有人說距(ju)離(li)產生美(mei)，這句話不但在(zai)生活中(zhong)適(shi)(shi)用(yong)，在(zai)PCB的(de)(de)鉆(zhan)(zhan)孔(kong)(kong)(kong)(kong)加工中(zhong)也同樣適(shi)(shi)用(yong)。本期(qi)我們就來聊聊PCB的(de)(de)鉆(zhan)(zhan)孔(kong)(kong)(kong)(kong)間距(ju)對PCB可靠性的(de)(de)影響。PCB單面(mian)板(ban)或雙面(mian)板(ban)的(de)(de)制作(zuo)都是(shi)在(zai)下料之(zhi)后(hou)直接(jie)進行非導(dao)通(tong)(tong)孔(kong)(kong)(kong)(kong)或導(dao)通(tong)(tong)孔(kong)(kong)(kong)(kong)的(de)(de)鉆(zhan)(zhan)孔(kong)(kong)(kong)(kong), 多層板(ban)則是(shi)在(zai)完成(cheng)壓板(ban)之(zhi)后(hou)才去(qu)鉆(zhan)(zhan)孔(kong)(kong)(kong)(kong)。傳(chuan)統孔(kong)(kong)(kong)(kong)的(de)(de)種類(lei)除導(dao)通(tong)(tong)與否簡單的(de)(de)區分外,以功能的(de)(de)不同尚(shang)可分:零件(jian)孔(kong)(kong)(kong)(kong),工具孔(kong)(kong)(kong)(kong),通(tong)(tong)孔(kong)(kong)(kong)(kong)(Via),盲孔(kong)(kong)(kong)(kong)(Blind hole),埋孔(kong)(kong)(kong)(kong)(Buried hole)(后(hou)二者亦(yi)為via hole的(de)(de)一種)。在(zai)我的(de)(de)上期(qi)文章(zhang)中(zhong)說到(dao)我們的(de)(de)常(chang)規(gui)的(de)(de)鉆(zhan)(zhan)孔(kong)(kong)(kong)(kong)，是(shi)通(tong)(tong)過鉆(zhan)(zhan)孔(kong)(kong)(kong)(kong)機械加工出(chu)來的(de)(de)。在(zai)實際(ji)加工中(zhong)鉆(zhan)(zhan)孔(kong)(kong)(kong)(kong)之(zhi)間的(de)(de)間距(ju)通(tong)(tong)常(chang)會(hui)影響鉆(zhan)(zhan)機的(de)(de)加工及成(cheng)品(pin)的(de)(de)可靠性。

那(nei)(nei)么孔(kong)(kong)(kong)到(dao)間距(ju)設計太近，有(you)哪些危害，如果(guo)是(shi)相同(tong)網(wang)絡的(de)(de)孔(kong)(kong)(kong)間距(ju)過(guo)近，通(tong)常(chang)會(hui)產生(sheng)破孔(kong)(kong)(kong)、鈹鋒等不良(liang)情況，影響板子的(de)(de)外觀及裝配。如果(guo)是(shi)不同(tong)網(wang)絡的(de)(de)鉆(zhan)(zhan)(zhan)(zhan)孔(kong)(kong)(kong)間距(ju)不足，會(hui)產生(sheng)破孔(kong)(kong)(kong)、鈹鋒、還有(you)芯吸(xi)效(xiao)應(ying)導致(zhi)的(de)(de)短路(lu)等不良(liang)情況。芯吸(xi)效(xiao)應(ying)具(ju)體是(shi)怎么產生(sheng)的(de)(de)呢？鉆(zhan)(zhan)(zhan)(zhan)孔(kong)(kong)(kong)在(zai)機械加(jia)工(gong)(gong)時，由于鉆(zhan)(zhan)(zhan)(zhan)孔(kong)(kong)(kong)的(de)(de)高速運轉和鉆(zhan)(zhan)(zhan)(zhan)頭對周圍板材(cai)產生(sheng)的(de)(de)壓力，會(hui)導致(zhi)板材(cai)內部的(de)(de)玻(bo)纖(xian)松動(dong)(dong)(dong)，鉆(zhan)(zhan)(zhan)(zhan)孔(kong)(kong)(kong)動(dong)(dong)(dong)作進給速度過(guo)大(da)，或鉆(zhan)(zhan)(zhan)(zhan)咀破損不夠鋒利(li)以致(zhi)拉松拉大(da)玻(bo)纖(xian)紗束，或材(cai)料本身纖(xian)維束有(you)缺口，過(guo)于疏松。加(jia)工(gong)(gong)中，過(guo)度除鉆(zhan)(zhan)(zhan)(zhan)污會(hui)使玻(bo)纖(xian)紗束的(de)(de)樹脂被溶掉，那(nei)(nei)么在(zai)后(hou)工(gong)(gong)序(xu)沉銅(tong)電鍍工(gong)(gong)序(xu)就會(hui)有(you)藥(yao)水順著松動(dong)(dong)(dong)區域進行滲(shen)透(tou)，造成短路(lu)的(de)(de)產生(sheng)。IPC-A-600G里面對于芯吸(xi)是(shi)有(you)這樣(yang)規定(ding)的(de)(de)：對于芯吸(xi)作用(yong)(B)沒(mei)有(you)減少(shao)導線(xian)間距(ju)使之(zhi)小(xiao)于采購(gou)文件規定(ding)的(de)(de)最小(xiao)值(zhi) ，芯吸(xi)作用(yong)(A)沒(mei)有(you)超過(guo)80mm[3.150min。鉆(zhan)(zhan)(zhan)(zhan)孔(kong)(kong)(kong)之(zhi)間也同(tong)樣(yang)適用(yong)。

還有一(yi)種不良就(jiu)是(shi)鉆孔設(she)計(ji)時間距過近(jin)會(hui)產生一(yi)個(ge)CAF效應。什(shen)么是(shi)CAF效應呢(ni)？CAF，也(ye)叫離(li)子遷移，全稱為導電性陽極絲(CAF：Conductive Anodic Filamentation)，指的是(shi)PCB內部銅(tong)離(li)子從陽極（高(gao)電壓(ya)）沿著玻纖絲間的微裂通道，向陰極（低電壓(ya)）遷移過程中發生的銅(tong)與(yu)(yu)銅(tong)鹽的漏(lou)電行為。當PCB/PCBA在(zai)高(gao)溫高(gao)濕的環境(jing)下(xia)(xia)帶電工作(zuo)時，兩(liang)絕緣導體間可能會(hui)產生嚴重的沿著樹脂(zhi)或玻纖界面生長的CAF，此現(xian)象將(jiang)最終導致絕緣不良，甚(shen)至短路失(shi)效。 它通常發生在(zai)過孔與(yu)(yu)過孔之間、過孔與(yu)(yu)內外層導線(xian)之間、外層導線(xian)與(yu)(yu)導線(xian)之間，從而(er)造成兩(liang)個(ge)相鄰的導體之間絕緣性能下(xia)(xia)降甚(shen)至造成短路，

CAF形(xing)成(cheng)(cheng)的(de)原因有以(yi)下幾點(dian)：1.常(chang)規FR4 P片(pian)是由(you)玻(bo)(bo)(bo)璃絲編織成(cheng)(cheng)玻(bo)(bo)(bo)璃布，然后涂環氧樹脂(zhi)半固化后制成(cheng)(cheng)；樹脂(zhi)與玻(bo)(bo)(bo)纖(xian)(xian)之間(jian)的(de)附(fu)著力(li)不足，或含浸時膠性不良，兩者之間(jian)容易(yi)出(chu)現(xian)間(jian)隙； 2、鉆孔(kong)等機械(xie)加(jia)工過程中，由(you)于切向(xiang)拉(la)力(li)及(ji)縱向(xiang)沖(chong)擊(ji)力(li)的(de)作用對樹脂(zhi)的(de)粘合力(li)進一步破(po)壞，鉆孔(kong)太過粗糙，造(zao)成(cheng)(cheng)玻(bo)(bo)(bo)纖(xian)(xian)束(shu)被拉(la)松或分離而(er)出(chu)現(xian)間(jian)隙； 3、距離較(jiao)近的(de)兩孔(kong)若(ruo)電(dian)勢不同，則正極部分銅(tong)離子在電(dian)壓驅動下逐漸向(xiang)負極遷移；高(gao)溫高(gao)濕的(de)環境下，使得環氧樹脂(zhi)與玻(bo)(bo)(bo)纖(xian)(xian)之間(jian)的(de)附(fu)著力(li)出(chu)現(xian)劣化，并促成(cheng)(cheng)玻(bo)(bo)(bo)纖(xian)(xian)表面硅烷偶聯劑的(de)化學水解，從而(er)在環氧樹脂(zhi)與玻(bo)(bo)(bo)纖(xian)(xian)的(de)界面上形(xing)成(cheng)(cheng)沿著玻(bo)(bo)(bo)纖(xian)(xian)增強(qiang)材(cai)料形(xing)成(cheng)(cheng)CAF泄露的(de)通路；