May 05, 2010 
Flexible and flex-rigid PCBs are being designed more and more with the prevalent ZIF interface, possessing a so-called ZIF contact (ZIF = Zero Insertion Force) for connection with a ZIF connector. Hence, appropriate one-stop system solutions make sense. For that reason, Würth Elektronik eiSos’ designers of the new ZIF connector: WR-FPC paid particular attention to closely attuning system relevant areas. The ZIF connector WR-FPC has been specifically designed for the best connection to flexible and flex-rigid PCBs by respecting the tight tolerances and parameters applicable in PCB manufacturing in the connector design. That way, customers get comprehensive and systematic counselling services, seamlessly tailored system components included. Würth Elektronik is offering a ZIF connector design-kit, particularly created for designers and offering a range of options for design test. The ZIF connector WR-FPC is available in different variants and for any kind of construction, either upright, horizontal, contacts on TOP or BOTTOM.
Faithful to Würth Elektronik’s tradition of close customer ties and proximity, difficult and complex customised solutions can be worked out as well. “We are not only service providers, but also consultants and trouble-shooters in the entire range of flexible and flex-rigid matters,” emphasizes Andreas Schilpp, product manager "3D" at Würth Elektronik, ”no single job will be released into production without full optimisation - even if in some cases it that could only be a double-check of the production panelisation for cost reduction purposes. For example, a well- thought-out use of base materials can help reduce costs whilst, at the same time, improving quality. At present, we are able to build about 80% of incoming jobs with Würth’s specifically qualified and standardised range of base materials, a selection of the many materials available in the market for rigid-flex PCB technologies. For the remaining 20%, we find specifically tailored solutions together with our customers.”
Various options for ZIF connections
Detachable connections between displays and PCBs, or between two PCBs are often created by using flexible printed circuits boards or by flexible flat cables and ZIF connectors.
Fine pitch-ZIF-connectors are available, for example, on a 1.0 mm and 0.5 mm pitch. The contact arrangement is created between the ZIF connector, which is soldered onto the PCB and the ZIF contact, which must fit exactly into the ZIF connector. This ZIF contact can be simply a flexible flat cable, a complex flexible PCB or a flex tail from a flex-rigid PCB.
There are various ways of creating ZIF contacts. Flex-rigid PCBs and the generally thinner flex PCBs are handled in different ways to attain the required 0.3 mm PCB thickness at the contact position: For flexible PCBs, a stiffener is affixed to the contact area; which means relatively high costs due to the necessary handling required at the adhesive bonding process. In contrast, for flex-rigid PCBs, Würth Elektronik utilises the simple and cost effective method of high-precision depth milling to obtain the required thickness.
Tolerances in production
Very small geometrical tolerances must be maintained in manufacture, so that the ZIF contact fits optimally in the ZIF connector. Due to this reason, only flat solderable surfaces are possible. For increased reliability and a longer lifespan, the solderable surface finish electroless nickel/immersion gold has proved itself favourably in practice.
The profile of a simple ZIF contact, with the technical requirements of a PCB profile with a tolerance range of ±0.1 mm, is created through routing. However, enhanced tolerances such as ±50µm can only be realised through the use of camera controlled routing or laser cutting.
Drawing specifications in the manufacturing documents
Without the corresponding specifications on the drawing and/or without dimensioning in the gerber data, a flexible tail is not recognisable as a ZIF contact. Therefore, the following details should be contained in the manufacturing drawing according to the data sheets of the connector manufacturer:
- Length and width of the contact area
- Thickness of the contact (stiffening material + flex material + copper) including the tolerance. Standard is 0.3 mm ± 0.05 mm
- Length of the stiffener. The stiffener must overlap with the flexible solder mask/cover-lay
- Toleranced distance of the first contact to the profile
- Ideally an indication of the possible areas for connecting tabs in the delivery panel
There are many ways to set connecting tabs in flex-rigid boards
In order to allow the insertion of the contact into the ZIF connector, the specified contact area in its length must not have any tabs at its side edges. Connection tabs must be placed beyond the connection area. There are a number of options here:
- If the customer agrees to extend the contact stiffener into the flexible area, stable tabs can be set in the stiffened area (FR 4 depth milling).
- If an extension of the stiffened area is not suitable, the tabs in the flexible area must be set directly beside the stiffener.
- Intermediate tabs: Here, the entire PCB’s material thickness remains unchanged, offering various advantages.
- Laser-cut micro tabs in the flexible area: Micro tabs can be applied for laser-cut flexible outlines. Rigid material underneath the flexible limb enhances the delivery panel's stability, and allows for easy manual de-panelisation.
- Ease of handling with "ears": An idea originating from PCB technology; highly efficient and cost saving.
Further options with a stable intermediate tab
Usually, for flex-rigid boards with an internal flexible layer, the ZIF contact is also located on the flexible core. That would require some complex special operation steps during the production process. For flex-rigid with the flexible layer located as external layer, a change of the contact side could become necessary. Such a change is made possible by via holes in the rigid area in between.
Rigid- flex boards in practice
”Making quality manufacturable and cost effective is Würth Elektronik’s top priority“, high-lights the flex-rigid PCB specialist Andreas Schilpp, ”an essential prerequisite is as much standardisation as possible. Through our internal, cross-departmental coordination of system solutions, we are in a position to almost standardise innovative technologies such as flex-rigid, and open up more and more areas of application. We have been able to prove this in many projects.”
Furthermore, Schilpp points out: “As this is also the case in many other areas: the earlier customers involve their suppliers in the design and development stages, the higher the chances of benefiting from system solutions become. Namely, this is because we are able to introduce our PCB production know-how at the right time. A fact that often proves advantageous and cannot necessarily be recognised by customers at the early stage of design“.
Flex-rigid multilayer boards are already proving their value in daily applications. For example, as sensors for controlling motors in roller shutters, awnings and Venetian blinds. In this case, internal resistors and heaters were also used. Here all the typical demands are met: high op-erational reliability and durability; simple assembly; resistance again oil, grease, humidity and aggressive environmental conditions as well as simple, reliable electrical connections using a flexible limb with a ZIF contact. Wiring and solder joints were replaced, high quality connections applied, and an assembly process for the heating resistors was avoided.
For another application, a special design and implementation of contacts was created using a semi-flexible FR4 PCB with a ZIF contact for use in standard ZIF connectors. This application was used in a module for the optical display of temperature and air quality in a sensor for measuring air quality. In order to achieve large bending radii, the single-sided PCB already had a thickness of 0.3 mm in the contact area, which meant that it could be directly plugged into the ZIF connector without the need for additional stiffeners or extra milling. Further ad-vantages were that cable wiring to the motherboard was no longer necessary, the assembly process was simplified and the additional benefit that a drying process before soldering was no longer required due to the base material used.
Flex-rigid technology was also employed in the re-design of an MDE device (mobile data entry), created for inventory administration. An adapter board with two ZIF contacts enabled the display to be updated smoothly and securely. Additionally, by laser cutting the flexible/ZIF contours, precise contact tolerances were achieved and an ingenious panelisation provided a stable assembly panel. In the flexible region, the PCBs could be removed by hand, without the needs for tools or special processing.
Particular advantages of flex-rigid PCBs
Flex-rigid technology is well-proven and reliable; and is particularly good in overcoming space and weight issues with spatial degrees of freedom. Careful consideration of flex-rigid solutions and a proper assessment of the available options at an early stage in the design phase can return significant benefits:
- Space requirements can be minimised by applying 3D circuitry
- By removing the need for connectors and cables between the individual rigid parts the board size could be reduced and overall system weight can be brought down
- Less solder joints assure higher reliability
- Handling during assembly is easier when compared with complete flexible boards
- Otherwise difficult connections can be created, thereby simplifying the assembling processes
- Integrated ZIF contacts provide simple modular interfaces to the system environment
- Test conditions are simplified. A complete test prior to installation becomes possible
- Logistical and assembly costs are significantly reduced with flex-rigid boards
Using flex-rigid technology to create ZIF contacts is becoming more and more widespread, as it offers a number of benefits:
- Savings in joints/components
- Improved reliability
- Cost savings in procurement and logistics
- Reduced installation size
- Less complexity and expense in design and development
About Würth Elektronik Circuit Board Technology (CBT)
Würth Elektronik is the leading PCB manufacturer in Europe, producing a wide spectrum of PCBs - from standard technologies through to pioneering system solutions - at manufacturing plants in Niedernhall, Rot am See and Schopfheim. The PCB specialist delivers application specific solutions in all technologies and is a driving force behind new technical innovations, for example, in the field of embedded active and passive components. The extensive PCB portfolio ranges from double sided PCBs and multilayers in all conventional technologies through to technically demanding PCBs such as HDI, flex-rigid and heatsink technologies.
About the Würth Elektronik Group
Würth Elektronik CBT is - alongside Würth Elektronik ICS (electro-mechanical and electronic system solutions), Würth Elektronik eiSos (passive and electro-mechanical components) and Würth Solar (CIS- photovoltaic modules and solar power systems) - a division of the Würth Elektronik Group, an internationally operating, independent subsidiary of the Würth Group. Beside their own R&D, Würth Elektronik is also involved in numerous external research communities, e.g. GloveNet or TIPS. In 2009 Würth Elektronik employed around 5,900 personnel and generated a turnover of 374 million Euros. Further information can be found at http://www.we-online.com .
