Recently came across a PCB leakage problem, the cause is a low-power products, had the whole uA current level, after aging at room temperature used for some time found their increased power consumption, individual consumption even reached prototype mA level. Carefully ruled out the components question, finally found a 5V voltage point, in a state of dormancy this product which is 0V, but it actually has about 1.8V voltage drop! Patiently PCB cutting line, surprised to find that no two on the PCB vias electrically connected to the test was able to hundreds of ohms resistance between each other. See the original design, two plate, pad pitch via pitch> 6mil, the hole wall spacing> 18mil, this design it is conventional drilling process in the PCB industry. Conductive ink wash away the impurity problem, remove the ink or the surface of the hole, measured through the hole between the resistance still exists! After puzzling for some time, he realized that "CAF effect" leakage problems caused!
What is CAF effect:
The CAF, called the anode conductive filament (CAF: Conductive Anodic Filamentation), refers to the internal PCB copper ions occurs from the anode (high voltage) between the channel along microcracks fiber glass to the cathode (low voltage) migration leakage behavior of copper and copper salts.
Following picture, of the two through holes of the two adjacent longitudinal polished, was placed under an electron microscope enlarged 100 times, the sheet was dark color, the bright part is the gold copper, can be seen between the two through holes, copper point, the presence of copper wire.
CAF generation mechanism:
1. Conventional FR4 PCB sheet material is woven glass filaments into glass cloth and semi-cured epoxy resin in the finished coating. Insufficient between the resin and glass adhesion when glue or impregnating poor prone gap therebetween , coupled with the drilling and other machining process, since the effect of the tangential force and the longitudinal impact force of adhesion of the resin further damage may cause glass fiber bundle is pulled loose or separated a gap. In hot and humid environment, the adhesion between the glass and the epoxy resin occurs more degradation, and contribute to glass surface chemical hydrolysis of a silane coupling agent , is formed along a path for electron migration glass reinforcement;
2. 基于上面的条件，此时距离较近的两个过孔若存在电势差，那么电势较高的阳极上的铜会被氧化成为铜离子，铜离子在电场的作用下向电势较低的阴极迁移，在迁移过程中，与板材的杂质离子或OH-结合，生成了不溶于水的导电盐，并沉积下来，由此两个绝缘孔之间的电气间距急剧下降，严重的甚至可以直接导通形成短路。
阳极：
Cu → Cu2++2e–
H2O → H++OH-
阴极：
2H++2e– → H2
Cu2++2OH– → Cu(OH)2
Cu(OH)2 → CuO+H2O
CuO+H2O → Cu(OH)2 → Cu2++2OH–
Cu2++2e– → Cu
在还没有意识到CAF效应导致的不良之前，我对于相互绝缘的两个过孔间出现阻值的现象感到不可思议，后来经过资料查询，才发现许多同行也为这个问题困扰过，甚至CAF效应已经是PCB业内一个较为热门的可靠性问题之一！
如何防止或减少CAF的发生？
1. 提高板材在抗CAF方面的能力。对于电路板基材工艺，可以从提高材料中离子纯度、使用低吸湿性树脂、玻璃布被树脂充分浸泡结合良好等方面进行提高。对于应用端的工程师，在板材选型时，可以考虑使用耐CAF板材。如下板材供应商生益的板材选型中，就有耐CAF的板材可供选型。
2. PCB的机械钻孔或镭射烧孔会产生高温，超过板材的Tg点时会融溶并形成残渣，这些残渣附着于孔壁会造成镀铜时接触不良，因此在镀铜前必须进行除渣作业，除渣作业中的浸泡处理会对通孔造成一定的侵蚀并可能带来渗铜问题，使后续的铜迁移现象更加容易；
3. PCB设计时，增加通孔间距，另外，由于CAF通道几乎沿着同一玻璃纤维束产生，因此，将相邻的通孔交叉发布有助于降低CAF的发生；
4. 对PCBA进行表面清洁处理，例如使用高压气枪进行灰尘清理，避免杂质残留导致不必要的杂质发生电解。另外，在PCBA表面涂覆三防漆，避免水汽的侵入，特别是在高温高湿的地理环境。
对于这个CAF问题导致的漏电问题，从一开始的困扰到后面的豁然开朗，这其中有两点让我有了更深的体会：
1. 对于一个bug现象的存在，当自己感到不可思议时，也请保持一种客观的态度面对，因为当前现象与已有认知的相去甚远，很可能只是你的知识体系里有盲区而已。碰到CAF现象时，我向PCB产商抛出“相互绝缘的过孔间为什么会有阻值存在”的问题，产商也觉得不可思议，但对方基于“自己做了几十年的板子也没有客户反应过这个问题”的经验性思维，始终没能客观地面对这个问题的存在，在这个前提下，即使有再好的配合力度，所谓的验证也就只能停留在了自证自己材料、制程属于行业规范的层面上，但CAF对于目前PCB行业来说本来就是一个无法100%规避的问题。由此迫使我找第三方的厂家进行剖片分析，显而易见地看到孔间的铜后，该问题才有了不容反驳的定论；
2. 应用端的电子工程师，对电子器件的认知，除了能用、会用的能力之外，还要对其基础材料有所认知。如同每天都在和电容、电阻、电感等器件打交道，但对于这些器件的制作工艺、基础材料组成却是有很多人都不自知的，而这正是这些器件电气特性的根本所在。例如在不同工作频率特性下，一个电容为什么会有阻性、感性、容性的成分所在；又或者一个铝电解电容反接为什么会爆浆，在反接电压未知的情况下，一定会爆浆吗？关于这些问题，后续我会整理出来进行记录、分享~
本文参考文献传送门：
http://www.anytesting.com/news/543325.html
《PCB漏电流失效案例分析》
http://www.edadoc.com/cn/TechnICalArticle/Show.aspx?id=1239
《谈谈钻孔间距对产品可靠性的影响》
https://mp.ofweek.com/ee/a345673623826
《印制电路板CAF失效研究》
http://www.researchmfg.com/2014/12/caf/
《电路板内微短路现象》
https://www.researchmfg.com/2015/08/caf-cause-solution/
《CAF形成的原因及改善对策》
http://www.researchmfg.com/2016/06/3d-x-ray-ct/
《3D X-Ray CT非破坏性立体扫描分析PCBA不良》
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