SMT007 Magazine

SMT007-Feb2019

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80 SMT007 MAGAZINE I FEBRUARY 2019 must be avoided because the baking process should not be repeated. Even one exposure to baking at these temperatures induces oxida- tion and intermetallic growth, which reduces the wetting ability of the connection surfaces. Intermetallic thickness has been shown to in- crease by approximately 50% when baking at 125°C for four days. Thicker intermetallic layers can lead to a reduction in solder joint integrity, and in extreme cases, reduce solder- ability. To fight this well-known effect, many sup- pliers of baking ovens provide an additional reduction of oxygen by means of a nitrogen atmosphere or vacuum during the drying pro- cess. Setting the clock back to zero for the component can take in excess of 72 hours, in- evitably bringing along considerable costs for nitrogen. Only a low-rest oxygen content of less than 13 ppm stops the oxidation. Long-term Storage of Obsolete Components Product lifecycles are very short with new models being released sooner than ever be- fore. Many manufacturers in industries includ- ing automotive, aviation, military, and avionics must guarantee the availability of replace- ment parts (including PCBs) for 10 or even 20 years. This demands the advance purchase and extended storage of components and materi- als. Further complicating the problem is that most components cannot be stored for more than a few years without very special handling procedures. Oxidation, intermetallics, and em- brittlement are just a few of the conditions that must be addressed. IPC/JEDEC J-STD-033D addresses a broad range of fundamentals regarding moisture sen- sitive devices and their proper handling, but very long-term storage is not addressed. Also, the obsolescence of components and ever-shortened product life cycles forces man- ufacturers to build up an appropriate stock for further production and spare parts supply. Various ageing processes on electronic com- ponents show that storage in dry packs (also under nitrogen atmosphere) cannot guarantee reliability in the future. Besides temperature, humidity is the stron- gest negative influence on the surface oxida- tion of metals. Due to oxidation, the compo- nents are more difficult to wet and cause more soldering defects, which can lead to failure of the entire assembly. Corrosion Protection Ensures Quality in the Long Term Two conditions must always be met for metal corrosion to occur. An oxidizing agent must be present, which is provided by the 21% oxygen content of the air. The second prerequisite is an aqueous solution that acts as an electrolyte. The electrolyte forms above a humidity of 10% relative humidity in the form of a thin, invis- ible water film on the metal surface [1] . As soon as one of these components of the oxidation reaction is missing, the corrosion stops. Three processes are common for this. Storage in a dry pack and/or under nitrogen removes both the oxidant and electrolyte from the storage atmosphere. Storage in a dry atmo- sphere only removes the electrolyte. But what is the more effective protection against oxida- tion? The dry pack is only suitable for short- to medium-term storage because there is a risk that outgassing will lead to corrosion effects on the contact surfaces. These are usually plasticizers and flame retardants, which can reach high concentrations within the bags. This effect does not occur with nitrogen and dry storage because the storage atmosphere is permanently filtered or replaced. Direct Comparison of Nitrogen and Dry Storage To determine the oxidation rate under the two storage conditions, a long-term test (over nine months) was carried out. At regular in- tervals (every four weeks) the samples were examined for oxidation by EDX analysis. In parallel, the same materials were stored as a reference under ambient air. For each material, there is a diagram show- ing the development of the percentage oxygen peak height compared to the respective main peak as a function of the storage time. This

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