chapter12 manufacturing-processes

The University of New South Wales

School of Electrical Engineering and Telecommunications

ELEC3017 ELECTRICAL ENGINEERING DESIGN

CHAPTER 12: SOME ELECTRONIC

MANUFACTURING TECHNIQUES

Lecture Notes Prepared by

Mr D. Williams,1 Prof. W. H. Holmes, & Dr A. P. Bradley

1 Chief Executive Officer, Associative Measurement Pty Ltd.

ELEC3017 ELECTRICAL ENGINEERING DESIGN 1 ELECTRONICS MANUFACTURING

ELECTRONIC ASSEMBLY TECHNOLOGIES

Nearly all modern electronic circuits are constructed by mounting most of the electronic components on a basically two dimensional substrate with conducting tracks to connect them. By far the commonest form for the board or substrate is a printed circuit board (PCB), though occasionally one of several hybrid technologies is used. PCBs in particular are described in more detail in this chapter.

These substrates have two important functions:

• They are the means for physically mounting electronic components;

• They are the means of interconnecting the electronic components.

There are three basic electronic component assembly (or mounting) methods for components on substrates:

1. Through-Hole Technology

The wire component leads are inserted into holes which have been drilled through the PCB (including the copper tracks), and then soldered to the copper tracks. Component insertion may be manual or automatic. Usually all soldering is carried out in a single operation using a wave solder machine.

Through-hole technology is still the dominant approach, but surface mount technology (see below) is likely to play an ever increasing role, offering much higher component densities and being more suitable for automatic manufacturing methods.

Figure 1. Through-hole, hybrid and surface mount technologies


2. Surface Mount Technology

The component leads in surface mount technology (SMT) are soldered directly to the surface tracks without drilling. The components are held in place before soldering using a dab of glue.

Special leadless components with pre-tinned attachment areas (not pigtails) are used, such as chip capacitors and chip resistors. High component densities are more easily achievable than with through-hole mounting. This process is especially suitable for automatic manufacture – indeed, it almost requires automatic techniques to be viable.

Some other acronyms often encountered in connection with surface mount technology (SMT) are SMA (assembly), SMC (component) and SMD (device).

The application of SMT components is generally limited to the smaller discrete devices and larger leaded components that can provide some form of mechanical compliance in their means of attachment. The problem arises because of the disparity in thermal expansion characteristics between the current types of substrate materials (such as FR4) and the components themselves. If the effects of this are not properly accommodated then the overall reliability of the assembled board is reduced due to the propensity for component to substrate soldered joint to fail under heat induced mechanical stress.

The increased circuit density with SMT carries with it an increased need to evacuate the heat produced by dissipative components and special heat sinks and/or improved board substrate materials have to be employed. Except for consumer applications where paper-based substrates are used, the predominant substrate used for double-sided PTH boards is glass-fibre epoxy material such as the ubiquitous FR4. There are improved versions of FR4 being developed and used for heat critical applications, while special applications require polyimide resins or, for RF applications, PTFE.

3. Hybrid Technology

A hybrid circuit uses a substrate (similar to surface mount assembly) on which there is a combination of film technologies and discrete component technologies.

The film technology is used to realize most passive components, especially the component interconnections (conductors), resistors and capacitors. Either thick or thin films may be used.

In thin film technology, the components are created by vapour deposition onto unmasked areas of the substrate, followed by selective etching or machining. In thick film technology, they are made by screen printing with a paste onto the substrate, which is then dried and fired, and finally machined (if required).

The advantage of film technologies is that they can realize many passive components in a very small area with a single manufacturing step. Automatic precision trimming is also possible.


The remaining components (active and passive) are then added to the substrate to complete the circuit. Although leaded components (as with through-hole PCBs) can be used, leadless components offer the most benefit, just as they do for surface mount technology.

PRINTED CIRCUIT BOARDS (PCBs)

The rest of this chapter is concerned with PCB design and technology, the most important electronics assembly method. We will concentrate here mainly on through-hole technology.

A printed circuit board consists of a sheet of insulating material with one or more layers of conducting copper tracks, either attached to the surface or buried within it. Occasionally other conductive metals or alloys are added for special purposes.

TYPES OF PRINTED CIRCUIT BOARDS

The major classes of PCBs are:

1. Single Layer Boards

These are boards having only one conducting layer. While this is one of the oldest types of printed circuit board, it is still produced in volume since it offers the most economical solution for a large range of consumer electronics products.

The components are usually mounted on the side of the board without the conducting tracks. This side is hence called the component side of the board, whereas the side with the conducting layer is called the solder side, since the component leads will be soldered on that side.

2. Two Layer Boards, with or without Plated-Through Holes (PTHs)

These are boards with two conductive layers, usually one on each side of the insulating substrate. Their main advantage over the single layer boards is that it is much easier to design them without crossing tracks (which require wire jumpers in practice.) Another advantage is that it is often possible to use areas of copper on the component side to provide either shielding or low-impedance earth or power supply connections. However, geometric constraints still often lead to long tracks, which are electrically undesirable (higher resistance and inductance, increased problems with cross coupling, etc.).

In the simplest cases there will be no interconnections between conducting tracks on opposite sides of the board except those provided by the component leads (about 5% of consumer product boards are like this).

However, the majority of two layer boards use plated-through-holes (PTHs) to connect conducting tracks on opposite sides of the board. PTH connections are usually made by depositing copper on the sides of the holes drilled to connect tracks on opposite sides of the board.


The double-sided board is readily adapted to surface mount techniques, sometimes having conventional components on one side of the board and surface mount devices (SMDs) on the other. However, as SMT components have become more readily available, more and more double-sided boards are being produced with SMT components on both sides of the board and with very little application of conventional through-leaded componentry.

3. Multilayer Boards

Multilayer boards can be used to further ease the geometric problems of interconnecting complex circuits, so that track lengths can be reduced and the density of electronic components can be much larger, especially with SMT components.

Also, special layers are often dedicated to earths, power supply rails or for shielding purposes. Such layers can greatly improve the electrical performance of the circuit. For example, with multilayer boards it is very easy to provide separate low impedance earths and power supply rails for analogue and digital components, and to use guard tracks and/or shields for sensitive analogue or high frequency connections.

Many computer and telecommunications products, as well as low volume special circuits, call for PCBs with layer counts from 4 to 18 or so, which are not always much more expensive than two-layer boards. Some manufacturers can produce boards with up to 60 layers. However, because of the increase in complexity and the increased requirements for capital equipment to cope with laminating, fine tolerance holes and inspection and testing, there are far fewer manufacturers of multilayer boards than of single and two-layer boards.

There are some special problems that can arise in the use of multilayer PCBs, especially the fact that the thermal expansion of rigidly bonded SMT components may possibly be mismatched to that of the substrate, which can lead to stress between the conducting tracks and the substrate, with possibly catastrophic effects over time (layer separation).

Multilayer boards are usually fabricated from a number of ‘cores’, each of which resembles a very thin double-sided board. Holes are not always drilled and through-plated at this stage unless the design calls for connections solely between the two layers of a core. The cores are assembled in a stack, separated from each other by one or more layers of partially cured substrate material called ‘pre-preg’, and with outer layers of copper foil.

Heat and pressure are applied to fuse the whole into a rigid board. Holes are drilled to provide, when plated with copper, for through connections (called ‘vias’) between layers and the copper tracks etched onto the outside layers. In this way a variable (usually even) number of circuit planes is interconnected to form a complex whole.

Choice of materials in multilayer PCBs is important because of the high temperatures during manufacture. Since many of the materials that may be used, such as epoxy resins, have high coefficients of thermal expansion, it is difficult to maintain fine mechanical tolerances, and there may be cracking of plated-through holes on cooling. Some modern


materials, such as polyimides and BT resins (see the appendices), alleviate these problems.

In addition to the above major classes of PCBs there are a number of others, including:

4. Flexible and Flexi-Rigid PCBs

The main function of flexible printed circuits is to provide connections between normal (rigid) PCBs. They replace the more usual cabling or wiring harnesses and provide an economical and compact interconnection method, especially useful for compact portable equipment.

Flexible circuits are constructed in a number of ways:

• Using additive plating methods to attach a conductor foil to a base flexible insulator material;

• Using conductive epoxy screened onto the flexible base substrate; or

• Using a three-part system whereby a copper foil is attached to the laminate with a separate adhesive and pressure is applied.

5. Multiwire Circuit Boards

The Multiwire process was invented by the Kollmorgen Corporation in the 1970s. It utilizes a method in which the circuit connections are made by individual insulated wires laid in a partially cured resin on a substrate base. When all the wires are laid and the adhesive is cured, holes are drilled through the wires and the substrate, and through-plated to make the required connections. The substrate may contain a copper layer which can be etched in the normal manner to provide a screen or power supply busses.

The wires are laid very accurately by numerically-controlled (NC) machines and being insulated the wire crossings possibly eliminate the need for ‘vias’

6. Moulded Circuit Boards

Research is being undertaken into new laminate materials to match SMT component thermal expansion characteristics.

Where quantities are sufficient, moulded PCBs based on a combination of injection moulding and copper plating can offer a solution. Commercially available thermoplastics such as polyarysulphone, polyethersulphone and polyetherimide permit the moulding of a complete board, including the holes together with other features such as bosses or recessed lands for SMT components.

The manufacturing technology for PCBs, including the materials used, is discussed in more detail in the appendices.


COMPONENT PLACEMENT

The components must be mounted on the boards before soldering, and this is a major bottleneck in the production process, as well as a major source of errors. This process is often called “stuffing” the boards. There are three methods:

• Manual

This method is time consuming and error prone.

To improve it, the components should be sorted into marked and sequenced storage bins and the PCBs should be screen printed with a masked legend which shows the locations and types of each component.

• Semi-automatic

Simple measures can greatly improve on the manual methods. For example:

∗ Preform (bend and/or cut) the component leads to the required shapes and sizes for insertion.

∗ Use location aids to show the assembler where each component should be placed. For example, a slide projector or a computer-controlled light beam or laser beam can be trained on the correct board location and source bin for each component in turn, or a small light may be turned on under each component.

∗ Leads can be automatially crimped after insertion to hold the components in place. At the same time, excess lead lengths can be trimmed before soldering.

• Automatic

The insertion head of a computer-controlled robot picks up the components, preforms them (if necessary), places them and inserts then. Three different types of inserters may be needed for axial components, radial components and ICs. Some difficult components may still have to be manually inserted.

Automatic placement is very commonly used in SMT, since it is easier to automatically place SMDs than through-hole components.

Components for automatic insertion are usually provided in reeled or taped form (‘bandoliers’), so that the mechanical and control problems of supplying components to the insertion head are reduced.


SOLDERING

The essential elements of the soldering process are:

• Solder

The soft solders for electronic assembly are usually tin-lead alloys which melt at about 185º C (e.g. 62% tin, 38% lead - the eutectic composition, which does not have a plastic phase between the solid and liquid phases). Its resistivity is about 10 times that of copper, and its thermal conductivity about one eighth that of copper - but because of the small thicknesses of solder in a joint, these differences are usually immaterial.

20/80 40/60 60/40 80/20 100/0 Proportion tin/lead alloy

320 300 280 260 240 220 200 180

Temper-ature

Solid

Liquid

Plastic

Eutectic point 62/38 183ºC

Figure 2. Phase Diagram of Tin/Lead Alloys

• Flux

This is needed to clean the surfaces to be soldered and so aid wetting. Even apparently clean metal surfaces usually have an oxide layer or are tarnished by atmospheric or other contaminants, which prevent proper contact of the solder.

Fluxes react with the tarnished surfaces to produce a pure base metal surface for the solder to wet. Common fluxes are:

∗ Organically soluble ones, such as gum rosin dissolved in an organic solvent. Their residues after soldering are not very corrosive, but should still be cleaned off later, as they attract dirt and contamination.

∗ Water soluble ones, such as various salts and acids. These are much more active than organic fluxes and therefore more suitable for heavily tarnished surfaces. Their residues are highly corrosive and must be removed after soldering.

• Heat

The solder must be heated at least to its melting point. Heating may be done in many ways, e.g. laser, hot air, induction (see below).


• Wetting

When the molten solder contacts the clean metal surfaces to be bonded, it forms a thin layer of an alloy of solder and metal, so that there is bonding between solder and metal at the atomic level. This is referred to as wetting, and is impossible with unclean metallic surfaces.

• Lead trimming

After soldering, the excess leads are cut off.

• Cleaning

This necessary both before assembly (to ensure wetting) and after soldering (to remove flux residues). Depending on the contaminant to be removed, either acqueous or solvent products are used for cleaning. These may be applied by brush, immersion, spraying, wave or ultrasonic means.

• Testing

The complete board is usually tested (see below).

• Conformal Coating

This is a thin transparent coat of material applied to the whole assembly after soldering to protect against humidity, dirt, atmospheric contamination, etc. It is essential for fine line or high reliability PCBs. Many materials may be used, including especially acrylics, expoxies, polyurethane and silicone. It is applied by brushing, spraying, dipping or flow coating.

SOLDERING PROCESSES

For very small or one-off jobs, hand soldering is still used. However, mass soldering is almost universal nowadays, even for very small production runs. There are two major divisions of mass soldering processes:

• CS - place components first, then solder them into place (e.g. wave soldering).

• SC - apply solder first, then place components, and finally apply heat (e.g. reflow of solder pastes). These techniques are becoming commoner for SMT, but cannot be used for through-hole circuits.


The most important processes used are:

• Wave Soldering

This is by far the dominant method, especially for through-hole circuits, but also for SMT. It is very suitable for assembly line mass soldering. There are three stages:

∗ Fluxing. Flux is applied in a liquid form (foam, spray or wave).

∗ Preheating. This is needed to dry and activate the flux, and to reduce thermal shock. Usually done by convection and/or radiation heating.

∗ Soldering. The board is moved over a wave of molten solder.

Afterwards the boards are cleaned and (possibly) conformally coated (see above).

Figure 3. The Principle of Wave-soldering.

• Reflow Soldering

This is only possible for SMT, not through-hole circuits. The three stages are:

∗ Apply solder and flux to the areas to be soldered, in the form of a solder paste.

∗ Place the components.

∗ Apply heat to melt the solder paste and wet the surfaces. This may be by conduction (hot liquid, hot plates or molten metal), convection (hot air, hot gas or hot vapour), or radiation (infra-red or laser).

The commonest methods of applying heat at present are infra-red radiation from above and/or below, and hot vapour convection (more accurately, condensation).

In vapour phase soldering, a liquid with a boiling point around 200-230 ºC, such as FC-70, is heated to boiling and the circuit board is


passed through its vapour. A benefit of this process is that the vapour removes flux residues, so that further cleaning may not be necessary.

Figure 4. The Principle of Hot-vapour Soldering.

PACKAGING

The complete production of electronic devices is essentially a multistage connection problem: interconnection between the elementary components (resistors, ICs etc.) and the PCB, interconnection between the circuit board and its housing and off-board components (e.g. front panel components, power supply sockets), etc.

Six levels of interconnection may be identified [1]:

1. On-device, where the parts of the IC are connected by minute etched wires or conducting tracks on the semiconductor surface.

2. Device-to package, where the IC is attached by thin aluminium or gold wires to its terminals.

3. Package-to-board, where the package terminals (usually flat or round leads or flat pads) are connected to the PCB. Sometimes this is a separable connection through the use of sockets, which themselves are permanently soldered to the board.

4. Board-to-board or on-board, where components are connected to boards or boards connected to other boards by permanent or separable connectors and cables.

5. Board-to-box (or cabinet), where the board is connected to its cabinet or components mounted on the cabinet using permanent or separable connectors or cables.

6. Box-to-box, where separable connections are made between cabinets with multiple contact cables and connectors.

So far we have mainly discussed level 3. Levels 1 and 2 belong to the more technological areas of electronics, which are treated in basic electronics courses.


Levels 4-6 are concerned with the electromechanical packaging of electronic assemblies. They mainly involve sheet metal work, mechanical mounting of components, adhesives, coatings and encapsulation for environmental protection, printing, etc. (Other relevant factors are discussed in other chapters, including connectors, cables, RFI-EMI-EMC and thermal management.)

Some important packaging considerations in levels 4-6 include:

• Sheet Metal Work

Most boxes or cabinets for electrical equipment are made from bent sheet metal, with punched or drilled holes as required. There is considerable art in the design of sheet metal boxes to make them attractive in appearance but at the same time rigid and easy and cheap to assemble, etc. Screws should be kept to a minimum. Self tapping screws are often used, but sometimes nuts are welded to the sheet metal to take normal bolts.

Much consumer equipment is nowadays mounted in moulded plastic cabinets.

• Mechanical Mounting of Components

Heavy components on PCBs, such as transformers, need to be supported by extra spacers (or stand-offs) near or under them, so that mechanical resonances etc. can’t damage the PCB.

Front or back panel components, such as potentiometers, switches, sockets for signals or power leads, displays, etc., can be mounted directly on the panels (or sometimes on an extra PCB just behind the panels), but this requires hand work to mount them and hand-installed leads to connect them to the main PCB.

It is often cost saving and neater to mount most of such components on the edges of the main PCB itself, but this requires close attention to the mechanical tolerances of their positions and of the corresponding holes through the panels. It also constrains the PCB layout considerably.

With PCB-mounted panel components such as switches or connectors it is necessary to make sure that the user cannot damage the PCB when activating them - for example, extra PCB stand-offs may be placed near these components to take the stresses, or mechanical stops or grommets on the panel may be arranged to limit the forces applied to the PCB itself. Tight tolerances of the panel holes also help.

• Adhesives

Adhesives have many applications in assembly, but a major problem is to select the right one for the particular surfaces to be glued together. The main adhesive families are acrylics, anaerobics, cyanoacrylates (‘super glues’), epoxides, phenolics and polyurethanes. They come as emulsions, solutions, powders, sticks, etc.

• Coatings

Cabinets must be coated for aesthetic and consumer reasons. But they must also be coated because they need protection from the environment. Depending


on what is being protected against corrosion, different coatings are used for protection:

∗ Aluminium or aluminium alloys: Use anodizing, blackening, plating, or spraying with zinc or aluminium.

∗ Ferrous metals: Use galvanizing, phosphates, plating, or spraying with zinc or aluminium.

∗ Organic materials (plastic or wood). Use cathodic sputtering, conductive paints, electroless plating followed by conventional plating, metal spraying, or vacuum evaporation of metals.

After protection, the cabinet is usually finished with a coating of some type for aesthetic or marketing reasons. A wide variety of finishes is available (powder coating etc.).

• Embedding, Impregnation and Potting

These are all variants of a process in which an assembly of components (e.g. a small PCB module) is embedded in a protective material, such as silicone or epoxy, at least 6 mm thick, which fills all the free space between the components. The aim is to improve the protection from the environment and to produce a standalone module. A mould is normally used for embedding, which can either be removed later or left in place (in the latter case the process is called “potting”).

• Printing

Cabinets or boxes usually have some printing on them to aid the user, or for marketing or aesthetic reasons. The commonest process is silk screen printing.

TESTING

COMPONENT TESTING

The component parts are often tested before assembly for conformance to specifications, especially regarding functionality, parameter values, dimensions and solderability.

The bare PCB (before component placement and soldering) is often tested for track continuity, shorts, accuracy and solderability. A bed-of-nails tester (see below) may be used for the first two tests, which are particularly important for multilayer boards. Visual testing is also used (see below).

CIRCUIT TESTING

Testing of completed circuits is done (prior to conformal coating) by means of test points located at circuit nodes. There may be many hundred test points. Often special pads (extended lands or vias) for test points are provided during the PCB layout. Test


points must be large enough (0.9 mm diameter) and not too near the components. They must be soldered or plated and on one side of the PCB.

Assemblies are usually tested on a bed-of-nails tester, which contacts all the test points using a matrix of spring-loaded pins. Many such testers are universal and programmable, allowing up to 50,000 test points, but some are dedicated. Sometimes a set of moving probes, controlled by a computer, is used instead.

In either case a series of computer-controlled in-circuit electrical tests is applied to the board, and the results are summarized on a test sheet. The statistics of the test results help greatly in the quality assurance program.

Visual inspection is also carried out to detect problems with solder joints, thermal damage, missing or misplaced components, etc. Automated optical tests are also sometimes used, in which a scanned image of the board is compared with the ideal ‘golden’ board or checked for compatibility with a set of rules.

Destructive tests are often used on a proportion of the boards, to check for features such as the integrity of PTHs, multilayer registration etc.

TESTING OF COMPLETE EQUIPMENT

Complete devices, including boxes, displays, controls etc., usually go through a number of tests, often after several days burn-in under cycled extreme environmental conditions (which aims at detecting and weeding out early failures).

The commonest tests are functional tests to see whether all the controls work and whether the performance is within specification. These tests are often also computer controlled.

REWORK

If the completed circuit fails the board tests (which should be a rare event in a quality manufacturing process), it is sent to a rework station, which must be manned by a skilled worker, to attempt to salvage it.

The automatic testing should have isolated the fault, though sometimes additional detective work may be needed to find the exact problem for rework. Hence the rework process mainly consists of replacing known faulty parts.

The faulty component must first be desoldered by heating the solder joints and removing the solder with a ‘solder sucker’ or ‘solder wick’. A new component is then manually soldered into place. Controlled temperature soldering and desoldering equipment is used, sometimes under controlled atmospheric conditions.

After rework the board is returned to the assembly line and retested.


COMPUTER AIDED MANUFACTURING

COMPUTER VERSION OF MANUFACTURING OPERATING SYSTEMS

Consider medium to high volume production systems.

Every production process (i.e. the unique production process required to make an individual product, such as the multilayer PCB process) fits in to the manufacturing plant as follows:

Quality Assuranceoperating system

Manufacturing controlsystem (CAM)

Product Nmanufacturing

process

Product Bmanufacturing

process

Product Amanufacturing

process

CONTROL HIERARCHYIN A DESIGNEDMANUFACTURINGENVIRONMENT

.....

..

The central block provides the data management function and is a very large computer program. Such an MIS (Management Information System) can cost millions of dollars for the software alone. It is also an extensive hardware system with many terminals distributed throughout a company's various ops areas.

A medium sized system might be as follows:

• 400 VT100 terminals, spread over 20 sites (e.g. in a printing system 5,000 characters are word-processed into and out of print-space formats every second at Cybernetics Cumberland in Parramatta)

• 30,000 line item component parts. (Electronics products have huge inventories compared with other products of similar product volume.)

• Production cycle management of 3 years.


• Production Capital Management of millions of dollars.

The types of CAM system are many and varied but they are always the backbone of the management system.

Many companies do not realize the effort required to maintain the discipline necessary to ensure accurate data on the system. A whole area of quality control of a successful company must be devoted to ‘INFORMATION ACCURACY’. Bad information is VERY expensive in control of large budgets.

Statistical S/W tools exist which process data entry for errors based on historical parameters.

All CAM systems have the following architectural form:

Purchasing (MRP) andfinance resource control,

Inventory

Financial monitoring,variance reports

Finished GoodsInventory andDistribution

Quality Controlregistration

Modelling,Batching and

Scheduling

COMPONENTS OF CAMSYSTEMS FORMANUFACTURING

Work in progress, Shoprouting, Change notes

TestMonitor

Costing

Order entryand encoding

COMPUTER AIDED MANUFACTURING (CAM) SYSTEMS

Older CAM systems were not ‘real time’ and critical functions of the company were limited in their operation. Good control systems result primarily in good cash management.

If you manage such a system as above as a manufacturing manager, your day will start as follows:

• Collect Work in Progress (WIP) figures from Controller (head Electronic Data Processing (EDP) guru).

• Is it bigger than yesterday? Why? Is it bigger than budget? Is it going to blow out further?


• Find reason for unusual/unplanned excess. Can look at variances but more usually you’ll find some purchasing shortage is the holdup of goods moving into finished goods inventory and not leaving WIP.

REMEMBER - ONE 20 CENT PART CAN HOLD UP A MILLION DOLLAR SHIPMENT.

A feature of the electronics industry is the number of change notes that are applied to a device under manufacture. The effectivity date of these change notes is important as it might rely on new parts, new test procedures, new construction techniques and they all must happen at once. Costs in redundant parts must also be taken into account.

The encoding, modelling and order entry system is a crucial part of the structure of a product and can determine its manufacturing cost, e.g. Summit Matrix switch ordering options.

The prevailing thoughts on inventory encoding is that parts should not have ‘Significant Numbers’.

Indented Bills of Material (BOMs) are assembly - subassembly - sub-subassembly ordered.

Change orders ideally travel with the BOM into assembly and test.

RESOURCE LEVELLING/WORK ROUTING

Automated timing of use and optimization of use of capital plant and labour resources.

VARIANCES, FINANCIAL MONITORING AND COSTING

Variances are differences between projected time/finance and actual time/finance.

Variances are measured against ‘standard costs’ which are originally estimates and eventually averaged history costs. Standard costs are a baseline accounting reference and costing may be done on Average plus Standard Deviation or other. Forward quotes for volume production depend on the accuracy of these figures.

Costing is meant to be a continuous process.

MATERIALS REQUISITION PLANNING (MRP), TRACEABILITY AND QUALITY ASSURANCE (QA)

A Materials Requisition Plan (MRP) must account for vendor qualification, lead times for parts arrival and is a significant cost centre in the manufacturing operation. A good MRP operation/system can achieve ‘JUST IN TIME MANUFACTURING’, i.e. no goods (= cash) sitting on the shelves, not earning interest and not being used to buy more parts. You have to pay in 30 days for your component parts - if you can build in


two weeks and get your customer to pay inside the month it will minimally effect your cash flow. If you are operating on bank finance the goods on the shelf are costing you high interest and eating into your profit margins.

Traceability allows you to transfer responsibility for faulty devices to the component supplier by being able to prove in law that a specific batch number/part number from a supplier had sufficient qualification (e.g. Underwriters Laboratory specification UL 930 concerning fire resistance). This tracing back through a manufacturing operation is cumbersome and requires a highly sophisticated CAM system. Essential requirement to manufacture ‘critical devices’ (life support devices, e.g. pacemakers, cardiac catheters, drugs). The legislation is laid down in the USA as GMP - Good Manufacturing Practice.

Many electronic components have so called ‘functional equivalents’, but they must be proven to be such by the production engineers who will supply a list of ‘qualified vendors’ to purchasing.

GOOD MANUFACTURING PRACTICE RULES

Specify the requirement of autonomous management of QA. The QA manager should NOT report to line managers (e.g. manufacturing managers) but DIRECTLY to the Chief Executive Officer (CEO), who has the total responsibility for Quality Manufacture.

The rules also specify QC procedures (there is a defined difference between QA & QC - you must know it!) involved with rework, testing and reporting (device histories and fault histories of ‘type’. Also a complaints file must be kept.

Prototyping Versus Simulating

Suppose that the design is basically complete. All that remains is to finally check it in a complete working circuit, the ‘prototype’.

But prototyping is costly and slow. It is also difficult to do in small enough ‘chunks’ without continuously building a simulation of the surrounding signalling structure. Some computer aided design programs (e.g. Daisy) facilitate hierarchical design modules which greatly assist prototyping.

It is better to use simulation for all deterministic design modules and only test out the really doubtful bits, assuming that you are a good judge of ‘doubtful bits’. Its also good to know what the limitations of your simulation system are to help in this decision.

Use of Programmable Logic

The consequence of making errors is vastly reduced by the use of programmable logic (PLAs, gate arrays, etc.). It is then possible to order PCBs whose connectivity need not change even after the discovery of major design errors.


BIBLIOGRAPHY1

[1] K. Brindley, Newnes Electronics Assembly Handbook. Oxford: Newnes (Butterworth-Heinemann), 1990.

[2] C.H. Harper (Ed.), Electronic Packaging and Interconnection Handbook. NY: McGraw-Hill, 1991

[3] F. Riley (Ed.), The Electronics Assembly Handbook. Bedford, UK: IFS Publications, 1988.

[4] P. Horowitz and W. Hill, The Art of Electronics. Cambridge: Cambridge University Press, 1989 (2nd edn).

1 Note: The figures in Sections 1-5 of these notes are taken from references [1] and [2].


APPENDIX 1. MATERIALS USED IN PRINTED CICUIT BOARDS

A1.1 BASE MATERIAL

The original material used in the manufacture of PCBs usually consists of a type of plastic laminate clad on one or both sides with copper foil. Three types of laminate are commonly available:

• Phenolic resin impregnated paper laminate; • Epoxy resin impregnated paper laminate; • Epoxy resin impregnated glass fabric laminate.

Other more exotic materials such as poly-tetra-fluoro-ethylene (PTFE or Teflon) or polyimide (Kapton) based substrates, are produced for special applications.

The same groups of material are also produced without copper cladding for use as additive materials.

The particular material to be used in an application is selected to give the best technical properties with optimum economy. The availability of a range of base materials such as those above, which differ in physical strength, resistance to combustion, electrical parameters, translucency and machinability enables the optimum choice to be made.

A1.2 SUBSTRATES AND RESINS

The PCB substrate consists of a combination of fillers and resin. The fillers are high-quality special class papers or glass-fibre fabrics; the resins are usually phenolics, epoxy resins or close derivatives.

For consumer applications the paper/phenolic resin combination has been extensively developed to improve its technical properties to the point where it enjoys a wide application because of an adequate performance coupled with a lower price than other resin types.

Teflon based substrates are commonly used in high radio-frequency applications where their special low-loss, low-dielectric constant and stable mechanical and thermal properties justify the use of such an expensive material.

Polyimide (Kapton) based substrates are used in flexible PCBs. Polyimide based flexible circuits have the ability to maintain their superior physical, electrical and mechanical properties over wide temperature ranges. The polyimide film has no known organic solvent and is infusible and flame-resistant.

BT resins (B-Triazine) are formulated from triazine and polyimide and have a high glass transition temperature, though less than that of polyimide alone. This product can be formulated to meet Underwriter’s Laboratory (UL) specifications and can provide good copper adhesion.

It is becoming increasingly common to use materials such as polyimides and BT resins for multi-layer boards with layer counts above 8. These offer superior thermal stability


and reduce the incidence of cracking of plated-through holes. The BT resins are suitable for use with SMT components.

Polyester film is a cheaper material with a restricted temperature range of -70 to +150 °C, which also finds use in flexible PCBs made primarily for low-cost consumer applications.

A1.3 COPPER FOIL

High purity (>99.5%) electrolytic copper is usually used for copper cladding of substrates to ensure outstanding conductivity and soldering properties. Typical copper-foil thicknesses are 18, 35, 70 and 105 μm (corresponding to 0.5, 1.0, 2.0 and 3.0 oz/sq. ft. respectively).


APPENDIX 2. PCB MANUFACTURING TECHNOLOGY

A2.1 INTRODUCTION

The material presented in this section represents a synthesis of the best techniques that were evident from a series of visits to local and overseas manufacturers.

A2.2 PCB DESIGN AND MANUFACTURING DOCUMENTATION

Originally very simple techniques were used for pattern generation and layout of printed circuit boards. Opaque tape and pre-formed outlines were laid by hand on clear film to form the desired copper pattern. For accuracy, these hand layouts were often made at twice full size and were reduced to normal size in the subsequent photographic stage which produced the photo tool masters. While still in use for simple boards, this method has largely been replaced by CAD systems of generating PCB layout artwork.

For modern, fine line, dense boards (particularly multi-layer), manual layout methods are neither accurate enough nor cost effective. The best of the CAD software and hardware available today will permit the transformation of the circuit diagram directly into a layout with the capability of generating production photo tools directly from associated photo and/or laser plotters. In addition the best systems permit the checking of the design against pre-established design criteria (design rules) which allows the design to be optimized to suit the particular manufacturing process. Using the same data files it is also possible to generate automatic inspection (Auto Optical Inspection (AOI)), testing and drilling instructions for the CNC drilling machines.

An important feature of modern PCB production is the ability to generate a netlist from a CAD circuit schematic so that functional connectivity on the PCB is a total representation of the designed and tested prototype or simulation.

Clients will typically supply board design information to the manufacturer in the form of a magnetic tape (or disk) file (usually in Gerber format). The manufacturer will then use his CAD equipment to check that the design meets the required design rules, make any adjustments necessary to meet production requirements and produce photo tools, drilling, inspection and test instructions. It is also common to generate all other production documentation required using computer systems linked to the CAD system.

A2.3 RAW MATERIALS

A2.3.1 PCB Substrate

As already mentioned, various grades of substrate material are available to a range of industry and military specifications. Almost universally, the factors of major interest are:

• Dimensional stability; • Low bowing and twisting; • Resistance to chemicals;


• Good punching/drilling properties • Copper adhesion and solderability.

A2.3.2 Pre-Preg Material

The situation for this material is pretty much the same as for substrate. Being a partially cured resinous material, shelf life is an important parameter to be monitored, apart from the basic chemical composition of the material.

A2.3.3 Copper Foil

Again similar principles apply to this material. Important for copper foil is its chemical purity which affects solderability and adhesion during bonding and mechanical imperfections.

A2.3.4 Chemicals

• Cleaning (acid, solvent and aqueous based) • Plating (copper, nickel, palladium, gold, tin and solder) • Etching • Screen printing (inks and solvents) • Masking (photo imageable liquid films)

A2.3.5 Photo Materials

Accurate photo masters and secondary masters (usually prepared on CAD equipment and laser photo plotters) of circuit layer copper patterns are required to ensure that the design moves through each production process with a minimum possibility of error.

At the accuracies demanded for multi-layer PCBs, any film materials used for primary and secondary masters must exhibit a high degree of stability under changing heat and humidity conditions. In many applications it is necessary to provide controlled environments for the creation and handling of photo films.

A2.4 BLANK PREPARATION

A2.4.1 Blank Sizing

Unless special arrangements have been made with suppliers to supply laminates, copper foil and pre-preg sheets already cut to size, then stock material must be cut to size.

Prior to cutting, usually by guillotine, the copper laminate panels are baked to remove any entrapped moisture, facilitate cutting and to stabilize the mechanical properties.

Registration holes (for later pinning) may also be punched or drilled at this time and lot number or other identity number stamped into the copper foil at the edge of the panel.


A2.5 PHOTO TOOLS PREPARATION

The optimum approach, and the one that most producers are working towards, is to have customers prepare their board design information on CAD equipment to design rule specifications prepared by the PCB manufacturer. The design information can be supplied on tape or disk (usually in a Gerber plotter format) and can be input directly to the manufacturer’s Computer Integrated Manufacturing (CIM) equipment.

Using the CIM equipment, the manufacturer can check for design rule violations and correct the artwork if necessary. This may require close consultation with the customer to resolve any conflicting specification requirements. Where required, pads can be augmented to increase the tolerance to drill runout and the circuit traces can be adjusted to meet impedance and capacitance control requirements.

A2.6 PHOTO PRINTING BLANK PANELS FOR ETCH

A2.6.1 Pre-Clean

Careful preparation of the copper surfaces of PCB cores and outer layers is vital to achieving proper dry film photoresist performance.

A number of techniques are used; pumice scrubbing; grit bristle brushing; very fine silicon carbide compressed pad brushing; and peroxide sulphuric etching.

A2.6.2 Dry Film Lamination, Exposure and Development

The next step is to coat the copper surface with a photo-resist film (photopolymerisible material), expose the film using ultraviolet light through the photographic mask (phototool) to harden the resist covering the desired trace pattern, and develop using appropriate chemicals to remove the unwanted resist material.

A2.7 ETCH AND STRIP

Once the desired copper pattern has been masked by the etch resist, the panel is placed into an etching bath where the unwanted copper is removed by chemical action.

Typical etchants used are ferric chloride, ammonium persulphate, ammoniacal etchants, sulphuric/peroxide and chromic/sulphuric acids.

The resist is next removed using typically, alkaline solutions or solvents.

The board is then thoroughly cleaned and rinsed to remove all traces of solutions or solvents and dried before being passed to the next processing stage.


A2.8 100% INSPECT

Panels are 100% inspected to ensure that the etched copper pattern meets quality requirements. In the more modern plants this inspection is carried out by Auto Optical Inspection (AOI) equipment.

A2.9 OPTICAL DRILLING OF JIG LOCATING HOLES

For improved accuracy in aligning cores during the lamination process, the drilling of the locating dowel holes can be done at this stage of the process using an optically aligned, computer-controlled drilling machine.

A2.10 BLACK OXIDE COATING

To ensure that the remaining copper surfaces adhere properly to the resins in the pre-preg material to be applied during layup and bonding, it is necessary to treat the etched copper pattern by forming a surface layer of copper oxide (typically black oxide).

A2.11 LAMINATION-LAY UP, PRESSING AND DEBOOKING

In the layup process, the individual etched core layers are next assembled in order with the interstitial layers of pre-preg and outer layers of foil (or sometimes board).

A2.12 X-RAY DRILLING OF JIG LOCATING HOLES

A computer-controlled drill using X-Ray sensing is used to produce new jig locating holes in each panel.

A2.13 TWO-SIDED PROCESSING

A2.14 NC DRILLING

Panels at this stage have all their through holes drilled using numerically-controlled (NC) drilling machines.

NC drills are capable of 200 plus hits per minute and table positional and repeatability accuracies of between 0.2 mil and 0.5 mil.

A2.15 DEBURRING

Due to the heat generated during the drilling operation, burnt resin, glass fibres and copper dust is smeared over the inside of the through hole.


A2.16 ELECTROLESS PLATING

The next step in the process is to deposit copper in the holes drilled in the preceding step so as to provide the basic inter-layer circuit connections. As a first step the holes are catalysed with tin chloride/palladium chloride solution and the whole board is plated by a non-electrolytic process. In this process a controlled autocatalytic reduction deposits copper ions onto the catalysed hole surface.

A2.17 OUTER LAYER PATTERN PRINTING, DEVELOPMENT, & TOUCH-UP

The next step is to coat the copper surface with a dry laminate photo-resist film, expose the film using ultraviolet light through the photographic mask (phototool) to harden the resist covering the desired trace pattern, and develop using appropriate chemicals to remove the unwanted resist material.

A2.18 PATTERN PLATING, STRIP RESIST, ETCH, & INSPECT

To provide a plating resist to protect the copper circuit pattern in the next etching process a coating of etch resistant metal is used.

A2.19 BOARD FINISHING

The objective of applying a finishing process to an etched board is to:

• Protect the copper track surface from corrosion and ensure that solder-ability is preserved in the required areas;

• Mask the copper tracks in areas that do not require to be soldered to prevent unwanted solder bridging during subsequent assembly and flow-soldering processes;

• Annotate the board surface with component reference nomenclature to assist in assay and/or service operations.

A2.20 ELECTRICAL TEST AND FINAL INSPECTION

It is important particularly with multi-layer boards to ensure that the circuit is fully functional before the customer adds value to the board in terms of assembled and soldered components.

A comprehensive electrical test on the bare board is usually carried out. The optimum form of testing uses a “clam shell” tester in which opposing “beds of nails” or sets of electrical contacts are applied to the board and under computer control a series of electrical currents are passed through the boards’ copper traces and the connectivity pattern verified. The latest techniques now use currents of sufficient magnitude to destroy copper traces of less than adequate cross section.


A2.21 QUALITY CONTROL AND QUALITY ASSURANCE

All overseas PCB manufacturing plants visited exhibited a dedication to quality control evident at top management level and all other levels in the plant. Many have adopted a Total Quality Management system, in which statistical process control methods are used to maintain control of critical production processes and reduce routine inspection.


chapter12 manufacturing-processes

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