PCB Cost Vs. Functionality Vs. Manufacturability
Eminent PCB manufacturers like Rush PCB Inc. know that a board’s stack up directly affects its cost. Additionally, there are fabrication variables that also contribute to the cost of a board. Therefore, many manufacturers are switching to additive manufacturing to circumvent the cost of a board, but without sacrificing its performance.
PCB manufacturers often come across complex layouts with deadlines that refuse any attempt at completion within the scheduled time. In such situations, they have two alternative solutions—to ask the customer for more time or charge them more—to meet the board’s requirements while conserving design intent. In a majority of cases, customers prefer to pay more rather than delay the project.
While it is extremely difficult for a layout designer to gauge the effect of their design on the manufacturing aspects of a board and estimate the influence on its cost, their decisions directly impact each step of the design. Although engineering factors motivate the component packages the designer will use, the overall size of the board, its layer count, and more, it is the financial motivation that typically acts as the common denominator. The situation is the same everywhere, and designers can feel frustrated when they know they can solve their layout problems with a small additional cost but cannot do so.
PCB Cost Analysis
The cost breakdown of a PCB includes the cost of its constituent material, procured components, and manufacturing costs. The manufacturing cost depends on the complexity of reproducing the design. For instance, the board may have a stack up with more layers, is particularly complex, or the board has features that are beyond standard manufacturing practices. These can either increase the cost because of the additional processing requirements or reduce the yield as the fabrication is more demanding. PCB cost analysis ultimately leads to whether or not the board is feasible to produce. Ultimately, for a feasible board, its cost, performance, and features all compete for engineering resources.
PCB Cost Breakdown
Inputs from many factors are needed for an accurate assessment of a board’s cost breakdown. Each of these factors may have their own subcategories and qualifiers. For instance, when deciding on the material to use, the designer must consider the functional purpose of the material and its influence on the overall performance of the board. They further break down the functions and conditions into the heat transfer properties, thermal ratings, mechanical properties, and signal transfer properties of each material. Furthermore, the material must be capable of withstanding vibrational stress and mechanical shock.
To begin a PCB cost breakdown, it is customary to consider the size of the board and the number of layers it has. The device that utilizes the board and the number of circuits required therein primarily determine the size of the board. On the other hand, there are a number of factors that decide the number of layers. These include:
- The loads the circuitry will require to handle
- The presence of high-technology components, like BGA
- Pin density of high-technology components
- The number of signal layers (dependent on pin density)
Variable Cost Drivers
Designers must thoroughly analyze the variables influencing the overall cost breakdown of the PC board. Some variables are critical enough to wield a greater influence. These are:
Number and Size of holes
As hole diameter decreases, their cost rises. For holes drilled with mechanical drill bits, the smaller the bit size, more is the difficulty in handling them. For instance, drill bits break more easily when drilling holes with a diameter of 0.5 mm or lower. Drill bits require sharpening more often when drilling large numbers of them.
Holes with super-small diameters require laser drills. Designs that require drilling in more than ten layers, and in hard-to-drill materials, will increase the PCB cost.
Width of Traces
Traces must have adequate width to avoid overheating and transient surges. If the manufacturer finds traces that can carry higher currents, they may enlarge the width of the trace, and add an extra solder mask. This can add to the PCB cost.
Custom Specifications
Irrespective of whether the board is small and has only one or two layers, or is large and fairly complicated, custom specifications will almost always raise its cost. The main reason for a cost increase is the requirement for production skills or special tools. For instance, a board with a higher-grade surface finish and longer shelf life will always be more expensive. Other features that increase the cost of a board include rounded edges, extra clearance for solder masks, and metal edging. Boards that must handle high-frequency signals, and those that are military-compliant will cost more as they must use higher-grade materials and undergo rigorous testing.
Common Features Influencing Board Cost
Layer Count
The more layers the board has, the higher will be its cost, while fewer layers will drive the cost down. Designers must build the stack up for the fewest number of layers possible without sacrificing power distribution or signal integrity.
Drilled Holes
Drilling is the most expensive and time-consuming process in board manufacturing. The risk is also higher, as even a small error can lead to board rejection and a great loss. As explained above, larger and fewer holes will cost less than smaller holes in larger numbers, and the requirement of laser drills, for extra-small holes, increases the cost even further. Cost-efficiency of a board increases with fewer number of drilled holes, and if the holes are of a larger diameter. The smaller the holes are, the higher the requirement of precision for placing them on the board.
Another constraint for holes is their aspect ratio. This is the ratio of hole depth to hole diameter. The higher the aspect ratio, the greater the cost. Designers can extract significant savings in high-volume lots by minimizing vias during routing. Reducing vias also improves signal integrity practices.
Through-Holes and Vias
A board can have through-holes, vias, micro-vias, and via-in-pads. All of them form vertical connections spanning the layers of the board. They are constrained by the aspect ratio, and the tight clearance required between the pad and via, especially under BGAs.
While through-holes and vias require only a single-step drilling, micro vias (more common in HDI), require drilling in each layer pair. Capping for via-in-pads adds to the drilling cost.
Trace Width
Cost increases if the trace width is wider or narrower than the default trace width. Most manufacturers use a subtractive method for forming traces. Here, the etchant removes the unwanted copper in each layer, while an etch resist protects the required final feature. The cost assumes an inverted bell curve, rising to the highest at the extremes due to the requirement of additional processing and lowered yields. The cost is the lowest at the center, representing the default trace width. Minimum trace width is typically at break-out routing for the tightest pitch, such as for BGAs.
Reliability Concerns
Class III PCBs command the highest cost, with the cost falling while moving to Class II and Class I. Certain industries require minimizing or eliminating service disruptions, while heightening defect tolerances. Examples are the aerospace industry which uses Class III boards, and the medical industry which uses Class II boards. The consumer electronics industry typically uses cheaper Class III PCBs.
High-Frequency & High-Speed Boards
Most high-frequency, high-speed boards, such as those for the use of RF/Microwave applications, are expensive because of their exotic constituent materials. Furthermore, they require rigorous EMI/EMC testing, which is extremely expensive for a single test. A board that fails EMI/EMC testing in the first instance, must undergo revisions and further evaluation.
PCB Functionality Vs. Cost of Manufacturability
Traditionally, the cost of manufacturability overrode the PCB cost breakdown. During a conflicting two-design option, the ease of manufacturability decided the issue as it cost less rather than optimizing the board functionality.
However, recent advances in additive manufacturing or even 3-D printing are reducing PCB production costs considerably. With additive manufacturing, it is possible to create 3-D solid objects from a digital file by depositing successive layers of material. With the scaling up of 3-D printing while it enters the electronic manufacturing industry, design is gaining in highly efficient functionality, and simultaneously, gaining a foothold over manufacturability as well.
The major advantage of 3-D printing is the elimination of tooling requirements. Lasers and metal drills are no longer required for creating vias and through holes. 3-D printing can create high-definition traces on unlimited numbers of multi-layered boards. These also measure up to the industry standards of strength, conductivity, dielectric properties, and thermal stability. The technology is capable of building single components with adequate mechanical strength and flexibility, if bending is required.
In the future, the advent of 3-D industrial printers, making their way into mainstream electronic production, will usher in new design freedoms. They will streamline functionality, casting aside the older manufacturing limitations. PCB cost breakdowns will then be limited to functional requirements, choice of materials, and digital file creation.
Conclusion
Rush PCB Inc. recommends avoiding exceptionally high costs from unique or custom PCB designs. For this, designers must calculate the PCB cost breakdown before they send their design for manufacturing. A PCB cost breakdown can determine the specifications of the design along with the steps necessary to fabricate the board. This exercise can easily prevent hidden cost overruns.