Table of Contents
Bonding Sheet, Sheet Adhesive, Pure Adhesive, FPC Adhesive?
In flexible circuits, Bonding Sheet, also called Sheet Adhesive, Pure Adhesive, or simply Adhesive, which consists of adhesive coated on release paper. The stand-alone adhesive is primarily used to bond flexible inner layers or rigid cap layers in multilayer lamination. It’s also widely used to bond flexible circuits to rigid-flex boards during the fabrication of rigid-flex PCBs, as well as to bond stiffeners and heat sinks.
What’s the Adhesive Material in Bonding Sheet?
Adhesives in bonding sheet used either to bind the metal foil to the base material to create a laminate, or to bind layers of laminate material together in multi-layer FPCs.
The most common adhesives used in FPC manufacturing include polyesters (PET), polyimides (PI), acrylics and modified epoxies, with each having their advantages and disadvantages, again depending on the application – see Table below. Polyester adhesives are cheap but perform poorly at high temperatures and have relatively low bond strength. They are widely used where the substrate material is polyester, and in applications where there are no extremes of temperature, or forces that will significantly stress the circuit. Although more expensive, polyimide adhesives perform exceptionally well at high temperatures and are popular within the defense, aerospace and satellite sectors, particularly for demanding multi-layer circuit applications.
| Property | Polyimide | Polyester | Acrylic | Modified Epoxy |
| Peel Strength (lb/in.) | 2.0-5.5 | 3-5 | 8-12 | 5-7 |
| After Soldering | no change | no change | 1-1.5x higher | variable |
| Low Temperature Flex | all pass IPC-650 2-8.18@5+ | |||
| Adhesive Flow | <1mil | 10mils | 5mils | 5mils |
| CTE | <50ppm | 100-200 | 350-450 | 100-200 |
| Moisture Absorption | 1-2.5% | 1-2% | 4-6% | 4-5% |
| Chemical Resistance | good | fair | good | fair |
| Dielectric Constant @100 kHz | 3.5-4.5 | 4-4.6 | 3-4 | 4 |
| Electrical Strength (kV/mil) | 2-3 | 1-1.5 | 1-3.2 | 0.5-1 |
Adhesive Thicknesses in Bonding Sheet
- 12μm (0.5mil)
- 25μm (1mil)
- 50μm (2mil)
- 75μm (3mil)
- 100μm (4mil)
In unit conversion of coverlay and bonding sheet, flex PCB manufacturers always use 1mil = 25μm (micron) when choosing adhesive thickness, we should take the trace thickness and remaining copper rate into consideration for ensuring the adhesive in selected bonding sheet can cover and bond the copper features fully.
Packaging and Storage Conditions
Bonding sheet is supplied on 24” (610mm) wide by 250 ft (76m) long rolls, on nominal 3” (76 mm) cores. Narrower widths or cut sheets are also available by special order. FPC bonding sheet should be stored in the original packaging at temperatures of 4 – 29 °C (40 – 85 °F) and below 70% humidity. The product should not be frozen and should be kept dry, clean, and well-protected
Bonding Sheet Performance
Taking DuPont™ Pyralux® FR Sheet Adhesive Properties as example. Please note FR0100 is flame retardant B-staged modified acrylic adhesive.
| Property | FR0100 Typical Value | Test Method |
| Dielectric Constant (Dk) 1MHz 10GHz |
3.6 2.9 |
IPC-TM-650 2.5.5.3 ASTM D2520 |
| Loss Tangent (Df) 1MHz 10GHz |
0.03 0.02 |
IPC-TM-650 2.5.5.3 ASTM D2520 |
| Peel Strength* (Adhesion to Copper) As Received, N/mm (lb/in) As Solder, N/mm (lb/in) |
1.6 (9.0) 1.6 (9.0) |
IPC-TM-650 2.4.9 |
| Adhesive Flow, mm (mil) | 0.10 – 0.15 (4 – 6) | IPC-TM-650 2.3.17.1 |
| Solder Float, 288°C for 10s | Pass | IPC-TM-650 2.4.13 |
| Volume Resistivity (Ω·cm) | >1015 | IPC-TM-650 2.5.17 |
| Surface Resistance (Ω) | >1015 | IPC-TM-650 2.5.17 |