Assembly of Printed Circuit Boards: (PCB Assembly Rules )
Printed circuit board assembly (PCBA) involves most common PCB assembly rules firmly attaching electronic components to a printed circuit board using tin-lead solder (in a wave soldering machine or applied as a paste and then spread in a low-temperature oven) or epoxy resin (processed in a low-temperature oven).
The type of PCB (single-sided, double-sided, multilayer, or flex) determines the density of components that can be attached to it. The choice of PCB assembly method is based on technology and reliability. The main methods include full surface mount technology (SMT), mixed technology (includes SMT and through-hole technology (PTH)), and bottom-side component mount technology.
Typically, modern electronic and computer assembly plants employ mixed technology whereby some components are surface mounted and other joints and components are soldered using through hole or solder drip technology.
The following is a description of a “typical” mixed technology using surface mounting, including adhesive attachment, wave soldering, and drift soldering. With mixed technology, it is sometimes possible to tap-solder surface mount components (SMCs) on the top side of a double-sided board and wave solder-SMCs on the bottom side.
This method is especially useful when you need to apply surface mount technology and technology and through holes on a single board, which is common in today’s electronics industry. The first step is to attach the SMCs to the top side of the board using drip solder technology. The next step is to insert the components through the through holes. The board is then flipped and the SMCs are attached to the underside of the board with adhesive. The final step is wave soldering of the components in the through holes and on the underside of the board.
Manufacturing of Printed Circuit Boards Using Mixed Technology Includes the Following Main Stages:
- Preliminary and additional cleaning
- Application of solder paste and adhesive (screen printing and placement (SMT and PTH))
- Introduction of components
- Processing of adhesive and flow of solder
- Fluxing (РТН)
- Wave soldering (РТН)
- Visual inspection and correction of defects
- Repair and replacement of faulty components
- Auxiliary operations – stencil cleaning
Environmental, Health and Safety Issues are Briefly Discussed Below for Each Process Step.
Pre- and post-cleaning
Commercial printed wiring (PWB) circuits are usually purchased from the PWB board supplier, where they are pre-cleaned with a deionized (DI) aqueous solution to remove surface contamination. Before ozone concerns arose, electronic manufacturers used chlorofluorocarbon (CFC), an ozone depleting substance, in the final or even pre-cleaning phase.
At the final stage of assembly of printed circuit boards, a “steam degreasing” with CFC was usually carried out to remove residues from wave soldering and from fluxes. It is one of the most common PCB assembly rules. The problems of preserving the ozone layer and strict restrictions on the production of CFСs have led to technological changes, which make it possible to exclude the cleaning process during the installation of printed circuit boards or use cleaning only with deionized water.
Solder Paste and Adhesive Application (Screen Printing and Placement) and Component
Injection the application of tin-lead solder paste to the surface of the PWB board allows the surface mount component to be attached to the PWB board, which is the core of SMT technology. The solder material acts as a mechanical bond for electrical and thermal conductivity, as well as a protective surface coating and improving solderability. Solder paste consists of approximately 70-90% non-volatile matter (weight by weight or weight per volume):
- tin-lead solder
- Mixture of modified resins (resin acids or weakly activated resins
- Activators (in the case of products that do not require cleaning, mixtures of amine hydrogen halides and acids, or simply carboxylic acids)
- Solvents (volatile matter) make up the remainder of the materials (usually a proprietary blend of alcohol and glycol ether).
The solder paste is applied through a stencil that is an exact copy of the surface pattern formed on the surface of the board (PWB). The solder paste is forced through the holes in the stencil to the pads on the PWB using a special knife that slowly moves the stencil. The stencil is removed, leaving the paste on the corresponding pads on the board.
The components are then injected into the PWB. At this stage, the main measures for environmental safety, health protection and safety of workers are related to maintaining cleanliness in the working rooms and personal hygiene of operators involved in applying solder paste to the surface of stencils, cleaning the device for applying paste and stencils. These are all common pre and post PCB assembly rules.
The concentration of lead in the solder and the stickiness of the dried paste necessitate the use of protective gloves, thorough cleaning of work surfaces, removal of contaminated cleaning materials and strict adherence to personal hygiene practices (regarding washing hands with soap before eating, drinking and applying cosmetics). The concentration of lead in the air is usually below the detection limit and with good personal and indoor hygiene, blood lead readings are normal.
Applying the adhesive involves automatically pouring small amounts of epoxy (usually a bisphenol A-epichlohydrin mixture) onto the surface of the PWB and “picking up and placing” the component and injecting it through the epoxy onto the PWB. In this case, a risk factor is mechanical arrangements for “grabbing and moving” components with a shuttle motion, presenting the risk of serious injury if the equipment is not properly protected and locked.
Adhesive Handling and Solder Leakage
Components attached by screen printing or adhesive application are conveyed by a mechanical fixed-height conveyor into the draw-off oven. In the furnace, the solder is “separated” by the repeated spreading of the solder paste heated to a temperature of 200 -.
The components attached with epoxy adhesive are also passed through an oven that is connected to the previous baking oven and typically operates at temperatures between 130 and… Solvent components, solder and epoxy pastes are removed during oven processing, but the tin-lead component does not volatilize. A cobweb-type residue is formed in the outlet pipe of the outflow furnace. A metal mesh filter can be used to prevent this. PCBs can sometimes enter the conveyor system and overheat in the oven, causing an unpleasant odor.
To form a reliable solder connection between the component lead and the PCB surface, the lead and the PCB surface must be free of oxidation even at high soldering temperatures. In addition, the molten solder must provide good wettability to the surface of the metals to be joined.
This means that the soldering flux must interact with metal oxides and remove them from the surface of the parts to be brazed, as well as prevent re-oxidation of the cleaned surfaces. Residues should be either non-corrosive or easily removable. Electronic soldering fluxes fall into three broad categories: resin-based fluxes, organic or water-soluble fluxes, and solvent-removable synthetic fluxes. New low solids fluxes.
Fluxes Resin based fluxes are most commonly used in the electronics industry as sprays or foams. The fluxer should be included in the wave soldering equipment or be a separate apparatus located at the entrance to the wave soldering device. As a base, fluxes of this category have natural resin or rosin, which is obtained from the resin of coniferous trees by distillation of liquid components (turpentine).
The resin is collected, heated and distilled; this removes particulate matter and results in a purified natural product. It is a homogeneous material with a uniform melting point.
Assembly of an electronic module consists of mechanically connecting parts and electronic components in a sequence that ensures their required location and interaction to meet the specified technical requirements. The structure of the electronic module includes a printed circuit board and electronic components placed on it, construction details; more and more often, software is included in the electronic module.
The printed circuit board is a structural part of the module. It acts as a carrier of components and anchorage for electrical conductors. If we do not follow proper PCB assembly rules, we won’t be able to get expected results. Soldering provides a mechanical connection of the module components to the board and electrical contact of the components with a conductive pattern The assembly of the module also includes the following operations: cleaning the board from residual flux and coating with a protective varnish.
The connection of two parts using a low-melting alloy is called soldering. Unlike welding, melting of the parts to be joined does not occur; the soldering process is more like gluing parts together, where a heated solder acts as an adhesive – an alloy with a sufficiently low melting point. Soldering is the primary way to permanently connect components to board conductors.
For a good soldering connection, the surfaces of the parts are prepared by applying a flux. Parts in the soldering area are heated by the soldering equipment to a temperature higher than the solder melting temperature. The solder spreads over the surface and displaces the flux. The surfaces to be joined are wetted. A fusion zone is formed during the diffusion of the solder and the surfaces of the parts to be joined.
Soldering the Component Lead
If the area of the conductive path of the board is large or the lead of a massive component is being soldered, then the time for heating the soldering area may take longer. For such rations, you need to use a soldering iron of higher power. Otherwise, the fusion zone will not form and “cold” soldering will occur, giving poor electrical contact.
If the wetting is insufficient, the solder first covers the surface and then collects, forming bumps and exposing the contact area on the board. This can be caused by deterioration of the properties of the flux, improper selection of soldering temperatures, contamination of surfaces, or release of flux solvent vapors.
Poor solder wetting of the component lead.
Solders and Fluxes
Solders different grades have different properties depending on the combination of tin, lead, bismuth, copper, zinc, cadmium, silver. Solders have components that form alloys with the metals to be joined. Only a few of the brands of solder are designed for assembling electronic modules. Currently, traditional and lead-free solders are used. Traditional solders are tin-lead alloys or similar ones.
Solder is produced in the form of cast ingots, rods, wire or a thin tube containing filler similar to a flux to facilitate soldering. In the molten state, the solder should ensure good wetting of the surfaces to be joined and ductility, a sufficiently strong mechanical connection of the parts after the soldering has cooled. Of traditional brands, POS 61 is used for mounting components on boards. Solders with a melting point below 450 ° C are called soft, if higher – hard solders.
The maximum melting temperature of solders for assembling electronic modules is 300 ° C. The eutectic phenomenon is used to lower the melting point of the solder. The ratio of metals in the alloy when the melting point becomes lower than any of the metals in the alloy is a eutectic. Solder is an alloy that is eutectic.
Flux– a non-metallic material used for chemical cleaning of the surfaces to be joined and providing bond strength in the soldering area. During brazing, the flux dissolves oxides and sulfides on the surfaces to be joined. Residual flux should not alter the electrical characteristics of the materials or cause corrosion.
The composition of the flux is due to the need for the adsorption of surface-active oxygen by the solder and the main substance of the flux, as well as its partial release, thereby changing the surface tension and wetting ability. The flux should be easily removed after the installation is completed, as the flux residues can become corrosion centers in the future.
Fluxes for Assembling Electronic Modules:
Rosin 10 … 40%, alcohol 60 … 90%,
Rosin 6%, alcohol 80%, glycerin 14%
Rosin 22%, alcohol 70%, aniline hydrochloric acid 6%, triethanolamine 2%,
Resin PN-9 or PN-56 15 … 50%, ethyl acetate 50 … 85%
Rosin 10 … 60%, ethyl acetate 40-90%
Rosin 10 … 60%, ethyl alcohol or ethyl acetate 40 … 90%
The assembly of an electronic module by installing components with leads in the holes of the printed circuit board and subsequent soldering is called lead wiring. This type of installation is the progenitor of modern technologies for the production of electronic modules. Output wiring appeared simultaneously with printed circuit boards.
The advent of PCB assembly rules, assembly using printed circuit boards later made it possible to automate the design and production of electronics. Now output wiring fades into the background, retreating to the installation of planar components, but there remain the categories of electronic devices, where lead wiring dominates over other technologies. These are power electronics, power supplies, high-voltage modules and others.
There are components that have no analogues in a planar design – connectors, relays, transformers for which the assembly can only be performed using the technology of output wiring.
Preparing the components for mounting is necessary to align the flex leads of the components. The shaping is done in such a way that the distance between the ends of the component leads corresponds to its place of installation on the board and the required distance between the board and the component is provided. The shape of the component leads depends on the installation option.
Component Leads are Molded and Installed on the Board
Forming of flexible leads should not damage them, violate the coating of the leads, bending is unacceptable at the point of junction of the lead with the body and is performed only at a distance not less than the permissible one. The molding method should exclude the rotation of the terminal relative to the component body. The glass insulators between the lead and the metal case of the component must be intact.
The simple constraints of the two dimensions R and L describe the allowable bending shape of the component lead during molding. The bending radius R of the terminal depends on the terminal diameter and is at least two terminal diameters.
The smallest gap between the entry point of the lead into the component body to the vertical axis of the molded lead L is in the range of 1 … 4 mm and depends on the type of the component body. After shaping, deformations and thinning should not appear on the leads. Compliance with these simple rules contributes to the safety of components and the reliability of the electronic modules.
Dimensions of the molded component terminal in the axial terminal package.
The component is installed in Option II, with a gap between the component body and the PCB.The lead length from the component body to the soldering area must exceed 2.5 mm. It is forbidden to form rigid leads of powerful transistors, diodes of medium and high power. Leads of components in packages that have multiple pins, such as chips in a DIP package, must not be molded.
Manual molding equipment.
The forming operation is carried out on manual devices and semi-automatic installations. Forming semi-automatic machines can perform straightening, stripping and trimming of leads. Semi-automatic machines can control the electrical parameters of the components, place the components in technological cassettes.
Components enter the molding machine from special belts, tubular cassettes or bulk. The required dimensions of the terminals are adjustable, the molding machines are equipped with various molding dies. The design of the forming dies ensures correct forming. For the forming equipment, there are automatic counters for the components fed from the belt. The productivity of automatic meters is up to 360,000 pieces per hour.
Automatic Molding Tool Performance:
Type of shell
From the placer
Manual feeding of components provides a typical forming productivity of about 1500… 3000 components per hour.
Options for Installing Output Components on a Printed Circuit Board
Option I – No clearance between component case and PCB. This option works well for installing components on a single sided board. The conductive pattern is located on the opposite side from the components, which eliminates contact between the component body and the conductive pattern. Reducing the length of the component leads reduces the susceptibility to electromagnetic interference and reduces the radiation of its own interference into the air.
The components withstand vibration well. The module height is reduced. It improves component cooling by transferring heat to the board, which increases reliability. The disadvantage of this installation option is the complexity of cleaning the module from the flux, ensuring the isolation of the component from the conductive pattern in the case of a double-sided board.
Lead Molding to Provide Clearance Between the Board and the Component Case
Option II – there is a gap between the board and the component body. Used for double-sided boards. This installation method helps to remove excess flux by washing, and the heating of the microcircuits during soldering decreases. The pad on the single sided board may be damaged if the component is loaded from above.
Option III – vertical installation. Pivot components are tighter. This option reduces manufacturability, increases the likelihood of a short circuit between the terminals, and increases the height of the module. When installing components vertically, the tilt angle of the component relative to the vertical axis should not exceed 15 °.
The installation of the components should make the marking easier to read. It is especially important to consider reading the polarity markings. Maximum ease of reading of the marking facilitates installation control.
Best PCB assembly rules for the assembly of electronic modules using output components can be done manually or using special automatic equipment.
The quality of the lead component soldering depends on the clearance between the lead of the component and the walls of the plated hole. The gap should provide capillarity, which facilitates the retraction of the solder into the cavity between the outlet and the walls of the hole and ensure the penetration of flux, the release of gases during soldering. The optimal gap is 0.3 to 0.4 mm when using lead solders and 0.5 mm when using lead-free solders for boards with a thickness of 1 to 3 mm with holes from 0.6 to 1.2 mm in diameter.
Manual Outlet Mounting
It is advisable to use modules in the following cases: the use of automatic equipment is unprofitable due to a small order volume or assembly of several prototypes of modules, boards are not suitable for automated installation, during the final installation of output elements after automatic installation. Today electronics is at a level of development that does not allow completely abandoning manual assembly operations. The installer carefully checks the appearance of each component prior to installation. If necessary, the leads are cleaned from oxides and the leads are tinned. It is possible to give the leads of each component, the shape that is most optimal for installation on a board, due to the design of the electronic module. Hand molding allows you to shape component leads to make markings easier to read.
Some PCB assembly rules for hand soldering. When installing, use a soldering iron with a pre-tinned tip. Depending on the mass of the component and the width of the track, it may take from a fraction of a second to two seconds to warm up the soldered area. When using tubular and wire solders, soldering is carried out with two hands. For best results, follow these steps.
To preheat the surfaces to be connected, touch the contact pad of the board and the component lead with the soldering iron tip at the same time. The solder on the tip of the soldering iron, applied when tinning the tip, promotes heat transfer due to the larger contact area of the tip with the soldering area.
It performed with the help of special equipment of two types: component installers and soldering machines. The advantages of automatic PCB assembly: reliability, cost reduction, high accuracy, speed, assembly of miniature elements, automatic control. Automatic machines allow for the changeover of production lines thanks to programming. The quality of automatic installation, as well as its cost, when using automated devices, is largely ensured at the design stage. Modern technologies allow placing components with a minimum distance from each other, up to fractions of a millimeter, but this is not always justified. Small distances make repairs and inspection of components and solder joints difficult. Installation of components is carried out using special assembly machines.
Output component installers are equipped with a set of assembly heads. In most machines, the heads have mechanical grips controlled by a servo drive. Standard component rotation angles are multiples of 90 °. It is possible to equip the machine with an assembly head with a free rotation angle. A number of machines have the ability to install wire jumpers on the board, cutting them just before installation. The rated capacity of modern assembly equipment reaches 20,000 … 40,000 components per hour. The installer’s productivity when assembling complex-shaped components can be ten times less.
The assembly machines are equipped with various loading devices – feeders. Components can be shipped glued to tape, reeled on a reel, or packed in a magazine box. Belt feeders are designed to feed components glued into a belt. Feeders from tubular cassettes are designed for microcircuits in DIP-package and complex-shaped components, have an inclined transport tray.
There are horizontal component feeders that do not slide freely on the inclined chute due to their design: weight, body shape, or protruding sharp points. Vibratory hopper feeders feed various components from the bulk and provide orientation of the components before picking up. Matrix (honeycomb) feeders for complex shaped components for feeding from matrix pallets, magazines.
The choice of soldering technology is carried out depending on the number of mounted elements, their location, assembly volume and complexity. Automatic assembly of lead-out components is performed on a selective assembly line or wave soldering.
originated in Great Britain in the fifties. The technology is used to solder the terminal components located on one side of the board. This is now the most common way to assemble large batches of electronic modules. Wave soldering allows the use of domestic lead-out components, thanks to which this technology has become widespread in the CIS.
Wave soldering technology offers unique performance for automated assembly of electronic components. At the same time, a number of operations are performed on the board: applying flux, preheating, washing off excess flux and drying. The board is in contact with the solder wave for a short time, which reduces the effect of high temperature.
Due to the fast heat transfer, wave soldering is very effective when installing components installed in plated holes. Disadvantages of the technology: a significant mass of solder permanently in the bath 100 … 500 kg, significant equipment dimensions of about several meters, high oxidation of the solder. The use of wave soldering technology puts forward certain requirements for the design of the board. Correct tracing of the conductive pattern reduces the likelihood of solder defects.
The boards have to be protected from the solder wave. For this, a layer of water-soluble film is applied to the board. To transfer the flux to the bottom surface of the board, it is foamed or sprayed. Soldering with molten solder is provided by a permanently present stationary wave. Boards with installed elements move across the wave. The best results are achieved by adjusting the conveyor slope and wave parameters. The angle of inclination of the conveyor is in the range 5 … 9 °. The speed of movement of the boards is selected based on the design of the assembled module, the soldering time of the components used, the pace of production, and the preheating temperature. The assembled modules move at a speed of about one meter per minute. The remaining excess solder is blown away with a narrow jet of hot air.
For the formation of skeletal rations and high resolution, the solder should be applied evenly in a thin layer on the soldering areas. Various waves of different profiles are used: plane wave or broad, secondary or “reflected”, delta wave, lambda wave, omega wave. The different number of waves allows the equipment to be divided into categories: with one, two and three waves.
Two Waves of Solder
With two wave technology, one wave is made narrow and turbulent. The energetic movement of the solder in the first wave eliminates the formation of voids in the solders caused by the evaporation of the flux. The second wave cleans the board from excess solder and completes the formation of the regular rations. Soldering temperature is in the range of 235… 260 ° С.
Lowering the soldering temperature creates a more gentle thermal regime for the parts of the assembled module. High temperature is required when using lead-free solders and mounting multilayer boards. To prevent oxidation of the surfaces to be joined, brazing is carried out in a nitrogen atmosphere.
The solder wave is created by mechanical and electromagnetic blowers, gas supply, ultrasonic vibrations. A mechanical blower works as follows. Molten solder is continuously pumped into the chamber with the nozzle using the impeller. The impeller is driven by an electric motor. The wave height is regulated by changing the speed of rotation of the motor shaft.
There is an easier way to create a wave of solder. For this, gas is used, supplied under pressure into a closed cavity. But this method also has disadvantages. Intense air movement through the solder leads to oxidation of the solder. The use of inert gas is economically unjustified.
When the solder contacts the conductive board pattern and component leads, a small amount of copper is dissolved in the solder. Small amounts of copper in the solder can destroy the eutectic of the alloy. The melting temperature of the solder rises, cold soldering occurs. To eliminate this phenomenon,
copper and bismuth are included in the composition of the solder. Copper is preliminarily added to the solder until saturation and a further increase in the copper content in the solder is impossible, while bismuth lowers the melting point of the solder.
Selective soldering appeared in the nineties. Selective soldering is a relatively new technology that allows selective wiring of terminal components only. The method requires a minimum of modifications to optimize printed circuit boards for this technology and allows you to mount most of the existing types of output components. Installation productivity is several times higher than manual installation.
The difference between selective soldering and wave soldering is that the board is heated only in the soldering area, as in soldering with a soldering iron. It is convenient to use selective soldering when assembling an electronic module containing planar and a small number of output components. Selective soldering is now becoming more common due to the reduced use of terminal components. When compared to wave soldering, selective soldering is more cost effective.
Compared to wave soldering, selective soldering does not lead to unnecessary heating of the board and allows using more types of components, reducing the likelihood of defects, reducing preparation for installation, eliminating the protective mask, and reducing the wear of the equipment for cleaning the boards. The introduction of selective soldering allows to reduce the number of operations with boards, reduce installation time, and reduce the amount of manual work.
Selective soldering is carried out in several stages. First, the flux is applied. Then the flux is heated for drying, activation and prevention of thermal shock during soldering. The last step is the application of solder. The whole process is automated and takes place in a special installation. The board automatically moves through all selective soldering steps, starting with the application of the flux.
Selective soldering technologies can be divided into two main types depending on the solder head used. One type of technology can be attributed to the use of a nozzle with a solder, over which the soldered board is moved and all points are soldered in turn. The second type can be attributed to the use of a tooling that generates mini-waves on several nozzles at the same time, located in the areas of rations. The first type of technology is more suitable for the production of small batches of electronic modules, the second for large batches. Brazing can be carried out in a nitrogen atmosphere.
The flux is applied by the three most common methods.
The flux assembly is similar to the head of an inkjet printer and allows you to apply flux in small batches. Unlike the printer, the flux applicator moves along the plane like a plotter pen. Only the soldered areas are fluxed. An electromagnetic pump without mechanical parts supplies the flux to the nozzle. The micro-droplet nozzle eliminates the ingress of flux to the board areas located around the soldering area. The accuracy of the flux application eliminates the cleaning operation.
Spray fluxing applies flux to the entire board. The number of flux application nozzles varies for each electronic module produced.
Dip fluxing is performed using a bath and an adapter with retractable nozzles. The adapter is made for each manufactured module individually. All areas of the rations are coated with flux at the same time. The application of flux by dipping is important in the manufacture of a large batch. The adapter with nozzles changes when switching to another board. The use of nozzles ensures a good application of the flux exactly to the soldering area. The flux is well applied even to hard-to-reach soldering points.
Preheating the Board
Infrared heating is performed using heaters with a medium to short wavelength range of radiation. The power of each heater is measured in kilowatts. Heating can be set according to the PCB design.
Heating by quartz radiators and directional convection devices. When using such radiators, the number of heat sources involved is set according to the board width.Soldering can be done with a single nozzle or with multiple mini-waves.
With single nozzle technology, the board is moved with high precision and positioned over the nozzle of the solder head. It is possible to control the parameters of each individual soldered joint: height of the nozzle, waves, time spent in the wave of solder and others. A strictly specified amount of solder is consumed for soldering in accordance with the program.
Selective Mini-Wave Soldering Nozzle.
Various nozzles with an inner diameter of 1.5 … 20 mm make the soldering process adaptable to the production of most possible electronic modules. If brazing is carried out in a nitrogen atmosphere, then nitrogen is supplied directly to the brazing zone.Multi-wave soldering provides improved performance. Multiple mini-wave solders all the necessary soldering points at the same time, and at the same time even hard-to-reach points are processed with high quality. When soldering with multiple mini-waves, the soldered surfaces are perfectly wetted, the formation of bridges is minimal.
Selective brazing is unique in that the application of the flux occurs pointwise and dosed, the flux burns out during the brazing process and no cleaning of the flux residues is required. This allows you to save on the technological process of cleaning the boards. This eliminates the cost of cleaning equipment. Systems for selective laser and hot gas soldering are being developed.
f you follow these PCB assembly rules properly, you will be able to build a perfect printed circuit board with extraordinary performance. For a reliable pcb assembly you can contact pcbmay.