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PCB-June2014

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58 The PCB Magazine • June 2014 by Michael Carano omg electronic cHemicals The Degrees of Nickel Hyper-Corrosion and Mitigation Strategies c o l u m n trouble in your tank Introduction In previous columns, I presented informa- tion on electroless nickel-immersion gold and possible concerns with black pad and brittle fracture. I am a firm believer (as well as a stick- ler) for tight process control. In addition, this also means that understanding the root cause or causes of defects must be pursued with vigor! As a case in point, in this month's column, I will present additional information about nickel hyper-corrosion by further defining the five de- grees of hyper-corrosion. This implies that cer- tain levels of the attack on the nickel are more detrimental than others. It should be noted that for purposes of this writing, I define hyper-cor- rosion as a spike or fissure in the nickel deposit evident after immersion gold plating. Finally the root causes of such attack on the base nickel will be presented along with strategies to miti- gate these effects. The Five Degrees of Hyper-corrosion As the title of this column implies, we have identified five degrees of hyper-corrosion. While somewhat arbitrary, the extent of the corrosion spikes or fissures are responsible for the rating given. We found it necessary to provide this in- put to the industry as we found that, all too of- ten, the OEM sees a tiny fissure in the nickel de- posit and makes the false assumption that the PCB will fail in some way. That is categorically false, and I will explain why. First, however, let's review the definition of each of the degrees of hyper-corrosion: 1. Level 1: Only a few spike-type defects and not on every pad observed. 2. Level 2: A few spike-type defects observed on most pads. 3. Level 3: More than a few spike-type de- fects and some spreader/spike defects on most pads observed. At this activity level, more than 99% of the solder surface has not degraded or shown signs of increased phosphorus and as such should not inhibit intermetallic forma- tion. 4. Level 4: More spreader/spike defects and some area black band defects on most pads ob- served. This activity level may degrade solder joint integrity. 5. Level 5: Mostly large areas of continuous black band on many pads observed. This level of defect activity will affect solder joint integ- rity.

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