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Two primary options for encapsulating the external circuit layers of a flex circuit: polyimide coverlay and flexible liquid photoimageable (LPI) solder mask. The two materials have very different capabilities and requirements.
Of the two, polyimide coverlay is the most commonly used and preferred solution throughout the industry. It provides a much more robust and durable solution with very good flexibility and a high dielectric.
Flexible LPI solder mask is essentially the same formula (with the addition of a flex agent) that has the same capabilities and is applied in the same manner as on rigid circuit boards.
Coverlay is a two part sheet form material comprised of a layer of polyimide and a layer of either epoxy or acrylic based flexible adhesive. The function of the adhesive is both to bond the polyimide to the flex circuit and to encapsulate the circuitry.
Coverlays are then aligned and laminated to the circuit surface under both heat and pressure.
Coverlay is available in variety of combinations of both film and adhesive thicknesses, the most common being a 0.5 mil film with a 0.75 mil adhesive.
The specific film and adhesive combination is typically a function of the following items:
1. Minimum bend requirements. Thinner coverlay may be required to meet tight bend requirements
2. Copper weight of the external layers. Min. of 1 mil of adhesive is required per OZ of copper to ensure complete encapsulation E: 1 OZ Copper = 1 mil adhesive
3. Cost
4. Dielectric Withstanding Voltage (rare)
As a sheet form material the plated through hole (PTH) and surface mount technology (SMT) etc. features are machined into the coverlay using one or more of the following methods: drilling, routing, laser cutting, knife cutting, or punch & die sets. The exact method(s) is dependent upon the feature shape, size, complexity, and quantity of parts being manufactured.
This in-turn imposes additional considerations beyond that of Flex LPI:
Larger min. annular ring to exposed feature requirements. Allows for material and manufacturing tolerances and potential adhesive squeeze out during lamination.
Larger min. web thickness between adjacent features. Prevents easily damaged thin sections/webs and allows for sufficient adhesive to ensure proper lamination and circuit encapsulation
Isolated “island” type features not available. Would fall out of layer after machining.
In flexible PCB designs with higher density SMT & PTH features and if the design allows, it is required to combine multiple feature openings into larger “ganged” openings to accommodate the above items.
Some flex circuit designs due to their density, complexity, and/or component requirements may require the use of both materials. Both are selectively applied to specific areas of the part and provide benefits of both materials within one design. The manufacturing files are created to ensure that the two materials overlap within the rigidized area(s). This results in a complete encapsulation of the circuitry without the creation of mechanical bend stress concentrators in the flexible areas. If this were allowed the reliability of the part may be compromised. While this methodology adds some cost, the design may mandate it and/or the benefits outweigh the added cost.
Two common solder mask design configurations are to either apply flex LPI in rigidized component areas and polyimide coverlay in the flexible sections or to add selective webs of solder mask in ganged coverlay openings.
Designs that have one or more of the following may require the use of coverlay throughout in specific areas or layers:
1. ZIF finger area Polyimide stiffener(s)
2. FR4 component area rigidizing stiffener(s)
3. Silver Ink Shield Layer(s)
4. EMI & RF Shielding film(s)
5. Pressure Sensitive Adhesives (PSA) or 3M adhesive or Tesa adhesive
The materials above may not sufficiently adhere to LPI and pass IPC quality control requirements.