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Recently, the diameter of through hole on printed circuit board (PCB) has become smaller with downsizing of electric devices. However, the drilling small diameter through hole includes some problems such as hole position accuracy and hole bending. Entry sheet is used to improve drill centering, drill guidance and reduction of burrs when drilling PCB. Series of drilling tests of PCB are carried out to investigate the effect of entry sheet on hole position accuracy, hole bending and chip-evacuation using three kinds of materials of entry sheet; aluminum, aluminum clad with lubricant and resin. The thrust during drilling entry sheet and the wear of the drill are measured. The hole position accuracy and hole bending examined after the drilling tests. The chips evacuation behavior out of drilled hole with the drill is filmed by a high-speed motion camera and the shape of chips produced is observed by a microscope. The amount of chisel wear and flank wear of the drill is not almost affected by the kinds of sheets. The hole position accuracy of entry side is worse as the maximum thrust during drilling entry sheet is increasing. The rate of increase of the amount of hole bending to the hit number of hole when using high rigid and high lubricant entry sheet decreases rather than that when using low rigid and low lubricant entry sheet. The evacuation out of drilled hole with the drill in case of drilling with aluminum clad with lubricant is better than that in case of drilling with aluminum. 1. 緒 言 パソコンや携帯電話に代表される電子機器の多くにはプリント配線板(Printed circuit board:以下PCB)が内蔵 されており,スルーホールと呼ばれる微小径の導通穴を用いて基板の上下で立体回路が形成されている.近年, 電子機器の軽薄短小化や高性能化に伴いPCB配線パターンも高密度化し,スルーホールも小径化かつ狭ピッチ化 の傾向にある.スルーホールの小径化や狭ピッチ化に伴いスルーホール間距離が減少し絶縁不良を起こしやすく なり穴位置精度向上が重要となる.スルーホール加工は,生産性や加工穴品質の観点から現在でもドリルを用い た加工が中心である.ドリル径が小さくなるに伴いドリルの曲げ剛性は低下し,ドリルの求心性の乏しさも加わ って加工穴は曲がりやすく穴位置精度悪化を生じやすくなること(平中他,1993)や加工初めのドリル食い付時 の位置ずれが穴の曲がりに影響すること(Galloway, 1957)が報告されている.ドリル加工はドリル先端部のチゼル 部は負のすくい角であり切削速度はほとんどゼロである.切削状態は切削というより押しわけ作用に近く,チゼ ル刃の作用は自己求心性に乏しい.ドリル先端が被削材に食い付いた直後のドリルのふれまわり挙動は歩行現象 と呼ばれており,数多くの研究がなされている(鬼鞍他,1988)(加藤他,1990).高い工具剛性のドリルを用いて微 小送りにすることにより,歩行現象を抑制でき位置偏差を小さくできるとしている(Katz and Poustie, 2001).一方,

Effect of entry sheet on hole position accuracy when drilling of printed circuit boards