During the COVID-19 lockdown, I attempted to make pizza several times. It was fascinating how much thought went into the baking process itself. As I consider myself more of an intuitive cook than a baker, the science of yeasted dough was difficult. There was no room for real-time changes – things like the moisture content, proofing, oven temperature, and so much more were locked settings from beginning to end. Once I had gained some confidence following a recipe, I immediately hit a setback. After trying to add too many toppings, I made a thick pizza that burned in spots and was raw in others. Without compensation for the variables I adjusted, the attempt was a failure.
PCB design is also a locked-in science, albeit one with much higher stakes than dinner. Designers and fabricators must ensure the design rules are crafted to maximize the ease of development and production. The PCB aspect ratio is arguably the most important aspect of board thickness after the stackup itself, defining the most efficient via size to squeeze maximal routing and placement from the board.
The PCB Aspect Ratio Is A Manufacturing Constraint on Design
PCB fabrication encounters many gives-and-takes during the design process. As board features and functionality tend to increase over time, with HDI becoming more prevalent in general, the number of board features has to rise. There are two general ways to accomplish this: increasing the board size (planar dimensions) or the thickness of the board with additional layers. Of these two methods, the second option is the far preferred as dimensioning in the z-axis is less constrained in fixtures than the board’s planar perimeter. However, increasing the thickness affects the scaling of a critical feature in the design — via hole diameter after plating.
The plating process requires gradual deposition of metals on the surface of the via hole to form the conductive barrel that connects the electrical layers of the board. Dense designs will want to use the smallest possible via diameter to avoid wasting space considering their overall prevalence in design. However, too small of a diameter for the depth drilled can result in problems during plating. As metal is deposited quicker towards the hole’s opening, the flow rate through the hole decreases. This scenario can lead to a via barrel of uneven thickness that may experience intermittent connectivity when passing over the section of incomplete plating. Worse still, the via barrel could plate shut completely. Not only would this create an open connection, but any trapped gasses or liquids would also be liable for violent release in later manufacturing steps if not corrected by costly drilling rework. With all that said, the plating process is entirely passive in terms of the board substrate – what steps can designers take to ensure the best possible outcome?
The solution: the aspect ratio. The aspect ratio relates the depth of the hole to the length of the plated circumference of the hole. Manufacturers will have different precisions based on the individual fabrication processes of the board, but most manufacturers will top out at a 10:1 ratio. As IPC suggests, no more than an 8:1 ratio and the value can differ depending on the board’s fabrication processes. Talk with a manufacturer to determine the best value for a specific project. While building out design rules, designers can get an idea of the target via size by selecting the smallest via that will fit within the tightest pitch component, typically an LGA or BGA. By using the smallest via pad and subtracting the 10 mils/.25 mm of the annular ring on either side of the hole diameter, designers are left with the minimum hole diameter that can be used alongside the board thickness to directly calculate the aspect ratio.
Microvias: When Standard Aspect Ratios Aren’t Enough
What about situations where an 8:1 or 10:1 via eats up too much routing space on the board with no possibility for expansion in the z-axis for additional signal layers? Laser-drilled microvias and blind and buried via structures offer a possible solution for the cost of additional fabrication work and time. Instead of using precise mechanical drill bits, a laser ablates the surface and some predetermined depth of the via hole. Thanks to the precise control available to laser drilling machines, aspect ratios can go up to as high as 20:1 for exceptionally dense designs.
As mentioned, the alternative to plated through holes requires additional fabrication, foresight and processing. Drilling and plating blind and buried vias must occur before lamination, where the individual layers are fused via elevated temperatures and pressure. Microvias, or the vias formed from laser ablation, will individually span a pair of layers and require one of two orientations in the design: stacked or staggered. Stacked microvias will line up consecutively from top to bottom layers along the same z-axis position. In contrast, staggered microvias can have different planar positions that need to be tied together with traces. Designers are allowed a bit more flexibility in a staggered structure, as the layers of the via no longer have to exist at a single point but can be spread throughout the design. Coupled with a via-in-pad fabrication option, designers can navigate dense designs with the smallest possible via structure.
Your Contract Manufacturer Can Balance Manufacturability with Design Density
Beginning designers can sometimes overlook the PCB aspect ratio, but it is a crucial point of synergy between the designer and fabricator that maximizes design space without sacrificing producibility. Incorporating the various needs of engineers, designers, fabricators, and even manufacturers can be difficult. Still, it is necessary as modern PCB design attempts to squeeze more chips, tracks, vias, and other board features into shrinking dimensions to fill the need for miniaturization. To help lead this design process, you want a contract manufacturer that knows the ins and outs of design for manufacturing PCB design and can guide production with this vision in mind. Here at VSE, we’re an eager team of engineers who build electronics for our customers. VSE and our partners maintain only the highest amount of quality and service throughout the design and manufacturing process of your PCB.
