The two primary options for encapsulating the external circuit layers of a flex circuit are Polyimide Coverlay and Flexible Liquid Photoimageable (LPI) Soldermask. The two materials however have very different capabilities and requirements.
Rigid-flex printed circuit boards are unique with their integrated construction of both rigid PCB and flex circuit technologies. Being unique comes with a number of unique requirements that should be reviewed and implemented during the rigid-flex PCB Gerber layout phase of the design process.
Flex and rigid-flex PCB constructions have many variations that allow for a wide range of applications and solutions. A significant difference to rigid PCB constructions is that uneven layer counts are allowed and frequently used. The primary reasons being reduced flex thickness, improved flexibility and reduced part cost.
An essential element of a flex or rigid-flex printed circuit board (PCB) design is verification that the construction will meet your mechanical bend requirements. Exceeding the minimum flex bend radius requirements creates the opportunity to exceed the physical properties of the copper circuitry resulting in failed parts and long term reliability concerns.
This blog post is intended to enforce the design support we offer at Epec in regards to our flex and rigid-flex PCB's. Technical design and engineering is one of our core fundamentals that allows us to help our customers meet there product requirements. The blog post will cover design areas that require special consideration.
Two distinct types of polyimide flex core material constructions are utilized in today’s flex circuit manufacturing. The difference is in the method used to physically attach the copper layers to the polyimide core. This results in a different set of material properties for each of the material types which must be factored in to a specific application to ensure the design meets all the requirements and is cost effective.
With rigid-flex printed circuit boards (PCB's), the transition from the rigid material to flex material (Transition Zone) can exhibit visual imperfections that, although acceptable, could affect the final part. These imperfections can consist of adhesive squeeze-out, protruding dielectric materials, crazing, or haloing. With the protruding dielectric materials (resin), the particles can have sharp glass-like qualities.
The copper thickness used in a flexible circuit board needs to be carefully selected in order to meet both the electrical and mechanical bend requirements of specific flex circuit design. Copper weight for a flex PCB is both the stiffest and most critical component.
In the flexible printed circuit board manufacturing, a flex circuit coverlay (aka coverfilm) is used to encapsulate and protect the external circuitry of a flexible circuit board.
A flexible circuit coverlay serves as the same function as soldermask that is used on a rigid printed circuit board. The difference with a flex coverlay is the added element of flexibility and durability it provides to the flexible circuit board.
Flex circuits offer multiple advantages for anyone considering their use in a future project or if you’re trying to decide on revamping your current design. The most prominent features include packaging, reliability, capabilities, and cost savings.