沉银板银层下界面空洞关键影响因素的研究

Study of the key factors affecting interface caves in IMAg PCBs

沉银板银层下界面处铜层空洞情况影响着其后续焊接可靠性，而目前业界对沉银后银下铜层空洞的成因和影响因素研究较少，本文作者结合沉银制程，对沉银后界面处铜层空洞形成机理及影响因素进行分析，指出沉银过程中原电池效应、反应速率过快、药水残留是导致沉银板银下铜层空洞的可能原因，并设计L8（26）正交试验验证相应因素对沉银后铜层空洞的影响。然后采用激光离子束研磨及场发射电镜观察银下铜层空洞，结果显示：微蚀剂是影响空洞的关键因素，不同微蚀剂微蚀后铜面粗糙程度和表面形态不同，沉银时铜面各处置换反应速率和原电池效应不同，局部反应速率过快处银离子来不及沉积完全和凹陷较大处原电池腐蚀显著，易产生铜层空洞。

Copper caves under silver surface affected the solder joint reliability, however, currently in the industry the relevant research subjects about the forming mechanism and influencing factors of interface copper caves were rare to see. So the possible forming mechanism and influencing factors of copper caves under the surface of silver was proposed by the author of this paper, which pointed out that factors such as the galvanic corrosion, the rate of replacement reaction, the residual chemistries might cause copper caves basically. Then the influence of corresponding factors on copper caves was studied by designing L8（26） DOE experiment. The copper surface morphology was inspected by CP and SEM after stripping silver. As a result, different types of microetching agent, as the key factors affecting copper caves, had different microetching effects （copper surface roughness and morphology） on copper surface, which meant during silver immersion the rate of replacement reaction and galvanic corrosion in copper surface varied correspondingly. Those areas where silver layer not completely immersed on copper surface resulting from the relatively fast reaction rate and where serious galvanic corrosion happened because of deeper depression, had greater chance to form copper caves