PFC has decades of flex circuitry experience across industries. Our Knowledge Center is a resource for the questions that arise when embarking on a new design, considering a flex circuit over other technologies, or are new to flex technology. Our cutting-edge technology and industry leading skills has made PFC the flex circuit experts. Our in depth understanding of the flex industry aids in creating a clear path to successfully launching flex technology for customers.
PFC’s high density capabilities offer customers a wide variety of custom flexible circuits
with features such as extended lengths, multilayer, non- traditional material
constructions, and specialized foils. The properties of flex-based materials enable
smaller applications with the benefit of reduced weight and optimized packaging
utilization. Our goal is to facilitate new designs that require smaller footprints and ultradense and precise conductor geometries.
Advances in materials and etching technologies have made HDI – high density flex geometries attainable. Standard copper clad-foil for HDI flex circuitry has 18-microns thick copper and, in some cases, thinner. Material choices are a large factor in PFC’s ability to provide cost-effective, high-density circuits.
High density via fabrication dramatically increases the I/O count. Mechanical drilling of vias is now being done as small as 0.004” (100 micron) diameter. Laser drilling can be done at 0.002” diameter (50 micron) on thin substrates. While PFC offers laser capabilities, the cost to laser each hole one by one becomes expensive.
|DESCRIPTION||HDI FLEX CIRCUIT|
|Microvias/pads diameter||50/200 µm|
|Thinnest copper||25 µm|
|Thinnest dielectric thickness||12 µm|
|Conductor width tolerance||+/- 20%|
|Artwork to soldermask tolerance||+/- 25 µm|
|DESCRIPTION||ULTRA-HIGH HDI FLEX CIRCUIT|
|Microvias/pads diameter||40/100 µm|
|Thinnest copper||9 µm|
|Thinnest dielectric thickness||12 µm|
|Conductor width tolerance||+/- 15%|
|Artwork to soldermask tolerance||+/- 25 µm|
Stiffeners are materials added to a flex circuit to rigidize areas. Rigidizing is recommended under SMT components, but not required. Other areas to consider rigidizing include:
Stiffener and coverlay termination points should overlap a minimum of .030” to avoid stress points. placing stiffener PFC can assist with the location of stiffeners, identification of the material and the required thicknesses.
Flex circuit design must consider the mechanical characteristics of bending and potentially cycling the circuit. Other mechanical aspects that affect a flex circuit are size, shape, thermal properties, and mechanical stability in key areas. Additional factors are reliability & stiffeners, and materials. Below are general guidelines for static and dynamic circuits:
“How much can a flex circuit bend?” This is the most frequent question we hear. The standard IPC answer is 10 times the thickness of the material. Section IPC-2223 offers reasonable information on bend radius calculations. PFC considers various factors when designing a flex circuit for high reliability.
Conductors should be staggered from layer to
layer and not stacked on top of each other to
ZIF connectors are a reliable low-cost interconnect solution. While it simplifies assemblies, detailed planning is important to designing the ZIF interface for a flex circuit. Every ZIF connector manufacturer will call out different dimensional requirements. Connector specifications will be needed for a quotation.
Electrical factors are considered when designing and producing flex circuits to ensure flex circuit electrical and mechanical aspects work cohesively. During our electrical analysis, we will find the best options for the impedance, shielding, bendability, and material. PFC provides an unmatched electrical solutions for flexible circuits.
Flex circuits are regularly used in high current/power applications. Some application examples of designs produced at PFC include bus bars, backplanes, and power distribution cables. Most of the high current applications PFC develops are to replace wire harnesses
The benefit of using flex circuit replacing a wire harness includes:
IPC has come out with a new specification on current carrying capabilities for PCBs and
flex circuits in 2010: IPC 2152. This new specification takes the place of IPC 2221 which
was developed in 1955!
The thicker the copper, the more current a flex circuit can carry. PFC has the ability to provide flex circuits utilizing up to 560-micron thick copper for power applications. Available copper
weights for power applications in microns include:
IPC 2152 contains information that can be used to develop current-carrying capacity
guidelines for individual designs. Through the use of computer modeling and
information within IPC 2152 and its appendix, current carrying capacity design
guidelines can be optimized for any variation in printed circuit technology. Until the
publication of IPC 2152, this was not possible with the available public information
If you are designing a high current flex, PFC urges you to obtain IPC 2152. You can
purchase the publication from the IPC online store.
The biggest design consideration for high current flex circuit applications is temperature
rise and material thermal expansion. Below is a list of relevant material expansion
This is the measure of opposition to time-varying electric current in an electric circuit. Simply put, impedance control is the slowing down of an electrical circuit. As flex circuit design and components become more complicated, smaller and faster, it becomes necessary to slow certain circuits down, allowing specific functions of components to perform before others. The increase in processor clock speed and component switching speed on modern flex circuit means that the interconnecting paths (traces) can no longer be regarded as simple conductors.
Physical characteristics of impedance trace:
Some PCBs may have multiple impedance requirements! More impedance
requirements mean more impedance coupons, which can decrease the amount of
usable panel space for PCBs.
Double-layer and multi-layer flexible circuits are ideally suited for providing interconnections that are specifically designed to provide desired levels of signal integrity. Construction techniques commonly referred to as “stripline” or “microstrip” are particularly well suitedfor these applications.
Applications with limitations to the electromagnetic and/or electrostatic interface may require shielding. This can be accomplished through multiple methods. Shields are designed and used for EMI and ESD considerations as well as impedance requirements. Shielding provides various advantages to a flex circuit.
Copper has the best shielding characteristics. Silver allows more flexibility and at a slightly lower cost. Shielding film is most flexible but a higher cost solution.
Our flex experts offer a vast knowledge of flex circuit materials. We provide a Materials Stack Up list of specific materials and thicknesses that will be used in the manufacturing of your circuit. This will include required adhesives, stiffeners and any special drilling requirements such as blind and buried vias.
|Adhesiveless Materials||DuPont APPanasonicEspanex|
|Coverfilm/ Covercoat/ Coverlay||Kapton (Pyralux), Espanex, DuPont-Black, Taiflex-Black&White|
|High Speed Laminates||TK- DuPointTaconic|
|Flexible Solder Mask||Various per IPC-840 Class 3|
|Rigid Solder Mask||Taiyo PSR|
|Metals||Copper 9-750 micron (.0008 - .030) thickBeryllium CoppyNickel Alloys|
|Finishes||Immersion gold over Electroless nickel Immersion TinHot air solder level (tin-lead)Hard goldSoft goldOSP (Organic Solderability Preservative)|
|Shielding Materials||Copper Silver paste/inkTatsuta|
|Stiffeners||PolyimideMachined Metal, Aluminum (cast)CeramicFR-4Black FR-4|
After a thorough analysis of the project, PFC will provide a list of specific materials and thicknesses that will be used in the manufacturing of the circuit. This will include: required adhesives, stiffeners and any special drilling requirements such as blind and buried vias.
Electronic equipment, in many cases, require a UL (Underwriters Laboratory) certification. In turn, the components that make up the electronic system must be certified, including flex circuits.
UL does not require the flex circuit itself to be approved or certified. However, it must be manufactured using UL approved flexible circuit material and manufacturing processes. In order to get an approval from UL, PFC provides UL with documentation and samples of a specific construction
UL will test and approve that specific construction. Once the construction is approved, PFC can use that specific construction on other circuits as UL approved construction.
PFC offers a full range of UL approved Dupont materials, including AP and FR laminates, coverlays, and a full range of Panasonic and Espanex materials. Additionally, PFC just had the full line of Taiyo “BN” soldermasks recognized for any color.
Visit the UL website for many of the approved constructions for PFC and more information on UL approved flexible circuit material.