Solder Masks and Flex PCB Fabrication
Flex PCB Fabrication
As with rigid PCBs, flex circuits require a number of layers to support their flexible and durable structures. Adding more layers adds to the cost of fabrication, as each additional layer requires more lamination materials and expertise to create. A high layer count can also affect the design and assembly process, increasing the time required for a prototype. To minimize costs, you can consider a number of factors during design and layout, including the layer count, panel utilization and surface finish options.
The conductive patterns on a flex pcb fabrication are known as conductors, and they are created on the copper foil through lithographic procedures. These conductors are surrounded by a dielectric layer of either Kapton or polyester, which in turn is bonded to the substrate by an acrylic or epoxy based adhesive film. The final product has a solder mask coating that protects the conductors and encapsulates the copper circuitry on the board. The conductors are then plated and soldered to the pads on the board.
In addition to protecting the conductors and encapsulating them on the board, the solder mask also plays an important role in preventing solder starvation. Uncovered pads are susceptible to wicking, where molten solder flows down the exposed track and starves the pad of sufficient solder for a reliable connection. The solder mask ensures that the plated copper is covered to prevent this from happening, and also makes it easier for inspectors to locate the copper pads and traces on the final assembly.
Solder Masks and Flex PCB Fabrication
A variety of different colors are available for the solder mask, with green being the most common option. Other options include black, white, red, yellow, blue, and clear (not technically a color). Other special colors like orange and purple are also available for companies looking to differentiate their products.
During fabrication, the bare copper is exposed to air and can easily oxidize, which leads to poor performance over time. The solder mask coating protects the exposed traces from air and other contaminants, extending their lifespan.
The thickness of the solder mask is an important factor in determining the maximum size of a flex PCB that can be produced. Thicker masks make it harder to accommodate smaller components, but they offer higher durability and flexibility. A thicker mask may also increase the cost of a flex PCB due to the increased production and labor costs associated with it.
The thickness of the solder mask is also a factor in determining whether or not the board can be fabricated on conventional equipment. Thicker masks require more advanced and costly equipment to apply, while thinner masks can be fabricated on standard equipment. This allows a broader range of companies to get started with flex PCBs and reduce their investment risk. A thinner mask can also be easier to inspect, making it a better choice for small or medium-volume orders. However, it is still critical to maintain the minimum solder mask expansion and minimal solder mask sliver requirements as stated in the IPC-SM-840C standards.