In the right application, flexible circuits can reduce wires, eliminate connectors and rigid circuits. They might also simplify component assembly and reduce wiring errors. But competitive products such as rigid circuits, wires and connectors also have their place in the interconnection packaging world. These substitute products all can offer cost effective solutions and may be the best overall packaging answer in any individual application. Knowing what drives the cost in a flexible circuit can help steer a design.
While many factors affect the cost of fabricating a flexible circuit, I would suggest there are three overarching cost drivers:
1. Physical Size:
Size matters, but it isn’t a simple calculation of square inch area. In the world of rigid printed circuit boards (PCB’s), prices are often quoted in price per square inch. This simplification can work reasonably well with a commodity grade product but even with rigid PCB’s, area based pricing is usually offered with a few “it depends” qualifiers.
The vast majority of flexible circuits are processed in rectangular panels. In the US, “standard” sizes are usually 12” X 24” and 18” X 24”. Fabrication of larger circuits is possible but special materials and equipment are required. Material optimization is accomplished as the individual parts are “nested” within the panel. Reverse nesting is a common term used to describe how two parts might be interlocked together with a 180 degree turn to allow best material utilization. Whether the panel has 100 individual parts or 2, the labor and materials per panel is about the same. Panel borders have rules defining “keep out” regions for tooling, and the required distance between adjacent circuits also affects nesting density. Further complications occur with automated component placement as assembly occurs on a subset of the manufacturing panel and conventions for assembly handling need to be accommodated. Deciding between maximizing circuits per panel vs. expedient assembly can be a balancing act between the competing interests of a fabrication house and a contract assembler.
Understandably, the more circuits nested on a panel, the lower the cost per circuit. In some situations a flexible circuit is in panel form through nearly the entire fabrication process. In other cases, the flex circuit may be singulated and have subsequent “personality” steps involved such as folding, connector assembly, or stiffener attachment. In the latter case, costs are driven by the value add to the individual part, so the cost formula changes.
2. Circuit Construction:
The number of conductive layers in a flexible circuit is also a major cost factor. Double sided (2 layer) circuits will often be priced at 1.5-2X vs. an equivalent sized single layer circuit. More expensive material costs are one reason, but the added capital equipment and processing steps are a bigger factor.
As 4, 6 and 8 layer circuits are priced, the estimated yield loss also becomes a significant variable. As additional layers are added to the circuit, sequential processing steps are required. It is not unusual to have 40 or 50 process steps during fabrication. And if each process is 99.5% yield, a part with 40 steps will only yield 82%. An error near the end of the process can be expensive…both in cost and delivery timing..
3. Volume:
Although there are niche circuit shops specializing in low volume and quick turn delivery, most fabricators are trying to cover engineering and overhead costs with low volume builds. Supplier support for low volume programs is often done as a necessity if higher volume quantities are to be captured. A lot charge for a low volume quantity of parts is pretty common. “Soft” tooling to cover part number costs is required as each circuit is custom requiring unique artworks, drill files, electrical test programs, inspection files and excising files. A non-recurring charge might also be expected.
Electroplated copper thicknesses are more variable with electroplated surfaces vs. non-plated copper. It is not
uncommon to see a 10% variation in a circuit’s copper plating thickness as current densities cause features to plate
at higher or lower rates. This variation can translate to a similar variation in the impedance. Pads only Plating of
circuitry requiring impedance control will result in less variability within an individual part and between multiple
product builds.
Another critical aspect of impedance control is the Relative Dielectric (ER) constant of the dielectric stack up. The dielectric stack up can consist of one or more layers of dielectric film bonded together with adhesives. The adhesive has a different dielectric coefficient than the dielectric film. Using adhesiveless laminates reduces the variation caused by adhesive and will normally result in a more consistent impedance.
To see the whole article with photos click the button below!
In the world of printed circuits, the relationship between cost and price can be pretty tenuous. With niche players segmenting the market by volume, delivery time, market desirability and targeted customer penetration strategies it is easy to understand why significant variation in pricing occurs among different suppliers. This variation can be further complicated by how busy a factory is at any given time. With this abundance of constantly changing variables, it is pretty difficult to make generalizations about product pricing.
To see the whole article with photos click the button below!