The ECAD Industry is Firing on All Cylinders. No Wonder Wages are Rising.
Results of the annual PCD&F design engineer salary survey. READ MORE…
I4.0 and Reshoring
Why they go hand in hand. READ MORE…
Creating a Documentation Package
Communicating the design intent, clearly and concisely.
READ MORE…
Designing an LVD/IR Printed Circuit Board
Will a “universal” footprint work for various-sized passives?
READ MORE…
Practical Statistical Tools for Process Engineers
A review of five useful tools and the basis for using each. READ MORE…
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The ECAD Industry is Firing on All Cylinders. No Wonder Wages are Rising.
Results of the annual PCD&F design engineer salary survey.
https://www.pcdandf.com/pcdesign/index.php/editorial/menu-features/16746-the-ecad-industry-is-firing-on-all-cylinders-no-wonder-wages-are-rising
I4.0 and Reshoring
Why they go hand in hand.
https://www.pcdandf.com/pcdesign/index.php/editorial/menu-features/16748-why-i4-0-and-reshoring-go-hand-in-hand
Creating a Documentation Package
Communicating the design intent, clearly and concisely.
https://pcdandf.com/pcdesign/index.php/current-issue/241-designer-s-notebook/16835-creating-a-documentation-package
Designing an LVD/IR Printed Circuit Board
Will a “universal” footprint work for various-sized passives?
https://pcdandf.com/pcdesign/index.php/editorial/menu-features/16837-designing-an-lvd-ir-printed-circuit-board
Practical Statistical Tools for Process Engineers
A review of five useful tools and the basis for using each.
https://pcdandf.com/pcdesign/index.php/editorial/menu-features/16838-practical-statistical-tools-for-process-engineers
Communicating the design intent, clearly and concisely.
The CAD system can do a lot for you; then you’re on your own. Eyeballing the layers two or three at a time will help you find the hidden traps. A camera-ready board is just the beginning. Documenting the PCB requirements with dimensions and other details, including hole chart(s), stack up diagram(s), a list of intentional shorts, etc., puts the necessary guardrails around the fabrication and assembly.
Fabrication notes will change with the technology used for the PCB. The general thrust of this list of example notes is for a multilayer board targeted for consumer electronics with components on both sides and controlled impedance on inner- and outer layers.
Flex circuits or high-reliability boards would have significantly different callouts. In any case, all the attributes of the drawing need traceability. When calling out a specific process, such as cleaning with “ultra pure water,” it should be accompanied by a requirement for a Certificate of Conformance. Otherwise, who could tell if the process was followed?
Note 1A (below) covers a lot of ground by itself. The notes that follow are best used to describe options within the spec and exceptions to the spec. For instance, you may call for “Class 2 (as shown)” but do not want to allow 90° break-out of the drilled hole from the pad. In that case, a note calling for “no break-out, tangency permitted” would be necessary and sufficient for that exception to the general Class 2 fabrication rules. If that option is requested, make sure it is possible by increasing the pad size so that the tolerance stack makes sense.
Figure 1. A gleaming high-tech sensor factory.
Standard fabrication notes (unless otherwise specified).
FIGURE 2. Assembly drawings come in two main types: SMT assembly and (like this one) second-operation drawings. Note that the details use third angle projection. (Source: Velodyne)
Assembly drawings. Another inspection document is the assembly drawing (FIGURE 2). The primary goal is to show what the finished assembly looks like. If you read my columns, you probably know that I insist that this is a what-is, not a how-to document. How-to documents are subject to frequent revision as the process evolves through continuous improvement. Don’t get yourself wrapped around that axle.
Projecting views. There will always be a topside plan view of the PCB outline. It may be augmented with a side or cutaway view to show details, such as the beveled edge for gold fingers. The US, Australia and possibly some other locations use what is called “third angle projection,” while “first angle projection” is the norm in Europe and elsewhere (FIGURE 3).
For third angle projection, imagine the board at the bottom of a bowl. The top and side views are created by sliding and rotating the view up the sides of the bowl, so the side seen in the edge view faces toward the plan view. On the other hand, first angle projection inverts the imaginary bowl and allows the additional views to slide off the bowl.
Figure 3. The graphics in the upper left corner of these views are included in the title block to establish the projection of various views. (Source: Practical Machinist)
Standard assembly notes (unless otherwise specified).
Formatting data. The industry has used Gerber data about as long as it has used computers to aid our design efforts. Gerber data has evolved to permit unlimited apertures. Originally, this was a mechanical process that involved aperture wheels with 24 different openings used to generate the phototools.
The aperture wheels evolved to an electronic equivalent. Every PCB design was taped out with a separate file for the apertures. The aperture list is now embedded with each artwork layer to make it more resilient. The information conveyed in this standard is just the raw geometry. File names supplemented with a readme.doc help the fabricator determine how to use the data.
Eventually, a de facto standard came along from Valor (now part of Siemens). It is native to the software used in the front-end of the fabrication process. The format is called ODB++ and it has some intelligence along with the geometry data. Fabrication and assembly data can be combined in a single archive. Most PCB factories prefer this type of data. This was acceptable when Valor was a standalone enterprise.
The ECAD industry has come together around a new specification that adds more intelligence and is not encumbered by commercial interests. This new spec is IPC-2581. Want a good time? Hook up with an outfit that will talk IPC-2581 with you. The artwork still must be camera-ready; it’s not doing your drawings. What it will do is a thorough extraction and allow the designer to define what goes to the manufacturing partner.
According to my colleague, Hemant Shah, “IPC-2581 not only makes design data handoff efficient, it also makes manufacturing the board more efficient. IPC-2581 connects to IPC-CFX to help with smart factory automation. With IPC-2581 revision C, manufacturing partners can provide technical queries in the 2581 format, making it very easy to cross-probe, track, resolve, approve and reject TQs.”
Many fabricators rely on Gerber data because they have a different CAM tool. A majority of high-capability shops use ODB++ and prefer it. It will be up to us, the design community, to help launch the new CAD-neutral IPC format. •
Related documents. Standards and specifications that cover PCB documentation and electronics data transfer include:
is a career PCB designer experienced in military, telecom, consumer hardware and, lately, the automotive industry. Originally, he was an RF specialist but is compelled to flip the bit now and then to fill the need for high-speed digital design. He enjoys playing bass and racing bikes when he’s not writing about or performing PCB layout. His column is produced by Cadence Design Systems and runs monthly.
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