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In this second part of a three-part series regarding testing flexible circuits, I will cover the testing of raw materials or components. The following are the common materials that go into a non-populated flex circuit:
- Copper foil
- Dielectric substrate
- Copper/dielectric laminate
- Dielectric insulators
For each of the above, there are several variations:
- Copper foil can be in different thicknesses and can be rolled annealed (RA), electro-deposited (ED) or high ductility electro-deposited (HDED). The copper surface can also come with different treatments. The variety of treatments often impart a different color to the foil. The treatments are generally used as ways to enhance bonding in subsequent lamination cycles.
- Dielectric substrates can also come in different thicknesses and compositions. Polyimide is by far the most popular film used to produce flex circuits. One of the most widely known polyimide film is DuPont’s Kapton®. Polyester, liquid crystal polymer, and PTFE (Teflon®) are also used in some specialty applications.
- Copper/dielectric laminate constructions come in many variations, as a result of combining the different copper foils with dielectrics. They also come with different adhesives that bond them together, including several adhesiveless systems. Films commonly range between .0005” and .005” in thickness. The most frequently used film is .001” thick, but as requirements vary, so will the film thickness.
- Dielectric insulators for cover films and solder masks also come in multiple thicknesses as well as material types. This would also include printable and photoimageable solder masks, which come in both film and liquid form.
In addition to raw materials, there are many consumable process materials used in the fabrication of a flexible circuit. These would include etchant chemistries, plating solutions, plating metals, resist material, developing solutions, press films and much more. These have test requirements as well, but are not specified in applicable IPC documentation.
There are two basic reasons for tests and specifications of raw materials:
1. To assure that the final flexible circuit product will meet the end user requirements.
2. To assure consistency of properties so the material can be processed with maximum efficiency. An example of a property that may make a difference to the circuit fabricator, but not to the end user, is etch-shrink of the laminate. Laminates used for flexible circuits tend to undergo small dimensional changes while being processed. This change is acceptable provided that they are consistent from lot-to-lot (predictable) and small enough so as to not adversely affect layer-to-layer alignment in multilayer circuits.
The vast majority of raw material test requirements are specified in IPC-6013, IPC-4202, IPC-4203, and IPC-4204. There are more than 24 different tests for materials. In some cases the supplier will certify compliance to a certain specification; in other cases the supplier will provide lot test data. There are also situations where the flexible circuit manufacturer will do its own incoming testing.
The following are some of the common test requirements for materials:
- Electrical Properties
- Dielectric constant
- Surface and volume resistance
- Insulation resistance
- Mechanical Properties
- Peel strength (as received, after solder float, and after temperature cycling)
- Flexural endurance
- Low temperature flexibility
- Dimensional stability
- Solder float
- Initiation tear strength
- Propagation tear strength
- Chemical Properties
- Chemical resistance
- Fungus resistance
- Thermal Properties
- Service temperature
- Temperature cycling
- Cosmetic Properties
- Wrinkles, creases, streaks and scratches
- Holes and tears
- Pits and dents
- Other Properties
- Moisture absorption
- Copper foil purity
Other testing is done based on specific customer or industry requirements, for example out-gassing specifications are common for products that need to operate in a vacuum. The automotive industry has some special chemical resistance testing that includes brake fluids, oils and gasoline. Many customers have their own unique requirements in terms of long term temperature aging, maximum service temperature and long term moisture absorption. In general, a flexible circuit supplier will work with customers on unique test requirements and create specifications for materials that are consistent with end use requirements.
Do material properties change after going through a flexible circuit fabrication process? Or is it possible that the raw materials meet the overall requirements, but the combinations and stack-up of materials do not? The answer to both those questions is yes. For example: Two properties that materials are commonly tested for are flexibility and thermal resistance. The ductility of copper can change under heat and pressure. Sometimes platen pressing at elevated temperatures and pressures is used for dielectric bonding. If the temperature and pressure are high enough, the ductility properties can change, although for typical processing parameters, the change is very small. Sometimes a given characteristic improves, such as thermal resistance, after exposure to temperature and pressure. In most cases, the material changes through processing are known and predictable. An experienced flexible circuit manufacturer will have test data to determine where potential issues might arise.
Flex testing done on strips of foil or laminate doesn’t necessarily translate to the final product. In the case of flexibility, the etched trace patterns, surface treatments and material stack-ups can dramatically change flex properties. It is important to test properties in the stack-up and configuration of the final product, just as the customer will perform reliability testing on the finished electronic package containing the flexible circuit.
Even though the raw materials may individually meet certain specifications, it is important to determine if the final product configuration meets those specs. So testing done at the finished circuit level is also needed. Tests specific to the finished circuit will be the topic of our next column on testing.
Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.