Method of paste printing using stencil and masking layer

DETAILED DESCRIPTION The present invention is directed to a method for depositing a solder paste or other conductive or non-conductive material on a contact pad or other designated region of a substrate.
The inventive method produces a more uniform bump of the deposited material than conventional techniques and prevents accumulation of the material in non-planar regions of the designated region or its surrounding boundaries.
In practicing the invention, a layer of resist is applied over the boundary of a region in which a bump of a deposited material is to be formed.
The resist layer is applied by a screen printing process in which the portion of the screen overlying the region is blocked so that no resist is deposited where the bump is to be formed.
The screen is open in the areas surrounding the blocked portion, permitting resist to be deposited over any non-planar surfaces surrounding the intended bump location.
This provides a substantially planar surface which may be used as a mask for direct paste deposition into the region where the bump is to be formed, or the planar surface may be used as a flat surface upon which a stencil may be supported.
After formation of the bump of conductive or non-conductive paste, the sacrificial resist layer is removed.
If a solder mask is present, the resist layer fills up the non-planar regions in the solder mask layer surrounding the pads, which assists in preventing undesirable accumulation of the later deposited paste in depressions present on the solder mask layer.
The resist layer can also function as a spacer, providing increased volume for paste deposition during stencil printing, and in the case of solder printing, for the solder to reshape during reflow.
The presence of the resist layer also facilitates the process of releasing the stencil from its position.
The inventive method preserves the nature of the pad surface since the resist is not in direct contact with the pads.
This ensures good bonding between the pad and bump material without additional-treatment steps