| Printed Circuit Board (PCB) is the fundamental component of all the electronic products. PCB bears the responsibility to hold the chips on it and connects to the outer circuits. Since the electronic and information industry becomes more and more important in today’s national economy, so do the PCB industry. Nowadays, from smart mobile phone to laptop, from all kinds of consumption cards to large scale control system, even in the defense industry, PCB is merely everywhere. In the year of2006, China produced PCB of12.1billion US dollars, surpassing Japan and becoming the No.1PCB manufacture country in the world.The manufacture process of PCB is very complicated, which involves more than100procedures and300different kinds of rare materials. There is saying among the manufacture industry that a man capable of managing a PCB factory is capable of managing any kind of factory. With the development of electronic&information industry and IC technology, the requirement placed on PCB is becoming more and more stringent. Getting finer, thinner, lighter and more functional is the future thrust of PCB. Meanwhile, with the implement of RoHS directive, the environmental impact of PCB is getting more and more attentioa In a word, new requirement and new rules bring the70year-old PCB industry new challenge, and by the same time, great opportunity for the China PCB industry.In PCB industry, reliability issue has always been perplexing the manufacturers. Among all the reliability issues, surface finish is no doubt a key topic. Surface finish is the part which contacts the outer environment, bearing the responsibility of protecting inner circuit from external attack, maintaining the solderability and wiring ability and forming electric connection. Since the surface finish is extremely thin and gentle, and required to be thinner in the future, the reliability issue of surface finish is therefore worthy of research.Based on the industry research and surface finish overview of PCB, this thesis focused on the failure analysis of two representing surface finishes of PCB. In the two cases, one sample was treated with ENIG (electroless nickel and immersion gold). It displayed poor wetting performance after hot air leveling. The other one was finished with a relatively novel technology ENEPIG (electro less nickel electro less palladium and immersion gold), which displayed unfavorable wiring ability. In the purpose of defining the failure mechanism, chasing down the root cause and improving the yield, a series of modern devices was applied which include3D-stereo microscopy, SEM, EDS, FT-IR, TGA and metallographicalpreparation devices. Finally, it was found that the inferior immersion gold was the root cause of the first sample. To be more specific, the coarse grain and the crack on it made it possible for the underlying nickel to diffuse upward and cover the outermost surface, preventing the solder from touching the gold layer. The second sample was proved to be failed because of the poor quality of the palladium layer. From the cross section of sample, it can be found there was tumor formed in some part of the palladium layer, a structure which induced stress into the gold layer covered on it during wiring procedure. Once the stress accumulated to certain extent, the gold layer fractured, exposing the underlying palladium to the wiring machine. Since the wiring parameter was set to meet the gold layer, it failed once it encountered palladium layer. In the two cases, corresponding counter strategies were both put forward. |
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