DELIVERING QUALITY SINCE 1952.

Best PCB Layout Practices for Successful Manufacturing

Clean and Clear Design Choices For Successful PCB Manufacturing

6.17.15

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Agenda

Epec’s front end workflow, from quotation through pre-production engineering.

The importance of complete information for accurate quoting and job setup.

Epec design rule checks and error reporting.

Automatic checks and repair of minor issues.

Design choices to avoid, and what to do instead.

Resources for success.

Q&A

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Epec’s Front End Workflow

Our workflow begins with an accurate quotation, based on complete and accurate information that you supply.

Once we receive a PO, we select an appropriate production facility, based on design complexity and delivery requirements.

Your order then moves to aqueue for pre-production CAM,where we perform design rulechecks, and manipulate yourdata files as required to ensurethat there will be no holdups oncethe boards enter the manufacturingstage.

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An Accurate Quote Starts With Complete Information

When we receive incomplete information, then find out later about important requirements that were not available to us at the time of the quote, we are forced to revise our quote – and it seldom gets cheaper.

This creates delays, and it’s an uncomfortable situation for everybody.

When you supply complete information, it enables us to price your product accurately right from the beginning.

This means that you can make your purchase with confidence that the pricing will not change after you have placed your order.

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Defining A “Complete” Data Set

A complete file set includes Gerber files for all layers, including:

– Copper layers– Solder masks– Component silkscreens– Solder paste layers– PCB outline– Gerber files should be in

RS274X format, with embedded apertures.

– Also supply an Excellon format (ASCII text) drill file, with tool codes embedded if possible.

– Decimal resolution format for all files should be 2:4 or higher. Avoid using 2:3 resolution.

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Include A Fabrication Drawing If Possible

A fabrication drawing defines the outline, key dimensions, and hole locations.

Use notes to specify PCB thickness, surface finish, material type, ink colors for soldermask and silkscreen, stackup and / or impedance requirements, panelization layout, etc.

The fabrication drawing may be a 1:1 Gerber, .DXF, or .HPGL file. PDF is acceptable, but we cannot import PDF into our CAM program, so it’s not ideal.

If you cannot create a fabrication drawing, supply a README-type text file with the same information.

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Other Information We Like

If possible, also supply an IPC-D-356 format netlist file. This will enable us to verify that your Gerber output files match the electrical requirements for the circuit.

Inform us of any special testing requirements, such as Hi-Pot, PPAP, automotive or aerospace specifications, etc.

If the product is ITAR controlled, or must be manufactured in the USA for some other reason, express this at the time of the quote -- don’t wait to say it on the PO.

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Initial Checks – Netlist, Measurements, Etc.

When your order moves into Epec’s CAM department, we first load the files, and check to be sure that we have received all of the information we need.

The layers are assigned types, and we create a reference netlist, which is the CAM program’s interpretation of the PCB’s electrical path.

If we received your IPC netlist file, we run a comparison of our system-generated netlist to your file. They should match 100%.

We then spot-check items like dimensions, hole quantities, etc. to be sure there are no obvious discrepancies.

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DRC: What Design Rule Checks Do We Run?

Once we are sure that the data files are complete and valid, we run a series of design rule checks for minimum spacing, minimum feature sizes, annular ring, etc.

Notice at right that many of these minimums increase with copper weight. Heavier copper requires wider design rules.

To find out whether your design complies with Epec’s design rules, you may view the sheet at right on our website: http://www.epectec.com/pcb/rigid/

Or, download our entire PCB design guide from this page:http://www.epectec.com/guides/

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What Happens When DRC Identifies Potential DRC Problems?

Our first priority is to keep your order moving, so if we find something small and easily resolved, such as a conflict between instructions on two different documents, we first try the phone.

If we find an inordinately large set of critical issues that the system has flagged during DRC, you will receive an email which may include a report such as the one at right. It explains what we’ve found, and why it’s a problem.

We make every effort to include all issues in a single message, rather than stopping to ask each question separately.

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Why We Ask So Many Questions

We do not like placing things on hold – each job on hold is another item for us to manage. So why do we do it?

The answer is that we are responsible for meeting all of the requirements stated in our customer’s documentation. This means that we need to resolve conflicts when they occur, because only one thing can be correct at a time. For example, we can’t certify that we have built a board that is both .047 thick per one document and .062 thick per another, or run a 3-ounce copper design with .004 copper-to-copper spacing.

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Automatic DRC-Based Enhancements – No Delay

The CAM program runs design rule checks by comparing the Gerber to an internal file populated with definitions of the PCB requirements, and flags any violations it finds.

If the number of violations is small, and easily corrected without fundamentally modifying the design, we typically perform the edits and continue.

Such edits include fixing common minor violations, like Refdes (Reference Designators) ink falling onto pads. This rejectable condition is shown at top right. The corrected version is below.

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Other Routine Fixes Based On Design Rules

One of the only changes we will make to the copper without first asking the customer is the fixing of “acid traps.” These are small slivers (typically sub-.008”) between etched features, as marked with the red X at top right.

If left unmodified, resist film may come loose, causing a bridge short.

Another failure mode is that etchant can become entrapped in the area of the small gap. Etchant which is not rinsed away can continue to etch the trace segment, eventually creating an open circuit.

At bottom right, we have filled the sliver.

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Potential Assembly Problems Which Do Not Show Up As DRC Violations

Here is a questionable design practice which is harmless to us, but can cause assembly defects.

Notice that 3 of the 4 circuits are much larger than the SMT pad terminations. Notice also that the mask openings (green) expose the wide traces. The result is that the solder paste will spread off the pad onto the wide trace,

resulting in poor coverage. This will not show up during automated checks – there is no actual violation.

Your best strategy here is to avoid using such a configuration in the first place.

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Not A Defect - Yet

Here is another problem waiting to happen, which will not be found by automatic checks – a grossly oversized thermal pad.

It is not technically wrong, and will not cause any problem with bare PCB production.

At assembly, however, the extra copper will act as a heat sink, and may cause a cold solder joint.

The inner diameter of a thermal pad should be approximately the same as the diameter of the external layer pads for the same hole (like those above and below the large one.) These will contain the heat more effectively during soldering.

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Resolution Errors

Back near the beginning, I mentioned not using 2:3 decimal resolution. The illustration at right shows one reason.

The Gerbers were exported at 2:3 resolution, while the drill was exported at 2:4.

Notice how the removal of the 4th digit has resulted in rounding errors. The drills are where they should be, but the pads are not centered around them – they are rounded to the nearest 1/1000th of an inch.

On designs with tight annular ring, this can cause literally hundreds of completely avoidable violations per layer. Always use at least 2:4.

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BGA Dog Bones And Assembly Shorting

If a BGA uses dog bones for escape routing, it’s usually not good to have soldermask openings larger than the pads at the via end of the connection, as shown top right. It’s too likely to result in solder shorts under the BGA after assembly.

Instead, either cover the via pads with mask, or use a smaller mask opening which leaves the via open, while partially covering the pad. Recommended opening is (Hole Dia + .006”.)

This effectively increases the distance any solder would need to migrate, and drastically reduces the likelihood of bridges forming.

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Incomplete Impedance Callouts Cause Delays

Many customers ask for controlled impedance, but do not provide enough specifics.

On the board at right, we were asked to control 5 different layers at 100 Ohms differential, but no spacing value was specified.

Each of the layers was made up of hundreds of traces, all the same size, many of which ran roughly parallel. The only way to locate the diff pairs was by highlighting the .005 D-code, zooming, and looking around. This was a tedious and unproductive exercise.

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Incomplete Impedance Callouts

We eventually identified some pairs visually, but on only 3 layers, not 5. It turned out that there were just a few true pairs on the entire PCB, hidden among all those .005 lines.

Per the customer documentation, we were still responsible for reporting the impedance on 5 layers.

We were obliged for this reason to place the order on hold until the customer confirmed that 2 of the layers did not require impedance control or testing.

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Information Required ForImpedance Control

When calling for impedance control, please provide these specifics:– Layer number(s) where traces

occur.– Trace size(s) and target impedance.– Spacing, if differential.– Reference plane(s).– Hint: If there are many non-

controlled traces the same size as the impedance traces, please reduce the non-controlled traces by .0001. This makes it simple for us to identify the impedance controlled traces, in the event that we need to make micro-adjustments to the Gerbers to dial-in the exact impedance.

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How To Participate In YourOwn Success

Double check your documentation for errors and for conflicting information.

Use your software developer for support in changing settings in your CAD program, or talk to other users.

Familiarize yourself with industry guidelines, in particular IPC-2221 (Design) and 6012 (Qualification and performance.)

Refer to Epec’s website. Read our blog, and download our Design Guide and Ebook.

Don’t be afraid to ask questions. Nobody was born knowing everything. It is easier to avoid a problem than it is to fix one.

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Our Products

Battery Packs Flex & Rigid-Flex PCB’s User Interfaces

Fans & Motors Cable Assemblies Printed Circuit Boards

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Design Centers & Technical Support

Battery Pack & Power Management – Denver, CO User Interfaces & Cable Assemblies – Largo, FL Fans & Motors – Wales, UK & New Bedford, MA PCB’s – New Bedford, MA & Shenzhen, China Flex & Rigid Flex – Toronto, Canada Cable Assemblies – New Bedford, MA & Largo, FL

Our Engineering and Design teams are ready to help our customers create world class and cost effective product solutions.

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Q&A

Questions?– Enter any questions you may have

in the Control Panel.

– If we don’t have time to get to it, wewill reply via email.

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Thank YouCheck out our previous webinars at www.epectec.com.

For more information email sales@epectec.com.

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