Incorporating the Principles of PCB Manufacturing to Optimize Design
It is not an overstatement to say that most successful endeavors are based upon either a solid foundation or lady luck. Of course, most of us will take success however it comes, yet relying on chance is akin to shutting your eyes and attempting to cross a busy street. Likely to be quite painful. On the other hand, plotting a course that is purposeful and guided by well-defined principles will help you avoid the pitfalls that will lead to failure.
There is a pretty well-known adage or principle that is mostly utilized in business management called the Pareto Principle. Essentially, this principle states that there is a proportional relationship between a cause and its effect that is not 1:1. This ratio is typically given as 20:80. In other words, a 20% effort impacts the result by 80%. The specific ratio is not fixed; instead, it is meant to represent that some inputs that cost very little in terms of resources can have a large impact on the output.
Today, the PCB design and development industry is highly competitive and it is rapidly becoming unacceptable simply to be able to produce boards. Instead, it is necessary to design and build high-quality boards more efficiently than other developers and OEMs. To achieve this level of performance requires that your design process be optimized. And one of the key factors is the incorporation of the principles of PCB manufacturing. Before, discussing the ways in which this can be done we first need to understand those principles and the underlying PCB manufacturing process.
The PCB Manufacturing Process
The importance of the PCB manufacturing process to the overall development of your design cannot be overstated. Turnaround time and board quality are just two of the factors that depend on your choice of contract manufacturer (CM) for fabrication and assembly services. PCB manufacturing consists of a formalized process, as listed below:
Board Fabrication Steps
Imaging - initially the board layout image is created.
Etching (inner layer) - or the removal of copper from all areas, except traces and pads.
Board layup - is where the board layers are arranged, aligned and bonded together.
Drilling - all holes, whether for vias or mounting are drilled next. Depending on the board size or height this may need to be done in steps, especially if laser drilling is used.
Etching (outer layers) - next, the outer layers must be etched. Copper resist must also be cleaned from these layers.
Via plating - is the filling of vias with a conductive material, typically copper.
Applying solder mask - to the board surface is done to protect against external contamination, such as oxidation. This also prevents issues; such as solder bridging, from occurring during assembly.
Silkscreen printing - is the adding of reference indicators, pin one indicators, polarity signs, labels and other imagery to the board surface.
Apply finish - this step, which is not always included, is meant to protect the board against external contamination and prepare the board for assembly.
PCB Assembly Steps
Apply solder paste - an initial layer of solder paste is applied to help keep SMD components in place for soldering.
Place surface mount devices (SMDs) - next the SMDs are placed on their footprints.
Solder reflow - this is where the SMD connections are made.
Rework (if necessary) - after inspection, if any SMDs are not secured or aligned properly on their pads, this is corrected.
Mounting through-hole components - here, the through-hole components are placed with leads through the board.
Soldering through-hole components - through-hole components are then mounted, most often through wave soldering.
Final soldering (if necessary) - if any connections are not secure, they are corrected.
Cleaning - is done to ensure there are no contaminants left on the board. However, it is not performed in all cases.
Applying conformal coating - the final layer of protection for the entire board surface is conformal coating. This protects the conductive traces and helps prevent clearance creepage.
Depanelization - during this step, routing or scoring is used to separate the boards into individual units.
As shown, the fabrication and assembly stages of PCB manufacturing define an orderly structured process. Yet, many of these steps depend upon specifications that are made during design. Therefore, your design decisions have a significant impact on the process and the results. These decisions are in some cases directives. For example, opting to use FR4 as the material for your boards. However, for your board manufacturing to have maximum impact on your development it should incorporate the principles of PCB manufacturing.
What is the Foundation of the Principles of PCB Manufacturing?
Knowing what steps are involved in the building of a circuit board provides the basis for understanding the principles of PCB manufacturing. In fact, instituting these principles relies upon your ability to make design decisions that will aid your CM and enhance the manufacture of your boards. The most effective way to do this is by adopting and utilizing the rules and guidelines of your CM’s process or employing design for manufacturing (DFM).
For PCB development, DFM can be defined as follows:
Design for manufacturing or DFM is the process of incorporating the fabrication and
assembly equipment capabilities, techniques and processes of your circuit board
manufacturer into your design to ensure manufacturability and help facilitate or ease
fabrication and assembly. This includes setting board parameters; such as clearances
and drill hole sizes, that fall within the tolerances specified by your manufacturer.
When the specifications are targeted to PCB assembly or PCBA as opposed to fabrication, they are referred to as design for assembly or DFA. Therefore, we can define DFA as follows:
Design for assembly or DFA is making design decisions that are intended to help
facilitate the assembly of a circuit board or PCBA. This includes choice of type of via(s),
annular ring sizes, settings for solder mask expansion, component spacing for good
thermal distribution and other options.
The total number of DFM and DFA or DFMA options available for specification is the basis of your ability to apply principles of manufacturing to your design, depending upon what data is available from your CM and most importantly, the capabilities of your PCB design software.
How are the Principles of PCB Manufacturing Incorporated Into Your Design?
The extent of utilization of DFMA is the primary means by which the application of PCB manufacturing principles are implemented for your design. However, the principles themselves are embodied by adopting a manufacturing-focused approach to PCB design. Adopting such a philosophy can assist you in making decisions that not only improve manufacturing directly but can also positively impact overall development and production; such as deciding when to design-to-cost or design-to-value.
So how is this philosophy where PCB design decisions are tempered by what effect they have on the manufacturing process actually manifested? It starts with understanding the art of building PCBs or the flexibility you have to direct manufacturing activities. By exercising this freedom to select manufacturing options, within DFM and/or DFA guidelines, you can eliminate or avoid PCB assembly and subassembly redesigns that will extend turnaround time and drive up costs. You should also take into account manufacturing overhead and production volume variance which improves manufacturing cost estimation. Making the most effective use of PCB manufacturing principles requires that you utilize advanced PCB design tools; such as 3D simulation for PCB systems.
Let’s explore these principles of PCB manufacturing in more detail.
The Art of Building PCBs
The art of creating the PCB design
You probably have never thought of designing circuit boards as art, but consider this. An artist takes an idea or request and brings it to life as imagery. An engineer or PCB designer starts with an idea or request, usually with some specific objectives for functionality, and creates a PCB design that can be represented as an image. True, you say, but the artist has freedom over the dimensions and material or canvas that is used.
Well, the designer can select the materials, stackup, components and how the traces will be routed. Now, there are limitations, which may be stipulated by your client, regulations and standards or the operating environment for your board. Yet, just as an artist does, the designer has the flexibility to create a design that reflects their own vision of what the PCB, when built, should be. The key is to know what attributes of the design are variable and what are the boundaries within which they must reside. These boundaries are set by your CM’s DFM and/or DFA rules and guidelines.
If you’d like to learn more about the art of building PCBs, read about it here.
Avoiding PCB Assembly and Subassembly Redesign
Another principle of PCB manufacturing that may seem somewhat obvious is to avoid redesign. Redesign of PCB assembly or subassembly is required when errors are detected or identified that can only be corrected by modifying design parameters or specifications. These errors may be detected prior to or during fabrication, component placement or assembly. Examples include the following:
Errors Requiring PCB Assembly or Subassembly Redesign
Common Design Errors
Inadequate spacing or clearance between components, traces and drill holes, drill hole missing, unconnected traces, trace width and copper weight violations and silkscreen overlapping soldermask.
Component not available, package and footprint mismatch, component sensitive to moisture or temperature.
Missing solder dams, components without lead traces, board edge clearance violation, insufficient thermal relief and reference or pin 1 indicators not clear.
The above list does not include all of the possible errors or problems that can halt manufacturing and many can be detected by having your CM perform a DFM check prior to fabrication. This will not prevent the redesign but may prevent some additional costs. Nevertheless, the best way to avoid the majority of these issues and redesign is to adhere to the rules of your CM’s DFMA.
To learn more about avoiding PCB assembly and subassembly redesign, read about it here.
Factoring Manufacturing Overhead and Production Volume Variance Into Your Design
During the development and production of PCBs it is always desirable to minimize or eliminate the need for any type of redesign. However, for all but the simplest circuits, completely eliminating design changes is not practical. For example, during prototyping, it is routinely required that several design⇒build⇒test cycles or iterations be performed to remove any errors and improve the design quality of the board.
This prototyping phase occurs during low-volume production, where the number of boards built may range from a handful to hundreds. The other production level, high-volume, occurs after development and board design is finalized. The number of PCBs produced here may be in the tens or hundreds of thousands, or even higher.
High volume PCB production
Low-volume and high-volume production have different manufacturing objectives and costs and optimization requires that you incorporate these into your design strategy and process.
For tips on manufacturing overhead and production volume variance, read here.
The Importance of Accurate Manufacturing Cost Estimation
Recognizing the variance in manufacturing overhead and how it affects the turnaround time for your boards and the cost is one thing. Implementing it is another. In order to fully measure the resources that producing your boards will require you need to perform accurate manufacturing cost estimation. Manufacturing costs are greatly determined by your CM, which are presented to you in the form of a quote. The accuracy of the quote is virtually entirely reliant upon the accuracy and completeness of the design information that you supply to your CM. Therefore, you should make sure to deliver a comprehensive design package a preferred format for your CM.
Read about ensuring accurate manufacturing cost estimation here.
Making Effective Use of 3D Simulation for PCB Systems
3D simulation of finalized PCB
Another essential for design that includes the principles of PCB manufacturing is tool utilization. Incorporating and leveraging manufacturing principles such that time and costs are minimized requires that your design be analyzed and well-vetted before submission to your CM. One of the best tools for this type of evaluation is 3D simulation for PCB systems. The ability to view and design in 3D provides you with a number of other advantages, such as better flex board design, in-depth multi-board design inspections and MCAD/ECAD collaboration.
If you’d like to learn more about using 3D simulation for PCB systems effectively, read about it here.
Software Requirements for Easy Principles of PCB Manufacturing Integration
The best or optimal board design process includes the incorporation of principles of PCB manufacturing. After all, the manufacturing process and its results determine the turnaround time, quality and costs for your boards. Understanding this significance enables you to realize the advantages of its implementation, provided you have the software tools necessary to do so. A key to maximizing utilization of manufacturing principles is a PCB design software package that allows for comprehensive DFMA inclusion.
Comprehensive DFM constraint management
As shown above, Cadence’s OrCAD Designer platform includes an extensive Constraint Manager that enables fast and efficient PCB design driven by DFMA requirements. The realization of constraints in real-time optimizes your implementation of principles of PCB manufacturing and your design.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.