The Complete Guide to Printed Circuit Boards and Electronics Packaging
Introduction and the Packaging Hierarchy:
In the vast landscape of modern electronics, the journey from a microscopic semiconductor die to a fully functional consumer device is a complex process that relies heavily on intermediate structures and interconnection technologies. Packaged integrated circuits (ICs) are seldom used in isolation, rather they almost always function as part of a larger assembly where multiple ICs and discrete components work together to perform the desired computing, communication or control tasks. The substrate that facilitates the final interconnections among all these completed chips, while simultaneously serving as the communication link between the outside world and the microelectronic circuitry within each packaged IC, is the printed circuit board (PCB). Beyond just holding ICs, circuit boards must also accommodate discrete circuit components. These include resistors, capacitors, inductors, high performance transistors, large capacitors, precision resistors and crystals used for frequency control. These discrete parts are necessary because they either take up too much valuable real estate on the limited silicon surface possess special power dissipation requirements that are impractical to integrate or simply cannot be implemented on a chip at all. For example, inductors are notoriously difficult to integrate onto the silicon surface, necessitating their placement directly on the board. The physical and mechanical infrastructure that holds, protects and interconnects these components is formally known as the electronics package. A well designed electronics package must successfully serve five critical functions: it must distribute power and facilitate signal interconnection between various components, provide structural support to maintain the physical integrity of the system, protect the circuits from physical impacts and chemical hazards present in the operating environment efficiently dissipate the heat generated by active circuits to prevent thermal damage and finally, it must minimize delays in signal transmission to preserve the high speed performance of the system.
To manage the complexity of large electronic systems containing thousands of components and millions of interconnections, the electronics package is organized into distinct hierarchical levels. The lowest tier, referred to as the zero level, pertains to the intraconnections that exist directly on the semiconductor chip itself the microscopic wiring between individual transistors and logic gates. Moving up, the packaged chip consisting of the IC enclosed in a plastic or ceramic carrier and connected to external package leads constitutes the first level of packaging. These packaged chips along with other discrete components, are then assembled onto a PCB using one of two primary technologies: surface-mount technology (SMT) or through hole technology, which is also known as pin in hole (PIH) technology. SMT has become much more widely adopted in modern industry, particularly for high volume, mass produced consumer electronics though both technologies are often employed on the same board depending on the component types involved. The assembly of components onto the PCB represents the second level of packaging. Following this, the assembled PCBs are connected to a chassis, rack or backplane framework which constitutes the third level. In major systems like large computers, PCBs are frequently mounted onto a larger printed circuit board called a back plane that contains conductive paths to permit interconnection between the smaller boards, this specific configuration is known as card on board packaging. Finally, the fourth level encompasses the internal wiring and cabling that runs inside the system cabinet to link various subassemblies together. It is important to note that for relatively low complexity systems, the packaging hierarchy may not include all of these possible levels.
PCB Structures, Types and Fundamental Materials:
A printed circuit board also frequently referred to as a printed wiring board is fundamentally a laminated flat panel constructed from insulating material. Its primary purpose is to provide the electrical interconnections between the electronic components attached to it. These interconnections are made through thin conducting paths known as tracks which are made of copper and run along the surface of the board or are sandwiched in alternating layers between sheets of insulation. In addition to tracks, copper areas called lands are also present on the board surface specifically for attaching and electrically connecting the leads of components. The widespread adoption of PCBs across virtually all sectors of electronics is driven by several distinct advantages. They provide a convenient and standardized structural platform for components. A board with correctly routed interconnections can be mass produced with remarkable consistency eliminating the variability and errors associated with manual hand wiring. The assembly process is highly efficient because nearly all soldering connections between components and the board can be accomplished in a single mechanized operation. Furthermore, assembled PCBs deliver highly reliable performance and in complex systems, each individual board can be easily detached for service, repair or replacement without disturbing the entire system.
The insulating materials used in PCB construction are typically polymer composites that are reinforced with glass fabrics or paper to enhance their mechanical strength and dimensional stability. The most widely used polymer is epoxy followed by phenolic and polyimide. For glass reinforcing fabrics, particularly in epoxy-based boards, E glass is the standard fiber type while cotton paper is commonly utilized as a reinforcing layer in phenolic boards. The materials chosen for the PCB structure must possess a specific combination of properties to ensure long term reliability. They must be electrically insulating to prevent shorts, strong and rigid to support heavy components, resistant to warpage to maintain flatness dimensionally stable under temperature variations, heat resistant enough to withstand soldering processes and flame retardant to meet safety standards. To achieve these last two characteristics, chemicals are often added to the polymer composite during manufacturing. The typical thickness of a single substrate layer ranges from 0.8 to 3.2 millimeters while the copper foil used for the tracks is around 0.04 millimeters thick.
There are three principal configurations of printed circuit boards. Single sided boards feature copper foil exclusively on one side of the insulation substrate, making them the simplest and least expensive form. Double sided boards have copper foil on both sides which allows for more complex routing but typically requires vias plated holes that establish electrical connectivity between the features on both sides. Multilayer boards are the most complex, consisting of alternating layers of copper foil and insulation material. These are utilized for highly intricate circuit assemblies where a large number of components must be interconnected with many routing paths that cannot be accommodated by just one or two copper layers. In the industry, a four layer configuration is the most common, but boards with up to 24 conducting layers are produced for the most demanding applications such as servers and telecommunications equipment. Multilayer boards can incorporate partial, buried or through hole vias providing immense flexibility for routing. One of the major benefits of double sided and multilayer configurations is that IC packages can be bonded to both sides of the board enabling much more compact and space efficient product designs.
Manufacturing of Starting Boards and Copper Foil Production:
The manufacturing process for PCBs begins with the production of the base materials. Single sided and double sided starting boards can be purchased from specialized suppliers who mass produce them in standard sizes. These base panels are then custom processed by a circuit fabricator to create the specific circuit pattern and board dimensions required for a particular application. Multilayer boards on the other hand, are fabricated from these standard single sided and double sided boards. The circuit fabricator processes these individual boards separately to form the required circuit pattern for each layer, and then bonds them together using additional layers of epoxy impregnated glass fabric. Processing multilayer boards is significantly more involved and expensive than the simpler types but the superior performance and space savings they provide for large systems justify the increased cost. The copper foil that forms the conductive pathways is produced through a highly specialized continuous electroforming process. In this method, a rotating smooth metal drum is partially submerged in an electrolytic bath containing copper ions. The drum functions as the cathode in the electrical circuit, causing pure copper to plate onto its surface. As the drum continues to rotate out of the bath, the newly formed thin copper foil is peeled off from its surface. This specific electroforming technique is ideal for producing the extremely thin, high-purity copper foil required for fine-pitch PCB applications.
The fabrication of the insulating starting boards involves pressing multiple sheets of woven glass fiber that have been thoroughly impregnated with partially cured epoxy resin. The number of glass fiber sheets used in the sandwich assembly directly determines the final thickness of the board. Copper foil is placed on one or both sides of the epoxy glass stack, depending on whether single sided or double sided boards are being manufactured. For single sided production, a thin release film is used on the side opposite the copper to prevent the epoxy from sticking to the press platens. The entire stack is pressed between two steam heated platens of a hydraulic press. The combination of high heat and high pressure compacts the layers and fully cures the epoxy resin, bonding the laminates into a single, rigid and strong piece. After pressing, the board is cooled and trimmed to remove any excess epoxy that was squeezed out around the edges during the lamination process. At this stage, the completed board consists of a glass fabric reinforced epoxy panel clad with copper on one or both surfaces, ready for the circuitization process. These panels are usually produced in large standard widths designed to match the handling systems of wave-soldering equipment, automatic insertion machines and other automated processing facilities. If the final electronic design calls for a smaller size, multiple individual circuits can be processed together on the same larger panel and then separated later into individual boards.
Detailed PCB Fabrication Processes:
The transformation of a bare copper clad board into a functional printed circuit requires the circuit fabricator to employ a diverse array of processing operations including cleaning, shearing, hole drilling or punching, pattern imaging, etching and various electroless and electrolytic plating techniques. Many of these processes must be conducted under clean room conditions to avoid microscopic defects in the printed circuits especially for boards designed with very fine tracks and close tolerances. The initial preparation phase involves shearing the board to its required size, punching or drilling tabs and slots and creating tooling holes. These tooling holes, typically about 3.2 millimeters in diameter are drilled or punched into the corners of the board and are specifically designed for positioning and aligning the board during subsequent processing steps. The entire fabrication sequence requires extremely close alignment from one process to the next and these tooling holes serve as precise references for locating pins at each operation. Three such holes per board are usually sufficient to achieve accurate registration across all steps. During this preparation phase, the board is typically labeled with a bar code for identification and traceability purposes. Finally, a rigorous cleaning process is employed to remove dirt, dust and surface grease from the board. While cleanliness requirements are not as stringent as those in IC fabrication, any small particle of dust or film of grease can cause significant defects in the circuit pattern or inhibit chemical etching and plating processes. Functional circuit holes are required in PCBs for several distinct purposes. These include insertion holes for accepting the leads of through hole components via holes which are subsequently copper plated to serve as conductive paths between different layers and holes used to mechanically fasten components such as heat sinks and connectors to the board. These holes are typically produced by drilling as this method yields cleaner and more dimensionally accurate results compared to punching. To increase production throughput, a stack of three or four panels may be drilled simultaneously in the same operation using a computer numerically controlled (CNC) drill press. For high volume production runs, multiple spindle drills are sometimes employed to drill all the required holes in a board in a single feed motion.
The use of standard twist drills for PCB drilling presents several unique and demanding challenges. The most significant issue is the exceptionally small hole size required in modern designs, drill diameters are generally less than 1.27 millimeters and some high density boards require holes of 0.15 millimeters or even less. Such small drill bits inherently lack mechanical strength and have a very limited capacity to dissipate the heat generated during cutting. The work material itself adds to the difficulty, the drill bit must first penetrate a thin, ductile copper foil and then proceed through an abrasive epoxy glass composite material. Each of these materials would normally require a specifically designed drill yet a single drill bit must suffice for both. Consequently, cemented carbide or coated carbide tools are heavily favored over conventional high speed steel due to their superior hardness and wear resistance. The practice of stacking multiple boards results in a high depth to diameter ratio which severely aggravates the problem of extracting chips from the hole. Additional stringent requirements include high positional accuracy, exceptionally smooth hole walls and the complete absence of burrs. Burrs are typically formed when the drill bit enters or exits the material so thin sheets of sacrificial material are often placed on top of and beneath the stack to prevent burr formation on the production boards themselves. To operate at peak efficiency, any cutting tool must be used at the correct cutting speed, which for a drill bit is measured at its outer diameter. For the very small drill sizes common in PCB fabrication, this translates to extremely high rotational speeds, sometimes reaching up to 100,000 revolutions per minute necessitating the use of specialized spindle bearings and high frequency motors.
Following hole formation, the circuit pattern must be transferred to the copper surface. This is achieved through one of two primary methods: screen printing or photolithography. Screen printing, which is the origin of the term printed circuit board involves placing a stencil screen containing the circuit pattern onto the board and squeezing liquid resist through the mesh onto the surface beneath. While this method is simple and inexpensive, its resolution is inherently limited and is normally only used for applications where track widths are greater than about 0.25 millimeters. For higher resolution, photolithography is employed. This process uses a light sensitive resist material that is exposed through a mask to transfer the circuit pattern. Photoresists are available as liquids, which can be applied by roller or spraying or more commonly as dry films. Dry film resists consist of three layers: a photosensitive polymer film sandwiched between a polyester support sheet on one side and a removable plastic cover sheet on the other. The cover sheet protects the photosensitive material from sticking during storage. Dry films are more expensive than liquid resists but form coatings of highly uniform thickness and are simpler to process. After removing the cover sheet, the resist film is placed on the copper surface and pressed onto it using hot rollers. The masks are aligned to the board using registration holes that correspond to the tooling holes. Contact printing is used to expose the resist, which is then developed by removing the unexposed regions of the negative resist. The remaining resist now covers the areas destined to become circuit tracks and lands leaving the open regions unprotected. Etching removes the copper cladding from these unprotected areas, typically using a chemical etchant sprayed onto the board surface in an etching chamber. Various etchants are utilized, including ammonium persulfate, ammonium hydroxide, cupric chloride and ferric chloride each with specific advantages and disadvantages. Process parameters such as temperature, etchant concentration, and exposure duration must be closely controlled to prevent over-etching or under etching. After etching, the board is thoroughly rinsed and the remaining resist is chemically stripped from the surface.
Plating is an essential step in PCB fabrication, particularly for double sided and multilayer boards where conductive paths must travel through the board thickness. Electroplating offers a high deposition rate but requires the target surface to be metallic and conductive. Electroless plating is slower but does not require a conductive surface. After drilling via holes, the walls consist of non-conductive epoxy glass insulation; therefore, electroless plating must be used initially to deposit a thin, uniform coating of copper on the hole walls. Once this thin conductive film is established, electrolytic plating is then employed to increase the coating thickness to between 0.025 and 0.05 millimeters. In some cases, gold is plated onto the board, typically as a very thin coating on edge connectors to provide superior electrical contact and corrosion resistance with a thickness of only about 2.5 micrometers.
PCB Fabrication Sequences and Finishing Operations:
The specific sequence of fabrication steps varies depending on the type of board being produced. The subtractive method, the most traditional approach starts with a fully copper clad board and etches away the unwanted copper leaving only the tracks and lands. The additive method starts with a board that has no copper cladding on its surface, instead the uncoated surface is treated with a chemical catalyst, often called a buttercoat which acts as a catalyst for electroless plating allowing copper to be built up only where the circuit pattern is defined. The semiadditive method represents a hybrid approach starting with an extremely thin copper film of 5 micrometers or less on the surface which is then selectively plated up to the required thickness.
For single sided boards, the subtractive method is typically used to produce the circuit pattern in the copper cladding. Double sided boards require a more complex sequence because the circuit tracks on both sides must be electrically connected via plated via holes. A typical fabrication sequence for a double sided board employs the semiadditive method: the board is drilled, the holes are initially plated using electroless copper, electroplating increases the hole thickness and finally the outer circuit patterns are defined and etched. Multilayer boards are structurally the most complex. In the first stage, the inner layers (logic layers from double sided boards and voltage layers from single sided boards) are processed to create their individual circuit designs. Thinner insulating substrates are used for these internal layers to ensure that the final assembled board does not exceed the required thickness. In the second stage, the individual layers are assembled together starting with a copper foil for the bottom outside surface, followed by the individual patterned layers separated by sheets of glass fabric impregnated with partially cured epoxy, and topped with a final copper foil for the top outer layer. The entire stack is bonded into a single integral board by heating the assembly under pressure to cure the epoxy, after which excess resin squeezed out around the edges is trimmed away. In the third stage, the bonded board, now resembling a thick double sided board undergoes its own processing sequence: additional through holes are drilled, these holes are plated to establish conduction paths between the outer layers and specific internal layers and photolithography and etching are used to form the final circuit pattern on the outer copper surfaces.
After circuitization, inspection and testing are crucial for quality control. Visual inspection, performed by human operators or machine vision systems detects visible defects such as open circuits, short circuits, and errors in drilled hole locations. Continuity testing uses an array of contact probes brought into simultaneous contact with specified track and land areas to quickly verify electrical connectivity. Finishing operations prepare the bare board for component assembly. A thin layer of solder is applied to the exposed copper tracks and lands to protect them from oxidation and contamination, either by electroplating or by passing the board over rotating rollers partially submerged in molten solder. A coating of solder resist is then applied to all areas of the board except the lands that will be soldered, this resist is chemically formulated to repel solder. Finally, an identification legend is screen printed onto the board surface indicating component placement positions and a bar code is often printed for production control.
Surface Mount Technology (SMT) Assembly:
Since the late 1980s, surface mount technology has become the dominant assembly process in PCB manufacturing. Traditional through hole technology presented inherent limitations regarding packing density: components could only be mounted on one side of the board and the center to center distance between lead pins in leaded components had a minimum of 1.0 millimeters and was typically 2.5 millimeters. SMT addresses these issues by soldering component leads directly to lands on the surface of the board rather than inserting them into plated holes. This fundamental change yields numerous advantages: components are smaller with leads closer together circuit densities are substantially increased, components can be reliably mounted on both sides of the board, smaller PCBs can be used for the same system, and the drilling of the many through holes is largely eliminated. In practice, the board surface area occupied by SMT components typically ranges between 20% and 60% of that required for equivalent through hole components. Despite its clear advantages, SMT has not completely supplanted through hole technology. The smaller size of SMT components makes them more difficult for humans to handle and assemble manually. Inspection, testing and rework are generally more challenging due to the finer scale and tighter component spacing. Furthermore, certain types of components are simply not available in surface mount form, forcing many assemblies to incorporate a mix of both SMT and PIH components. The basic sequence for PCB assembly is largely identical for both technologies: components are placed on the PCB, soldered, cleaned, inspected, tested and reworked. However the details of placement and soldering differ significantly.
Two primary methods are used for SMT placement and soldering: solder paste with reflow soldering and adhesive bonding with wave soldering. In the solder paste method, a suspension of solder powders in a flux binder is applied to the board surface by screen printing or syringe dispensing. The paste serves three critical functions: it provides the solder material typically 80% to 90% of the total paste volume, supplies the flux to clean the surfaces and acts as a temporary adhesive to hold the components in place. After paste application, automated or semiautomatic placement machines position the components. Automated machines operate under computer numerical control, retrieving components from tape reels or magazines and placing them using a high speed x-y positioning system and suction nozzles, achieving cycle rates of up to 30,000 components per hour. Semiautomatic machines assist a human operator with a high-resolution vision system, guiding them to the correct location and achieving rates of about 1,000 components per hour. Once all components are placed, a low temperature baking operation is performed to dry the flux binder, reducing gas escape during subsequent soldering. Finally the reflow soldering process heats the assembly, melting the solder particles and forming high-quality mechanical and electrical joints.
In the adhesive bonding method, adhesives such as epoxies or acrylics are used to affix components to the board surface instead of solder paste. The adhesive is applied by brushing through a stencil using an automatic dispensing machine or via a pin transfer method where pins are dipped into adhesive and then pressed onto the board. The components are then placed using the same types of placement machines used in the solder paste method. After placement, the adhesive is cured using heat, ultraviolet light or a combination thereof. With the components securely bonded, the board is passed through wave soldering for the actual electrical connection. However, SMT wave soldering presents technical challenges, including components being uprooted from the board, components shifting position during the process and larger components creating shadows that inhibit proper soldering of neighboring smaller parts.
Through Hole Technology and Combined Assembly:
In printed circuit assemblies utilizing through hole technology, the lead pins of components must be inserted into the plated through holes of the board. In double sided and multilayer boards, the hole walls are typically copper plated to facilitate soldering giving rise to the term plated through hole. After insertion, the leads are soldered into place and the boards undergo cleaning and testing, with defective boards being reworked if possible. The component insertion process involves several distinct stages. For many components, the leads must first be preformed which involves bending initially straight leads into a U shape suitable for insertion into the holes. Automatic insertion machines are widely used for high volume production. Components are loaded into these machines in reels, magazines or other carriers that maintain proper orientation. The insertion operation itself is accomplished by a work head designed for a specific component configuration. Components are classified into three basic categories: axial lead components such as resistors, capacitors and diodes, radial lead components such as LEDs, potentiometers, resistor networks and fuse holders and chip carriers like dual in line packages. Because the configurations of these categories are so different, separate insertion machines with specially designed work heads are required for each. A high speed x-y positioning table accurately positions the board beneath the work head before each insertion. Once the leads are pushed through the holes, they are clinched (bent on the underside) to mechanically secure the component to the board until soldering, and cropped (cut to length) to prevent the leads from bending and causing shorts with adjacent tracks or components. The three types of insertion machines can be linked into an integrated assembly line using a conveyor system, with a central computer tracking each board and downloading the correct programs to each workstation.
For soldering inserted components, wave soldering is the predominant method. In this mechanized technique, PCBs containing inserted components are conveyed over a standing wave of molten solder. The conveyor is positioned so that only the underside of the board, where the component leads project through the holes, contacts the solder. The combination of capillary action and the upward force of the solder wave causes the molten solder to flow into the clearances between the leads and the plated holes, creating a robust joint. The enormous advantage is that all solder joints on the entire board are completed in a single pass. Hand soldering where an operator uses a soldering iron to make connections one at a time, is much slower and is generally reserved for small lot production, rework or for adding delicate components that would be damaged in the harsh wave soldering environment. However hand soldering has distinct advantages: heat is localized to a very small target area, the equipment is inexpensive compared to a wave soldering machine and energy consumption is significantly less.
Modern PCB assemblies almost always combine both SMT and PIH components on the same board. In addition, SMT components can be populated on both sides of the board whereas PIH components are normally limited to the top side only. This complexity requires a carefully tailored assembly sequence. When SMT and through hole components are placed on the same side, a typical sequence involves reflow soldering the SMT components first then inserting and wave soldering the through hole components. For the most complex assemblies with SMT components on both sides, the sequence must be adjusted to protect components already attached from the heat of subsequent reflow or wave soldering processes. Design considerations for such boards include ensuring that wave soldering is used only on one side that all through hole components are inserted from that same side, and that surface mount devices on the insertion side are reflow soldered while those on the lead side can be wave soldered. IC packages should be oriented in the same direction to facilitate automated placement and to ensure proper solder flow. The spacing between ICs is primarily dictated by the need to remove heat during operation, requiring sufficient clearance for forced airflow and convection. Additionally, adequate space must be left around each IC package to allow for reworking and repair without disturbing adjacent devices.
Cleaning, Inspection, Testing and Rework:
After soldering, the printed circuit assembly is inevitably contaminated with various foreign substances including flux residues, oils, greases, salts and dirt particles. Some of these contaminants can cause long term chemical degradation of the assembly or interfere with its electronic functions particularly by increasing leakage currents on the board surface. To address this, one or more chemical cleaning operations must be performed. Traditional cleaning methods include hand cleaning with appropriate solvents and vapor degreasing using chlorinated solvents. However growing environmental concerns regarding these chemicals have motivated the industry to search for and adopt effective water based and environmentally friendly cleaning alternatives. Following cleaning, the PCB assembly must be thoroughly inspected for defective solder joints. Visual inspection is conducted to detect board substrate damage missing or damaged components, soldering faults and other observable quality defects. Inspection of soldering quality is considerably more challenging for surface mount circuits. SMT assemblies are generally more densely packed, the solder joints are significantly smaller, and the geometries differ from those in through-hole assemblies. One specific problem arises from how SMT components are held in place during soldering by solder paste or adhesive which does not provide the secure mechanical fastening of clinched leads found in through hole assemblies, leading to a higher risk of components shifting. Additionally, the finer pitches in SMT increase the likelihood of solder bridges forming between adjacent leads resulting in short circuits. To automate this tedious process, machine vision systems are increasingly used to perform these inspections.
Functional testing of the completed assembly is critical to verify its performance. The board design must include dedicated test points convenient locations in the circuit where probes can make reliable contact. Individual components can be tested by contacting their leads applying input test signals and measuring the outputs. More sophisticated procedures include digital function tests where the entire circuit or major subcircuits are exercised using a programmed sequence of input signals while outputs are monitored to simulate operating conditions. Another widely used approach is the substitution test where a production unit is plugged into a mock-up of the actual working system and energized to perform its functions. If the assembly performs satisfactorily, it passes and is unplugged for the next unit to be tested. A final critical test is the burn in test, which is applied to assemblies susceptible to infant mortality. This involves operating the boards under power for an extended period, typically 24 or 72 hours, sometimes at elevated temperatures such as 40 degrees Celsius, to force latent defects to manifest as failures during the testing period rather than during early service life. Boards that survive burn in are expected to have a long service life. The smaller scale of SMT introduces challenges in circuit testing as well; contact probes must be physically smaller and more probes are required because assemblies are more densely populated. Designers sometimes add extra lands whose only purpose is to provide a test probe contact site, but this counteracts the goal of achieving higher packing densities.
When inspection and testing reveal a defective component or a faulty solder joint, it is almost always more economical to repair the assembly rather than discard it along with all the other functioning components. Common rework tasks include solder touch up to repair minor faults, replacement of defective or missing components and repair of copper film that has lifted from the substrate. These tasks are inherently manual operations, demanding skilled workers using specialized tools such as small bit soldering irons, magnifying devices, and precision instruments for grasping and manipulating small parts. Rework of surface mount assemblies is notably more difficult than for conventional PIH assemblies due to the smaller component sizes and tighter spacing requiring the use of specialized tools and exceptional operator skill.
Electrical Connector Technology:
Printed circuit board assemblies must ultimately be connected to backplanes, racks, cabinets and other external systems through various cables and connectors. The growing ubiquity of electronics across nearly all product categories has elevated electrical connection technology to a critical engineering discipline. The overall performance and reliability of any electronic system depend heavily on the reliability of these individual connections. Connector technology is primarily applied at the third and higher levels of the electronics packaging hierarchy. Two fundamental methods exist for making electrical connections: soldering, which is the most widely used technology in electronics, and pressure connections which rely on mechanical forces to establish electrical continuity. Pressure connections are further divided into two categories: permanent and separable. Permanent connections involve high pressure contact between two metal surfaces where one or both parts are mechanically deformed during the assembly process. The three primary permanent connection methods are crimping, press fit technology and insulation displacement. Crimping is extensively used to assemble wires to electrical terminals. The operation involves the mechanical deformation of the terminal barrel around a stripped wire end inserted into it. Crimping is performed using specialized hand tools or automated crimping machines, and the terminals are supplied either as individual pieces or on long strips that can be fed into the machine. When properly executed, a crimped joint exhibits both low electrical resistance and high mechanical strength ensuring a durable connection.
Press fit technology is similar to mechanical interference fits but uses components specifically designed for electronics. It is widely used to assemble terminal pins into metal plated through holes in large PCBs. The connection relies on an interference fit between the terminal pin and the plated hole. Terminal pins fall into two categories: solid and compliant. Solid pins are rectangular in cross section and are designed so that their corners press into and even slightly cut into the metal of the plated hole to form a reliable connection. Compliant pins function as spring loaded devices that conform to the hole contour while pressing firmly against the hole walls to achieve excellent electrical contact without causing damage to the plating. Insulation displacement is a method where a sharp, prong shaped contact physically pierces the wire insulation and squeezes against the conductor to form an electrical connection. This technique is commonly employed to make simultaneous connections between multiple contacts and flat cable known as ribbon cable which consists of numerous parallel wires held in a fixed arrangement by the surrounding insulation. Ribbon cables are frequently terminated with multiple pin connectors used widely in electronics to connect major subassemblies. To make the assembly, the cable is placed in a nest and a press drives the connector contacts through the insulation and against the metal wires, simultaneously making all connections and significantly reducing wiring errors and assembly time.
Separable connectors are specifically designed to permit disassembly and reassembly allowing them to be connected and disconnected multiple times over their service life. When mated, they must provide metal to metal contact between components with high reliability and low electrical resistance. These devices typically consist of multiple contacts contained within a plastic molded housing designed to mate with a compatible connector, individual wires or terminals. They are used for electrical connections between various combinations of cables, printed circuit boards, components and individual wires. A vast selection of connector types is available and design engineers must consider several critical factors when choosing among them: the power level, overall cost, the number of individual conductors involved, the types of devices and circuits to be connected, physical space limitations, ease of attaching the connector to its leads, ease of mating with the corresponding terminal and the expected frequency of connection and disconnection cycles.
The principal connector types include cable connectors, terminal blocks, sockets and low or zero insertion force connectors. Cable connectors are permanently attached to the ends of cables and are designed to plug into and unplug from a mating receptacle. A common example is a power cord connector that plugs into a wall outlet. Other styles include multiple pin connectors and mating receptacles used for signal transmission between electronic subassemblies as well as styles used to attach PCBs to other subassemblies. Terminal blocks consist of a series of evenly spaced receptacles that allow connections between individual terminals or wires often attached by screws or other mechanical fasteners to permit easy disassembly. A socket in the electronics context is a connection device mounted directly to a PCB, into which IC packages and other components can be inserted. Sockets are permanently attached to the board by soldering or press fitting but they offer a separable connection method for the components, allowing them to be conveniently added, removed, or replaced without soldering making them a valuable alternative to direct soldering in many applications. One significant challenge with pin connectors and PCB sockets is the insertion and withdrawal forces which increase proportionally with the number of pins involved. For high pin count components, these forces can become so high that they risk damaging the connector, the board, or the component itself. To address this, special connectors have been developed with low insertion force (LIF) or zero insertion force (ZIF). These incorporate mechanisms that reduce or completely eliminate the force required to push the mating halves together or to pull them apart protecting both the components and the assembly tools.
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