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Acceptable etchback, being positive or negative,exhibits the evidnece that resin smear has been removed form the innerlayer cooper/drilled hole interface.There is data,pro and against,that positive etchback is more reliable than negative etchback and vice versa.This all depends on what typs of copper plating,copper foil.and weight of the foil being utilized.Excessive positive etchback as well as excessive negative etchback are not Target Condition.Excessive etchback,in both instances,has an adverse effect on the reliability of the plated through hole life.
The etchback process.also known as positive etchback,is utilized to etchback the dielectric material.The evidence of resin material being etched back theorizes that all resin smear has been removed and in addition,a three way than one.The drawbacks of positive etchback are that it creates rough holes which could create plated through hole barrel cracks.Excessive positive etchback also contributss to stresses that might create foil cracks.
The negative etchback process utilizes a smear removal process.In most cases,a similar process as used for the positive etchback,but not as aggressive is used for negative etchback.the smear removal process is followed be etching back the internal copper toil.The benefits for utilizing negative etchback are that the process dose not create a stress point at the internal plane.as done by positive etchback,and it results in a very smooth/uniform copper barrel hole wall.The smooth hole wall and negative etchback are beneficial especially for utilization of copper plating,for hi-reliability long term life applications.The drawback of negative etchback,if excessive,is that it may create innerlayer separation due to entrapped air pockets/contamination.
This section is not intended to peove or disapprove which etchback process is preferred.There are many board manufacturers such as pcbquick that are very successful in utilizing both the positive and negative etchback processes.It is up to the Individual designer/user,depending on the material,copper plating,copper foil and application,to specify what etchback process should be utilized.
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