Pressure Sensitive Adhesives: Guidelines for Flexible Circuit Boards

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Dec 3, 2020|Product Innovations and Design, Technological Advancements and Materials

Flexible circuit boards have revolutionized the electronics industry, making it easier for designers to implement complex circuitry in tight or contoured spaces. These flexible circuit boards are composed of many layers of material, which can be bonded together with various adhesives depending on the application. 

Today, we’re going to take a closer look at the role each PCB adhesive plays and provide a detailed overview of how manufacturers use these adhesives to build PCB components. 

PSA Stiffener Bonding

Frequently, there are components of a PCB that must be rigid so they can function correctly. Circuit board stiffeners provide extra strength and protection while making it easier to attach components or interconnects. 

When stiffeners are added to a flexible circuit board, there are two different ways they can be attached: 

• Heat and Pressure Thermal Bonding: The favored method for bonding stiffeners to circuit boards is with pressure and high heat, using the same adhesive used to attach coverlays. The resulting bond is permanent and exceptionally strong, so it’s preferred for most applications. 
• Pressure bonding: As an alternative, you can make this bond using only a PSA, such as 3M or Tesa Tape. Manufacturers use this method for applications where you can’t use a flexible adhesive to make the necessary bond. 

Pressure Sensitive Adhesives

Pressure-sensitive adhesives (PSA) are available in many types, each of which is well suited to specific circuitry functions. These adhesives are generally quite flexible and easy to work with, making them a superior choice for most applications. These adhesives all possess superior bond strength, and in some cases, the bond gets stronger over time. 

Unlike some adhesives, which present issues when bonded to specific materials, PSAs can be used to adhere flex circuits directly to a substrate or other material, increasing their usefulness even further. 

Read on as we take a closer look at some of the different pressure-sensitive adhesives and how PCB manufacturers use them.

Thermosetting Adhesive Films

These PCB adhesives can be epoxy or acrylic-based, and they’re supplied as a flexible tape. The manufacturer applies light pressure and heat to tack the components in place. You must take special care to ensure enough heat and pressure to make the film tacky enough for the initial adhesion but not so hot that it begins the curing process. 

Once the components are joined, they’re placed in a press or autoclave. From there, you can apply additional heat and pressure to finalize the bond. 

Thermosetting adhesive films are most often used to bond layers of a circuit together. While they can be used to attach stiffeners or supports, it involves an extra lamination step, making them more costly and time-consuming to use than some other adhesives. 

Liquid Adhesives

Liquid epoxy adhesives are available in one and two-part varieties, and they provide exceptional bonding strength. However, the nature of this type of PCB adhesives makes it very difficult to apply the adhesive uniformly, so they’re typically only used for specialized applications, like for pressure alleviation at the edge of a PCB. 

Thermoplastic Adhesive Films

Thermoplastic adhesives are particularly popular as they offer many advantages over other adhesives. These adhesives are fully-polymerized resins, and there’s no treatment necessary to facilitate adhesion. These pressure-sensitive tapes are also easy to rework in the event of an issue with the board, making it easier to troubleshoot and make repairs.

UV Curable Adhesives

Ultraviolet adhesives are a useful option for a variety of applications. These adhesives cure in seconds once they’re exposed to UV light, and you never have to worry about rework resulting from the adhesive migrating to solder pads or holes. 

The “on-demand” curing of these adhesives makes them ideal for bonding components, tacking wires and components, strain relief, and component ruggedization. 

Openings for Stiffeners

PCB stiffeners will have a variety of different openings present, and each serves a specialized function. There are holes for elements, mounting holes, as well as unsupported holes. Let’s take a closer look at the importance of each kind and any special considerations that must be made as they’re built.

Element Holes in Stiffener

Element hole openings support through-hole mounting of other components, such as double in-line package deals. These holes need to be .010-.015” larger than the PCB’s through-holes. Design standards dictate that these through-holes should be .010” larger than the lead. 

These tolerances give you a bit of wiggle room if there is any misregistration or motion between the support and the PCB that could occur during the lamination or bonding processes. Following these standards also ensures that the component lead can access the through-hole. 

Assembly Mounting Holes

For mounting holes in the stiffener, the holes should be slightly smaller or equal to the hole diameter in the PCB itself. Sizing the holes in this manner ensures that the PCB and stiffener can stand up to tension. If the PCB is being mounted directly to a substrate without a stiffener, you can disregard this rule of thumb.

Unsupported Mounting Holes

Holes that aren’t reinforced by a stiffener present a unique challenge because there’s very little to support the board. If the mounting holes are unsupported, as much copper as possible around the hole should be preserved to lend additional strength. Preserving as much copper as possible is practical whenever holes are being added to a PCB or stiffener, but it’s especially crucial with unsupported holes. 

You’ll want to closely consider copper-to-edge tolerances to provide enhanced durability to the PCB. Doing so ensures that the PCB can remain functional even at its minimum dimension. Many companies will punch or pre-drill holes before etching or imaging during circuit board manufacturing. Creating these cut-outs provides a practical clearance between the hole and minimum dimension.

Conclusion 

The versatility of PCBs makes them a wise choice for practically any application. You’ll find a broad range of adhesives, each of which is well suited to certain applications. Depending on the type of circuitry you’re bonding and the characteristics you need it to have, you should have no trouble finding the right PCB adhesive for your needs.