what are Rigid PCB and Flexible PCB
Rigid PCB Flexible PCBFlexible PCB (also variously referred to around the globe as flex circuits, flexible printed circuit boards, flex print, flexi-circuits) are members of electronic and interconnection family. They consist of a thin insulating polymer film having conductive circuit patterns affixed thereto and typically supplied with a thin polymer coating to protect the conductor circuits. The technology has been used for interconnecting electronic devices since the 1950s in one form or another. It is now one of the most important interconnection technologies in use for the manufacture of many of today's most advanced electronic products.
Some Similarities and Differences Between Rigid PCBs and Flex Circuits
When designing rigid PCBs, certain design rules must be followed, including minimum hole sizes, minimum space and trace width, minimum distances to board edges, and copper and overall design thicknesses. Additionally, many manufacturing process steps are shared between rigid and flexible PCBs. Such process steps include the drilling and plating of holes and vias, photo imaging and development, the etching of copper traces, pads, outlines, and planes, and the heating (baking) of the circuit boards for the purpose of removing moisture from the PCBs. At this point in the manufacturing process, rigid PCBs head to the solder mask station while flex circuits go to the coverlay station.
IPC Standards for Rigid and Flexible PCBs
The list of IPC standards below applies to rigid PCBs and flex circuits. Take note that this list is not exhaustive, and additional IPC standards may need to be considered. You should consult the ipc.org website for a full list of available IPC standards.
IPC-2221A, Generic Standard on Printed Board Design
IPC-2223, Sectional Design Standard for Flexible Printed Boards
IPC-4101, Specification for Base Materials for Rigid and Multilayer Printed Boards
IPC-4202, Flexible Base Dielectrics for Use in Flexible Printed Circuitry
IPC-4203, Adhesive Coated Dielectric Films for Use as Cover Sheets for Flexible Printed Circuitry and Flexible Adhesive Bonding Films
IPC-4204, Flexible Metal-Clad Dielectrics for Use in Fabrication of Flexible Printed Circuitry
IPC-6013, Qualification and Performance Specification for Flexible Printed Wiring
Size and Weight Reduction Benefits to Flexible Circuitry
Generally, flexible circuitry is the go-to solution for manufacturers who need:
· Wiring solutions that fit where rigid boards cannot.
· Thin, lightweight products that are nonetheless durable.
· Miniaturized versions of existing technologies.
· Three-dimensional packaging geometry.
· A low number of device interconnects.
· Shock and vibration resistance.
These benefits point to flexible circuitry options as an ideal solution for mobile consumer electronics. Enterprising circuit board amateurs who take apart their smart phones or laptop computers will find a wealth of flexible circuitry inside any modern device on the market.
In the case of mobile devices, the use of rigid circuitry would result in a device too large, too heavy, and too fragile to conveniently carry around. This was the case with the Osborne I, the first fully powered mobile computer, which weighed in at an intimidating 24.5 pounds.
Size and weight reduction represent only one half of the flexible circuit story, however. They are also ideal for high temperature and high-density applications.
High Temperature and High-Density Applications
In many cases, flex circuits are made of polyimide or a similar polymer. This material dissipates heat better than most rigid circuit board materials. For this reason, flexible circuits can be placed in inconvenient locations where heat would impact the performance of a rigid circuit board.
Flexible circuit boards can be designed to withstand extreme temperatures – between -200° C and 400° C – which explains why they are so desirable for borehole measurements in the oil and gas industry.
In fact, because of these conditions, and the need for small, unobtrusive devices in most industrial environments, flexible circuits represent the first choice for engineering design in most industrial sensor technologies.
High temperature resistance comes usually comes with good chemical resistance and excellent resistance to radiation and UV exposure as well. Combined with the ability to control impedances in high density circuit board designs, flexible circuit designs offer many benefits to manufacturers.
Why Not Make All Circuit Boards Flexible?
Flexible circuit boards are certainly useful, but they are not going to replace rigid circuit boards for all applications. Cost efficiency is the main obstacle to implementing an exclusively flexible circuit board design in a consumer product. Rigid circuit boards are less expensive to manufacture and install in a typical automated high-volume fabricating facility.
Typically, the ideal solution for an innovative product is one that incorporates flexible circuitry when necessary, and employs solid, reliable rigid circuit boards where possible to keep manufacturing and assembly costs down.
Some manufacturers even use hybrid rigid-flex printed circuit boards expressly for this purpose. This is common in laptop computers and medical devices, where rigid circuit boards can be connected to one other using ribbon-like flexible circuits. These boards can be compounded and designed to meet any number of engineering needs by focusing on the respective strengths of each circuit board base technology.