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Successful Flex Circuit Assembly

There are key differences between flexible PCBs and rigid PCBs requiring alternative design rules and handling methods. One general area that is often a particular concern occurs when flexible circuits require component assembly. Many contract manufacturers are reluctant to mix rigid and flexible circuits on the same assembly line as the handling and fixturing requirements can be quite different. Characteristics allowing a flexible circuit to be flexible can often present learning curve challenges when component or mechanical assembly is required. 

The following are some common issues experienced when assembling flexible circuits.

General Handling Damage

Rigid printed circuits provide structural component support and protection not inherent in flexible circuits. This can often be accommodated on a flex circuit with selective stiffeners, generally either polyimide or glass epoxy. In addition, both surface-mounted and through-hole components can cause handling damage if proper precautions are not taken during assembly. Assembled circuits loosely placed in storage trays can experience damage due to jostling or sliding. Custom inexpensive in-process and shipping storage trays are frequently adopted to minimize the chance of handling damage.

Misaligned Solder

Solder printing with a stencil is the most common method for discrete placement of solder paste and can often be done while the flexible circuit is in a multiple-up array. Compensation of the stencil may be required as flexible circuits are less dimensionally stable versus the rigid board counterpart. A myriad of factors determine if this is necessary including circuit construction, component types, circuit/array size and circuit feature density. Solder paste misalignment causes incomplete solder joints, resulting in opens or intermittent continuity.

Fracturing Due to Bending/Creasing

A flexible circuit is usually folded and/or creased as part of its final installation or during its assembly process. There are a few design features that might cause issues during assembly or installation:

• If the bend is too close to a solder joint, the probability of a solder fracture increases significantly. While the copper on a flexible circuit is flexible, the intermetallic layer between the solder and copper is not. Adequate distance between the solder joint and bend area can be chosen strategically by coordination with a flex circuit applications engineer.
• With a double-sided flex construction, one needs to be concerned about the “I-beam” effect. This occurs when top- and bottom-side traces are directly aligned on opposite sides of the dielectric. This forces the outside layer copper trace into a tension mode and increases the probability of trace fracture.
• The copper type will also have an impact. Rolled annealed (RA) copper is generally better equipped to withstand folding or creasing than electrodeposited (ED) copper. It is advisable to make sure RA copper is used for a part used in a dynamic flex working environment and insure the grain direction is parallel to the fold line.

Stiffener/Connector Alignment

Many times, an FR-4 (glass epoxy) stiffener is used to support a connector. It is a mistake to have the stiffener hole sizes exactly match the flexible circuit openings. Misalignment will result in problems with the connector assembly or the access to the through-hole connector. The stiffener needs to have larger openings to allow for placement tolerances.

Packaging/Shipping/Unpacking   

Flexible circuit assemblies are more prone to damage due to handling. Parts loosely packaged in a bag can experience damage during shipping or when parts are removed from the package. Special packaging trays or foam bags will usually provide enough protection to assure the part arrives to the assembly line without damage.

With proper designs and handling methods, flexible circuit assemblies have proven to be as reliable as an assembly of a rigid PCB. Since each part is a custom design, nuances of materials, fabrication processes, and end environment create an individually unique performance situation. Good communication between the flexible circuit manufacturer and the end user can go a long way to ensure minimal assembly issues. An experienced applications engineering support staff should guide a customer to define a robust interconnection part number.