IEC 61249-3-5:1999

Overview

IEC 61249-3-5:1999 specifies requirements for transfer adhesive films used in the fabrication of flexible multilayer printed boards and flex-rigid printed boards. It defines acceptable materials, construction, thickness ranges, mechanical and electrical performance after curing, packaging, marking and designation. The standard is intended for manufacturers, purchasers and quality engineers working with adhesives for flexible printed circuit (flex) assemblies.

Key topics and requirements

• Materials and construction: Adhesive layers are based on B‑staged epoxide, acrylic or polyester (PET) resins. Films are supplied on a removable paper carrier with a polymeric release film that must remain adhered until lamination.
• Thickness and tolerances: Adhesive thickness range is 12.5 μm to 75 μm with a permissible tolerance of ±20%; preferred thicknesses are 25 μm and 50 μm. Rolls are the only form of delivery.
• Electrical performance (after curing): The standard requires minimum electrical performance after damp‑heat conditioning, including minimum surface/volume resistivity criteria, relative permittivity ≤ 4.5, dissipation factor ≤ 0.05, and dielectric strength ≥ 60 kV/mm.
• Bond and mechanical properties: Peel‑strength requirements to copper are specified (different values for acrylic/epoxy and polyester bases) for as‑received, after heat shock and after dry heat. Blistering and peel behavior after thermal stress are addressed.
• Adhesive flow and clearance filling: The document specifies acceptable adhesive flow during lamination and clearance‑filling behavior so that voids are avoided in critical test areas.
• Flammability: Flammability behavior of the adhesive films (and their effect when combined with base materials) must be agreed between supplier and purchaser or demonstrate no increase in base material flammability.
• Packaging and marking: Rolls on cores (ID ≥ 75 mm), standard lengths (50, 75, 100 m), width tolerances, maximum splice count (3) and required label data (manufacturer, adhesive ID, thickness, flammability, batch, dimensions, date).

Applications and users

• PCB manufacturers producing flex and flex‑rigid multilayer assemblies
• Adhesive film suppliers and formulators specifying product performance
• Procurement, inspection, and quality assurance teams requiring acceptance criteria
• Design engineers selecting adhesives for lamination, via encapsulation and flexible interconnect stacks

Related standards

• IEC 61189-2 (test methods for materials)
• IEC 60249 series (base materials for printed circuits)
• IEC 60050 (electrotechnical vocabulary)

Keywords: IEC 61249-3-5, transfer adhesive films, flexible printed boards, flex‑rigid printed boards, adhesive thickness, peel strength, lamination, electrical properties, flammability, adhesive flow.