A PCB is a component like any other electronic component, except for the fact that it is not on the shelf waiting for you to buy it, but will be produced from scratch when you order it. In my eyes, it's the most important component, as it is the carrier or foundation in any electrical product connecting most other components. With this responsibility on “its” shoulders you might want to go that extra round before production, to make sure all data is correct and the design feasible. Poor design might lead to poor connection and failures—not the result you want.
Questions about PCB design are a "never-ending story." Strangely, after years of PCB production and development, lots of the questions and issues are still the same, just only affected by the changes related to PCB technology.
Rather Too Much Than Too Little
PCB designers and engineers who are not frequently designing might find it not only helpful, but crucial, to seek help one time too many rather than too little.
The consequences and costs of not asking can be high. My experience is that the earlier involvement, the better the product. When possible, involve the right team from when the idea for a product or printed circuit is hatched. When you have the right knowledge of PCBs onboard, you can make decisions based on knowledge and experience. This helps make sure the PCB process is as flawless as possible.
Get Another Set of Eyes
Design reviews allow others to check the features and function of the PCB design and inspect the interconnection of the various circuits. Don’t assume that everything is fine until you’ve had someone double-check your work. When a qualified official design review is performed, you might discover errors early in the process—ones that you might not have discovered until later in the production process, or even worse, not discovered at all.
Communicate and Look Outside Your Box
PCB design is a long and collaborative process, but when engineers get so focused on their part of the puzzle and forget to share information, errors and frustration might develop. Designers should be careful to communicate constantly, and above all, not just share thoughts and improvements with each other, but include every member of the project.
If you can avoid these common mistakes of narrowing down the involvement, you might be able to shave time and money off of your PCB design, and generate a more consistently high-quality project with a clear path from design to production.
These initial basic phases are usually related to function, technology, choice of materials, organization of layers and stackup, and electrical design requirements. The cost factor is not to be forgotten either. I have mentioned all this earlier in various contexts, and I am sure I will mention it hundreds of times in the future.
Search for Mistakes Like the Eagle Searches for Prey
However, here's a small sidestep focusing on the fact that when you as a designer have placed the last connection on a design, successfully ran a design rule check (DRC) without any error messages, and your design is still not optimized for production.
I have explained this numerous times. At this phase in the project, it's time to bring out the eagle eyes and study your masterpiece. Search for improvements and mistakes, as the eagle searches for its prey, to clean up your layout. This clean-up aims to improve the design for production and thus also improve the board itself and provide a better production yield. Better yields save time and money.
As a designer, it is important to know the most critical processes, and understand the consequences of your choices at each step, as these are crucial to obtain the best possible conditions for a successful PCB production. It will benefit and generate a more consistently high-quality project.
Here Are Some of My Related Tips
Some of these choices may be in your CAD software library. For example, how you have defined SMT pads in your pad-stack? I recommend that square/rectangular pads shall be defined with rounded corners (Figure 1). It is electrically good, as well as beneficial for PCB production, assembly, and soldering.
What to Look Out For
Improved routing by increasing the distance between conductors, vias, and pads where possible. Move the connections so they are centered (Figure 2). Figure 3 shows more samples where it is possible to increase distance between pads and traces, also between differential pairs. In all the samples, traces are too close to the mechanical drilled hole.
How Do You Connect Neighboring Pads?
It should not look like a short circuit on the finished board (Figure 4).
Prevent Unwanted Flow of Solder Paste During Soldering/Vias That Are Not Covered With A Solder Mask
Move the via further away from the SMD pad, so you ensure that there is room for a solder mask in between the SMD pad and via pad. An alternative is to reduce the solder mask opening to be slightly bigger than the drilled via hole. Consult the IPC document IPC-4761 Design Guide for Protection of Printed Board Via Structures, to find the solution for your design. Or just ask.
Critical Signals That Require a Reference Plan
Make sure the connections are sufficiently far from neighboring pads so that they really get the desired reference copper plane below or above them (Figure 6).
Uneven—Unbalanced Copper Distribution Can Give Bow and Twist
Layer-to-layer copper coverage: Within several layers causing low pressure areas through the bonding process of the board. In Figure 7, all features in red are balancing patterns on the inner layers. On this 14-layer board there were a total of eight nearly similar layers.
Same Net Spacing
Electrically nothing is wrong with this (Figure 8), but during automatic optical inspection (AOI) this will cause problems and delay in production. In the sample, the same net distance is below 45 mm.
Unwanted Angles—Causing Acid Traps
In these sharp angles, chemicals from the processes can remain and cause reduced or broken connections over time (Figure 9).
Some Parameters Can Be Set and Taken Care of During Post-Processing
Do you keep or remove unused pads and vias? There is no rule without exceptions, but my general recommendation is to keep pads on all layers for through-hole components and screw holes.
Allow unused via pads to be removed if there is no risk for low pressure area. Do not remove all unused via pads in local areas where there is a high proportion of vias, such as typical BGA areas. BGA areas may require higher temperatures during soldering. This can promote delamination.
Teardrops or Snowmen?
My general recommendation is to enable this feature.
I have been working with PCBs for decades. In the past year, I have held numerous seminars and webinars, talked to customers, improved designs, scrapped designs, helped students, made hundreds of drawings, explained the basics, experienced advanced technology and materials, and pushed the PCB design to its limits.
However, even as advanced as the technology might currently be, sometimes it's great to get back to the basics, ask the simple questions, and before thinking about the cool and advanced features, make sure that what you are thinking of designing, actually is designable. Just ask, and you will get an answer. And make sure the source is reliable, because that's what you expect from your PCB.
This column originally appeared in the June 2021 issue of PCB007 Magazine.