rigid flex pcb

How to Reduce the Cost of Your Flex Circuit

Flexible PCBs and Flex circuits have revolutionized the field of electronics ever since their inception. Not only did they made it possible to create electronic circuits of dynamic nature (circuits that need to bend constantly), but they also allow designers much more freedom in design, since flex PCBs can be fabricated and installed in 3D.

Compared to some other PCB technologies and techniques, flex circuit designing and fabrication have matured quite a bit. But there are still some hindrances in its mainstream uses. But they are relatively easy to overcome or compensate for, if you do your due diligence, and work with experienced fabricators. This allows you to conceptualize a highly efficient circuit at a reasonable price.

Flex PCB Cost Consideration

One of the issues that prevent some people from using flex circuits in their design, despite its distinct advantages is the cost. But there are some ways to reduce the cost of your Flex circuit.

1. Keep the number of layers in check. Adding a higher number of layers in a flex circuit increases design complexity, and multiple cycles, thin in turn drives the cost.

2. Efficient panel utilization can significantly reduce the cost of your Flex circuit manufacturing. Fabricators usually charge using the formula: Piece part cost = Panel Price / No. of Parts on Panel. Simply put, the larger the number of parts you can put on a panel, the less your cost will be. If you can design without wasting a lot of panel space, your cost can turn out to be significantly less. This also means that working with fabricators that have a substantially more extensive than typical panel size, can help you design a more cost-friendly flex PCB.

3. It would be best if you also kept in mind that fabricators typically employ a one-inch outside border (on all sides) for tooling holes, so account for that as well. This reduces a 20″X 80″ panel to 18”x78” of usable panel space.

4. When it comes to material selection, limiting yourself to the materials stocked by your fabricator is a very cost-effective option. But you can only do it for straightforward flex designs, where there are no significant electrical or mechanical considerations that fall outside of the scope of some of the flex materials. This isn’t usually a problem because most manufacturers now keep high-quality materials like PI, handy.

5. The thickness of the substrate and copper layer can also impact the cost of your flex. In a lot of cases, it’s seen the PI thickness below 1-mil or upwards of 3 or 4-mil can usually drive the cost up. But that may vary from manufacturer to manufacturer and your design complexity in general. Similarly, going beyond the typical norms of copper layer weights (0.5 to 1 oz) can have a drastic impact on the cost. As well as the flex capabilities of the circuit.

6. Even if you are an experienced designer, communicating with your fabricator early on can allow you to understand the cost drivers better. They can especially help you with material selection.

7. Use blind and buried vias only when absolutely necessary. While they do offer a significant amount of design superiority, they also add to the cost of a flex. Whenever possible, stick with plated through holes.

8. A smaller plating aspect ratio for plated through holes and vias can lead to cracks when the circuit is bent and require costly do-overs. You can ask your fabricator about the aspect ratios they have had the most success with, and that can deliver the necessary result without the copper fracturing.

9. Surface-mounted components are an essential part of PCBs now. But while they sit quite well with rigid HDIs, they are a significant cost driver for the Flex circuits, especially if you require them on both sides. This adds to the processing time and to the cost.

10. A flex PCB with a uniform number of layers throughout the circuit will be more cost-effective then flex with dissimilar layers. This presents a problem explicitly when drilling and finishing plated through holes in the different layer areas of the flex.

11. The wrong choice of stiffeners can also be a costly mistake. Two of the most commonly used stiffener materials are FR-4 and PI. While they both have their merits, demerits, and respective costs, how they are added to the flex is a major cost driver. For example, if you use an FR-4 stiffener, and asked for it to be pasted using a thermal-set adhesive, the fabricator will have to add an extra step in the process, i.e., a press-cycle for thermal curing of the adhesive. If you had opted for the pressure-sensitive FR-4 adhesive instead, it could have reduced the cost. Conversely, if you use the pressure-sensitive adhesion method for a PI stiffener, it would cost more because the thermal set adhesion of a PI stiffener can be accomplished in the press cycle for the coverlay.

12. Size of the traces and holes are also significant cost drivers. Even if a fabricator can drill a minute sized hole, it doesn’t mean you have to design with that in mind. A common trend is that cost increases when traces are thinner than four mils or hole sizes smaller than ten mils. But some fabricators may have advanced tools, and assembly lines that permit hole and traces size much smaller than these, without adding any cost.

13. Anything over a simple surface finish adds substantial cost to the fabrication. If possible, keep the flex design to a surface. You may find that increasing the size of the flex (if there are no mechanical constraints) compared to adding components on both sides of the flex might be a less costly option.

Final Words

No matter how costly designing and fabricating flex is, there are applications where you simply can’t do without it. This is why it’s important to understand all the factors that can help you reduce the cost of your flex design, understanding the flex design guidelines of your fabricator, and working with fabricators who have years of experience working on thousands of PCBs for various applications. So they are familiar with most conceivable issues that can occur in fabricating a flex circuit and can guide you in designing the most efficient and cost-effective flex circuit for your needs.

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