Nine Cost Drivers for Rigid Circuit Boards
For PCB designers and engineers, optimization is key. Optimization of time, cost, and even effort. Sierra strives to provide customers with high-quality PCBs and excellent service. This includes best practice tips for designers. When designing your next circuit board, you need to watch out for some crucial cost drivers.
From the concept to the PCB fab and assembly, several factors impact the price of your board. Typically, the mechanical and/or electrical engineers determine the board requirements, such as the size, the applicable industry standards, the mechanical and electrical constraints, and material properties, so as to ensure that the board meets its targeted performance.
Once the engineers have a mechanical design that works and a functional schematic, the PCB designer has to do the CAD layout. When the layout is completed, the PCB manufacturer can start building your board. It goes without saying that the complexity of your design will have the biggest impact on the final cost of your circuit board, but the price will also mostly depend on:
The board size
The mechanical engineer has to determine the size and the shape of the PCB – also called the PCB outlines. A first drawing is sent to the design team who can reduce the board outlines if possible. This is the first way to save money since a smaller area means a reduction in material cost. Here, the cost of your board is a real estate issue – just like a home, the bigger it is, the more it costs. For example, imagine a 2’’ x 2 ‘’ board. Now imagine a 4’’ x 4’’ board. The surface area has been multiplied by four, therefore the base price (for the material) will also be multiplied by four.
The board shape
Rectangular boards are cheaper than curved ones for two main reasons: more machine time is required to curve the outlines and you have to pay for the waste.
The number of layers
Naturally, more layers mean higher cost. An additional two layers will increase costs by about 25%. As you add more layers to your design, you add more material, like prepreg and copper, and more production steps, like etching, pressing and bonding cycles, etc. And once produced, the layers have to be inspected: It is more work to inspect eight layers than it would be to only inspect four. An alternative to reduce layer count can be the use of HDI (high density interconnect). But be careful, if you are not an experienced designer in HDI, it would benefit you greatly to first understand the key aspects of HDI technology. You can read our article about the HDI cost advantages.
The copper foil weight
In general, the thinner the copper is, the less expensive the circuit board is. Using thick copper on your inner layers will require more prepreg during lamination to fill up the gaps between the areas made up of copper. More than ½ oz of copper on the inner layers and more than 1 oz of copper on the outer layer increases the PCB cost. Another disadvantage of using thicker copper is that you have to maintain greater spaces between traces, and you could also need thicker prepreg materials between two adjacent layers.
However, if you use very thin copper (less than ¼ oz), it adds extra cost – processing very thin copper is expensive.
The tighter your trace/space, the more difficult it will be to reliably etch your traces and pads. Take into consideration whether a wire bonding or HDI design will be more cost-effective in the long run. Sierra can manufacture sub 3/3 trace/space.
The hole size and type
Smaller mechanical hole sizes are more difficult to manufacture. They also require smaller drill bits, which cost more. And when you request a hole smaller than 6 mils, it usually has to be laser drilled, which drives up the cost.
HDI technology uses blind and buried vias, which add considerably to the cost of a circuit board. They are more difficult to drill than through-holes and also add lamination steps. Only use them when you don’t have any other options available, like in an HDI design where it actually makes sense to have these kinds of vias due to the PCB size constraint. If you’re having trouble routing traces, adding another two layers to the stack-up will be cheaper than requiring blind or buried vias.
The standard drill bit size is 8 mils, whereas the advanced drill bit size is 5 mils. And the R&D drill bit size could be less than 5 mils.
The drill to copper
Drill to copper is exactly that: the distance from the edge of the drilled hole to the nearest copper feature on a layer (pad, pour, trace, etc.). The smaller the drill to copper, the more expensive the PCB manufacturing process is.
Having a controlled impedance means designing and producing very specific and uniform trace widths and space. More expensive materials with specific dielectric properties have to be picked to ensure the targeted electrical performance will be met. Test coupons have to be fabricated to make sure that the PCB manufacturer met the standard 15% tolerance – sometimes it is even a 5% tolerance. More work, more surface area for the coupons and more testing drive up the price of the board.
Do not specify controlled impedance unless it is absolutely necessary. For more information, you can read our article explaining why controlled impedance really matters.
When you pick a panel option, keep in mind that it is just like the board size: The bigger the surface area is, the more it will cost you. So you even pay for the waste part (in faded green) that is thrown to the bin after assembly. If possible, place the boards on the panel closer to each other to reduce waste and cost.
To sum up, the hard cost drivers you should take into account during the concept stage are the PCB outlines, the layers and their traces/space and vias. Carefully select the types of material you need and try to avoid waste as much as possible. Finally, keep in mind that reducing machine time (for both manufacturing and assembly) will reduce cost.
You are now all set, it’s time for your idea to become a real circuit board!
For more information on design rules, check with our DESIGN ASSISTANCE team.