| Drill smear occurs during drilling of multilayer printed circuit boards when some epoxy is transferred from an epoxy layer to a copper layer. The smeared epoxy adheres to the copper layer and insulates the copper plane from the plating causing board malfunction. Multilayer printed circuit board drill smear was investigated experimentally and analytically to develop a method to estimate and reduce the occurrence of smear.;The literature available on drill bit wear, drilling mechanics, drill temperature and board smear was reviewed. The effect of drilling speed, drilling cycle, drilling depth and drilling feed on board smear was studied experimentally. A computer simulation of the drilling process was developed to estimate the drill temperature and board smear under variable drilling speeds, drilling cycles, drilling depths and drilling feeds. The effect of drilling conditions on drill temperature and board smear was studied with the computer simulation to establish a method for reducing smear. Two types of cooling methods were tried experimentally to determine their effectiveness for reducing smear. Several types of drill geometry, drill carbide, drill coating, diamond drill bits and backup materials were evaluated.;Drill smear was related to drill temperature and flank wear. Drill smear was reduced by reducing rotational drilling speed, bottom dwell time, drilling depth, drill torque, number of hits and by increasing the inward and outward feeds. The drill torque was reduced by reducing the width of the drill margins, the flank wear and the drill-board friction. |
