Finding the Perfect World for Multilayer PCB Design
I have always been a huge Clint Eastwood fan. Dirty Harry always seemed to be a little too quick on the draw for an urban twentieth-century environment, but The Man with No Name—he was perfect for the ruggedness and lawlessness of his time. Obviously, I always rooted for him to beat up the bully, outdraw the outlaw, or get the girl, but the most intriguing thing was that he was usually quick to make a decision. Of course, the results varied. Sometimes they were good, sometimes bad, and sometimes pretty ugly.
The process of creating a PCB can have similar outcomes depending on the decision to use or not to use a multilayer PCB. A number of PCB designers incorporate multilayer boards when it is not necessary. Doing so complicates the design, increases fabrication cost and makes field repair or modification virtually impossible.
In most of the Man with No Name movies, the plot proceeds from some dastardly deed(s) or intolerable burden being inflicted upon the victims by the villains and ends with my hero righting the wrong in the end. Following the same outline, let’s look at what can go wrong in the PCB design and development process first and then see what factors we need to incorporate into our decision to design a multilayer PCB or not.
Your Multilayer PCB Heartbreak Ridge
From a PCB design perspective, it can be quite tempting to use multiple layers when we design PCBs. After all, the notion that “smaller is better” seems to be pervasive throughout electronic design these days. However, there are significant reasons to avoid the smallness trap unless doing so is a primary design consideration. For example:
Design Complexity: When designing multilayer PCBs, it is imperative to have all through holes and vias line up properly. Mistakes can impact current flow and create mounting issues. Additionally, using odd numbers of layers or different thicknesses for internal layers may cause bowing or twisting of the board making it unmountable or being relegated to test-only status. Bad. For communications applications, where various signal types are being routed across multiple layers performance issues may arise due to crosstalk or unmatched impedances. Bad.
Increased Fabrication Cost: Manufacturing multilayer PCBs is significantly more expensive than other boards. More material is required, more time is required, and technicians must be highly skilled. Simply going from two to four layers can increase the fabrication cost by 100%. Bad.
Difficult or Impossible Bench Repair: As with any manufacturing or fabrication process, small errors do occur. Especially for boards with odd numbers of layers or variable-sized layers. With single (or 2-layer) PCBs, these can usually be easily repaired, and the board may still be usable. If the problem lies with an internal layer, this is practically impossible and the board(s) are useless. Bad.
Another issue that is sometimes overlooked is the increased heat due to the increased layers, which may not show up until the product has been in the field for a while. If severe enough for product failure, this may result in the three R’s: recall, redesign and remanufacture. Ugly.
As we can see, opting for a multilayer PCB can result in severe problems, if not done properly. However, there is a bright side. If we are deliberate in our selection and the design process, we can ride off into the sun just like Clint always does.
Absolute, Multilayer Power
Now, if you are willing to spend the extra time on design and fabrication, pay the higher cost, and follow the best multilayer PCB design guidelines, then you should proceed with the multilayer PCB option. Especially, if size is the primary concern. Let’s assume that it is not. What reasons are to justify using multilayer PCBs?
Functionality: Multilayer PCBs allow for the incorporation of more complex circuitry. Therefore, more functioning can be built into a smaller package. The marketing advantages of this are quite evident. Good.
Durability: Due to care and additional considerations that must be incorporated into multilayer PCB design and development, they are better constructed components and are highly reliable. Good.
Mountability: The smaller the PCB, the easier it is to mount. It requires less mounting holes and may be placed in more locations within the larger system. In fact, it may allow for reduction of the size of the overall product, which can increase portability, storage options and marketability. Good.
Multilayer boards carry a higher cost, longer fabrication time and require more expertise to design and manufacture. Therefore, deliberate consideration should be undertaken prior to determine if multilayer is the way to go. When deciding on whether to use multilayer PCBs, do not simply follow the “smallness” crowd. The results may be bad or quite ugly. Instead, base your choice on sound decisions that soberly weigh the advantages and disadvantages to ensure a good result.
No: Some PCB designs are simply better suited for single-layer layout. A good example is a computer motherboard. These boards typically have many ports or assorted sizes, not to mention detachable parts that need to be accessible. This does not render the use of multilayer design impossible; however, it is impractical as the benefits of reduced size do not outweigh the disadvantages of complexity and increased cost.
Yes: The most common reason to opt for a multilevel PCB design is to reduce the footprint of the board. In situations, where the design flexibility is not bounded by the number of inputs and outputs, fanout requirements for single ICs (i.e. MPUs, FPGAs, etc.) or similar trace length limitations then multilayer may be the way to go. This may be especially true when the product allows for the use of flexible circuits, which remove much of the precision difficulty from the fabrication process while giving more freedom to the designer.
If you are unsure and want to discuss which PCB would be best for your design project, the experts at Altium are just a phone call away.