As an EDR professional, you are one of a team of people, both within your company and outside it, who are involved in creating a product. The module you are just starting on aims to give you a base of information about the materials and processes involved that will be sufficient to enable you to understand the problems and issues from the point of view of your colleagues in manufacture. Only with that background knowledge will you be able to contribute meaningfully to discussions and create effective designs.
This page is by way of a reminder of where electronic design realisation fits within the wider scope of the product design process. If you are already comfortable with this concept, and feel that you can already answer the questions “Where does the design realisation process fit in?” and “What does design involve?”, then jump to the next page: there are no SAQs. But we do suggest that at least you look at the mindmaps linked to the text, as these neatly summarise the issues.
Design realisation is a process which is mainly carried out at a product’s inception, but has implications throughout life, and parts of the process may be repeated both during volume manufacture and to meet service requirements at the end of life.
The product life cycle is a concept you will meet again in the module Design for eXcellence, and has a number of stages which merge into each other, and sometimes overlap:
These last two stages may involve a number of iterations, until a satisfactory prototype has been produced.
Electronic design realisation is involved in the development/prototyping activity, in setting the scene for successful acceptance testing and volume manufacture, and ensuring that service, maintenance and eventual removal from the field are accomplished.
The product life cycle from concept to end of life varies widely, depending on the industry and nature of the product. However, whether the product is a car or a mobile phone, market pressures are forcing the life cycle to contract, with designs needing to be developed very fast, and perhaps made in volume for only a relatively few weeks before the design is superseded by an apparently improved version.
This shortening of the life cycle has implications for EDR, in forcing:
These are issues to which we will return in Design for eXcellence, but must be kept in mind from the earliest stage. Although computer aids are available, the actual work involved in design is, if anything, greater, with the result that designs are rarely the work of individuals, but involve professionals working together, sometimes even “hot desking” where designs are worked on round the clock, passing designs and software licences from country to country as the earth turns.
Whilst some jobs are totally new designs, in fact quite a lot of work of any design department involves more fragmented and in some ways less satisfying work:
These last two involve similar challenges in issue control and documentation, but the drivers are different. For this reason, designs produced internally are normally better controlled than those driven by customer needs, where major changes to the requirement may result in what are almost “botches” in order to convert a perfectly designed but non-working product into one that will meet end-user needs, albeit at some risk to long term reliability.
Totally new designs, as distinct from incremental improvements, fall into two categories:
Any new design involves some element of risk, but the use of emerging technology needs very careful management control at all stages during the process.
So far we have looked at the types of design job that the EDR professional might tackle, but perhaps not looked in enough detail at what design involves. Particularly in a small company your expertise in board design may have to be supplemented by knowledge over a very wide range of issues.
In order to illustrate a number of the areas in which the designer may be involved, we would like you to imagine taking a new computer out of its packaging, and opening it up. If you have a fairly high-speed web connection, then you can follow this activity on video, but you will be able to follow the discussion without images if you are broadly familiar with the insides of PCs. Of course, if you feel brave enough, you may even want to take a computer apart and look for the points we mention!
We start with the outer packaging – the cardboard box so beloved of “box shifters”. This provides protection against handling, and bears a label which indicates what it is, how to handle it, and perhaps verifying that it complies with the legislation necessary for the contents to be sold legally within the destination country. It is not a simple box: typically there will be an outer shell, internal packing which conforms to the computer inside, and probably a plastic bag. This last is to help prevent cosmetic damage to the case and the ingress of fluids, although the seals are rarely hermetic.
There are similar issues relating to styling when we come to take the computer out of its box and look at the case. For any enclosure we have a range of materials and manufacturing methods available, and a choice of finishes. Materials and finishes are inter-related, but the choice is often determined by our requirements or product styling. Even more than with the final packaging, because the computer enclosure will be visible to the end user for an extended life, having something which is easy on the eye, easy to keep clean and appropriately functional is very important. Whilst this kind of design is normally given to industrial designers with an artistic rather than technological bias, decisions on the enclosure can have a profound impact on the EDR task. If we look at this computer, then we can see some of the issues that hopefully will have been borne in mind during the development of a specific enclosure:
The enclosure is also important in three other ways:
We have taken the case off, and removed some of the mechanical assemblies protecting the circuitry. At this stage you will see clearly that the computer is not just a single circuit. Instead, the entire circuit function has been ‘partitioned’, separating it into different areas, to which different techniques have been applied in the interest of both functionality and cost.
The first level of partitioning is functional separation, for example, taking high voltage/high power elements in the power supply and ‘realising’ this as a separate module.
The concept of modularity is important in the computer industry, and also in applications such as test equipment. Although less apparent, there is usually some degree of modularity at the design level in most applications, if only to allow design reuse – if you have a circuit element which works well, and need the same function in a new design, why reinvent the wheel?
For the computer, modularity is also useful for the reasons that:
Most importantly, modularity encourages the use of existing modules, both those from previous designs and standard modules sourced from elsewhere. This cuts the time involved in development, and allows the cost benefits of volume production to be realised.
Circuit partitioning also has to take account of mechanical constraints, such as those resulting from product styling choices. For example, the main part of the circuit may have to be positioned within a volume which does not allow power or output elements to be integrated.
Circuit partitioning, as with the choice of certain kinds of enclosure, will also be affected by the existence of external standards, both for the industry and at a component level.
Circuit partitioning has an impact on the technology which is used – one can generally integrate all the components, but doing so needs a higher level of technology and higher cost than allowing a little more space and partitioning thecircuit into modules. The consequence is that partitioning impacts substantially both on the cost of manufacture, and on the extent of the design task, and hence on the time to market.
On the printed circuit board itself, there are a number of inter-related factors which the layout engineer has to balance. The cost of the assembly is made up of the cost of the board, the components and the assembly costs, including an allowance for reject parts. The board specification, in terms of the fineness of the features, the number of layers, and the surface finish, is determined by the number and type of components, and by the use of any sub-assemblies.
Usually it is cheapest to perform as much integration as possible in the silicon chip. Provided that the components are standard, it is generally very much cheaper to have complex functions in silicon, and relatively few support components. The reason is not just the cost of support components, but the higher integration density which results. As you will see in a later unit, the components used affect both the assembly techniques and the density of connections on the PCB.
It is worth making the point that, once you are working near the limit of density which is easily obtainable, doubling the density will substantially more than double the cost. On products such as a computer, where space is at a premium, the size of the board will be optimised for cost, with as much complexity as possible in silicon and on daughter boards, and with an even spread of components. This approach also helps the designer to make the product easier to rework.
The impact of increasing the component and interconnection density beyond the optimum can be seen historically in the comparatively high cost of laptop computers compared with desk top devices: only more recently, where a greater level of integration on the silicon has been possible, and the cost of smaller components has reduced, have laptops become a commodity item at a price which is not cripplingly more than the larger machine.
Designing a printed circuit board equates to making a whole series of compromises, based on discussions with the electronic engineer about the range of components available, and any options there may be. At the same time, the designer has to consider how the board is to be mounted and supported within the enclosure. For a computer, this is a relatively trivial exercise, unless the equipment is designed for a harsh environment, subject to vibration and shock.
One final aspect to be considered by the designer is how changes are to be implemented. Changes are inevitable, and can often be planned for. However, the re-tooling cost can be substantially avoided if it is planned from the outset to implement expected changes either in software, or using pluggable devices, such as ROM. For minor changes, in particular for correcting errors, or configuring boards for specific applications, movable jumpers and switches are helpful. For trickier changes, in order to make use of boards of an earlier design, then the use of ‘wire adds’ is not uncommon. However, it must be remembered that this kind of assembly work needs skill and time, and therefore incurs costs.
The computer is not just a set of boards within an enclosure, but embraces interconnections, and this is generally part of the job of the EDR professional. Some of these connections will be external to the enclosure, made by chassis mounting sockets; other connections are internal, either between boards, or between boards and individual components, using cables of various kinds.
Whilst most computers are a bit of a ‘rat’s nest’ of cables, in much other equipment more use is made of ‘harnesses’, which are collections of terminated wires of predetermined length – this is the way that a car is wired, for example. As well as single conductors, which may be solid or stranded cores, depending on the application, a variety of special cables are used: co-axial types for high frequency; ribbon cables to provide multiple connections.
The connectors themselves are a topic all on their own! Their specification will depend not only on their function, but on how many insertions they are designed to withstand during normal working life. A very few connections are made once only, as with plug-in modules as an alternative to solder connections; other connectors such as those between daughter board and mother board are designed to be made only a limited number of times; others are used frequently. The specification affects the design, the materials and of course the cost.
The physical format of connectors may be either 'double part', with a separate plug and socket, each of which is permanently connected (mostly by soldering) to the board or cabling, or 'single part', as in the case of memory modules, where one half of the connector is formed by the conductive pattern on the module. Yet other connectors are feed-throughs with hard wiring to both sides.
So boards have to be connected together, and choosing the right connector is an important part of the EDR process. However, cabling and connectors are the only way of interconnecting. As you will know if you have ever disassembled a printer mechanism, there are requirements for flexible connections in the printer head which are generally met using a flexible form of printed circuit board. We will be saying a little about the options in Materials of Electronic Assemblies.
So, by looking at a computer, we have covered most of the areas where the EDR professional operates, either alone or (more often) in conjunction with an electronic designer, test engineers, and stylists and packaging experts. There are, however, some elements which are not covered in this example. These include:
Having looked at this typical product, you will have some flavour of the design task in a real product, embracing enclosure, electronics and interconnections, with more than a passing glance at software development and test design. Many of these tasks are inter-related, and the task of the EDR professional is to take a holistic view of the situation – don’t get bogged down in the detail of the trees and forget the wood. Typically the EDR professional has to make decisions, each of which will have some implication for cost of performance. The process is a series of questions:
a) What is the requirement?
b) What are the options? – that is all the options, not just the most obvious.
c) What are the criteria for evaluating the options? – the criteria may differ according to whether you are an electronic engineer, production manager, or customer.
d) Do I have all the information needed for this evaluation? – “all” is the ideal, but typically engineers ask the question “do I have enough information?”.
e) What appears to be the best option?
Then comes the crunch, verifying that what appears to be the best option is compatible with the constraints of timescale, costs and technology.
The decision process is characterised by its iterative nature – you go round the loop more than once – and by the need to involve other people in the decision making process. Frequently you need to compromise, there being no perfect solution which exceeds everyone’s requirements. In some cases, one even ends up with “least worst”. Of course the decision has to be made now, if not yesterday, responding to the fact that most EDR professionals are working in an increasingly pressured environment.
On the left hand side of the mind map at the top of the page we tried to indicate the relationship between the EDR professional and the other people involved in the design process. This has been repeated below.
Note that the links between EDR and electronic design and test are particularly strong, but good two-way communication is also needed to the board fabricators and those carrying out assembly. This can be a challenge, now that fabrication is almost always carried out by independent houses, and assembly is increasingly sub-contracted to specialists.
From the diagram, you will notice a number of implied comments on, perhaps even criticisms of, the way in which information flows to EDR within many companies: