As mentioned in the first part of this unit, the most common and best-defined Design for Xs are Design for Manufacture and Design for Assembly. The emphasis during the next section will be on these, but the section following will give some explanation of other Design for Xs.
In the electronics industry Design for Manufacture includes Design for Fabrication (bare board), Design for Assembly and Design for Test/Testability. Design for Assembly will include inspection, repair and rework clearances. These will be covered in greater detail in the Units Design for Assembly, Design for Fabrication and Design for Test.
DfM of electronics products are a study in their own right because of the complexity of PCB assemblies, the tight tolerances on component, board and process and the reliability demanded of the products for low profit margins.
The list below shows a typical DfM review list. It includes aspects from PCB fabrication, assembly, repair and test.
With relevant background knowledge, the guidelines on this list seem obvious. But how do we get this knowledge?
The obvious step is to involve all design functions in the other aspects of the realisation process. This will aid the design functions to appreciate the integrated and concurrent aspects of PCB design. To help develop this, designers should take time to visit board shops and assemblers with the aim of talking to the fabricators and assemblers to ask how PCB design decisions can impact fabrication and assembly yields and costs. They will usually have examples to demonstrate design decisions that have a large impact on final product cost.
These meetings and discussions should be repeated several times each year. This is because of the continuous changes that are taking place in the evolution of Surface Mount Technology: new component package designs are released frequently; new PCB materials are being developed; new board finish choices are being developed. Contract Assembly companies are constantly faced with upgrading or adding new equipment that may impact the assembly process.
This knowledge would be used to continually update the designers’ guidelines. It is important to understand that the capabilities of assemblers or fabricators are the processes and dimensions they can achieve. The guidelines may give you this information but this does not mean that these are to be used for every design. Designing every PCB so that it pushes the fabricators’ or assemblers’ processes to their limits is uneconomic because the closer to these limits a design gets then product yield (and therefore cost) and quality are compromised. For example, if a PCB fabricator can provide boards with 4 thou tracks, then use this as a minimum limit in your designs. Use this only when necessary, because the thinner the track the more likely there will be breaks in the track. These poor yields will be charged to you.
The fabrication process consists of many specialised processes such as etching, drilling and plating. Each fabricator has limitations to each process that, when stretched, affects yield and quality. The fabricator will incorporate any scrap costs into quotations, so it is in the interest of designers to minimise these costs. PCB fabricators are keen to encourage incorporating their process capabilities into a design and will offer guidelines to designers. Some of these are available on the internet.
Typical guidelines could be:
As with PCB fabrication, the assembly process is complicated and poorly designed boards result in poor yields, high rework and low quality. The designer should consider the assembly process during design to reduce these.
Some guidelines to consider are:
There is little point in developing an advanced product if manufacturing cannot adequately test it. Early involvement of test engineers in the product design process is essential so that a clear understanding is obtained of how the product will be tested during the production processes.
Areas that might require clarification include:
A simplified PCB design process is shown in Figure 1. The information given to EDR engineers would be product schematic diagrams, a critical component list (with recommended board placement) and possibly a Bill of Materials. The EDR engineer then places the critical components as requested and then places the remaining components. The engineer would then route the interconnections between the components. Finally, information such as silkscreen and solder mask is added to the PCB. During the process several analyses would be carried out. Think about the stages of the process and describe the analysis for each stage, especially with respect to DfX.
Do make sure that you do this activity and look up the answer before reading further.
Design for Manufacture is concerned with optimising the manufacturing for cost, quality and productivity. DfM guidelines explain the effect the design has on manufacturing. They are intended to be used by designers during the design phase in conjunction with product functional and aesthetic specifications and, of course, other Design for Xs relevant to the product. If correctly followed, they should result in a product that is inherently easier to manufacture.
Some sample guidelines are presented below:
The objective of DfA is to identify product concepts that are inherently easy to assemble and to favour product and component designs that are easy to grip, feed, join and assemble by manual or automatic means. The main aims of DfA are:
DfA may be carried out manually or with the support of computer based systems available as conventional programs or expert systems. A DfA analysis could be carried out at the concept stages and DfA evaluation on completed designs or prototypes. The former is obviously preferable because changes are easier, but sometimes the DfA engineer has to work with what he is given!
Here is a list of the most commonly quoted guidelines for DfA:
One of the other DfXs has been given the title ‘Design for Low Cost’. It uses the DfM and DfA approaches, but has the specific aim of reducing the cost of the product, rather than just improving its manufacturability.
To give you insight into DfLC, we should like you to review the DfM guidelines for Mechanical Assemblies given above, and suggest how they might be used to provide some guidelines for Design for Low Cost that the PCB designer could use.