PCB Blog

Surface mount technology (SMT) is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards (PCBs). Electronic devices so made are called surface-mount devices or SMDs. In the industry it has largely replaced the previous construction method of fitting components with wire leads into holes in the circuit board (also called through-hole technology).

An SMT component is usually smaller than its leaded counterpart because it has no leads or smaller leads. It may have short pins or leads of various styles, flat contacts, a matrix of balls (BGAs), or terminations on the body of the component (passives).

Surface-mount technology was developed in the 1960s and became widely used in the late 1980s. Much of the pioneering work in this technology was done at IBM. Components were mechanically redesigned to have small metal tabs or end caps that could be directly soldered to the surface of the PCB. Components became much smaller and component placement on both sides of the board became far more common with surface-mounting than through-hole mounting, allowing much higher circuit densities. Often, only the solder joints hold the parts to the board, although parts on the bottom or “second” side of the board are temporarily secured with a dot of adhesive as well. Surface-mounted devices (SMDs) are usually made physically small and lightweight for this reason. Surface mounting lends itself well to a high degree of automation, reducing labor cost and greatly increasing production rates. SMDs can be one-quarter to one-tenth the size and weight, and one-half to one-quarter the cost of through-hole parts.

Multiwire is a patented technique of interconnection which uses machine-routed insulated wires embedded in a non-conducting matrix. It was used during the 1980s and 1990s. (Augat Inc., U.S. Patent 4,648,180)

Cordwood construction can give large space-saving advantages and was often used with wire-ended components in applications where space was at a premium (such as missile guidance and telemetry systems). In ‘cordwood’ construction, two leaded components are mounted axially between two parallel planes. Instead of soldering the components, they were connected to other components by thin nickel tapes welded at right angles onto the component leads. To avoid shorting together of different interconnection layers, thin insulating cards were placed between them. Perforations or holes in the cards would allow component leads to project through to the next interconnection layer. One disadvantage of this system was that special nickel leaded components had to be used to allow the interconnecting welds to be made. Some versions of cordwood construction used single sided PCBs as the interconnection method (as pictured). This meant that normal leaded components could be used.

Before the advent of integrated circuits, this method allowed the highest possible component packing density; because of this, it was used by a number of computer vendors including Control Data Corporation. The cordwood method of construction now appears to have fallen into disuse, probably because high packing densities can be more easily achieved using surface mount techniques and integrated circuits.

Resource: Wikipedia.org

Patterning (etching)

The vast majority of ‘printed circuit boards’ are made by adhering a layer of copper over the entire substrate, sometimes on both sides, (creating a “blank PCB”) then removing unwanted copper after applying a temporary mask (e.g. by chemical etching ), leaving only the desired copper traces. A few PCBs are made by adding traces to the bare substrate (or a substrate with a very thin layer of copper) usually by a complex process of multiple electroplating steps.

There are three common “subtractive” methods (methods that remove copper) used for the production of printed circuit boards:

1. Silk screen printing uses etch-resistant inks to protect the copper foil. Subsequent etching removes the unwanted copper. Alternatively, the ink may be conductive, printed on a blank (non-conductive) board. The latter technique is also used in the manufacture of hybrid circuits.
2. Photoengraving uses a photomask and chemical etching to remove the copper foil from the substrate. The photomask is usually prepared with a photoplotter from data produced by a technician using CAM, or Computer-aided manufacturing software. Laser-printed transparencies are typically employed for phototools; however, direct laser imaging techniques are being employed to replace phototools for high-resolution requiements.
3. PCB Milling uses a 2 or 3 axis mechanical milling system to mill away the copper foil from the substrate. A PCB milling machine (referred to as a ‘PCB Prototyper’) operates in a similar way to a plotter, receiving commands from the host software that control the position of the milling head in the x, y, and (if relevant) z axis. Data to drive the Prototyper is extracted from files generated in PCB design software and stored in HPGL or Gerber file format.

“Additive” processes also exist. The most common is the “semi-additive process. In this version, the unpatterned board has a thin layer of copper already on it. A reverse mask is then applied. (Unlike a subtractive process mask, this mask exposes those parts of the substrate that will eventually become the traces.) Additional copper is then plated onto the board in the unmasked areas; copper may be plated to any desired weight. Tin-lead or other surface platings are then applied. The mask is stripped away and a brief etching step removes the now-exposed original copper laminate from the board, isolating the individual traces.

The additive process is commonly used for multi-layer boards as it facilitates the plating-through of the holes (vias) in the circuit board.

Lamination

Some PCBs have trace layers inside the PCB and are called multi-layer PCBs. These are formed by bonding together separately etched thin boards.

Drilling

Holes, or vias, through a PCB are typically drilled with tiny drill bits made of solid tungsten carbide. 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.

When very small vias are required, drilling with mechanical bits is costly because of high rates of wear and breakage. In this case, the vias may be evaporated by lasers. Laser-drilled vias typically have an inferior surface finish inside the hole. These holes are called micro vias.

It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect only some of the copper layers, rather than passing through the entire board. These holes are called blind vias when they connect an internal copper layer to an outer layer, or buried vias when they connect two or more internal copper layers.

The walls of the holes, for boards with 2 or more layers, are plated with copper to form plated-through holes that electrically connect the conducting layers of the PCB.

Solder plating and solder resist

The pads and lands to which components will be mounted are typically plated, because bare copper oxidizes quickly, and therefore is not readily solderable. Traditionally, any exposed copper was plated with solder. This solder was a tin-lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the EU, which restricts the use of lead. Other platings used are OSP (organic surface protectant), immersion silver, immersion gold over electroless nickel, and direct gold. Edge connectors, made on the sides of some boards, are often gold plated.

Areas that should not be soldered to may be covered with a polymer solder resist (solder mask’) coating. The solder resist prevents solder from bridging traces and thereby creating short circuits between nearby component leads. Solder resist also provides some protection from the environment.

Screen printing

Line art and text may be printed onto the outer surfaces of a PCB by screen printing. When space permits, the screen print text can indicate component designators, switch setting requirements, test points, and other features helpful in assembling, testing, and servicing the circuit board.

Screen print is also known as the silk screen, or, in one sided pcb’s, the red print.

Test

Unpopulated boards may be subjected to a bare-board test where each circuit connection (as defined in a netlist) is verified as correct on the finished board. For high-volume production, a Bed of nails tester or fixture is used to make contact with copper lands or holes on one or both sides of the board to facilitate testing. A computer will instruct the electrical test unit to send a small amount of current through each contact point on the bed-of-nails as required, and verify that such current can be seen on the other appropriate contact points. For small- or medium-volume boards, flying-probe testers use moving test heads to make contact with the copper lands or holes to verify the electrical connectivity of the board under test.

Populating

After the PCB is completed, electronic components must be attached to form a functional printed circuit assembly, or PCA. In through-hole construction, component leads may be inserted in holes and electrically and mechanically fixed to the board with a molten metal solder, while in surface-mount construction, the components are simply soldered to pads or lands on the outer surfaces of the PCB.

Often, through-hole and surface-mount construction must be combined in a single PCA because some required components are available only in surface-mount packages, while others are available only in through-hole packages.

Again, JEDEC guidelines for PCB component placement, soldering, and inspection are commonly used to maintain quality control in this stage of PCB manufacturing.

After the board is populated, the interconnection between the traces and the ICs may be tested by boundary scan techniques. 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.

Protection and packaging

PCBs intended for extreme environments often have a conformal coat, 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. Some are engineering plastics sputtered onto the PCB in a vacuum chamber. Mass-production PCBs have small pads for automated test equipment to make temporary connections. Sometimes the pads must be isolated with resistors.

Resource: Wikipedia.org

Most PCBs are composed of between one and sixteen conductive layers separated and supported by layers of insulating material (substrates) laminated (glued with heat, pressure & sometimes vacuum) together.

Layers may be connected together through drilled holes called vias. Either the holes are electroplated or small rivets are inserted. High-density PCBs may have blind vias, which are visible only on one surface, or buried vias, which are visible on neither.

The inventor of the printed circuit was the Austrian engineer Paul Eisler (1907–1995) who, while working in England, made one circa 1936 as part of a radio set. Around 1943 the USA began to use the technology on a large scale to make rugged radios for use in World War II. After the war, in 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army.

Before printed circuits (and for a while after their invention), point-to-point construction was used. For prototypes, or small production runs, wire wrap can be more efficient.

Originally, every electronic component had wire leads, and the PCB had holes drilled for each wire of each component. The components’ leads were then passed through the holes and soldered to the PCB trace. This method of assembly is called through-hole construction. In 1949, Moe Abramson and Stanilus F. Danko of the United States Army Signal Corps developed the Auto-Sembly process in which component leads were inserted into a copper foil interconnection pattern and dip soldered. With the development of board lamination and etching techniques, this concept evolved into the standard printed circuit board fabrication process in use today. Soldering could be done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machine. However, the wires and holes are wasteful since drilling holes are expensive and the protruding wires are merely cut off.

In recent years, the use of ’surface mount’ parts have gained popularity as the demand for smaller electronics packaging and greater functionality has grown.

In electronics, printed circuit boards, or PCBs, are used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive substrate. Alternative names are printed wiring board (PWB), etched wiring board, and switchboard. After populating the board with electronic components, a printed circuit assembly (PCA) is formed.

PCBs are rugged, inexpensive, and can be highly reliable. They require much more layout effort and higher initial cost than either wire-wrapped or point-to-point constructed circuits, but are much cheaper, faster, and consistent in high volume production.

Resource: Wikipedia.org

Circuit boards were created in the mid-1930s, most likely by Austrian Paul Eisler. During World War II the United States produced them on a massive scale for use in war radios. During this period the invention remained confined to the military sector, and it wasn’t until the end of the war that it became available for widespread commercial use.

Commonly known as PCBs (printed circuit boards), circuit boards consist of an insulator (usually fiberglass), with threads of conductive material serving as wires on the base of the board. The insulator may consist of one or numerous layers of material glued into a single entity. These additional layers may serve a number of purposes, including providing grounding to the board.

The threads on the surface of a circuit board are usually copper, created either by laying down individual lines mechanically, or by coating the entire board in copper and stripping away excess.

Since the 1980s most circuit boards use surface-mounted components. These components are designed with small tabs, and are made to be easily soldered in place on the board with very little hassle. In modern circuit board production, this process is usually performed by placing the cool solder mixture, and baking the entire board to melt the components in place. Prior to the creation of surface-mount technology in the mid-1960s, all circuit boards used wire to attach components to the board. With the need for wire eliminated, circuit boards have become lighter and more efficient to produce.

A number of additional technologies may be applied to circuit boards for specialized uses:

• flex circuit boards, for example, are designed to be quite flexible, allowing the circuit board to be placed in positions which would not otherwise be practical, or to be used in wire systems
• circuit boards for use in satellites and spacecraft are designed with rigid copper cores to conduct heat away from the sensitive components and protect them in the extreme temperatures they experience
• some circuit boards are designed with an interior conductive layer to carry power to various components without the need of extra traces.

Resource: WiseGeek.com

A PCB is a printed circuit board, also known as a printed wiring board. It is used in electronics to build electronic devices. A PCB serves two purposes in the construction of an electronic device; it is a place to mount the components and it provides the means of electrical connection between the components.

A PCB starts out as a thin, non-conducting sheet of material. The most common material used is a glass fiber epoxy laminate material. A thin layer of copper is then chemically deposited on each side of this material.

The next step is to “print” the connection diagram onto the PCB. The connection diagram is the wiring required to connect the components. In the very early days of electronics, these connections were in fact done with wires. This is the reason PCBs are also sometimes referred to as printed wiring boards. The “printing” is usually done by photographically transferring the image to the PCB. This image is “printed” with an acid resistant material.

Next, the PCB is put into an acid bath. The acid bath removes the copper from the board, excepting the areas protected by the resistant material. This process leaves the connections or wiring “printed” on the PCB. Next, holes are drilled in the board to allow the components to be mounted to the PCB and the PCB itself to be mounted to the case protecting the electronics. Finally, a protective coating is applied to the board to prevent corrosion of the copper traces.

The above process describes what is referred to as a doubled sided or two layer PCB. It is possible to make PCBs of almost any number of layers by repeating the process above and laminating the resulting PCBs into a single PCB. The PCB inside of the computer on which you are reading this document is a minimum four layer PCB and more likely to be an eight or even 12 layer PCB.

It is also more common now for parts to be soldered directly to the board, eliminating the need for holes into which leads (wires) from the components must be inserted. A component soldered to the board is logically called a surface mount component, and the PCB is a surface mount PCB. This technique allows a large increase in the number of components to be mounted on the same PCB, or the same number of components to be mounted on a much smaller PCB. However, surface mounting also requires better quality control on the manufacture of the PCB.

A PCB is now used in almost every electronic product available. It is by far the most common method currently used to connect and assemble the components of almost any electronic device.

Resource: WiseGeek.com