It is often said that sports imitate life. In football, for example, the quarterback is the most important player. This is not because he is the best athlete, although he can probably throw the ball farther than any other player. Instead, the importance lies in the fact that he must not only know his role, but also that of the other ten players on the field. Otherwise, the team would likely fall victim to ineffectiveness, turnovers or defeat.
This same responsibility falls upon engineers and circuit board designers. Lack of knowledge of how your boards are made and the essential PCB design steps to take will likely lead to long turnaround times, errors that require redesign or the failure of your board to meet its objectives once in the field. Just as it is imperative to know and use PCB design terminology as we discussed in Part 2 of this series, knowing how your boards are built also begins with understanding the most important PCB manufacturing terminology, which is defined in this article.
List of PCB Manufacturing Terminology
Acceptance quality level (AQL)
The acceptance quality level (AQL) or acceptable quality limit (AQL), as defined by ISO 2589-1:1999 Sampling Procedures For Inspection By Attributes -- Part 1: Sampling Schemes Indexed By Acceptance Quality Limit (AQL) For Lot-By-Lot Inspection, this is the percentage or ratio of defective units per tested lot that is considered acceptable. This can be applied to components, board materials, PCBs and PCBAs and is a useful metric to assess the quality optimization of your board’s development processes.
For PCBAs, a CAD file is a computer-generated that contains data and information for the manufacture of the board. Most design file software packages have a native CAD file format. For manufacturing, this information is most often converted to either multiple Gerber files or a PCB design file, such as ODB++ or IPC2581, that includes the necessary information and imagery for fabrication and assembly in a single file.
Common formats include; *.brd, *.dfx, Specific data for fabrication, component procurement (if a turnkey service) and assembly is extracted from the single file. Today, virtually all PCB design software can be classified as electronic design automation (EDA) tools, as the generation of the PCB file(s) required by contract manufacturers to build the boards is mostly automated. The standard format for this data is either.
Conformal coating, which is applied to your board after assembly, protects against contamination during storage and/or installation. There are many threats that can negatively affect PCBA operation; such as temperature extremes, vibration, and nearby electronics. However, one of the most significant threats to your board is moisture and the most effective means of addressing this threat is conformal coating protection that is applied after assembly; however, proper usage is guided by regulatory standards.
Design for manufacturing (DFM)
Without a doubt, the best thing you can do to ensure the manufacturability of your board design is to apply your CM’s design for manufacturing (DFM) rules and guidelines, which define acceptable ranges for your board specifications. And optimizing DFM usage leads to the most efficient board build process and highest quality PCBAs.
The most used and oldest design file format used in the PCB industry is the Gerber file, where each file provides data and information about a specific layer or aspect of the board design. However, other formats; such as OBD++ and IPC-2581, are becoming more common.
For manufacturing, there are three IPC classifications that stipulate the quality level to which circuit boards must be built based on where and how they will be used. Additionally, all processes and stages of PCBA development are guided by standards established by the IPC Association Connecting Electronics Industries.
Non-recurring expense (NRE)
When developing a new PCBA, as opposed to revising or modifying an existing board, there are typically one time or non-recurring expenses (NREs) for research and development (i.e. design, prototype manufacturing, and testing) that will not be necessary once the product is proven and on the market. These costs are essential elements of the time and cost trade-offs inherent to new product development that can and should be optimized for custom PCBA production efficiency.
Panelization (and depanelization)
Panelization is the process of transferring your design on sheets for manufacturing. In most cases, multiple copies of the design are used to maximize the use of material. The design of these panels or the layout of your boards on the sheet is significant, as it determines the number of runs required, the method of depanelization or board separation, and the amount of material used and wasted. To facilitate depanelization, panelization design guidelines should be followed, especially when common PCB edge connectors are used.
PCB manufacturing can be divided into two types: development, which includes proof-of-concept and prototyping, and production that can be low-volume or high-volume, where the goal is to reliably manufacture boards with an acceptable AQL and yield rate.
Prototyping is the cyclic development process of design-build-test (DBT) to remove any errors and refine the board design before transitioning to production. During this stage, the objective is to achieve the highest quality design, which is significantly influenced by the PCB prototype iteration speed.
Routing is the drawing of copper traces on your PCB layout to form all necessary connections and paths for current. Although PCB routing techniques are applied to the PCB layout during design, they are bounded by the capabilities of your CM. For example, routing the current paths cannot violate manufacturing PCB trace width and spacing restrictions as provided in your CM’s DFM guidelines.
Spin a board
Spinning a board refers to the PCBA development process, which consists of three stages: design, manufacture or build, and test (DBT). For all but the simplest of designs, multiple iterations of this process are required to optimize the design prototype service. Essentially, each iteration is a “spin” of the board.
A test coupon, which is a small part on the panel identical to the PCB, may be used to avoid destructive testing of usable boards. For some circuit board applications, it is necessary to validate the structural integrity or other physical aspects of the fabricated PCB. To assist with these circuit board testing methods, a test coupon may be used. Additionally, if you require time domain reflectometry (TDR) testing, a coupon is needed for each layer as the impedance may vary.
Manufacturability of your circuit board depends upon your design specifications falling within the PCB tolerances of your CM’s equipment and processes. Choosing values that fall well within the DFM-specified range of parameters is a key design for reliability attribute that will lead to the longest operational lifecycles for your boards.
Turnkey PCBA manufacturing, where the entire board build process (fabrication, component procurement, and assembly) is handled by a single CM, is the simplest method of board build for the designer. Not only is turnaround time reduced, but centralized management is a more optimal PCB prototyping paradigm.
Next to minimizing the cost per unit, the objective of PCBA production is to maximize the yield rate, which is the ratio of usable boards to the total produced that can be improved by selecting board parameters that fall near or at the center of the range of PCB tolerances or process window for a particular manufacturing step.
|Tempo's Custom PCB Manufacturing Service
Our database is much broader now that we have added PCB manufacturing terminology, which can be used for more effective communication with your CM. Open, accurate and transparent communication is one the keys to efficient manufacturing by eliminating time-wasting prototype PCB quote delays. At Tempo Automation, we provide quotes in hours as opposed to days and fully built boards in days as opposed to weeks.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.