Sorting the myriad options for adhesive-based and adhesiveless foils.
QUESTION: I AM designing my flex stack-up and am curious what base foil to use. There are several options with and without adhesive. Which is correct for my application?
Before we can select a base material, we have to review the end-application and the performance requirements of the finished flex-circuit to find the best material fit. Our decision should be based on desired electrical and mechanical attributes first, and as always, an eye toward finished cost.
Base foils come in two basic types: adhesive and adhesiveless. With so many options available, it can be confusing as to which is right for your application.
Adhesives also come in several different material types, but for the purpose of this discussion we will ignore these differences and look at how the two substrate types directly impact the final product, both in electrical/mechanical performance and the bottom line.
One design tenet bears repeating: what is good for a flex-circuit mechanically is bad for it electrically. And, of course, what benefits electrical performance will have a negative impact on mechanical performance. Let’s start with a review of adhesivebased foils with these two attributes in mind.
It does not seem that long ago when the only materials available were adhesive-based clad versions. Generally there are fewer steps and complexity involved in the production of adhesivebased materials; therefore, they are typically priced lower than a comparable adhesiveless product of the same thickness. The adhesive is also typically less expensive than polyimide, further reducing the cost for a given thickness.
We highly recommend adhesive-based foils where lower cost and final circuit forming aredesired. In forming, the adhesive permits a little bit more “give” than the same thickness of polyimide due to its greater elasticity. This in turn permits forming to a smaller radius without undue stresses. Higher polyimide content within the flex stack-up tends to resist a form, as the polyimide has significantly greater tensile strength compared to common adhesives. FIGURE 1 represents a typical adhesive-based substrate cross-section. Suitable for a wide variety of applications, its main weakness is a relatively lower dielectric constant of the adhesives when compared to a pure polyimide. This makes it less desirable for impedance control applications. Forming and flexibility properties are ideal for most needs.
A recent development is the availability of base materials with thinner adhesives; these too can help contain costs, as less material often makes for a lower
price. We have seen no reduction in peel strength when using a thinner adhesive (within the base laminate, not cover layers). Thinner adhesives can also permit greater flexibility for high cycle applications.
FIGURE 2 shows a common adhesiveless construction. The term “adhesiveless” is a bit of a misnomer. In the case of most common adhesiveless materials, the polyimide core is made of three distinct layers, an inner-cured polyimide core and two outerlayers of B-stage polyimide. These are then laid up with copper “cap” layers and laminated together. The distinct layers that result can sometimes be seen under high-magnification crosssections. In the case of extremely thin polyimidedielectrics the polyimide may be cast directly to the copper layer.
Generally, all-polyimide substrates are more expensive than their adhesive-based counterparts. The big advantage to an adhesiveless construction is its superior dielectric constant when compared to adhesivebased systems. This does, however, come at a cost, both financial and in elasticity. An “all-polyimide” substrate will always be stiffer, making forming difficult. The thicker the polyimide, the more resistant to forming it will become. If too much polyimide is used in the stack-up, there may be a need to use heat-forming to reach the desired final architecture. As we increase the amount of polyimide in the stack-up the price increases as well; the relationship is not always linear. Commonly available thicknesses usually come in 25μm increments, with a maximum of 125μm from most manufacturers.
When calling out any base materials on a drawing, it is highly recommended to use their IPC-4202 designations. These codes spell out exactly what material type and thicknesses are required. Avoiding calling out of any brand names will also give a supplier greater latitude in finding cost-saving options. Remember, calling out IPC requirements ensures the materials are manufactured to a high set of standards. Adding the stack-up view to all mechanical drawings is an excellent way to assure you and your manufacturers know exactly what is needed. Isometric drawings are also important, as they provide critical input to the flex designer on what materials may work best.
While there is a difference in materials, early consultation between your design engineers and the flexcircuit manufacturer will ensure you get exactly what is required for both performance and cost.