How to Select the Best PCB Design Parameters

September 17, 2020 , in Blog

Laying out a new PCB is a complex task with design parameters that come from many different places. Internal policies and checklists, client specifications, government regulations, and industry best practices can all be considerations during the design process. One often-forgotten source of guidelines is your CM. Their early input can help steer your design towards a high performing, more reliable, or easily manufacturable board. High performance and reliability can even be at odds with each other, and a good CM can help guide your design to achieve what you want most. It’s also important to know the limits of your CM’s capabilities. It may be difficult for them to easily manufacture your layout, resulting in low yields or expensive late design changes. Some regulations or goals may also require the use of complex fabrication techniques or advanced materials that require special equipment. Engaging your CM at the beginning of the design process will help you develop your PCB more efficiently and effectively.

 Your CM can help you determine design parameters like drill hole size based on their capabilities.

Building the Best PCB

Everyone always wants their PCB to be the best, but what does "best" actually mean? It’s easy to say the best board should be fast, reliable, and easy to manufacture. One quick conversation with a CM will quickly show that some of those goals are mutually exclusive. Involving your CM early on in the design process can help you focus on the most important objectives for your board. Here we’re going to discuss performance, reliability, manufacturability, and special considerations.

High Performance and HDI

Cutting edge PCBs strive to be smaller and faster, which often requires using HDI (high density interconnect). Packing all those components into a tiny area means using complex design techniques and manufacturing processes. It’s important to check with your CM early on to ensure they can accommodate these requirements so that you don’t end up with a board they can’t build. Some key topics to discuss with your CM include via types, stackup, and hole drilling.

Via Types

There are many different types of vias which all come with their own advantages and disadvantages. It’s generally best to use the simplest type of via possible, which would be the average through via. That may be good enough for the average PCB, but for HDI boards you will probably have to consider using things like buried vias or via in pads. Via in pads are the most sophisticated type and are often used to break out BGAs or other components with lots of hard to get to pads. It’s best to have very small holes for via in pads, which may mean using a laser drill. If you’re planning on using special kinds of vias in your design, make sure your CM has the capability to fabricate them.

Stackup and Hole Drilling

As mentioned in the previous paragraph, high-performance boards often need very small drill holes. These come with their own intricacies. For example, if your CM is going to drill using a laser aspect ratio, requirements may mean the hole can only be drilled through the outer layers. In addition, lasers are limited by depth. This means that it may take multiple drilling processes to complete your board. More processes mean more complexity, more time, and ultimately, more money. It may also be necessary for layers to be successively bonded together as they are completed. All of this extra fabrication requires specialized equipment and can come at an added cost. It’s better to talk with your CM before you specify a convoluted design so that you’re sure they’re the right partner with the right machinery to build your board.

Reliability

On the other side of the coin of high performance is reliability. When designing a board that must go the distance, it’s important to include a margin in every step. That can mean spacing out components, using tried and true materials, and minimizing manufacturing defects.

Reliable Design

If you’re designing for the long haul, then it’s important not to push the envelope. HDI boards try to squeeze components into the smallest package possible. This can create signal integrity issues from crosstalk. Clustered chips and circuits can also lead to hot spots and eventual thermal failure. In order to avoid these problems, the most reliable PCBs often leave space for their ICs to breathe and minimize interference. Another sometimes forgotten weak point in boards is materials. Many designers let their CMs specify their laminate or other elements they don’t consider critical. Even if you don’t have all your materials picked out be sure to discuss recommendations with your CM. If they know the kind of board you want to build, they may be able to select something more appropriate than their default choice.

Reliable Fabrication

The most reliably designed board will fail if it’s riddled with manufacturing defects. You can get out ahead of assembly problems by talking through tolerances and fabrication techniques with your CM. If you know your CM’s ranges for drill hole size you can specify the middle of the range and be more confident that they will hit the mark. Your CM may also be able to analyze prototype or first-run boards for defects and identify problems that can be solved with design changes. If your CM doesn’t know the goal of your design and you don’t know their capabilities, building your PCB can be an exercise in futility.

Special Considerations

As if you didn’t already have enough to worry about, there are lots of other unique conditions that can add to your PCB design parameters. Government regulations are one of the big ones. If you’re going to send your board to Europe you’ll likely have to comply with RoHS and use lead-free assembly. Lead-free solders require higher temperatures during reflow which can stress sensitive components. You may want to go the extra mile and make your board halogen-free as well. Halogen-free boards require using uncommon materials that some CMs may not be able to use. If you have to comply with any special regulations or odd requirements, be sure to mention them to your CM.

It’s fairly common to decide your PCB design parameters with no input from your CM. This can be like the blind leading the blind as you may not know your CM’s capabilities, and they may not know the primary objectives for your board. Are you pushing for performance, reliability, or do you just want it made as quickly and economically as possible? Discussing your board and your design goals with your CM early on can help you determine the optimal parameters for your PCB. Maybe they can tell you what fabrication techniques they have that can minimize footprint for an HDI board. Perhaps they can recommend reliable materials that will work well with a board that has a long lifetime. Regardless of the application, clear communication with your CM will always make your design process more efficient. The problem is talking with your CM isn’t always enough, you need to make sure you’ve chosen the right one.

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Here at Tempo Automation, we strive to make your board the best possible, whatever best means for you.

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.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on selecting the best PCB design parameters to optimize your PCB development process, contact us.

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