the Flexperts
Flex Material Thickness
A little information goes a long way – but can carry added cost.
“My company has traditionally specified the finished thickness for each flex printed circuit (FPC) layer, and total thickness. This is because it’s understood some material layer thicknesses (i.e., adhesives) change during the manufacturing process due to compression and curing. As a purchaser of FPCs, we are less concerned with the initial raw material thickness than the finished thickness.

“We have received feedback, however, that the FPC market in general specifies the raw material thickness used in FPC fabrication, and not finished thickness. The assertion was nearly all customers purchasing FPCs follow this rule to minimize miscommunication. Is this common practice?”

Answer: The level of detail we see on customer drawings is all over the map, but the majority of customers that do specify individual materials will indicate the raw material thicknesses and then the overall finished circuit thickness.

I typically recommend you only specify what is truly critical to the electrical and mechanical function of the circuit. This usually includes minimum finished copper thickness, overall circuit thickness (in each area with unique stack-up), and dielectric spacing between layers (when there is controlled impedance). Any additional material stack-up requirements that are specified will only add cost and possibly reduce the supplier base that is willing to build your parts. Following are the different variables in play:

Finished copper thickness. You will notice the emphasis on finished copper thickness. Many factors during processing can affect finished copper thickness on each layer, and these factors can increase or decrease the final result. For this reason, never assume the starting copper weight is exactly what you will have on the finished circuit on any given layer. Fabricators use multiple copper cleaning steps during processing, and each will slightly reduce the copper thickness. Conversely, copper plating will typically increase the copper thickness. To further complicate the situation, if panel plating vs. button plating is not specified on the drawing, the fabricator has some liberty to use whatever method works best for them. So, if you assumed they were going to panel plate, and they instead button plated, your presumed finished copper thickness could be significantly less than expected. For any application where finished copper thickness is critical to the function of the circuit, it is always best to specify finished copper thickness. On the other hand, if you do not have impedance, high current, or voltage drop requirements, copper thickness is not critical to the function and you can just specify starting thickness.

Flexible adhesive films. You are correct that adhesive layers do change thickness as the material flows to fill between conductors. The exact amount of change will vary between manufacturers and the methods they use for lamination. But in reality, if your conductors are fully encapsulated, and the circuit meets overall thickness requirements, why do you care? It is much better to only specify what really matters, and give the vendor some freedom to meet those requirements in the most cost-effective way.

This cross-section specifies the material manufacturer’s P/N, which indicates “before processing” material thicknesses
Figure 1. This cross-section specifies the material manufacturer’s P/N, which indicates “before processing” material thicknesses. It also indicates total thickness over stiffener area, rather than a specific stiffener thickness.
Flexible dielectric films. Thermosetting polymer dielectric films like polyimide (most common) are very stable during lamination and other processing and will change little, if any, from the starting thickness. Thermoplastic polymer dielectrics (much less common) can change considerably during lamination. If a final dielectric layer thickness for a thermoplastic polymer is critical, note the final value on the drawing.

Stiffeners. Specifying stiffener and/or stiffener adhesive thickness can cause real heartburn for your fabricator. The same issues that apply to dielectric adhesives also apply to stiffener adhesive. Also, raw FR-4 laminate tolerances increase as the overall thickness of the FR-4 increases. The fabricator will know the typical tolerances and how to deal with them. You are best specifying overall circuit thickness, including the stiffeners, and let the fabricator determine the best stiffener and stiffener adhesive thicknesses to meet your requirements.

In reality, if the overall circuit thicknesses are within the specified range, and the circuit functions as it should electrically and mechanically, do you really care what any of the individual layer thicknesses are? It is important to ensure you are not over-specifying your flex design to where you are adding cost without adding value. If a particular thickness or feature will have a noticeable impact on the function of the circuit in its application, by all means specify that on the drawing or procurement documentation. If not, leave it off.

Any specific features you add to the drawing must be verified by the manufacturer and will carry a price tag. Review your drawings prior to release, and for each dimension and feature specified ask, “Does this really need to be on the drawing?

Ed.: The authors will speak on flex circuits at PCB West in October.

Mark Finstad Headshot
MARK FINSTAD
Mark Finstad is senior application engineer at Flexible Circuit Technologies (flexiblecircuit.com); mark.finstad@flexiblecircuit.com. He and co-“Flexpert Nick Koop (nick.koop@ttmtech.com) welcome your suggestions.