What is Adhesiveless?
‘Adhesiveless’ is really a misnomer. The term refers to flexible metal clad laminates
where the metal cladding (usually copper) is bonded to the base dielectric
(usually polyimide film, a.k.a. Kapton) with an adhesive (usually polyimide resin) other
than acrylic. The term may also refer to covercoat materials that use a polyimide
resin as the adhesive instead of acrylic. In both cases ‘acrylicless’ would probably
a more descriptive term.
What are the benefits of Adhesiveless materials?
High Reliability – in combination with no-flow prepregs and bikini covercoat construction, acrylic can be completely eliminated from the rigid sections, greatly reducing the potential for plated-through hole failures.
High Peel Strength/Resistance to Rework – adhesiveless laminates exhibit higher peel strengths and will endure more solder rework cycles than acrylic based flex laminates.
Lower Dk – adhesiveless laminates typically exhibit lower Dk properties than acrylic laminates of equal thickness.
Why is acrylic “bad”?
High Z-axis CTE/Low Tg – leads to plated though hole failures such as cracked barrels and post separation.
Low Peel Strength/Resistance to Rework – laminates that use acrylic as the bonding agent exhibit lower peel strengths and will endure less solder rework cycles than adhesiveless flex laminates.
Laminate voids – acrylic materials are more prone to laminate voids
When is it appropriate to use acrylic adhesives?
Type 1 -3 (all flex) PWB’s – because it remains flexible after curing, acrylic adhesives must be used as the bonding material in all flex constructions.
Type 4 (rigid-flex) PWB’s – Acrylic adhesive should NEVER be used as the bonding material in a rigid-flex construction. However, because it remains flexible after lamination, acrylic is still the most common adhesive layer for covercoats. A bikini construction is highly recommended when using covercoats containing acrylic.
TERMINOLOGY
LAMINATE: Dielectric material pre-clad with a metal foil (usually copper) on one or both sides. Typical PWB dielectrics include epoxy/glass, polyimide glass and polyimide film.
A.K.A .Core; Pre-Clad; C-stage
ADHESIVELESS: Flexible metal clad laminates where the metal cladding (usually copper) is bonded to the base dielectric (usually polyimide film, a.k.a. Kapton) with an adhesive (usually polyimide resin) other than acrylic.
KAPTON: Dupont trade name for unreinforced polyimide film
COVERCOAT: An adhesive (usually acrylic) coated dielectric (usually polyimide film) that serves
A.K.A .Coverlay; Coverfilm
NO FLOW PRE-PREGS: Prepreg that has reduced resin flow characteristics. Used primarily as the bonding agent in rigid-flex PWB’s.
BIKINI COVERCOAT: The technique of terminating covercoat layers just inside the rigid portion of a rigid-flex PWB to reduce or eliminate acrylic adhesive in the rigid sections. Bikini covercoat;
A.K.A.: Window covercoat, Cut back covercoat
CTE: Coefficient of thermal expansion. A measurement of the tendency of a material to expand or contract as a reaction to temperature change. Usually expressed as per degree C or F over a given temperature range.
DK: Dielectric constant.
TG: A.K.A. T sub G‘; Glass transition temperature
PEEL STRENGTH: A measure of the bond strength between the base laminate and metal cladding
POST SEPARATION: A.K.A. Interconnect separation, Hole wall pull away.