1. Field of the Invention
The present invention relates to an apparatus and a method of manufacturing a plug. More particularly, the present invention relates to an apparatus and an improved method of manufacturing a plug in a plated-through hole.
2. Description of the Related Art
Plated-through holes (PTH) are widely used in many applications such as printed circuit boards (PCB), Multi-Chip Module--Laminate (MCM-L), ceramic substrates, thick film devices and integrated circuit packaging carriers such as ball grid array (BGA), chip scale package (CSP) and so on.
Solder masks (S/M), epoxies, or conductive pastes such as silver, copper or silver-copper mixed pastes are commonly used to plug the plated-through holes. Currently, a stencil printer is used to form plugs in the plated-through holes.
During the printing step, a stencil is aligned to a printed circuit board. A large amount of paste is applied on the stencil by an operator. A squeegee is used to plug the plated-through holes with the paste. Plugs are formed in the plated-through holes. However, the plugs usually have a problem of void formation, which voids affect the reliability of the packages and decrease the surface uniformity of the plugs.
Because the printing step is performed in atmospheric conditions, air mixes into the paste during the printing step. Bubbles are formed in the paste. Thus, voids are formed in the plug because of air. Further, the paste itself also causes the voids. The paste usually consists of flux and solvent, and is mixed in atmospheric conditions. Air is easily mixed into the paste, so voids are easily formed in the plug when the paste is used to plug the plated-through holes. Furthermore, the paste is applied on the stencil during the printing step, in a large amount. The solvent mixed in the paste continually evaporates. As a result, the viscosity of the paste continuously changes during the printing step. The surface uniformity of the plugs is worse because the paste viscosity continuously increases during the printing step.