. The prototype is often constructed using techniques such as wire wrapping or using a breadboard, stripboard or perfboard, with the result being a circuit that is electrically identical to the design but not physically identical to the final product.

Electroless copper plating is a chemical process that deposits an even layer of copper on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing copper salts and a reducing agent such as formaldehyde.

References

  1. "PCB Vias: An In-Depth Guide". ePiccolo Engineering.
  2. "PCB design: A close look at facts and myths about thermal vias".
  3. Gautam, Deepak; Wager, Dave; Musavi, Fariborz; Edington, Murray; Eberle, Wilson; Dunford, Willa G. (2013-03-17). A review of thermal management in power converters with thermal vias. 2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC). Long Beach, California, U.S.A: IEEE. doi:10.1109/APEC.2013.6520276.
  4. "Castellated Holes / Edge Plating PCB / Castellations". Hi-Tech Corp. 2011. Archived from the original on 2016-05-26. Retrieved 2013-01-02.
  5. C. Hillman, Understanding plated through via failures, Global SMT & Packaging – November 2013, pp 26-28, https://www.dfrsolutions.com/hubfs/Resources/services/Understanding_Plated_Through_Via_Failures.pdf?t=1514473946162
  6. C. Hillman, Reliable Plated Through Via Design and Fabrication, http://resources.dfrsolutions.com/White-Papers/Reliability/Reliable-Plated-Through-Via-Design-and-Fabrication1.pdf
  7. "Plated Through Hole (PTH) Fatigue calculator". DfR Solutions. Retrieved 2017-12-17.
  8. "Progress and Application of Through Glass Via (TGV) Technology" (PDF). corning.com. Retrieved 2019-08-08.

Further reading