Flexible Circuits: The Basics

Just getting started with flexible circuits?  Here are few “basics” to keep in mind.   Designing flexible circuits is not difficult, but does require adjustments to ensure that you achieve the flexibility and reliability that you are expecting.

Benefits of flexible circuits:

• Solution to a packaging problem
• Reduce assembly costs – elimination of connectors and solder joints
• Replacement for PCB and wires – simplify system design, reduces number of interconnections and eliminates human error.
• Reduce weight and space
• Dynamic flexing
• Thermal Management/ High Temp Applications
• Aesthetics

Design Considerations:  Conductor Pad Design and Filleting:

• Pads should have tie-downs (rabbit ears).  Tie-downs are captured by the coverlay to anchor the copper and prevent separation of copper and base materials.
• Filleting:  All pads should be filleted to reduce stress points during flexing.

Bending and Folding Considerations:

• Radiused Traces – help to alleviate breaking during folding and bending
• I-Beam  Effect- when conductors are routed directly on top of one another, stiffness is increased in fold areas.  A better alternative is to stagger conductors, alternating their location to retain maximum flexibility.
• Fold Lines – tick marks in copper or silkscreen help identify designated location for bending or folding.
• Circuit Trace Width – this should not change in bend areas and the transition should be at least .030” from fold line.
• Bend radius – very general guideline of 10 times material thickness will work with most applications.
• Button Plating – increased flexibility by plating thru holes but not traces.

 Shielding:

• Solid Copper – this is the most common method of shielding.  This can be done on one or both sides of the circuit or to cover selective conductors.
• Crosshatched Copper – crosshatching will help the circuit to retain its flexibility and can be done in selective “flexing” areas or entire layers.
• Conductive Silver – not recommended for dynamic- flexing applications, but can be substituted for copper shield in certain applications.

 Stiffeners:

• FR4 – normally used to give added rigidity under a connector area.  These can be bonded with pressure sensitive adhesive or thermal set adhesive.  Often used as a carrier panel for automated assembly.
• Polyimide – often used to give added thickness under conductor fingers to meet ZIF connector requirements.  Can be used to give added strength to high wear areas or to identify “fold” areas.  Outline can blanked or routed at the same time as the flex to meet tight tolerance requirements.
• Location:  Stiffener and coveraly termaination points should overlay a minimum of .030” to avoid stress points and to reduce the chance of traces breaking.

Please contact us if you have questions or if we can provide any additional information to assist with your flexible circuit needs.

www.omnipcb.com

ENEPIG Solderable Finish

ENEPIG:  Is this the universal finish?

We are asked quite often about the availability and benefits of ENEPIG.  As I was reading a recent article in PCB007 by Mike Carano that discusses various solderable surface finishes, I thought our customers would appreciate the information presented and wanted to specifically highlight the information presented on ENEPIG.  A link the entire article is attached below.

ENEPIG 

If hyper-corrosion and black pad are of concern, ENEPIG is a solution. Here, the gold deposits onto the palladium, not the nickel. There is no hyper-corrosion effect as there is with gold over nickel. ENEPIG is often referred to as the “universal finish,” capable of good solderability and wire bondability. However, one must look at this more expensive finish in the context of the circuit board and its intended use/environment.

One area in which ENEPIG has found use is the IC substrate market. ENEPIG can function as one finish for both wire bonding and solder attachment. While it is true that ENIG can perform the same functions, ENEPIG is more robust with respect to gold wire bonding. Typical plating thicknesses for this three-metal-stack over copper are as follows:

• Au Layer: 0.03-0.06 micron;
• Pd Layer: 0.10-0.50  micron; and
• Ni Layer: 3.0-6.0 micron.

 The nickel present on the surface benefits from a Pd or Au protective layer to improve solderability by reducing brittleness and oxidation of the solder joint. The basic idea is to achieve improved solderability and wire bonding at reduced palladium and gold thicknesses. For the majority of ENEPIG systems, the palladium is deposited as an electroless reaction. Commercial palladium systems are based on one of two reducing agents, hypophosphite or formate. The former will co-deposit 1-6% phosphorous into the deposit, while the latter is nearly 100% pure palladium.

There is no industry specification for ENEPIG, although one is under development. A key component of any specification is the verification of both solderability and wire bondability at varying palladium thicknesses. Again, lower thicknesses of both palladium and gold will enhance the economic viability of this finish as long as the solderability/wire bonding requirements are met.

Mike Carano’s full article discussing factors to be considered when choosing a solderable surface finish,  a review of the latest solderable finishes and special considerations when using these finishes can be found at:  http://www.pcb007.com/pages/zone.cgi?artcatid=0&a=85243&artid=85243&pg=1.

Mike will be joining us for a PCB Coffee Talk webinar session later this year discussing surface finishes.