Typically, PCBs account for 90% of the total material cost of an electronic product. Developing PCB cost models can vary depending on the accuracy level needed. Consumer products are sensitive to cost variation, whereas new technology products are less sensitive.
The electronic design cycle and its implementation in PCBs is divided into several steps. For most current electronic design activities, computer aided engineering (CAE) is used to document the design and provide the basis for electronic analysis and iterations of the design. Its function is also to physically partition the design into distinct electronic groupings or models that are then incorporated into each PCB. It also acts as a data source for further steps in the cycle. Figure 6.5 shows the steps involved in the PCB design, cycle which are:
• The logical design phase of matching the product specification requirements by completing the electronic circuits design, selecting the components, and documenting the circuit connectivity.
The analysis phase, in which the design is checked out to produce the optimum performance in terms of minimizing errors in connectivity, loading, and race conditions, optimizing testability and conformance to specification. This is usually performed using analysis tools for analog and digital simulation and modeling to verify the functionality of the electronic design. In addition, the design review concept at this phase is important to ensure both the technical validity of the PCB design, its connectivity to other PCBs in the product, and its suitability for manufacturing. The design review is a good alternative in the absence of effective analysis tools, especially in today’s complex design environments.
• The PCB layout phase uses computer aided design (CAD) techniques to physically place the components and their interconnections to each other and to the outside world. This function deter- mines the tooling and manufacturing environments for the PCBs and their future cost.
• The supporting and follow-on processes, which include activities such as device library creation, prototype PCB fabrication, assembly, and testing.
The alternatives in the design and layout processes include the selection of process factors for the components, layout, fabrication, assembly, and testing technologies. These factors affect the overall product cost and quality differently, as follows.
Component technology affects the component count directly and hence the PCB layout space required, the assembly production rate, and the reliability estimates of the product. These technologies include the following:
1. Through-hole (TH) components, which have leaded terminals to attach them to holes drilled in the PCBs.
2. Surface mount technology (SMT) components, which are leadless or have low-profile leads to attach them to the surface of the PCBs.
3. Printed circuit materials, which can include single and multilayer plated-through PCBs as well as one sided, nonplated holes.
These components have different footprints (spacing), production rates, assembly equipment investment, and required support.
PCB layout offers a clear choice of faster development time versus fabrication costs. Two layers or several levels of multilayer fabrication technology are some of the alternatives presented in the PCB layout phase. As the layer count decreases, there is a proportional effect on the cost and reject rate of PCB fabrication, but an inverse relationship to the time required to completely lay out a complex electronic design.
Fabrication strategy is dependent on the desired physical and electrical characteristics of the PCBs, as well as the maturity of the design and the time required for completion. Multiple alternatives are available such as PCB materials, layer count, hole and line specifications, and construction technologies. Many design engineers are not aware of these choices and do not fully understand the cost-benefit ratios of each.
PCB assembly strategy is influenced by the selection of the component technology in the design phase and the machine complement on the production floor. The chosen technology dictates a particular set of assembly operations. Several levels of manual versus automatic production processes can be used, depending on the physical electronic components chosen for the design.
Test strategy allows for logical and physical interconnection between the PCBs and the test systems. Additional target test points and test circuits influence both the layout timing and the physical constraints of the design.