My frugal nature means that I tend to be a slow shopper. I don’t go to the mall too often, but when I do, I spend too much time weighing the costs and benefits of two different pairs of jeans. Such neurotic behavior will probably annoy your spouse, but it pays to be neurotic when you are debating which material and board thickness to use in your PCB.
Most designers consider the fabrication costs associated with FR4 PCBs to be a rather simple consideration. If I double the thickness of my board, then ideally I should double the fabrications costs. But once you start including real design considerations, this doubling idea does not always hold.
When it comes to your PCB manufacturer, you’ll have to consider many facets of the circuit board to make sure your copper, solder mask, and other materials get applied correctly. Don’t cut costs and effectiveness with your PCB fabrication because of a lack of knowledge, learn how to get your printed circuit fabricated with the materials you desire.
FR4 Manufacturing Costs
The manufacturing costs for FR4 boards are a linear function of the board thickness. If you increase the thickness of the board, you generally increase the fabrication costs by a proportional amount. Once you start taking into account the presence of vias in multilayer boards, the number of layers present in the PCB, and fabrication steps, it becomes more difficult to make such broad statements about fabrication costs.
Higher aspect ratio via holes require longer drilling time and increases tooling wear on the drill bit. Printed circuit boards, especially multilayer PCBs, with smaller area also tend to require that any vias have smaller diameter in order to fit the required number of connections.
Both of these factors decrease throughput, which increases fabrication costs. Opting for an overall thinner board decreases the via aspect ratio for a given via diameter, and can decrease overall fabrication costs. Thinner boards with HDI routing that are used in high speed mobile devices require a large number of vias. The fabrication costs increase linearly with the number of vias in the board.
Using a thinner board can also change manufacturing costs as automated fabrication machinery may not be usable with thin boards. Some pick & place machines can break thin boards that have large area, and more specialized machinery would need to be used with these boards. This could limit your manufacturing options, or may even require that boards be moved around the facility by hand, which then increase your per-board manufacturing costs.
Pick and place machines can be optimized for in your manufacturing process
Costs and Panelization
Your fabricator is going to produce multiple boards in panels, so designers need to consider their board dimensions and their manufacturer’s capabilities before designing their PCBs. The cost to produce a single FR4 panel will be fixed for a given thickness and layer count. The right panelization scheme can increase board yield, and thus reduce the cost per board. Consider the following simple example:
Suppose you calculate that your board needs to have 12 sq. in. of area to fit all of your components. Your manufacturer can work with 16 in. by 20 in. panels. If you make your boards 2 in. by 6 in., you can fit a total of 21 boards in each panel. But if you instead make your boards 3 in. by 4 in., you can now fit a total of 25 boards per panel. If your form factor requirements will allow, it would cost less to produce the 3 in. by 4 in. boards.
Panelized PCBs prepared for assembly
If the board dimensions and form factor are very strict requirements, then an alternative design choice that would reduce costs in the above example is to produce thinner 2 in. by 6 in. boards. This may not be an option if your circuit board is already very thin and you would lose out on the cost savings. Keeping in mind the potential of your materials in relation to your circuit board will ensure a greater awareness of how to address your PCB fabrication costs.
Other Design Aspects That Affect Cost: FR4 Alternatives
The list of design aspects that can affect cost (with the exception of legend printing) are too long to list. In short, every design aspect has an effect on cost. The design aspects relating specifically to FR4 that affect fabrication costs involve the material properties of FR4 itself. Given the widespread use of FR4, materials manufacturers know how to modify some of the important material properties of FR4, albeit at a certain cost to board designers.
Since FR4 can become electrically and mechanically unstable above the glass transition temperature, opting for a thicker board made from high TG FR4 may be a better option in devices that will run at high temperatures or will be thermally cycled frequently. Your initial costs may be higher, but your electronic components will thank you later.
RF losses do not become appreciable in FR4 until you start working in the GHz frequency range. At high frequencies (above 2.4 GHz), FR4 becomes very lossy and a specialty board material may be preferable. Obviously, specialized FR4 blends are more expensive and might be preferable in low power RF and high speed applications.
Alternative materials that are specialized with high glass transition temperature and low RF losses are available, although they are more expensive. These specialized materials can cost in the neighborhood of 20% more than standard FR4, but the material cost could be offset by using a thinner board.
Selecting the thickness, layer stack, and panelization scheme for your FR4 boards create a number of design tradeoffs. But if you’re looking for design software that is compatible with any of your board decisions and can allow you to have easy communication with your manufacturer, then consider Altium Designer. Altium Designer 18.1 has powerful CAD tools and rules checking features that make it easy to design your device around the right board thickness.
If you want to learn more about how Altium Designer can help you reach your design goals, talk to an Altium expert today.
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